2720 Power supply - replaced capacitors, still hums.
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Author:  patrickvf [ Wed Nov 27, 2013 3:29 pm ]
Post subject:  2720 Power supply - replaced capacitors, still hums.

I have replaced the capacitors on the 2720 power supply about two days ago, and there is still a hum present. I'm not sure what to do next. The hum is either coming from the VCO module when it's not activated by a CV/trigger signal (however the hum seems to change in tone from output to output (no, it's not the vco waveforms, it's hum as in line hum or something)), and it's present at the VCA and VCF too. So changing the caps didn't work, now what?

Author:  patrickvf [ Sun Dec 01, 2013 2:16 am ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

So... any ideas? I replaced the caps but that didn't fix it. I'm utterly and completely mystified.. :?

Author:  toneman [ Sun Dec 01, 2013 12:40 pm ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

Grounds between modules and gnds at amp are probably not the same.

U are probably not actually listening to the power supply hummmm.

U are "listening" to a ground problem(maybe).

U really need to dissassemble the system, then re-assemble one module at a time. Start with the VCOs because they are the signal source. Then the VCF because it is a signal modifier. By dissassembling the modules from the case, you will see how they are wired(mis-wired) and can also check for poor solder connections which, I suspect, is your most likely culprit.

Again, each module should be gone over with a good light and magnifier.
U shouldn't just ADD solder to solder connections. Better to remove some solder then add some new solder. Use the correct electronic solder that has built in flux. The built in flux will help clean the connection as you solder.

welcome to DIY!

Author:  patrickvf [ Mon Dec 02, 2013 3:30 pm ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

So you mean completely disassemble the modules, as in removing all components and solder them back on? Or were you talking about just remove the face plate and check the wiring between that and the circuit board? I think I'm just gonna start out with reinforcing the solder joints, before I go on with extensive surgery.. I'm certain some of the caps need replacing.. I think..

Author:  patrickvf [ Mon Dec 02, 2013 6:25 pm ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

Hmm I was thinking should I check the rectifiers and diodes on the power supply? Could a dying rectifier or diode be at least part of the problem?

Author:  toneman [ Mon Dec 02, 2013 10:11 pm ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

I doubt that the pwr supply diodes are bad(unless some are obviously burnt). Replacing the ps caps was the "obvious" repair. But, as U have already determined: it didn't fix the hum. This is assuming, of course, that you installed the caps with good solder technique.

Troubleshooting equipment with bad grounds is impossible to do. If you don't have a scope (oscilliscope) use a voltmeter. Be aware that improperly grounding a scope or a meter will not help the issue. Bad grounding of your test equipment will only confuse the problem, not solve it. Disconnect the pwr supply from the pwr buss. measure unloaded pwr supply with a DC meter. do you have solid voltages greater than +9, -9 and +18? This is a totally unregulated power supply, so the voltage will NOT be exactly +9, -9 and +18.

To answer your previous question: YES, it would be time well spent to disconnect EACH module from the pwr buss and check out one at a time AFTER you verify that the pwr supply is working properly. While U have the modules out, look over EACH one for cold solder connections and solder bridges. It really sounds like the person(s) who built the 2700 were not very well versed in proper soldering techniques or, at the very least, totally unfamiliar with proper grounding protocal for multiple modules sharing a common power supply.

Without knowing that the pwr supply is functioning correctly, and knowing if each module is grounded properly at the PS, U really can't do anything else except make 60cycle noise. AFTER the PS is working, and gnds checked and confirmed, the LFO is the next module to get going because it is MUCH simpler than the VCO and can give a varying voltage that you can measure with a meter. An analog meter would work really well for measuring the AC and DC components of the PS and the LFO.

Given the circumstances, I Don't think there is much more to suggest.

Author:  patrickvf [ Tue Dec 03, 2013 1:32 am ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

Ok, I'll give it a shot. Thanks for all your help! One thing I probably should have mentioned is that whoever worked on the power supply last, put power blocks on it that are way too big for the board. The outer one is only screwed down on one end and tilted to the side to make both fit, nearly (or completely) touching each other at the paper insulation around the spool o_O. It also looks much.. uh.. rougher (for lack of a better word) compared to the other Paia's PS. Hmm come to think of it, I should switch these out and see if there's still a hum present..

Author:  Billb3 [ Sat Jan 04, 2014 9:55 pm ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

have you been able to resolve this problem?

Author:  patrickvf [ Wed Jan 08, 2014 9:45 pm ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

Nope, didn't get around to do that yet, and I don't think that I'll have the time to do such extensive work in the near future, but that doesn't mean that I'll never get to it. But instead I decided to switch the power supply with the one from the Paia that I gutted (was crappily put together, to say it mildly). That one has much cleaner solder work to it and the transformers don't look like one is trying to shove the other off the board. However, the hum persists, but I'm starting to notice a pattern - The hum is mainly present in the VCA, VCF and maybe (have to check for that again) the VCO, so when the next opportunity occours, I will switch out at least the VCA and VCF modules, and I will check the power rails with my multimeter. Hopefully this will at least take me closer to a solution unless the other supply turns out to have issues as well. Upon googling around I came across a thread on Muffwigglers where somebody posted about a similar problem with his 27/4700 and found that the 18V rail was modulating? Can anyone confirm this as a possible cause (and provide a solution)?

PS: I wonder if a voltage regulator on each of the sources might help dealing with the hum? No?

Author:  toneman [ Thu Jan 09, 2014 7:32 pm ]
Post subject:  Re: 2720 Power supply - replaced capacitors, still hums.

Don't know what U mean when U say "power block"(?) Wondering if U mean the left-most module in the old ancient 2700 system w/keyboard(??) The one with 3 knobs and some 1/8 jacks and some pin jacks(?) If so, that's a totally unregulated powersupply! yech! Let me teach U about cheap, easy, simple 3 terminal regulators, and show U how to build your own stable, regulated bi-polar (or tri-polar) powersupply that will the be the best thing for your ancient synth since mojo. [get an account at photobucket] lmk........ 8-)

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