I have a POOGE modified (a la Walt Jung) Hafler DH-200 that was working fine, but having reviewed the old article from Audio Amateur, I was going over it, refreshing my mind about the layout and checking connections. Based on a comment in the article, and thinking it might be a good idea, I loosened all the screws on the output transistors, dabbed a little contact cleaner on the screw thread only, and tightened them down again.
When I fired it up, only to check it (without any input or speaker loads), I heard an unusual amount of hum, then noticed that very quickly the heat sink on one side started getting very hot. I shut it off immediately, and hooked up inputs and speakers to verify. I ran it for another 15 seconds, with music, and clearly the left channel was now out (right played okay). The left had an intermittent light popping sound. Around this point the protection circuit popped in.
I shut it off and found the heat to be coming apparently from just one upper transistor on one side. Everything else seemed to stay cool (except the heatsink taking up the heat). Maybe the screw tightening thing was a foolish thing to do with 25 year old transistors? Has anyone heard of this before? (Actually, this side always ran a little hotter than the other one.)
I have built kits and changed out caps, but otherwise I am not a very experienced DIY'er. Never did transitors. I have a meter which is supposed to test transistors. I've never used it, but I thought of pulling the two MOSFETS on that side of the heat sink and checking them with the meter. The operation might kill them both, but I presume I would have to replace them as a matched pair.
--1) Okay, so first of all, does this seem like a likely scenario--that I set up a vulnerable 25 year old transistor for failure?
--2) How can I verify that this is the problem?
--3) If it is, advice about a good place in the US to get these MOSFETS for the DH-200. The numbers on these are: 571134, and under that 8044 (I noticed the numbers on all the output transistors are not exactly the same, but each pair are--maybe having to do with matched pairs? Anyway, these are the ones of concern.)
Thanks if you can help.
Michael
When I fired it up, only to check it (without any input or speaker loads), I heard an unusual amount of hum, then noticed that very quickly the heat sink on one side started getting very hot. I shut it off immediately, and hooked up inputs and speakers to verify. I ran it for another 15 seconds, with music, and clearly the left channel was now out (right played okay). The left had an intermittent light popping sound. Around this point the protection circuit popped in.
I shut it off and found the heat to be coming apparently from just one upper transistor on one side. Everything else seemed to stay cool (except the heatsink taking up the heat). Maybe the screw tightening thing was a foolish thing to do with 25 year old transistors? Has anyone heard of this before? (Actually, this side always ran a little hotter than the other one.)
I have built kits and changed out caps, but otherwise I am not a very experienced DIY'er. Never did transitors. I have a meter which is supposed to test transistors. I've never used it, but I thought of pulling the two MOSFETS on that side of the heat sink and checking them with the meter. The operation might kill them both, but I presume I would have to replace them as a matched pair.
--1) Okay, so first of all, does this seem like a likely scenario--that I set up a vulnerable 25 year old transistor for failure?
--2) How can I verify that this is the problem?
--3) If it is, advice about a good place in the US to get these MOSFETS for the DH-200. The numbers on these are: 571134, and under that 8044 (I noticed the numbers on all the output transistors are not exactly the same, but each pair are--maybe having to do with matched pairs? Anyway, these are the ones of concern.)
Thanks if you can help.
Michael
I cannot help much with your amplifier issues, but I will suggest patience. 12 hours is not long enough to wait for a reply before asking again. Remember that this is a holiday weekend, so responses will be slower than usual.
It sounds like you may have knocked something else loose or moved something into contact with something it shouldn't touch. My DH500's gate stopper resistors have long exposed leads. Check in the area of the hot transistor for a lead touching something it is not soldered to. Reheat the solder joints in the area of your efforts in case you cracked some.
It sounds like you may have knocked something else loose or moved something into contact with something it shouldn't touch. My DH500's gate stopper resistors have long exposed leads. Check in the area of the hot transistor for a lead touching something it is not soldered to. Reheat the solder joints in the area of your efforts in case you cracked some.
Unscrewing the devices after 25 years will probably have damaged the micas.
The standard DH-200(both factory and kit model) had 2sets of Hitachi 2SJ49/2SK134 Mosfets with room for 3 sets. 2SJ50/2SK135 can be used, old Siemens BUZ900/901 or the new Magnatec BUZ900/901
New ones cost, scout another DH-200 for parts.
Do a search,must be plenty of info on DH200s here.
The standard DH-200(both factory and kit model) had 2sets of Hitachi 2SJ49/2SK134 Mosfets with room for 3 sets. 2SJ50/2SK135 can be used, old Siemens BUZ900/901 or the new Magnatec BUZ900/901
New ones cost, scout another DH-200 for parts.
Do a search,must be plenty of info on DH200s here.
Michael,
I agree that you first should check the mica insulator under the apparently overheating MOSFET output device.
If you had a Variac and a couple of meters you could test this channel using MOSFETs from the other channel by ramping up the voltage very slowly while monitoring the DC offset and bias current draw. But, if not done properly this could be dangerous.
I use a bona fide Hafler MOSFET checker here and can tell in a matter of seconds if a MOSFET is still good.
There are a variety of spots to check voltages and troubleshooting strategies to follow and I am sure some othes here will chime in with some of them.
Do you have the schematic and a good VOM and perhaps a Variac?
I have some extra MOSFETs here if you need them.
Good luck........
I agree that you first should check the mica insulator under the apparently overheating MOSFET output device.
If you had a Variac and a couple of meters you could test this channel using MOSFETs from the other channel by ramping up the voltage very slowly while monitoring the DC offset and bias current draw. But, if not done properly this could be dangerous.
I use a bona fide Hafler MOSFET checker here and can tell in a matter of seconds if a MOSFET is still good.
There are a variety of spots to check voltages and troubleshooting strategies to follow and I am sure some othes here will chime in with some of them.
Do you have the schematic and a good VOM and perhaps a Variac?
I have some extra MOSFETs here if you need them.
Good luck........
Thanks for the help. If the mica insulator is cracked, will I be able to see it easily by pulling that MOSFET off the heat sink? (I am trying not to do anything rash until I get a good idea of what I am doing.) It gets hot so fast on that one, I think it must be shorting.
I don't have a Variac, but I have thought of rigging a series of cords with various resistors (properly insulated) to achieve a similar effect of cutting the line voltage and raising it in stages, if necessary. I have a new auto-ranging digital multi-meter and an old tiny analog mini-meter. The digital unit also has a transistor test section. I have to know whether the transistor is NPN or PNP type, and which leads are the emitter, base and collector. These are not things I know off hand, or whether this can even test a MOSFET, but I sure enjoy learning about this stuff. (The meter has testing holes for the leads, and says: "LCD display will show the approximate hFE value at the test condition of base current 10uA and Vce 3.2V.")
I built kits years ago as a kid (mostly Dynaco, not Haflers) and have just started to work on these old amps. I own both a modded DH-200, and an unmodded one that I would like to POOGE as well. Since I can see what I am doing by direct comparison, as well as following the instructions, I thought this would be a good way for me to get going again, and then maybe eventually be able to build an amp from a design on this forum. But the transistor blowout threw me for a loop!
At this point I am concerned about turning the amp on again without removing that MOSFET. I've considered changing out the complete heat sink with one from the other amp--but of course that's a risk without checking first.
There is one other thing I did that I am wondering about, when I checked over the amp. I replaced the old twin-lead line cord with a grounded one, and I attached the green ground lead of the new line cord to the amp chassis. This was probably coincidental, and I doubted this would cause a problem on one channel (and I actually disconnected it again before turning the amp on the second time--MOSFET still got hot).
Again, I really appreciate the help, and am particularly glad to hear from Dick West, who I see from reading the forums, has a lot of experience with these old Haflers. Yes, I might need a couple of those MOSFETS! I do have the original kit building instructions printed out, which has a schematic, such as it is. But I am kind of in second grade still on reading schematics--willing to learn, though. I also have a copy of the Audio Amateur POOGE instructions.
Given my limitations, as well as what I have on hand, what would you suggest?
I don't have a Variac, but I have thought of rigging a series of cords with various resistors (properly insulated) to achieve a similar effect of cutting the line voltage and raising it in stages, if necessary. I have a new auto-ranging digital multi-meter and an old tiny analog mini-meter. The digital unit also has a transistor test section. I have to know whether the transistor is NPN or PNP type, and which leads are the emitter, base and collector. These are not things I know off hand, or whether this can even test a MOSFET, but I sure enjoy learning about this stuff. (The meter has testing holes for the leads, and says: "LCD display will show the approximate hFE value at the test condition of base current 10uA and Vce 3.2V.")
I built kits years ago as a kid (mostly Dynaco, not Haflers) and have just started to work on these old amps. I own both a modded DH-200, and an unmodded one that I would like to POOGE as well. Since I can see what I am doing by direct comparison, as well as following the instructions, I thought this would be a good way for me to get going again, and then maybe eventually be able to build an amp from a design on this forum. But the transistor blowout threw me for a loop!
At this point I am concerned about turning the amp on again without removing that MOSFET. I've considered changing out the complete heat sink with one from the other amp--but of course that's a risk without checking first.
There is one other thing I did that I am wondering about, when I checked over the amp. I replaced the old twin-lead line cord with a grounded one, and I attached the green ground lead of the new line cord to the amp chassis. This was probably coincidental, and I doubted this would cause a problem on one channel (and I actually disconnected it again before turning the amp on the second time--MOSFET still got hot).
Again, I really appreciate the help, and am particularly glad to hear from Dick West, who I see from reading the forums, has a lot of experience with these old Haflers. Yes, I might need a couple of those MOSFETS! I do have the original kit building instructions printed out, which has a schematic, such as it is. But I am kind of in second grade still on reading schematics--willing to learn, though. I also have a copy of the Audio Amateur POOGE instructions.
Given my limitations, as well as what I have on hand, what would you suggest?
There are so many possible variables it is difficult to give too much advice. But, here is a list of steps I might follow.
1. Get a Variac. You can pick up one with a 5 amp capacity off an eBay auction for around $50. It is an excellent investment for a POOGER.
2. Study the schematic for the DH-200 and the DH-220. Learn the difference between NPN and PNP transistors. Also, learn the difference between N-channel and P-channel FETs (output MOSFETs).
3. Study the voltage chart in the DH-220 manual and get a feel for which points should have the same voltages in the DH-200 circuit.
4. MOSFETs are very rugged but I believe the 2SJ49s are more prone to failure than the 2SK134 devices. Did you perchance switch them around when you were messing with that channel?
5. Capacitor failure is frequent given the age of your amp. The two filter caps on the PCB probably should be replaced or at least the power supply rails voltages checked carefully.
6. You can measure many voltages on the bad channel AFTER removing the MOSFETs. This way there is less chance of a problem creating more problems. This is a circuit somewhat similar to an op-amp and feedback from the MOSFETs is required to keep some voltages settled and in spec, but at least you can get a ball park feel for how the circuit is operating.
7. You can check the viability of the suspect mica insulator with your VOM set to measure continuity. Obviously, there should be no continuity between the MOSFET case and the underlying heat sink.
8. What is the serial number of your amp? Which version of PCB does it have? Personally, I would not mess with the older phenolic PCBs. Their traces were glued on so too much soldering heat will dislodge them. Also, the driver transistors on the PCB (the ones with the small metal tops) may have cold soldered joints because the phenolic does not allow a hot enough solder bath in manufacture. If you have phenolic you should reflow all solder joints.
9. There are many voltage points to check but you can't do this if a MOSFET is overheating so you have a catch 22 except to remove the MOSFETs and do some sleuthing with the PCB by itself, with a Variac to carefully control voltage applied.
Good luck. . . BTW, I gave up POOGing 15 years ago. My favorite amp here has a pair of PCBs from Musical Concepts applied to a DH-200 chassis and transformer. The more recent PCB upgrades from Musical Concepts make for a stellar performer far superior to the the DH-200 or DH-220! If you have good soldering skills you can install the kit from MC.
My $0.02
1. Get a Variac. You can pick up one with a 5 amp capacity off an eBay auction for around $50. It is an excellent investment for a POOGER.
2. Study the schematic for the DH-200 and the DH-220. Learn the difference between NPN and PNP transistors. Also, learn the difference between N-channel and P-channel FETs (output MOSFETs).
3. Study the voltage chart in the DH-220 manual and get a feel for which points should have the same voltages in the DH-200 circuit.
4. MOSFETs are very rugged but I believe the 2SJ49s are more prone to failure than the 2SK134 devices. Did you perchance switch them around when you were messing with that channel?
5. Capacitor failure is frequent given the age of your amp. The two filter caps on the PCB probably should be replaced or at least the power supply rails voltages checked carefully.
6. You can measure many voltages on the bad channel AFTER removing the MOSFETs. This way there is less chance of a problem creating more problems. This is a circuit somewhat similar to an op-amp and feedback from the MOSFETs is required to keep some voltages settled and in spec, but at least you can get a ball park feel for how the circuit is operating.
7. You can check the viability of the suspect mica insulator with your VOM set to measure continuity. Obviously, there should be no continuity between the MOSFET case and the underlying heat sink.
8. What is the serial number of your amp? Which version of PCB does it have? Personally, I would not mess with the older phenolic PCBs. Their traces were glued on so too much soldering heat will dislodge them. Also, the driver transistors on the PCB (the ones with the small metal tops) may have cold soldered joints because the phenolic does not allow a hot enough solder bath in manufacture. If you have phenolic you should reflow all solder joints.
9. There are many voltage points to check but you can't do this if a MOSFET is overheating so you have a catch 22 except to remove the MOSFETs and do some sleuthing with the PCB by itself, with a Variac to carefully control voltage applied.
Good luck. . . BTW, I gave up POOGing 15 years ago. My favorite amp here has a pair of PCBs from Musical Concepts applied to a DH-200 chassis and transformer. The more recent PCB upgrades from Musical Concepts make for a stellar performer far superior to the the DH-200 or DH-220! If you have good soldering skills you can install the kit from MC.
My $0.02
Hi MichaelP,
A couple things may have happened here. The hot mosfet was probably okay, the cold one(s) on the other polarity would be the suspects here.
You may have over tightened the device and cracked the die inside and / or cut the insulator causing a short. It's possible the insulator for the screw is missing or damaged, so the screw may short against the heatsink.
The capacitors are suspect as mentioned.
Whenever you disturb a power transistor, it's a good idea to clean everything up and use fresh grease and insulators. Take care not to over tighten the parts when you reinstall them.
-Chris
A couple things may have happened here. The hot mosfet was probably okay, the cold one(s) on the other polarity would be the suspects here.
You may have over tightened the device and cracked the die inside and / or cut the insulator causing a short. It's possible the insulator for the screw is missing or damaged, so the screw may short against the heatsink.
The capacitors are suspect as mentioned.
Whenever you disturb a power transistor, it's a good idea to clean everything up and use fresh grease and insulators. Take care not to over tighten the parts when you reinstall them.
-Chris
Solution?
Okay, I think I've worked out what is happening. Of all the excellent suggestions above, the one that turned out to be most immediately useful is also, I suppose, the simplest--checking continuity.
When I put an ohmmeter on the heat sink or ground of the unaffected side of the amp (contacting the heat sink itself is not easy, I had to find a place where the paint is scraped off to get a reading, and even scrape a little more off--so what was ultimately easier was finding any good ground point in the amp),... when I check the good side by getting a solid contact on the MOSFET case and the other lead to a clear ground contact or solidly on the heatsink, on each of the 4 MOSFETS I get a reading of around 800-1000 ohms, after a few seconds. This actually seems like a gradually increasing reading, which may be an artifact of my meter, or more likely an effect of measuring resistance which builds up on the transistor from initially around 700 ohms to 1K ohm or more. I found the same readings on all the MOSFETS on the right side of my POOGED amp, as well as roughly the same on all the MOSFETS on both sides of my original unmodded DH-200.
When I put the meter on the problematic left side, I got almost no resistance. Maybe .1 or .2 ohm, essentially a short. I got the same reading on all 4 MOSFETS, not just the "hot" one--which threw me at first. Slowly, I backed off each of the screws on the upper MOSFET that had gotten hot so quickly. The top screw had no effect, and I snugged it up again. I backed off the lower screw, and everything changed immediately. This took only about 1/4 turn of the screw. Once the pressure was off, all 4 MOSFETS gave a "normal" reading. TIghtening or loosening the screw slightly gave a very clear result on all 4 of the cases, which obviously have continuity between them.
Crossing many fingers, I plugged the amp in and fired it up. No smoke, no heat. I wired the speakers and preamp, and played some music. Eureka! She seems none the worse for wear. Well, possibly minimally worse on the right channel, possibly the softest of humming on the right side now, which in any case is unnoticable and totally swamped by even the softest music. Actually, that might only be my apprehension. The faint hum is probably no different than before. I've been told these MOSFETS are sturdy, but with all this shorting and the high temps, I would have thought they would have been toast.
By the way, the modded DH-200 sounds significantly better than the non-modded one, and is much quieter too, even though the mod was done many years ago.
Something I haven't mentioned yet that added to the evidence is that I realized upon closer observation that this MOSFET has had a problem before with the previous owner, and a fix was applied. The insulator (inside the heat sink) is clearly not original, and some kind of makeshift one has been fabricated. It is oversized and obviously does not do the job exactly as it should. Tightening the lower screw down to just after it starts to bite solidly results in a short. So the question arises, do I need a new insulator, or even a new MOSFET?
As I see it, I have three choices: 1) I could do a MOSFET replacement. 2) I could see if it is possible to replace only the insulating materials on this MOSFET. I haven't tried removing it yet, which will require unsoldering leads, so I don't entirely understand yet where the short is made. Clearly, these are designed to have thermal contact to the case, but electrical isolation--I don't know how that is accomplished because the screw goes through part of the transistor case, then through the heatsink and the insulator.
Lastly, I could 3) just leave things as they are and remind myself NEVER to tighten that screw again. With luck, I might get decent service from this amp for years more without fooling with it any more. It is actually pretty musical.
Dick, the PCB is one of the older types, not the newer green one I note you mentioned in one of your posts. These PCBs are pretty beat up already and won't take much more working on them. The transformer has beginning signs of heat breakdown too, so possibly I should use this a little longer until it fails or I'm feeling a need--and the capability--to do something totally new with the case. The serial # is 3111035.
The above is more than enough for now, though I still have some other thoughts rumbling around. For one thing, I keep wondering how this amp's sound compares to the mod with the Musical Concepts boards, and other options I've heard mentioned on these pages. And I need to tackle my other DH-200 next. Does someone have an extra insulator lying around?
Again, many thanks!
Okay, I think I've worked out what is happening. Of all the excellent suggestions above, the one that turned out to be most immediately useful is also, I suppose, the simplest--checking continuity.
When I put an ohmmeter on the heat sink or ground of the unaffected side of the amp (contacting the heat sink itself is not easy, I had to find a place where the paint is scraped off to get a reading, and even scrape a little more off--so what was ultimately easier was finding any good ground point in the amp),... when I check the good side by getting a solid contact on the MOSFET case and the other lead to a clear ground contact or solidly on the heatsink, on each of the 4 MOSFETS I get a reading of around 800-1000 ohms, after a few seconds. This actually seems like a gradually increasing reading, which may be an artifact of my meter, or more likely an effect of measuring resistance which builds up on the transistor from initially around 700 ohms to 1K ohm or more. I found the same readings on all the MOSFETS on the right side of my POOGED amp, as well as roughly the same on all the MOSFETS on both sides of my original unmodded DH-200.
When I put the meter on the problematic left side, I got almost no resistance. Maybe .1 or .2 ohm, essentially a short. I got the same reading on all 4 MOSFETS, not just the "hot" one--which threw me at first. Slowly, I backed off each of the screws on the upper MOSFET that had gotten hot so quickly. The top screw had no effect, and I snugged it up again. I backed off the lower screw, and everything changed immediately. This took only about 1/4 turn of the screw. Once the pressure was off, all 4 MOSFETS gave a "normal" reading. TIghtening or loosening the screw slightly gave a very clear result on all 4 of the cases, which obviously have continuity between them.
Crossing many fingers, I plugged the amp in and fired it up. No smoke, no heat. I wired the speakers and preamp, and played some music. Eureka! She seems none the worse for wear. Well, possibly minimally worse on the right channel, possibly the softest of humming on the right side now, which in any case is unnoticable and totally swamped by even the softest music. Actually, that might only be my apprehension. The faint hum is probably no different than before. I've been told these MOSFETS are sturdy, but with all this shorting and the high temps, I would have thought they would have been toast.
By the way, the modded DH-200 sounds significantly better than the non-modded one, and is much quieter too, even though the mod was done many years ago.
Something I haven't mentioned yet that added to the evidence is that I realized upon closer observation that this MOSFET has had a problem before with the previous owner, and a fix was applied. The insulator (inside the heat sink) is clearly not original, and some kind of makeshift one has been fabricated. It is oversized and obviously does not do the job exactly as it should. Tightening the lower screw down to just after it starts to bite solidly results in a short. So the question arises, do I need a new insulator, or even a new MOSFET?
As I see it, I have three choices: 1) I could do a MOSFET replacement. 2) I could see if it is possible to replace only the insulating materials on this MOSFET. I haven't tried removing it yet, which will require unsoldering leads, so I don't entirely understand yet where the short is made. Clearly, these are designed to have thermal contact to the case, but electrical isolation--I don't know how that is accomplished because the screw goes through part of the transistor case, then through the heatsink and the insulator.
Lastly, I could 3) just leave things as they are and remind myself NEVER to tighten that screw again. With luck, I might get decent service from this amp for years more without fooling with it any more. It is actually pretty musical.
Dick, the PCB is one of the older types, not the newer green one I note you mentioned in one of your posts. These PCBs are pretty beat up already and won't take much more working on them. The transformer has beginning signs of heat breakdown too, so possibly I should use this a little longer until it fails or I'm feeling a need--and the capability--to do something totally new with the case. The serial # is 3111035.
The above is more than enough for now, though I still have some other thoughts rumbling around. For one thing, I keep wondering how this amp's sound compares to the mod with the Musical Concepts boards, and other options I've heard mentioned on these pages. And I need to tackle my other DH-200 next. Does someone have an extra insulator lying around?
Again, many thanks!
With the mileage and obvious bad previous maintenance of this Hafler i'd remove all of the TO3 devices and reinstall them with fresh insulators and thermal paste. If you take the mosfets out you also have the chance to check if they are all OK, there are plenty posts here on diyA on how to inspect a mosfet with an MM.
Time and heat age mica insulators and dries out thermal paste, thermal transfer between all of the Hitachi mosfets and the heatsink is likely to be below the optimum. Solder joints also degrade over time, reflowing solder on all joints is a treatment for +20 year olds that would prevent trouble in that department for the next 20. A DH-200 is fairly simple and modular, so it is not a huge undertaking.
TO3 micas insulators are dirt cheap but i gathered mica quality in the US is poor compared to the grade that is common overhere.
In contrast, aluminium oxide insulators are much cheaper and of higher quality in America. The Avid Thermalloy Al-Ox. insulators i got thanks to the highly appreciated help of Mark Gulbrandsen do a great job on my Krell KSA output stages.
I've been using that kind of insulators for Hitachi mosfets output devices also in the 1980s. The Hitachi devices are an extinct breed, i'd give them the best possible care iiwy.
Mosfets do not suffer from secondary thermal breakdown, the resistance between drain and source becomes high when the device is overloaded. That limits dissipation and protects the mosfet from killing itself if the current short situation doesn't last too long. Once cooled down the drain to source resistance goes back to normal and they are ready for showtime again. The case of a TO3 is the source connection of the mosfet. My experience with Hitachi power mosfets is that they are indeed incredibly stirdy devices, no idea what a short does to the remaining life expectancy of those transistors. Everybody was fooling around with Hitachi based DIY stuff overhere in the 80s, i've salvaged many Hitachi TO3 mosfets from horrible diy projects. With exception of a few that had really gotten the trash can treatment they worked like new in a bunch of my fumblings.
Time and heat age mica insulators and dries out thermal paste, thermal transfer between all of the Hitachi mosfets and the heatsink is likely to be below the optimum. Solder joints also degrade over time, reflowing solder on all joints is a treatment for +20 year olds that would prevent trouble in that department for the next 20. A DH-200 is fairly simple and modular, so it is not a huge undertaking.
TO3 micas insulators are dirt cheap but i gathered mica quality in the US is poor compared to the grade that is common overhere.
In contrast, aluminium oxide insulators are much cheaper and of higher quality in America. The Avid Thermalloy Al-Ox. insulators i got thanks to the highly appreciated help of Mark Gulbrandsen do a great job on my Krell KSA output stages.
I've been using that kind of insulators for Hitachi mosfets output devices also in the 1980s. The Hitachi devices are an extinct breed, i'd give them the best possible care iiwy.
Mosfets do not suffer from secondary thermal breakdown, the resistance between drain and source becomes high when the device is overloaded. That limits dissipation and protects the mosfet from killing itself if the current short situation doesn't last too long. Once cooled down the drain to source resistance goes back to normal and they are ready for showtime again. The case of a TO3 is the source connection of the mosfet. My experience with Hitachi power mosfets is that they are indeed incredibly stirdy devices, no idea what a short does to the remaining life expectancy of those transistors. Everybody was fooling around with Hitachi based DIY stuff overhere in the 80s, i've salvaged many Hitachi TO3 mosfets from horrible diy projects. With exception of a few that had really gotten the trash can treatment they worked like new in a bunch of my fumblings.
Hi Michael,
I agree with what Jacco has said. I will add that you need to disconnect this unit now. Do not run it at all until you have remounted the transistors. That one you loosened will overheat if you don't do this.
You must clean all the old grease and debris from the heatsink and transistors before attempting to remount them. The transistors can be static sensitive, so handle them as litle as possible, never by the two leads.
Keystone is one manufacturer that sells TO-3 mica insulators (100 for about $4 USD). I think Radio Shack still sells them in single kits, MCM, Mouser, Newark and DigiKey should as well. New grease should be available from the same sources.
-Chris
I agree with what Jacco has said. I will add that you need to disconnect this unit now. Do not run it at all until you have remounted the transistors. That one you loosened will overheat if you don't do this.
You must clean all the old grease and debris from the heatsink and transistors before attempting to remount them. The transistors can be static sensitive, so handle them as litle as possible, never by the two leads.
Keystone is one manufacturer that sells TO-3 mica insulators (100 for about $4 USD). I think Radio Shack still sells them in single kits, MCM, Mouser, Newark and DigiKey should as well. New grease should be available from the same sources.
-Chris
Michael,
As I read your last message I think you are talking about the socket that mounts on the back side of the heat sink. You can loosen the screws that hold the heat sink to the chassis and also remove the 3 small screws that hold the PCB to the heat sink. Then, you easily can get at the back side to unsolder and install a new, proper, socket for that MOSFET. The other advice about cleaning and regreasing everything and install new mica insulators is spot on!
I did many, many mods to my previous 2 DH-200s (back in the 80s) and still have the original TAA article describing the POOGE process. Later, when I graduated to the Musical Concepts recent PCBs, I realized how much time had been wasted doing DH-200 mods. The MC boards really make a wonderful difference in sound all up and down the musical scale.
Your PCBs (phenolic) do not have through plated holes and their traces are very fragile to work with for modding.
BTW, your amp was sent from the factory the 11th week of 1981 and it was a kit, not factory wired.
If you listen to a lot of rock and heavy metal, mod the DH-200. If you like string quartets and piano concerti and Diana Krall, get the MC PCBs and install them. And, be sure to get the pair of power supply filter caps from MC. You won't be sorry.
Once again, my $0.02
As I read your last message I think you are talking about the socket that mounts on the back side of the heat sink. You can loosen the screws that hold the heat sink to the chassis and also remove the 3 small screws that hold the PCB to the heat sink. Then, you easily can get at the back side to unsolder and install a new, proper, socket for that MOSFET. The other advice about cleaning and regreasing everything and install new mica insulators is spot on!
I did many, many mods to my previous 2 DH-200s (back in the 80s) and still have the original TAA article describing the POOGE process. Later, when I graduated to the Musical Concepts recent PCBs, I realized how much time had been wasted doing DH-200 mods. The MC boards really make a wonderful difference in sound all up and down the musical scale.
Your PCBs (phenolic) do not have through plated holes and their traces are very fragile to work with for modding.
BTW, your amp was sent from the factory the 11th week of 1981 and it was a kit, not factory wired.
If you listen to a lot of rock and heavy metal, mod the DH-200. If you like string quartets and piano concerti and Diana Krall, get the MC PCBs and install them. And, be sure to get the pair of power supply filter caps from MC. You won't be sorry.
Once again, my $0.02
I will take Jacco's advice to heart. Actually, I was about to order some stuff from Parts Express. They have an item listed as:
TO-3 SILICON INSULATORS 10/BAG for about $2.
Does that sound correct? Two bags is enough that I could repack all the MOSFETS on both my DH-200's. I also need to check if this is the thermal grease I want:
NTE 303 THERMAL COMPOUND $2.33.
(These are some of those items for which PE has almost no info listed. I also have on hand some thermal grease for CPU processors--I suppose I could use that.)
Do I just coat the hole and transistor bottom with this after cleaning off the old grease, and then replace the transistor? Just so I don't get things mixed up (I have not yet pulled a transistor on these amps), are the MOSFETS on one side of the heat sink the same, and the two on the other side the same--but the two sides different from each other? (NPN vs. PNP??) I will try to label them and put them back where they were--but just in case! I also will try to find the forum posts on testing the MOSFETS with a MM.
Then there's that socket. I am really not sure if I am finding the right thing. I will check tomorrow with PE if they think this is the correct item. --Does it seem to you like this could be what I am looking for?:
NTE 209 3 TO SOCKET $2.22 EA.
Dick, I am the latter type of listener, definitely not rock but classical and yes even Diana Krall. So I see now I need to consider carefully what I will do with my second DH-200. The TAA mod (I have the article too), or buy the MC PCB's. Mmmmmmm.
TO-3 SILICON INSULATORS 10/BAG for about $2.
Does that sound correct? Two bags is enough that I could repack all the MOSFETS on both my DH-200's. I also need to check if this is the thermal grease I want:
NTE 303 THERMAL COMPOUND $2.33.
(These are some of those items for which PE has almost no info listed. I also have on hand some thermal grease for CPU processors--I suppose I could use that.)
Do I just coat the hole and transistor bottom with this after cleaning off the old grease, and then replace the transistor? Just so I don't get things mixed up (I have not yet pulled a transistor on these amps), are the MOSFETS on one side of the heat sink the same, and the two on the other side the same--but the two sides different from each other? (NPN vs. PNP??) I will try to label them and put them back where they were--but just in case! I also will try to find the forum posts on testing the MOSFETS with a MM.
Then there's that socket. I am really not sure if I am finding the right thing. I will check tomorrow with PE if they think this is the correct item. --Does it seem to you like this could be what I am looking for?:
NTE 209 3 TO SOCKET $2.22 EA.
Dick, I am the latter type of listener, definitely not rock but classical and yes even Diana Krall. So I see now I need to consider carefully what I will do with my second DH-200. The TAA mod (I have the article too), or buy the MC PCB's. Mmmmmmm.
Silicon insulators have to be torqued very evenly and not overtorqued, they're very fragile imo.
Thermal and isolation properties of silicon pads have improved much in 15 years. Thermal resistance went down from 0.5 C/W to 0.15 C/W or less( <1.45 C/mK), electrical isolation up from ~2.7kV to above 6kV. Thickness of silpads went down from 0.25mm to less than 0.20mm, downside is that they damage even easier so they'll have to be handled gently.(bare in mind that i'm a klutz and a brute, i'm not allowed near any wine glass)
For the TO-3 socket you'd have to check what boss height you want.
I think you can get TO-3 sockets cheaper than PE lists,
here's a file with types and pictures=>
www.keyelco.com/pdfs/prod22-trans-sockets.pdf
Thermal and isolation properties of silicon pads have improved much in 15 years. Thermal resistance went down from 0.5 C/W to 0.15 C/W or less( <1.45 C/mK), electrical isolation up from ~2.7kV to above 6kV. Thickness of silpads went down from 0.25mm to less than 0.20mm, downside is that they damage even easier so they'll have to be handled gently.(bare in mind that i'm a klutz and a brute, i'm not allowed near any wine glass)
For the TO-3 socket you'd have to check what boss height you want.
I think you can get TO-3 sockets cheaper than PE lists,
here's a file with types and pictures=>
www.keyelco.com/pdfs/prod22-trans-sockets.pdf
Hi Michael,
Jacco is dead right. He has pointed you to the Keystone website. You can find a dealer list there. I still use mica and grease.
You need to clean the mounting surfaces with a solvent, like lacquer thinner (cheap at a pain store). Get rid of all residue. The grease would be applied evenly to the transistor, stick the insulator on and push out the air bubbles, apply grease evenly to the insulator. Then you mount the transistor. I find an artist's #2 paintbrush works very well for applying grease.
Jacco's tightening advise is valid no matter what you use. Do not overtighten as most people seem to do.
For the socket, don't sweat the boss height. Get long ones. If they are too long, use a razor blade to trim to height. Mount the socket and run the blade along the surface so it trims to the surface. That's the safest way. Even then, all you really need is the screw to be centered. So a short one would locate things okay as well. Minor detail not worth worrying about.
-Chris
Jacco is dead right. He has pointed you to the Keystone website. You can find a dealer list there. I still use mica and grease.
You need to clean the mounting surfaces with a solvent, like lacquer thinner (cheap at a pain store). Get rid of all residue. The grease would be applied evenly to the transistor, stick the insulator on and push out the air bubbles, apply grease evenly to the insulator. Then you mount the transistor. I find an artist's #2 paintbrush works very well for applying grease.
Jacco's tightening advise is valid no matter what you use. Do not overtighten as most people seem to do.
For the socket, don't sweat the boss height. Get long ones. If they are too long, use a razor blade to trim to height. Mount the socket and run the blade along the surface so it trims to the surface. That's the safest way. Even then, all you really need is the screw to be centered. So a short one would locate things okay as well. Minor detail not worth worrying about.
-Chris
Okay, it looks like my best bet for small quantities is to order this from Mouser. They have the Keystone TO3 mica insulators #534-4636. I'll get 20 for $2.40, and the Keystone socket #534-4601 @ $1.56. The boss height on these is .09, which is the largest on this series. Well, the heatsink is almost 1/4" thick, so no way is that going through the heatsink all the way--I gather it is just there to position the socket and the screw, right? I've got about 3-4 small tubes of cpu heatsink grease around, which should be plenty to do 8 or even 16 transistors (both Haflers), on both sides of the mica. I've got both laquer thinner and acetone for cleaning the heatsink, as necessary. And what the heck, I'm finally going to get a static control wrist strap, after all these years (even though I've never fried anything yet by handling it).
Jacco, is mica a better insulator thermally and electrically than silicon? BTW, I'll watch out for wine glasses. No drinking and driving or remounting transistors.
BTW, I will be doing some rewiring on the other DH-200. In the old POOGE articles, they used fancy multiple-small wires, a little like Litz wiring (I have this on my modded one), but I've noticed most people just use primary wire inside an amp nowadays. Do you guys have any opinions on the wiring controversey? I was thinking of getting fancy and using a few pairs of CAT5 wire pairs in a braid on the leads to the input and output terminals from the PC board, but just use regular 16 gauge primary wire to the power supply. (The original was 18 gauge wire all around.)
Jacco, is mica a better insulator thermally and electrically than silicon? BTW, I'll watch out for wine glasses. No drinking and driving or remounting transistors.
BTW, I will be doing some rewiring on the other DH-200. In the old POOGE articles, they used fancy multiple-small wires, a little like Litz wiring (I have this on my modded one), but I've noticed most people just use primary wire inside an amp nowadays. Do you guys have any opinions on the wiring controversey? I was thinking of getting fancy and using a few pairs of CAT5 wire pairs in a braid on the leads to the input and output terminals from the PC board, but just use regular 16 gauge primary wire to the power supply. (The original was 18 gauge wire all around.)
Hi Michael,
For now, just fix it. Don't use CAT-5 wire for your high current leads. There is no reason why it would be better. If you are truly worried, get some fine strand wire in a slightly thicker size and use that. Much better and you still may not hear a difference.
Well, you might if you listen with a bunch of friends. Wine or beer would help bring out the finer detail.
-Chris
For now, just fix it. Don't use CAT-5 wire for your high current leads. There is no reason why it would be better. If you are truly worried, get some fine strand wire in a slightly thicker size and use that. Much better and you still may not hear a difference.
Well, you might if you listen with a bunch of friends. Wine or beer would help bring out the finer detail.
-Chris
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