Went ahead and did these items:
a) remove R34/R35 (3k9 2W resistors found slightly above and between the two 15V regulators)
b) remove Q4/5 (MJE340/350 with heat sinks)
c) remove D10/D13 (1N4007 found next to Q4 and Q5, respectively)
Q4 and Q5 were dead^3... Q4 (IIRC) was cracked also.
I still haven't replaced the zeners yet. I have a bunch of them, so I may select some (matched).
Also, I mentioned I do not have a variac. Any recommendations (if you think I should use one to bring this thing up)? Cheap is good, or at least give me min. specs. I looked at some 10A ones.
a) remove R34/R35 (3k9 2W resistors found slightly above and between the two 15V regulators)
b) remove Q4/5 (MJE340/350 with heat sinks)
c) remove D10/D13 (1N4007 found next to Q4 and Q5, respectively)
Q4 and Q5 were dead^3... Q4 (IIRC) was cracked also.
I still haven't replaced the zeners yet. I have a bunch of them, so I may select some (matched).
Also, I mentioned I do not have a variac. Any recommendations (if you think I should use one to bring this thing up)? Cheap is good, or at least give me min. specs. I looked at some 10A ones.
Hi Chris,
What condition is the PCB in where any components have failed? Pay attention to these areas.
A variac is something you really ought to have. It's something that you'll use for years to come, plus it will save you lot's of blown up stuff.
I use a 2.5 ampere model on the bench, I also have a 10 ampere one for the odd time I need it. I picked up a B&K AC power supply / leakage tester for a song on Eeekbay. It even works. I forget the actual amount I paid, but it was probably around $40 USD I think. This one is a model 1655, good for about 4 amperes, plus it includes an isolation transformer as well. You should be able to find something similar for $100 or less fairly easily. The other variacs are just transformers. The small one I built into a box with current and voltage meters, the big one has a socket and no meters at all.
I'd try for a 5 ampere model. This size should be more reasonable to mount in a box while being able to cover 98% percent of your needs. I just made that number up, but it will cover most of what you need. If you see a 10 ampere model cheap, may as well buy it. Try to make sure the brushes are okay, ask if the wiper moves smoothly. This should avoid you getting a dead or burnt one. Also, does it smell burned and sweet? That's the overheated odor. For safety in keeping it running, see if you can find a supplier that sells replacement brushes. I've never needed to replace any, but it would be nice to be able to save one that lived a rough life in case it becomes yours.
Don't bother selecting the zener diodes. They will only conduct once everything has gone wrong, so matching will be the least of your problems at that point in time.
-Chris
What condition is the PCB in where any components have failed? Pay attention to these areas.
A variac is something you really ought to have. It's something that you'll use for years to come, plus it will save you lot's of blown up stuff.
I use a 2.5 ampere model on the bench, I also have a 10 ampere one for the odd time I need it. I picked up a B&K AC power supply / leakage tester for a song on Eeekbay. It even works. I forget the actual amount I paid, but it was probably around $40 USD I think. This one is a model 1655, good for about 4 amperes, plus it includes an isolation transformer as well. You should be able to find something similar for $100 or less fairly easily. The other variacs are just transformers. The small one I built into a box with current and voltage meters, the big one has a socket and no meters at all.
I'd try for a 5 ampere model. This size should be more reasonable to mount in a box while being able to cover 98% percent of your needs. I just made that number up, but it will cover most of what you need. If you see a 10 ampere model cheap, may as well buy it. Try to make sure the brushes are okay, ask if the wiper moves smoothly. This should avoid you getting a dead or burnt one. Also, does it smell burned and sweet? That's the overheated odor. For safety in keeping it running, see if you can find a supplier that sells replacement brushes. I've never needed to replace any, but it would be nice to be able to save one that lived a rough life in case it becomes yours.
Don't bother selecting the zener diodes. They will only conduct once everything has gone wrong, so matching will be the least of your problems at that point in time.
-Chris
I used a 2.5A one (~ 500W) for many years. Then I tried to test a big linear amp at full wack - it still hisses when I turn it on. I have replaced it with a higher current rated unit (5kW) but really the trick is to use it properly.
You use a Variac to power up your newly built/repaired amplifier. You adjust the bias, offset etc with a dummy load attached but with the signal input shorted out. Once you've confirmed correct operation then you swap over to the wall plug and apply a signal.
That way you only need a unit rated to cope with the quiescent current. So you could easily get away with a reasonably sized and priced unit. It is quite convenient to be able to run any amp at full power straight from the variac. But if you can cope with a little inconvenience you get to spend the saved money on something else - music for example.
TIP: if you get a variac, do put it in a box with a separate ammeter and voltmeter. It is very very useful to be able to monitor the mains input current as you slowly wind up the mains voltage. This frees up your other DMMs to monitor circuit function in the DUT.
A variac is pretty useful but not absolutely necessary. You can power up just the output stage and test it if you have a dual power supply, preferably with a tracking output and adjustable current limit. You would want at least +/- 30V but you only need 1A of output current for what I have in mind. If you are buying equipment & don't have such a PSU, then I would see this kind of purchase as offering greater long term benefit than a variac. Both would be best, but it is your money we are spending...
I do have a bench power supply, dual tracking but only about +/-25Vdc, and nowhere near 1A; maybe 0.25A per rail (I'll need to check it). I also have an amb labs sigma22 PSU built up for another project at +/-30Vdc, but no current limiting on this either. This should easily supply one channel at quiescent current.
The σ22 Regulated Power Supply
For a variac, I was looking at something like this: POWERSTAT 3PN116B VARIABLE TRANSFORMER VARIAC - eBay (item 250671468031 end time Oct-21-10 20:07:12 PDT)
I agree that a better bench PSU would be money better spent, particularly one that did current limiting and would go higher than 25Vdc. If need be I suppose I could always do the light bulb trick on initial power up of the amp.
One possibility:
Lambda LPD-423A-FM Dual DC Power Supply 0-60V 1A TESTED - eBay (item 370442604537 end time Oct-14-10 15:15:20 PDT)
The σ22 Regulated Power Supply
For a variac, I was looking at something like this: POWERSTAT 3PN116B VARIABLE TRANSFORMER VARIAC - eBay (item 250671468031 end time Oct-21-10 20:07:12 PDT)
I agree that a better bench PSU would be money better spent, particularly one that did current limiting and would go higher than 25Vdc. If need be I suppose I could always do the light bulb trick on initial power up of the amp.
One possibility:
Lambda LPD-423A-FM Dual DC Power Supply 0-60V 1A TESTED - eBay (item 370442604537 end time Oct-14-10 15:15:20 PDT)
Hi Chris,
That would certainly do it for you, maybe a little large and expensive.
I hold about the opposite view on variacs. You can use a variac to create a variable output supply from any receiver or amp supply. I have many regulated power supplies, but when testing out a power amp circuit, I'll connect the power supply from an old Marantz 1180DC (that was unrepairable) to the variac to create a high current power supply at the voltage I need. This has worked extremely well for many years.
It's too easy to build a tracking, variable low voltage power supply from junk box parts if you need one, but you can't whip together a variac very easily. I will agree with Alan wholeheartedly that you can get by with a lower current variac for the bulk of your tasks. If you see a big one later on at a great price, pick it up. Otherwise, get one rated for a few amperes and put it into a box with a voltmeter and ammeter. A front panel breaker (fuses cost too much) will also be a great idea. Don't forget a power switch and pilot lamp so you know when it's on.
The light bulb trick sucks as far as I'm concerned, compared to a variac anyway. Tried it and found enough in the way of issues with the light bulb. One is having a known input voltage that doesn't vary all over.
Keep your eyes open and visit hamfests and flea markets. You can also find a variac inside most hipot testers. Recently I saw one for $20. Had I not been drowning in variacs at the time, I would have bought it. In the mean time, perhaps you can borrow one, or be allowed to use one on a bench to check the amp out?
From my earlier post ...
-Chris
That would certainly do it for you, maybe a little large and expensive.
I hold about the opposite view on variacs. You can use a variac to create a variable output supply from any receiver or amp supply. I have many regulated power supplies, but when testing out a power amp circuit, I'll connect the power supply from an old Marantz 1180DC (that was unrepairable) to the variac to create a high current power supply at the voltage I need. This has worked extremely well for many years.
It's too easy to build a tracking, variable low voltage power supply from junk box parts if you need one, but you can't whip together a variac very easily. I will agree with Alan wholeheartedly that you can get by with a lower current variac for the bulk of your tasks. If you see a big one later on at a great price, pick it up. Otherwise, get one rated for a few amperes and put it into a box with a voltmeter and ammeter. A front panel breaker (fuses cost too much) will also be a great idea. Don't forget a power switch and pilot lamp so you know when it's on.
The light bulb trick sucks as far as I'm concerned, compared to a variac anyway. Tried it and found enough in the way of issues with the light bulb. One is having a known input voltage that doesn't vary all over.
Keep your eyes open and visit hamfests and flea markets. You can also find a variac inside most hipot testers. Recently I saw one for $20. Had I not been drowning in variacs at the time, I would have bought it. In the mean time, perhaps you can borrow one, or be allowed to use one on a bench to check the amp out?
From my earlier post ...
-Chris
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Chris,
In answer to your question about the condition of the PCB, it is charred and delaminated somewhat under the two trimpots (VR1 and VR2). I posted some pics (post 20, and one on previous page) which you can click on and zoom in some (sorry, my camera is only 2MP and I suck as a photographer 🙂). I guess until it is determined that the output devices are good or not there is not much point in dealing with this if it does not affect that determination. Once (if) the devices are known good, then perhaps I can see about getting a replacement board or clean up this one somewhat. Maybe it really makes no difference?
As for the variac, I don't really do much repair work that would require it. I got by without one on my Counterpoint SA5.1 (which I am not using now and getting ready for sale). Still, one could be useful. A better bench supply would probably be more useful to me, but then again, other that finishing one or two projects I've had going for awhile (slated for this winter), I don't have any others on the horizon.
In answer to your question about the condition of the PCB, it is charred and delaminated somewhat under the two trimpots (VR1 and VR2). I posted some pics (post 20, and one on previous page) which you can click on and zoom in some (sorry, my camera is only 2MP and I suck as a photographer 🙂). I guess until it is determined that the output devices are good or not there is not much point in dealing with this if it does not affect that determination. Once (if) the devices are known good, then perhaps I can see about getting a replacement board or clean up this one somewhat. Maybe it really makes no difference?
As for the variac, I don't really do much repair work that would require it. I got by without one on my Counterpoint SA5.1 (which I am not using now and getting ready for sale). Still, one could be useful. A better bench supply would probably be more useful to me, but then again, other that finishing one or two projects I've had going for awhile (slated for this winter), I don't have any others on the horizon.
Hi Chris,
Okay, have a close look at that board. If you don't need anything in that area, just deactivate that analog computer. Also, there should be relays that short the gate drive to ground during startup. Make sure they are okay. You should be able to measure a lowish resistance from gate to ground with the unit off. There may be a series resistor (100 ohms?) in series with the relay contacts.
If you can score a variac, I think you'll find out just how useful they are. I use mine pretty constantly. That's the small one - 2 amperes.
-Chris
Okay, have a close look at that board. If you don't need anything in that area, just deactivate that analog computer. Also, there should be relays that short the gate drive to ground during startup. Make sure they are okay. You should be able to measure a lowish resistance from gate to ground with the unit off. There may be a series resistor (100 ohms?) in series with the relay contacts.
If you can score a variac, I think you'll find out just how useful they are. I use mine pretty constantly. That's the small one - 2 amperes.
-Chris
Still haven't changed the zeners out yet 😛alm:
Took some measurements on the FETs tonight, some of which bother me a bit. Both channels gate to GND measure 250 ohms for the banks that have the 50 ohm gate stopper, and 20 ohms for the bank with the 20 ohm gate stoppers.
However, source to GND is different from good channel to bad. Good channel measures a solid 27.71K, whereas the bad channel measures 2.56M (and moving around a bit). Not sure if the SA100 clone schematic I have is accurate, but it appears the only path to ground that I see is thru the zeners? Not sure if this indicates anything WRT to the state of the FETs in the bad channel or not?
Took some measurements on the FETs tonight, some of which bother me a bit. Both channels gate to GND measure 250 ohms for the banks that have the 50 ohm gate stopper, and 20 ohms for the bank with the 20 ohm gate stoppers.
However, source to GND is different from good channel to bad. Good channel measures a solid 27.71K, whereas the bad channel measures 2.56M (and moving around a bit). Not sure if the SA100 clone schematic I have is accurate, but it appears the only path to ground that I see is thru the zeners? Not sure if this indicates anything WRT to the state of the FETs in the bad channel or not?
Hi Chris,
I am concerned simply due to the parts you found that were blown. The analog computer section should never see damage. Funny readings on the outputs are not good either. Try shorting the gate to the source, then measure the resistance between the source and drain. This should measure close to open, if not completely open.
-Chris
I am concerned simply due to the parts you found that were blown. The analog computer section should never see damage. Funny readings on the outputs are not good either. Try shorting the gate to the source, then measure the resistance between the source and drain. This should measure close to open, if not completely open.
-Chris
Chris,
The devices are still in the amp; drains are all bussed together and the sources are all bussed together. Good channel is ~1M, bad is ~3M D-S. Do I need to pull the MOSFETs, or at least one?
The devices are still in the amp; drains are all bussed together and the sources are all bussed together. Good channel is ~1M, bad is ~3M D-S. Do I need to pull the MOSFETs, or at least one?
Hi Chris,
Is it possible to desolder the gate and source leads? If it isn't, just disconnecting the drain connection will allow you to check these out. If one is shorted gate - drain, the others will measure normally since all the gates and sources will be shorted together anyway.
Just look at the circuit, sit back and think about what you may want to check. Then figure out some ways to do that. Many times troubleshooting is more a matter of relaxing and thinking about the issues at hand. There are typically more than one way to check things. That also explains why it appears that technicians may disagree on an approach to service. We all have our ways of doing things, and one way is often only a better way for the technician that suggests it. Alan and I are both experienced, and his suggestions are sometimes more valid for you than mine might be. We aren't really disagreeing with each other. We just have different ways of approaching a problem.
I would invite Alan to weigh in as well. He may be seeing something I hadn't considered simply due to what each of us has seen with these (or not seen).
-Chris
Is it possible to desolder the gate and source leads? If it isn't, just disconnecting the drain connection will allow you to check these out. If one is shorted gate - drain, the others will measure normally since all the gates and sources will be shorted together anyway.
Just look at the circuit, sit back and think about what you may want to check. Then figure out some ways to do that. Many times troubleshooting is more a matter of relaxing and thinking about the issues at hand. There are typically more than one way to check things. That also explains why it appears that technicians may disagree on an approach to service. We all have our ways of doing things, and one way is often only a better way for the technician that suggests it. Alan and I are both experienced, and his suggestions are sometimes more valid for you than mine might be. We aren't really disagreeing with each other. We just have different ways of approaching a problem.
I would invite Alan to weigh in as well. He may be seeing something I hadn't considered simply due to what each of us has seen with these (or not seen).
-Chris
Hi Chris,
Yeah, was hoping Alan might weigh in over night. From my years as a technician, I realize there are different ways to attack a problem, but thanks for reminding me. Haven't worked as a tech for 20+ years 🙂
The output devices are in TO-3 sockets such as these:
4600 Keystone Electronics IC & Component Sockets
The drain and source pins are soldered to buss bars on the PCB. The gates are individual connections for each device. Since the drain connection depends upon the device screws for contact, I could remove the screws to disconnect the drains, but leave the devices in place for the time being..
I had mentioned the ohmmeter differences earlier in the thread (actually before I had grabbed the device datasheets so wasn't sure which pins I was looking at), but no one commented on that. At this point, I am not too hopeful that the devices are OK, but who knows?
Chris
Yeah, was hoping Alan might weigh in over night. From my years as a technician, I realize there are different ways to attack a problem, but thanks for reminding me. Haven't worked as a tech for 20+ years 🙂
The output devices are in TO-3 sockets such as these:
4600 Keystone Electronics IC & Component Sockets
The drain and source pins are soldered to buss bars on the PCB. The gates are individual connections for each device. Since the drain connection depends upon the device screws for contact, I could remove the screws to disconnect the drains, but leave the devices in place for the time being..
I had mentioned the ohmmeter differences earlier in the thread (actually before I had grabbed the device datasheets so wasn't sure which pins I was looking at), but no one commented on that. At this point, I am not too hopeful that the devices are OK, but who knows?
Chris
Hi Chris,
Certainly every technician has their way of doing things, and also the order in which they look at different things. I didn't comment simply because I like to be absolutely sure that the measurements are done the way I expect them to be. You are the only one that can comment on that. I also have to admit being bad for not looking back in a thread. Too many things on my mind at one time.
I'd loosen the drain screws only so they don't make contact (check first), then proceed with testing. At least you shouldn't disturb the thermal compound. It's probably hard as rock, and about as useful by now if the part is disturbed.
-Chris
Certainly every technician has their way of doing things, and also the order in which they look at different things. I didn't comment simply because I like to be absolutely sure that the measurements are done the way I expect them to be. You are the only one that can comment on that. I also have to admit being bad for not looking back in a thread. Too many things on my mind at one time.
I'd loosen the drain screws only so they don't make contact (check first), then proceed with testing. At least you shouldn't disturb the thermal compound. It's probably hard as rock, and about as useful by now if the part is disturbed.
-Chris
Been resting 😀 ?
There is a mute relay on the main board. It is a 2 form C type. One pole connects the gate drive line of the n-channel via 100R to a common point that is then connected by another 100R to ground. The second pole connects the gate drive line of the p-channel via 100R to the same common point. You can think of this as a Y-connection from the two gate drive lines to ground with 100R in each leg of the Y.
Now to the point. When the amp is not powered, the relay is closed this circuit is connected. The relay and the resistors can get damaged. The first thing you can do is gently remove the relay from the DIP socket. Depending on vintage, it may have a small wire strap holding it in place. That was to cater for violent couriers. Just snip that off - it is overkill for ground transport but was a real boon for amps 'drop' freighted by air to here.
By removing the relay, you are isolating the MOSFET circuit a bit more. If you also remove those gate Zeners (actually, you only need to snip one lead of one Zener on each bank) and the two fuses on the APC board. There are also a couple of diodes (D40/D41) on the APC board that provide back emf protection. Depending on the polarity of the DMM these can upset measurements.
I'll pull the relay and the zeners and see if anything changes.
Typo, the 20ohm gate stopper bank is 220 ohms gate to ground, not 20 ohms.
Hi Alan,
Yes, that would be the most common type of damage when one or more outputs are damaged. I think those relays were one of the better ideas that Michael had actually. Something we don't normally see in other designs for tube hybrid amps.
Good to see you by the way.
-Chris
Yes, that would be the most common type of damage when one or more outputs are damaged. I think those relays were one of the better ideas that Michael had actually. Something we don't normally see in other designs for tube hybrid amps.
Good to see you by the way.
-Chris
Regarding the relays: these ground the gate signals out when the amp is off or warming up? I am measuring 220/250 ohms gate to ground on both channels, which would seem consistent with what Alan was saying in 100 + 100 + gate stopper (20/50). So that portion would appear to be fine? Also I am using one of the RCA jack grounds as my reference point, I assume that is fine.
Yes and yes.
And also when the thermal switches trip and/or a properly functional APC senses a condition that exceeds its threshold.
Hi Chris,
The relays have the gate circuits connected to the normally closed contacts, which is good since it requires power to open the circuit. It fails "safe" in other words.
-Chris
The relays have the gate circuits connected to the normally closed contacts, which is good since it requires power to open the circuit. It fails "safe" in other words.
-Chris
I pulled the relay and no change, other than the gate->GND went up to 300 ohms on the side with the 50 ohm gate stoppers. I replaced the zeners on that side, and no change to source->GND.
I then removed the screws from the upper FET on both channels at that side of the boards (rear of amp, 50 ohm gate stopper side). Did not remove the devices. Sorry, I didn't read the devices to see if they are the N or P channel.
I verified that the D was no longer connected to the rail on the PCB. Shorting the G-S and measuring S-D:
Good channel: 3.06M with the + lead on the S and the - lead on the D (case of device). Open when I reverse the leads.
Bad channel: 3.32M + on S, - on D. Open when I reverse the leads.
So, from this test, it appears that the FET is good (or at least I can't tell if it is bad in the bad channel).
S on either channel is connected directly to the amp output for that channel. Still getting 27K on good channel S->GND, and 2.6M on bad channel S->GND. So something other than the devices is causing this. Since I don't have the schematic, I have no idea what else it could be? Any suggestions as to how to proceed? Should I check all of the devices on the bad channel? If the DC resistance at the amp output is supposed to be ~27K, then I don't see how a bad FET could cause it to increase into the M ohm range.
I then removed the screws from the upper FET on both channels at that side of the boards (rear of amp, 50 ohm gate stopper side). Did not remove the devices. Sorry, I didn't read the devices to see if they are the N or P channel.
I verified that the D was no longer connected to the rail on the PCB. Shorting the G-S and measuring S-D:
Good channel: 3.06M with the + lead on the S and the - lead on the D (case of device). Open when I reverse the leads.
Bad channel: 3.32M + on S, - on D. Open when I reverse the leads.
So, from this test, it appears that the FET is good (or at least I can't tell if it is bad in the bad channel).
S on either channel is connected directly to the amp output for that channel. Still getting 27K on good channel S->GND, and 2.6M on bad channel S->GND. So something other than the devices is causing this. Since I don't have the schematic, I have no idea what else it could be? Any suggestions as to how to proceed? Should I check all of the devices on the bad channel? If the DC resistance at the amp output is supposed to be ~27K, then I don't see how a bad FET could cause it to increase into the M ohm range.