Hi all -- I'm currently working on an HR-40 that is in questionable health. When it plays, it plays great. When it's not playing great, it's popping and humming. Judging from this and after replacing one tube that was obviously bad, I've concluded that I should probably replace the filter capacitor on the power supply. This capacitor is a big block with three contacts. It's not clear to me from the schematics which capacitor(s) this should be replaced with. If anyone has any experience with such a fix -- I'm all ears!
Yes! I did grab them from captain foldback -- super helpful, but while I have no issue with getting into electronics to make basic repairs, I'm not too adept at reading schematics and working from them to determine proper capacitance. I believe I can replace this block with a big can capacitor. Was just hoping to find someone with some experience as I feel like I'm missing some critical info.
I've repaired some HR-40 amps. It does fill me with some degree of dread to read of someone not really knowledgeable about electronics trying to get in there, but I guess it's your life.
While I can't say they never fail, the oil-block capacitors Hammond used for the power supply filtering in these amps are pretty reliable. To be honest, I can't tell what's going on in your video other than it sounds serious. Guessing it's the power supply filter capacitor is just that -- a wild guess. Something is arcing, or there's a bad connection that's getting made and then failing. Could be any number of things: bad tube, failing transformer winding, etc...
With these amps, I would start out by replacing all the paper dielectric capacitors in the amp. They will be the axial ones in metal shells or dipped in wax, and most will be mounted on the terminal strip in the middle of the amp.
Of course, replacing them may not fix the problem. When trying to fix amps this old, IMO, you need a comprehensive understanding of how they work and what things to look out for. There are simply too many 60+ year old parts in there that might fail for someone to watch a video and say, "Replace this one part, and it will be good as new."
I'm going to take a wild-ass guess (because I just can't help myself). I would look at the paper foil caps, it almost sounds like the cap breaks down, discharging itself, charges back up and repeats. I would think a bad power supply filter would do something that sounded worse. If you have a voltmeter you could monitor the B+ and see if it is doing a little dance and similarly, look at the voltage on each end of the paper-foil caps and see if it pulses. Better, like David says, just replace them and see if that fixes it. They should be replaced anyway.
Ok ok -- I'm heeding the warnings above and took the amp to a professional repair shop. After removing it from the tone cabinet and inspecting the circuitry it's apparent that previous work has been done on this amp. Well, fingers crossed.
On another note, I did trip across a PR-40 with an asking price of $299. I've heard good things about these cabinets and their necklace reverb. I checked it out in person -- the condition of the cabinet and electronics are very good.
That's probably for the best. In some ways, the Type LR amp from an HR-40 is a very simple tube amp, but with an amp behaving that way and ~65 years old, you need someone with all the tools (tube testers, etc...) to check it over.
One interesting thing about many of these Hammond amps of this era is that they use a variation on cathode bias known as "back bias." They use a voltage-dropping resistor to put the center tap of the high voltage PT winding at a negative voltage with respect to the chassis ground and then apply that voltage to the output tube grids. To the untrained eye, it can look like fixed bias, but it isn't.
I've been rebuilding a C3 and PR40 for one of our forum members and posted a little clip to demo them. Search YT for “Hammond C3 and PR40 Test”. I guess if I'm willing to embarrass myself for one member, might as well do it for everyone 8) The C3 TG is recapped, but the AO28 and PR40 are unmodified.
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