Greetings,
I am new and green to the electronics repair world. Not new to the receiving end of high end audio. Grew up around Threshold, Maggies, Macintosh, etc.
Started building speakers crossover networks successfully, reading, you tubing, asking questions.. Buying more and more bench test equipment..
I have a Hafler DH 500, (with the PC-10)
Had some troubles with the right channel. Made sound but was noticeably not as clean as the left side. Blew a speaker fuse on a couple occasions.. (most likely my fault by using this amp to test my speaker builds)
I purchased the driver cap mod/repair kit, and just got the 2 24,000uf caps,
After completing the right channel driver (PC-10), I re-ran new wires and connected everything up to find this loud Hum/Buzz noise while the amp was in idle with no source. The moment I connect the speakers I could hear this hum.
(was not there prior to the attempt to repair)
I read, researched and read through the manuals, I am green on exactly where to test via meter,
Not certain where to place the leads to capture what reading is needed..
I figured I would continue to finish the caps (electrolytics) only on the driver boards and the filter caps (these are the two big ones?)
Anyways, I hooked everything back up and found the hum was far less noisy but still present. I connected a source to see if I am getting any music through it and it does play music.. Very loud I might add... Read a bit about the Bias or DC offset (PC-19 has both pots, but the PC10 only has one, not certain which one this is DC or Bias.. I think it is the bias setting). I didnt want to screw around with this just yet until I knew what readings I am needing.
Found some and the right channel is substantially higher then the left side (just testing it through the speaker outputs, once again do not know if this is correct spot for reading values)
I did a bit of turning carefully and was able to bring that buzz / hum down a bit but still present..
I was re-reading some of my paperwork and on one of the pictorial diagrams I have it shows a black wire connecting the RCA grounds (which are chassis grounded) to the buss rail connecting the two large caps(filter?)
I used a jumper wire and WOW it went away, the volume level from the music went down quite a bit BUT it seems to be a more accurate volume level per my pre-amp...
I know this is far from done, my relay is clicking off far easier now..
I have the PC-9 board parts as well.. Have not started to put those on yet..
Any ideas? I am buying a fully restored Adcom GFA 555, I have a NAD 3150 and (Sorry Mr. Pass) I have a Threshold Stasis S300 with a right channel out... (Dont worry I will leave this particular repair to the pros) but point being I am all in, this has become an addiction and I am not going to throw in the towel.. I may ruin somethings in the process of learning and unfortunately do not have someone in person I can consult with..
Any advice is greatly appreciated..
I am new and green to the electronics repair world. Not new to the receiving end of high end audio. Grew up around Threshold, Maggies, Macintosh, etc.
Started building speakers crossover networks successfully, reading, you tubing, asking questions.. Buying more and more bench test equipment..
I have a Hafler DH 500, (with the PC-10)
Had some troubles with the right channel. Made sound but was noticeably not as clean as the left side. Blew a speaker fuse on a couple occasions.. (most likely my fault by using this amp to test my speaker builds)
I purchased the driver cap mod/repair kit, and just got the 2 24,000uf caps,
After completing the right channel driver (PC-10), I re-ran new wires and connected everything up to find this loud Hum/Buzz noise while the amp was in idle with no source. The moment I connect the speakers I could hear this hum.
(was not there prior to the attempt to repair)
I read, researched and read through the manuals, I am green on exactly where to test via meter,
Not certain where to place the leads to capture what reading is needed..
I figured I would continue to finish the caps (electrolytics) only on the driver boards and the filter caps (these are the two big ones?)
Anyways, I hooked everything back up and found the hum was far less noisy but still present. I connected a source to see if I am getting any music through it and it does play music.. Very loud I might add... Read a bit about the Bias or DC offset (PC-19 has both pots, but the PC10 only has one, not certain which one this is DC or Bias.. I think it is the bias setting). I didnt want to screw around with this just yet until I knew what readings I am needing.
Found some and the right channel is substantially higher then the left side (just testing it through the speaker outputs, once again do not know if this is correct spot for reading values)
I did a bit of turning carefully and was able to bring that buzz / hum down a bit but still present..
I was re-reading some of my paperwork and on one of the pictorial diagrams I have it shows a black wire connecting the RCA grounds (which are chassis grounded) to the buss rail connecting the two large caps(filter?)
I used a jumper wire and WOW it went away, the volume level from the music went down quite a bit BUT it seems to be a more accurate volume level per my pre-amp...
I know this is far from done, my relay is clicking off far easier now..
I have the PC-9 board parts as well.. Have not started to put those on yet..
Any ideas? I am buying a fully restored Adcom GFA 555, I have a NAD 3150 and (Sorry Mr. Pass) I have a Threshold Stasis S300 with a right channel out... (Dont worry I will leave this particular repair to the pros) but point being I am all in, this has become an addiction and I am not going to throw in the towel.. I may ruin somethings in the process of learning and unfortunately do not have someone in person I can consult with..
Any advice is greatly appreciated..
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Hi Sly! Not familiar with the amp youre working on, im a Pro sound guy. Never heard of hafler, but i am familiar with medium level amp repair. i was repairing an arcam reciever that had a loud hum. turns out it was a small input filter cap hidden on a board somewhere (noticed it was domed, replaced it, boom no more hum). so just a little background about caps and inductors (if you know then just skip through this explanation)
Capacitors stop the flow of dc and allow the flow of Ac, inductors stop flow of ac and allow dc. so knowing this. 60hz is pretty close to dc (dc is 0 hz) so we can use capacitors to reduce the lower frequencys (and inductors the reduce higher frequencys).
What i would suggest is pull out the dmm (digital multimeter) and start checking your capacitors (using the capacitance setting). if you dont know which caps are part of your power supply circuit. check them all. a loud hum or buzz could certainly be caused by dead caps that are shorted or not working. (or caps that have the polarity backwards).
Also there could be an internal grounding issue which is causing your problem. if you could check grounds relative to a schematic that would be of good use. which is where your grounding of the RCAs is compensating for. (which is fine, if its part of the documents of the amp it is safe and a permanent fix)
As far as Bias and dc offset goes. you can measure dc offset just by turning ur dmm to DC and hooking it up to the binding posts of the amp channel in question. if you get a dc voltage greater than 50-60mV then theres an issue. a little bit greater is ok maybe but something like a few volts, will fry speakers by heating them up. TOO HIGH OF BIAS CAN CAUSE DC OFFSET
as far as bias goes, there are usually test points inside the amp somewhere (that the manufacturer specifies) and these points should be able to tell you the current bias voltage, a lot of amps have different bias voltage set points. (my current project crown xs1200 is 3mV) so check with the service documents for that. The idea is that you check the voltage of the test points, and adjust the bias pot to match the test point voltage with the voltage of the service manual spec. but you can turn the bias voltage all the way down without damaging anything. turning the voltage high could cause dc offset and a lot of wasted energy (as heat) which can fry output devices. Careful touching the heatsinks with your hands as some of them have voltage potential to ground. best to use a thermometer or something that can tell u temp and not your hand.
Capacitors stop the flow of dc and allow the flow of Ac, inductors stop flow of ac and allow dc. so knowing this. 60hz is pretty close to dc (dc is 0 hz) so we can use capacitors to reduce the lower frequencys (and inductors the reduce higher frequencys).
What i would suggest is pull out the dmm (digital multimeter) and start checking your capacitors (using the capacitance setting). if you dont know which caps are part of your power supply circuit. check them all. a loud hum or buzz could certainly be caused by dead caps that are shorted or not working. (or caps that have the polarity backwards).
Also there could be an internal grounding issue which is causing your problem. if you could check grounds relative to a schematic that would be of good use. which is where your grounding of the RCAs is compensating for. (which is fine, if its part of the documents of the amp it is safe and a permanent fix)
As far as Bias and dc offset goes. you can measure dc offset just by turning ur dmm to DC and hooking it up to the binding posts of the amp channel in question. if you get a dc voltage greater than 50-60mV then theres an issue. a little bit greater is ok maybe but something like a few volts, will fry speakers by heating them up. TOO HIGH OF BIAS CAN CAUSE DC OFFSET
as far as bias goes, there are usually test points inside the amp somewhere (that the manufacturer specifies) and these points should be able to tell you the current bias voltage, a lot of amps have different bias voltage set points. (my current project crown xs1200 is 3mV) so check with the service documents for that. The idea is that you check the voltage of the test points, and adjust the bias pot to match the test point voltage with the voltage of the service manual spec. but you can turn the bias voltage all the way down without damaging anything. turning the voltage high could cause dc offset and a lot of wasted energy (as heat) which can fry output devices. Careful touching the heatsinks with your hands as some of them have voltage potential to ground. best to use a thermometer or something that can tell u temp and not your hand.
Outstanding advice! Thank you.
I just purchased another one of these AMPs DH 500, found a screaming deal on it from a retired audio tech who repairs things like this on the side now. He has all the skill and knowledge. Went to check it out listened to it and sounded amazing.
My thinking is to not only enjoy this one while continuing to work on my other one. And using the confirmed true and tested one as a reference. But get this,
I (not the smartest I know) wanted to give the new one a little cleaning using a little air and electronics cleaner that is safe to use on equipment. I simply removed one of the drive boards to clean behind and put it back. I upgraded the big filter caps on the new one as well while I was at it since I had an extra pair of 24,000uF. Hooked it all back up and turned it on!!
To my sad surprise I got a freakin hum in this one now. Shoot me! After walking in circles contemplating throwing in the towel with this hobby as I obviously am getting signs it isn't for me.
I took a closer look and found these little tiny black washers on the bottom of the chassis. Recognizing these from my old one, " what the heck do these go to?" After looking closer I found these were used as a little spacer for the drive boards since I only took one side off from the big heat sink I noticed the other one I didn't touch had them which was placed between the PC board and the heat sink they attach to. I took it off and replaced all these spacers and to my happy surprise quiet as a mouse.
So not certain this will fix the hum in my other one but it certainly helped this one. Perhaps it is non conductive keeping any residual noise interference from flowing into the PC board. Idk but first thing tomorrow it's off to the hardware store to buy these for my old one.
I will still continue inspecting the caps as you suggested but as long as grounding the RCAs (which the new or newer one has the RCAs grounded to the chassis stock) the difference on the one I worked on as I stated before was grounding and additional lead to the buss rail connecting the big filter caps.
I'm hoping after adding the spacers I will not need to do that anymore.
I just purchased another one of these AMPs DH 500, found a screaming deal on it from a retired audio tech who repairs things like this on the side now. He has all the skill and knowledge. Went to check it out listened to it and sounded amazing.
My thinking is to not only enjoy this one while continuing to work on my other one. And using the confirmed true and tested one as a reference. But get this,
I (not the smartest I know) wanted to give the new one a little cleaning using a little air and electronics cleaner that is safe to use on equipment. I simply removed one of the drive boards to clean behind and put it back. I upgraded the big filter caps on the new one as well while I was at it since I had an extra pair of 24,000uF. Hooked it all back up and turned it on!!
To my sad surprise I got a freakin hum in this one now. Shoot me! After walking in circles contemplating throwing in the towel with this hobby as I obviously am getting signs it isn't for me.
I took a closer look and found these little tiny black washers on the bottom of the chassis. Recognizing these from my old one, " what the heck do these go to?" After looking closer I found these were used as a little spacer for the drive boards since I only took one side off from the big heat sink I noticed the other one I didn't touch had them which was placed between the PC board and the heat sink they attach to. I took it off and replaced all these spacers and to my happy surprise quiet as a mouse.
So not certain this will fix the hum in my other one but it certainly helped this one. Perhaps it is non conductive keeping any residual noise interference from flowing into the PC board. Idk but first thing tomorrow it's off to the hardware store to buy these for my old one.
I will still continue inspecting the caps as you suggested but as long as grounding the RCAs (which the new or newer one has the RCAs grounded to the chassis stock) the difference on the one I worked on as I stated before was grounding and additional lead to the buss rail connecting the big filter caps.
I'm hoping after adding the spacers I will not need to do that anymore.
Hello,
those certainly sound like insulators to me. some heatsinks have rail voltage to them, so this heatsink being connected to the board without insulation could be causing some leaking current into components, which would then be amplified in your drive board and output board and boom there comes your source of noise. great trouble shooting. you can measure the heatsink voltage with refrence to ground. meaning stick a lead of your dmm on the heatsink and the other lead on chassis ground (all chassis must be grounded) or it is possible the heatsink has no potential and is insulated from the output transistors. in this case an ohms test between heatsink and ground or chassis should be performed (after the voltage test) if the ohms test shows ~0 or less than 1 ohm. than you know the heatsink does not have rail voltage.
Either way, putting those insulators in is a good idea because that is how it is designed.
those certainly sound like insulators to me. some heatsinks have rail voltage to them, so this heatsink being connected to the board without insulation could be causing some leaking current into components, which would then be amplified in your drive board and output board and boom there comes your source of noise. great trouble shooting. you can measure the heatsink voltage with refrence to ground. meaning stick a lead of your dmm on the heatsink and the other lead on chassis ground (all chassis must be grounded) or it is possible the heatsink has no potential and is insulated from the output transistors. in this case an ohms test between heatsink and ground or chassis should be performed (after the voltage test) if the ohms test shows ~0 or less than 1 ohm. than you know the heatsink does not have rail voltage.
Either way, putting those insulators in is a good idea because that is how it is designed.
Got it! I will proceed with testing.
Another thing I noticed is the hum is present in the right channel when no source is connected. Only speakers are connected. Once I turn it all off and hook up my pre amp then turn it all back on there is no noise. Perhaps the pre amp is assisting with the grounding.
Something else I realized when I was messing around with it, no source connect (trying to identify the hum/buzz) I carefully moved my hand between the right channel drive board and the massive transformer. I heard a difference in hum as I moved my hand between the two with out touching anything. This was the same both on my identical project Hafler DH 500 and the newest one I purchased from a retired audio tech. This guy lives close enough for me to contact very supportive and has all the pro bench test equipment which he varified distortion levels, validated watts per channel etc. has anyone considered insulating the transformer? I.e. A heavy fire / melt proof rubber? I'd say 8"-9" wide somewhat thick and made a round sleeve then slid around the entire transformer to assist with interference?
I assume since both are old components the transformers might be going bad causing the EMF disturbance. But could this also be a bad move insulating this part since I assume heat disapation would be subject and/or insulation resulting more dangerous temperatures developing?
P.S.
In my original searching I found manuals that had the PC19 schematics but had a very hard time finding one with the PC 10 board. I believe there is another thread on here where someone is asking about the differences and two POTs on the PC-19 whereas the PC-10 has 1. I got an original manual with the new one that has the build plans and schematics for the PC-10 board. When I get to a scanner I'll scan this great information and publish on here to assist others if needed.
I'm new to this site BUT LOVE IT!! Has been my number one go to for learning and figuring this stuff out. And not to mention the fact that people like Mr. Pass actually takes the time to posts things. That's not saying anything short to everyone else who has designed equipment and/or has outstanding knowledge frequents to assist others. That says a lot of the community.
Thank you Singaporerock!
Another thing I noticed is the hum is present in the right channel when no source is connected. Only speakers are connected. Once I turn it all off and hook up my pre amp then turn it all back on there is no noise. Perhaps the pre amp is assisting with the grounding.
Something else I realized when I was messing around with it, no source connect (trying to identify the hum/buzz) I carefully moved my hand between the right channel drive board and the massive transformer. I heard a difference in hum as I moved my hand between the two with out touching anything. This was the same both on my identical project Hafler DH 500 and the newest one I purchased from a retired audio tech. This guy lives close enough for me to contact very supportive and has all the pro bench test equipment which he varified distortion levels, validated watts per channel etc. has anyone considered insulating the transformer? I.e. A heavy fire / melt proof rubber? I'd say 8"-9" wide somewhat thick and made a round sleeve then slid around the entire transformer to assist with interference?
I assume since both are old components the transformers might be going bad causing the EMF disturbance. But could this also be a bad move insulating this part since I assume heat disapation would be subject and/or insulation resulting more dangerous temperatures developing?
P.S.
In my original searching I found manuals that had the PC19 schematics but had a very hard time finding one with the PC 10 board. I believe there is another thread on here where someone is asking about the differences and two POTs on the PC-19 whereas the PC-10 has 1. I got an original manual with the new one that has the build plans and schematics for the PC-10 board. When I get to a scanner I'll scan this great information and publish on here to assist others if needed.
I'm new to this site BUT LOVE IT!! Has been my number one go to for learning and figuring this stuff out. And not to mention the fact that people like Mr. Pass actually takes the time to posts things. That's not saying anything short to everyone else who has designed equipment and/or has outstanding knowledge frequents to assist others. That says a lot of the community.
Thank you Singaporerock!
I recently came into a dh-500 as well, and have been having fun upgrading the capacitors on the driver boards. I also noticed the hum present with no input cables while turned on. the noise is much less with the steel cover in place I have also noticed.
The sensitive area appeared to be centered around the input capacitors.
Congrats on the new gear/hobby.
The sensitive area appeared to be centered around the input capacitors.
Congrats on the new gear/hobby.
Thanks Phase!
Yes it is very addicting. I just finished fixing my Magnepan MG2s. Removed all the old glue and rewired. They sound incredible now. The bass section does not vibrate against the Mylar film anymore. Relatively easy.
Back to the Hafler DH500. I am still curious on a couple things. What is the difference on the PC10 versus the PC19 boards. I have two Haflers one with PC10 and the other with the 19.
What exactly does the PC9 board?
I've seen a lot of mods for this specific AMP and just purchased all MUSE gold line caps to replace the driver boards, transistors and most of the semiconductors from Mouser.com. As mentioned above I upgraded the PC19 one with the 24k power caps. But I've seen this with an additional CAP and what looks like a resistor on the top of each of the two large caps. What would be the best approach to figuring out the values for them?
Last is now that I have 4 20k caps id like to install them on my other Hafler but do I need to make any adjustments with the Diode Bridge or anything else by jumping the capacitance up to a combined 80k?
I'm going all the way with these Amps. Even considered Musical Concepts options. Toroidal transformer/ dual true mono. Etc.
Appreciate any input on anyone of these questions.
Yes it is very addicting. I just finished fixing my Magnepan MG2s. Removed all the old glue and rewired. They sound incredible now. The bass section does not vibrate against the Mylar film anymore. Relatively easy.
Back to the Hafler DH500. I am still curious on a couple things. What is the difference on the PC10 versus the PC19 boards. I have two Haflers one with PC10 and the other with the 19.
What exactly does the PC9 board?
I've seen a lot of mods for this specific AMP and just purchased all MUSE gold line caps to replace the driver boards, transistors and most of the semiconductors from Mouser.com. As mentioned above I upgraded the PC19 one with the 24k power caps. But I've seen this with an additional CAP and what looks like a resistor on the top of each of the two large caps. What would be the best approach to figuring out the values for them?
Last is now that I have 4 20k caps id like to install them on my other Hafler but do I need to make any adjustments with the Diode Bridge or anything else by jumping the capacitance up to a combined 80k?
I'm going all the way with these Amps. Even considered Musical Concepts options. Toroidal transformer/ dual true mono. Etc.
Appreciate any input on anyone of these questions.
Yup feeling a little stupid now that I realize I could of read this in the manual I do have..
Was reading up some more about the Power cap bypass.. Seen both pros and cons to doing that..
I am hoping just replacing the elec caps with the MUSE gold, and the matching semiconductors should result a decent result.
Was reading up some more about the Power cap bypass.. Seen both pros and cons to doing that..
I am hoping just replacing the elec caps with the MUSE gold, and the matching semiconductors should result a decent result.
If the 4 20,000 uf (microfarad) caps are intended for the power supply, make _absolutely_ sure they are oriented correctly for polarity. If you get one backwards it can hurt you if you're nearby from the explosion. I'd suggest you take _lots_ of pictures and drawings before, during and after the rework. Then have someone look over your work before powering up.
Make a Dim Bulb Tester and use it. Google the phrase for the details.
Lastly, on the upgrades available. Both Musical Concepts and Van Alstine have upgrades - major and minor at that - available for all the Hafler gear. I've installed the MC driver board and power supply upgrades for the past 20 years. They're excellent and you've got good support from the owner, John Hillig, too.
The downside to the extensive upgrades is that you really don't have a Hafler anymore and the investment you'll make will likely not be valued by others if you choose to sell it. This means that you need to plan on keeping the amp for years to get the return on investment. The upgrades are modular so you can do them as you go, which makes the initial investment lower _and_ you gain experience working with upgrades from the instruction manual they supply.
There are other threads here at DIYAudio and other sites so do some Googling for more perspectives.
Hope that helps.
Cheers,
David
I was aware that the smaller transistors needed to be matched in order to provide adequate dc offset. I have left those in mine since that characteristic is fine. Might want to handle the original ones with care in case they are better than replacements.
The only capacitors that were truly visibly failing on mine were on the relay driver board, the rest were within tolerance, just bad quality as provided in 1981.
the pc-10 boards are somewhere in between the pc-6, and the pc-19(big surprise huh?), and have an adjustment potentiometer for dc offset on later ones, along with bypasses on several capacitors, an inductor was introduced fairly early on that is part of the output.
Not sure if any of the overall gain structure was changed throughout the different versions, but they all look to be very similar in design.
I just took mine off the break in setup after a couple weeks, and it's very nice, the Russian polystyrenes are starting to smooth out. The slight hum is now gone too, bonus!
The 100uf decoupling caps were replaced with some mundorf ones left over from another project. I added another one to each rail on each board for the outputs as well, along with a .01 polystyrene. These were easy to mount on the pads that the wires attach to and helped to balance out the overall response and detail.
I dont plan on selling this to upgrade, so am not too concerned about "keeping it original", life's too short for that imo. I must admit it looks a bit goofy with the 4, 1uf Siemens film caps hanging of the boards now, a pair on each side of the boards like two water pails on a stick across ones shoulders...
The only capacitors that were truly visibly failing on mine were on the relay driver board, the rest were within tolerance, just bad quality as provided in 1981.
the pc-10 boards are somewhere in between the pc-6, and the pc-19(big surprise huh?), and have an adjustment potentiometer for dc offset on later ones, along with bypasses on several capacitors, an inductor was introduced fairly early on that is part of the output.
Not sure if any of the overall gain structure was changed throughout the different versions, but they all look to be very similar in design.
I just took mine off the break in setup after a couple weeks, and it's very nice, the Russian polystyrenes are starting to smooth out. The slight hum is now gone too, bonus!
The 100uf decoupling caps were replaced with some mundorf ones left over from another project. I added another one to each rail on each board for the outputs as well, along with a .01 polystyrene. These were easy to mount on the pads that the wires attach to and helped to balance out the overall response and detail.
I dont plan on selling this to upgrade, so am not too concerned about "keeping it original", life's too short for that imo. I must admit it looks a bit goofy with the 4, 1uf Siemens film caps hanging of the boards now, a pair on each side of the boards like two water pails on a stick across ones shoulders...
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PC 10 Schematic and Component list
Been meaning to add this.. Didn't realize I could got to the advanced reply to actually be able to upload a JPEG..
Anyways this was actually not the easiest thing to find on the net. I found a lot of manuals but did not find one that was for the DH500 with the PC10 boards.
This is actually from the original manual I have that came with...get this the latest Hafler DH500 I purchase that has the PC 19 boards.
Anyways.. Hopefully this will help someone down the line
Been meaning to add this.. Didn't realize I could got to the advanced reply to actually be able to upload a JPEG..
Anyways this was actually not the easiest thing to find on the net. I found a lot of manuals but did not find one that was for the DH500 with the PC10 boards.
This is actually from the original manual I have that came with...get this the latest Hafler DH500 I purchase that has the PC 19 boards.
Anyways.. Hopefully this will help someone down the line
Thank you for the great advice. Yeah don't plan on selling these. I figured it is a relative decent entry level project for me. I have two of them so not out one if something goes wrong. Building the light bulb limiter right now actually. Should of did that before I started this project. Luckily I have been lucky thus far.
Found one of the semi conductors slightly loose on one of the boards. Bought some silver bearing solder and using that here on out.
Probably not going to do the multi cap thing. Instead just buy another pair of the 24kuf for this one. Upgrade the elect caps with those niccion muse. A pair of gold rca and outs. Should be plenty decent for my needs at this point.
Now as for my Threahold S/300... I'll send over to someone near who specializes in amp repair as opposed myself just getting into It.
Then maybe ill refocus back on building some speakers to drive with these Amps.
I got a taste and now I'm hooked.
The DH-500 is pretty straightforward, but you do need basic troubleshooting skills and a scope otherwise you're more likely than not to damage more than is bad now.
Among the things that need to be changed in a *working* DH-500 are:
Ground buss between the caps - much heavier gauge, or a wide flat bit of PCB.
The ground point ought to be the *current* null along the ground buss.
The relay that handles the speakers is woefully under-rated.
I have changed them to TWO relays of similar type/size with larger contacts
or more contacts PER relay in parallel. The transistor that pulls the relays in ought to be upgraded in wattage and heatsinked.
There are some other things too, this is from now long in the past memories.
There are other threads here on the DH-500, good idea to review them first.
Pretty sure there may be a pic of the mods I have done up from a decade back.
Matched transistors are not required as I recall *except* in the outputs (mosfets - Hitachi), and then per rail, although both rails being similar is a plus.
The power switch is often arc'd through after years of use.
There ought not be any hum to speak of with nothing on the inputs, and certainly nothing with the inputs shorted. FYI.
_-_-
Among the things that need to be changed in a *working* DH-500 are:
Ground buss between the caps - much heavier gauge, or a wide flat bit of PCB.
The ground point ought to be the *current* null along the ground buss.
The relay that handles the speakers is woefully under-rated.
I have changed them to TWO relays of similar type/size with larger contacts
or more contacts PER relay in parallel. The transistor that pulls the relays in ought to be upgraded in wattage and heatsinked.
There are some other things too, this is from now long in the past memories.
There are other threads here on the DH-500, good idea to review them first.
Pretty sure there may be a pic of the mods I have done up from a decade back.
Matched transistors are not required as I recall *except* in the outputs (mosfets - Hitachi), and then per rail, although both rails being similar is a plus.
The power switch is often arc'd through after years of use.
There ought not be any hum to speak of with nothing on the inputs, and certainly nothing with the inputs shorted. FYI.
_-_-
very much appreciated Bear! I will certainly upgrade the ground buss and look into the relay..
The advice on the lightbulb limiter was something I should of done from the start! now I am having a lot easier time identifying areas that need attention. for example,
I built my lightbulb imiter, hooked it up to the hafler I am working on and the light came on and stayed on. So I turned it off and started by removing the rail fuses, turned it back on and the light bulb came on imediately but dimmed out fast. Replaced the two fuses on the left side (facing the front panel), flipped the switch and everything was ok. proceeded to the next pair of fuses, when all the fuses are in place except for the one closest to the transformer no light on the bulb. As soon as I connect that last fuse furthest to the right the light comes on and stays on.. So I snipped the lead going to the drive board leaving the lead connecting to the relay circut the bulb does not light up. So now I know it is somewhere in that drive board.
Id have more specific details but dont have my notes or schematic with me at work right now. All rails exept for the last one read 95.8v... A bit low as it should be 97v i believe.
I have a little scope, function generator and a smaller reg. power supply. My power supply will not give enough power to balance out the board (as I read somewhere else that it takes a good amount for the DC to balance out).
I have pretty much every single part for this amp brand new from diodes, transitors to resistors and caps. I am sure I could easily just replace everything on this with brand new but I really want to hone my skills by idenitifying the culprit causing this issue. (thinkng its one of the semi conductors on the drive board that has the little heatsinks), noticed one of them was slightly loose on the board, I re-soldered it but I have new ones to replace them all with if needed.
Using my other working Hafler as my reference, with the understanding that one has the PC19.
As far as mods are concered, your right I do need to learn to crawl before I learn to walk. I am certain I will be satisfied with the end result sound wise.. I read about people going "cap crazy" or "re-capping frenzy" I agree I had a taste of that initially.
My working Hafler drives the hell out of my magnepans MGiic,
Have a NAD T 547 which I tried using as a pre-amp for my hafler and amazingly my old NAD 1020 pre-amp as simple as it is sounds incredibally better.
Less is better, I do not need all the bells and whistles of a full surround sound integrated amp.
Long story short, I am taking a step back on my approach and trying to keep this simple as possible with quality parts.
Appreciate the advice nonetheless!!
Hurray! I think I found the problem. So last night I decided to start replacing all the transistors on the drive boards (starting with the board in question) after removing the differentials I checked them on my HfE on my Meter and found Q1-Q6 where way off some actually gave no reading at all. Doubled checked the values on the new ones ensuring they matched and popped them in. Then I starting testing the resistors around the bias Pot and those were pretty fried as well. R1 coming right off the input was off as well.
I went ahead and continued replacing the two wire wound resistors and going to move on to the rest of the semi conductors on the drive boards. I'll also replace all diodes and the remaining resistors while I'm at it.
Took Bears advice and used a piece of PCB cut it to size and drilled a few holes for the buss rail. I should have the entire board finished tonight then I'll move on to the other side.
So far all parts where easily ordered through mouser.com as mentioned above using quality elec. caps.
Two things though, once I finish one drive board before re-connecting the wires to the fets, input, output, 90v +&- I wanted to give it a little bit of power using a regulated power supply on the positive and negative rails, connect a function generator to the input and my scope to the output. My equipment is minimal and my regulated power supply is a VDC. It will only go up to 25vdc. Understanding the DC offset will balance out as it gets closer to 90v but is it ok to start at 1.5vdc, then 3vdc, etc. slowly increasing the power so I can at least see something on the output? Or am I just injecting DC current into something designed to offset it?
Also what is a semi easy way to get a round about idea on the power transistors or fets attached to the heat sink? Not certain what type of measuring tool / adjustment could be used to get an idea on them being ok.
Is it temperature readings? Would there be a resistance that could be compared to the rest hopefully identifying one or crossing fingers none being off or different than the rest.
I'll post pictures of my progress shortly.
I went ahead and continued replacing the two wire wound resistors and going to move on to the rest of the semi conductors on the drive boards. I'll also replace all diodes and the remaining resistors while I'm at it.
Took Bears advice and used a piece of PCB cut it to size and drilled a few holes for the buss rail. I should have the entire board finished tonight then I'll move on to the other side.
So far all parts where easily ordered through mouser.com as mentioned above using quality elec. caps.
Two things though, once I finish one drive board before re-connecting the wires to the fets, input, output, 90v +&- I wanted to give it a little bit of power using a regulated power supply on the positive and negative rails, connect a function generator to the input and my scope to the output. My equipment is minimal and my regulated power supply is a VDC. It will only go up to 25vdc. Understanding the DC offset will balance out as it gets closer to 90v but is it ok to start at 1.5vdc, then 3vdc, etc. slowly increasing the power so I can at least see something on the output? Or am I just injecting DC current into something designed to offset it?
Also what is a semi easy way to get a round about idea on the power transistors or fets attached to the heat sink? Not certain what type of measuring tool / adjustment could be used to get an idea on them being ok.
Is it temperature readings? Would there be a resistance that could be compared to the rest hopefully identifying one or crossing fingers none being off or different than the rest.
I'll post pictures of my progress shortly.
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