Hello all, looking for assistance/advise with the following:
Problem Summary
There is an annoying (power supply?) hum during quiescent (no input signal) conditions that's also noticeable during quiet musical passages. Also, the relay intermittently fails to pull in on power-up. When this happens, a sharp slap on the cover will coax the relay into behaving. Otherwise, the amplifier still works well. Crystal clean sound at rock-concert volume.
Background
I assembled the unit in 1974 and it's provided many decades of enjoyable listening. The problems started a little over a ten years ago. I opened it up in 2010 and performed a major overhaul: reworked poor solder connections, cleaned up the wiring, etc. With the problem still uncorrected, I took it to a local repair shop. He found damaged land on PC-30 (the power supply board) and laid in some new solder. This greatly reduced the hum's level but it's still present. I recently found another PC-30 from someone parting out a unit on eBay. Just installed the new (to me) board with no improvement.
I started looking online but cannot find anyone with the same issue. I did find instructions for setting the bias and DC offset. Just did that - no help. Any ideas?
Also...
I've read some threads on this site and noticed that many members are hobbyists who enjoy rebuilding audio gear. I'm more of the pragmatic sort. I'd like to find and correct this problem - that's all. I have a Fluke Scope Meter and I'm willing to do reasonable troubleshooting but if it comes down to replacing all the capacitors or changing out dozens of other components, I'll just go buy a new amplifier. Not going to throw too much $$ at a 46 year-old unit.
Thanks in advance.
Problem Summary
There is an annoying (power supply?) hum during quiescent (no input signal) conditions that's also noticeable during quiet musical passages. Also, the relay intermittently fails to pull in on power-up. When this happens, a sharp slap on the cover will coax the relay into behaving. Otherwise, the amplifier still works well. Crystal clean sound at rock-concert volume.
Background
I assembled the unit in 1974 and it's provided many decades of enjoyable listening. The problems started a little over a ten years ago. I opened it up in 2010 and performed a major overhaul: reworked poor solder connections, cleaned up the wiring, etc. With the problem still uncorrected, I took it to a local repair shop. He found damaged land on PC-30 (the power supply board) and laid in some new solder. This greatly reduced the hum's level but it's still present. I recently found another PC-30 from someone parting out a unit on eBay. Just installed the new (to me) board with no improvement.
I started looking online but cannot find anyone with the same issue. I did find instructions for setting the bias and DC offset. Just did that - no help. Any ideas?
Also...
I've read some threads on this site and noticed that many members are hobbyists who enjoy rebuilding audio gear. I'm more of the pragmatic sort. I'd like to find and correct this problem - that's all. I have a Fluke Scope Meter and I'm willing to do reasonable troubleshooting but if it comes down to replacing all the capacitors or changing out dozens of other components, I'll just go buy a new amplifier. Not going to throw too much $$ at a 46 year-old unit.
Thanks in advance.
Is this hum a new problem? Or from ten years ago, or always been there? Is it on both channels equally?
Have you measured the level of ripple on the supply rails? Checked the bias?
Have you measured the level of ripple on the supply rails? Checked the bias?
Logically if the hum is 60Hz continually I would check the smoothing capacitors at the rectifier output first but if you changed to board to no effect it should have eliminated that section but if heavy ripple change -
C304/5/6/7.
Rectifier diodes aren't perfect and can go faulty .
Scope the output for ripple but do the same for the output of the power supply ,after that it would be a slower progress but work from the power supply forward using the probe on your scope .
The relay has either dirty contacts or bad soldering joints (dry joints ) which could also apply to various capacitors .
Fault tracing in an amplifier isn't a "5 minute job " it requires patience and a logical approach checking each section in turn.
C304/5/6/7.
Rectifier diodes aren't perfect and can go faulty .
Scope the output for ripple but do the same for the output of the power supply ,after that it would be a slower progress but work from the power supply forward using the probe on your scope .
The relay has either dirty contacts or bad soldering joints (dry joints ) which could also apply to various capacitors .
Fault tracing in an amplifier isn't a "5 minute job " it requires patience and a logical approach checking each section in turn.
Starting point with hums is work out/measure if it's mains 50/60Hz or rectified 100/120Hz.
Oscilloscope, mobile phone app or audacity (and others) can detect freq.
EDIT: The relay is a separate issue (most likely). Probably tired components in the protect circuit though maybe rising DC offset, should measure dc offset, when relay does not engague, to ensure offset is ok.
Oscilloscope, mobile phone app or audacity (and others) can detect freq.
EDIT: The relay is a separate issue (most likely). Probably tired components in the protect circuit though maybe rising DC offset, should measure dc offset, when relay does not engague, to ensure offset is ok.
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Thanks for the early replies.
For the first 30 years, during quiescent operation if you put your ear up to a speaker, you could barely hear a hiss. The hum appeared around 10 years ago and has been present ever since.
I looked at the DC rail with the scopemeter and don't see anything I would characterize as ripple. Just a little noise which I see even when the meter leads aren't connected to anything. I will take a screenshot/photo and post it as it's possible I don't know what ripple looks like on a scope.
I did clean the relay's contacts - no help. I will check the solder joints to it.
For the first 30 years, during quiescent operation if you put your ear up to a speaker, you could barely hear a hiss. The hum appeared around 10 years ago and has been present ever since.
I looked at the DC rail with the scopemeter and don't see anything I would characterize as ripple. Just a little noise which I see even when the meter leads aren't connected to anything. I will take a screenshot/photo and post it as it's possible I don't know what ripple looks like on a scope.
I did clean the relay's contacts - no help. I will check the solder joints to it.
Ripple looks like a 60 hz or 120 hz waveform, not random high frequency hash that the atmosphere is full of. 16 millisec (60 hz) or 8 msec (120 hz) period. The 60 hz would be sine waves, the 120 hz would be rectified half sine wave. (all points down, all round parts up).
this amp is repairable by amateurs. It uses leaded parts on circuit boards, not surface mount ****. It also has a dozen rubber sealed electrolytic capacitors in it, that are of the same reliability as 46 year old tires. Not even Pebble Beach exhibits antique million dollar cars with 46 old tires on them.
You also may have a problem with an oxidized ground screw or connection. These just need removal & replacing to scrape the oxide off.
As for replacing, you haven't bought a consumer product lately have you? I bought a Sanyo 32" TV in 2017, and 16 months later it stopped turning on. The problem was a bad plated through hole on the input board. Not repairable since it is multilayer & untraceable. Used board was $45, but I didn't get the reliability I expected from a new $250 product.
In terms of decent new amps, see this thread: Who makes half decent power amps these days?
the recommended Bryston 300 w and 600 W amp are both selling for $6695 bing tells me. think that is worth maybe changing a dozen to 18 rubber capacitors?
BTW if you use the aluminum electrolytic capacitors rated >3000 hours service life, you won't have to do it again in 7 years as I did with 500 hour capacitors from a Houston TV parts store (sterling). Newark & digikey will tell you the service life of the capacitors they sell if you ask for it. The electrolytics are the ones with a minus on one end or a plus on the other. Or ones with "voltage NP" labeled on them, which means non-polar. The other capacitors are film or disk caps, which last forever.
As far as repairs, I've examined a few projection TV's on the curb repaired by "professionals". (I use the speakers, sometimes resistors). One of them had a used electrolytic capacitor spliced in place of a failed one on the power board. When you take your car to the shop, do you expect used tires in repair? My 1980 Mercedes clutch cylinder was "repaired" by the local pro with a used master cylinder filler hose, which failed 2 weeks after I picked it up. diy or send it to the dumpster. I hired a pro that week because it is 6 degrees outside the first week of January.
There are honest repairmen out there. I don't know any. Most are obsessed with quoting the lowest possible cost for the job so they get the job. Not shipping product that might last another 5 years.
Happy shopping. Maybe repairing diy with new long life parts.
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Hi,
I would try removing and cleaning the ground lugs located at the bottom of one of the large filter capacitor brackets and tighten securely. My two cents.. Your mileage may vary....
I would try removing and cleaning the ground lugs located at the bottom of one of the large filter capacitor brackets and tighten securely. My two cents.. Your mileage may vary....
Connect your scopemeter to the speaker posts at minimum/zero volume. Slowly increase volume, 50/60Hz or 100/120Hz. Repeat for other channel, same magnitude?
Thanks for the additional replies. All comments/suggestions duly noted.
Based on Indianajo's description, I can now confirm that there is absolutely no ripple in the DC rail. I will follow up on everyone's suggestions, starting with the low hanging fruit. Give me a day or two.
Based on Indianajo's description, I can now confirm that there is absolutely no ripple in the DC rail. I will follow up on everyone's suggestions, starting with the low hanging fruit. Give me a day or two.