Anyone ever seen decoupling caps go leaky? PPG wave problem.

I have a faulty PPG Wave 2.3 PROZ board here, (yes, the dreaded PROZ board) that is pulling the 5V regulator down to about 4.3 Volts when the machine is first turned on. From then on the regulator voltage slowly rises to about 4.9V over about 10 minutes.

Now the 5V regulator on a PPG is a 5A device in a TO-3 case, so it's no sissy, but nothing is getting warm on the PROZ board, even though there is a lot of current being pulled. All that is on the board really are chips and decoupling caps, a lot of which are integrated into chip sockets (about 1/3 of the IC's are socketed).

As no chips are getting warm I am kind of suspecting a bad decoupling cap, and I am wondering if anyone has ever seen a monolithic cap do this kind of thing before?

I have seen ceramic decoupling caps do many strange things - including making circuits oscillate... But if a cap is dragging down the voltage, then it should get very hot, shouldn't it?

Best regards,

Mikkel C. Simonsen

[quote author="mcs"]I have seen ceramic decoupling caps do many strange things [/quote]

The PPG Wave synths are capable of strangeness beyond any of our wildest dreams...
and I'm not talking about sound creation either. :wink:
When repairing them you don't need a service manual as much as you might need a wand and a cape.
A pointy hat with moons and stars on it might do well also.

sure any 'lytic cap has the potential to leak. They tend to short internally when they do too. I suppose you could shotgun the board and try it again.

[quote author="producer4000"][quote author="mcs"]I have seen ceramic decoupling caps do many strange things [/quote]

The PPG Wave synths are capable of strangeness beyond any of our wildest dreams...
and I'm not talking about sound creation either. :wink:
When repairing them you don't need a service manual as much as you might need a wand and a cape.
A pointy hat with moons and stars on it might do well also.[/quote]


I have been working on PPG's since the 80's when I did warranty repairs on them, and they never cease to throw up nasty problems. Could be worse though, it could be a Waveterm.

What is happening on the input side of the regulator? This might indicate whether current is being drawn and whether regulation is being lost due to low voltage on the input side.

If a de-coupling capacitor, or other component was defective - I would expect to see or smell burning.....

JG

I don't have any PPG problems ...... since I bought the PLUGIN !!!

:wink:

[quote author="AudioJohn"]What is happening on the input side of the regulator? This might indicate whether current is being drawn and whether regulation is being lost due to low voltage on the input side.

If a de-coupling capacitor, or other component was defective - I would expect to see or smell burning.....

JG[/quote]

Funny you should say that. I checked, there is plenty of input voltage to the regulator, but there is a single, large 10,000 microfarad filter capacitor in between it and the rectifiers, and every time I see one of these caps in a Wave it reminds me of when one failed once in the workshop, we heard a very loud bang, and suddenly there was a perfectly round dent punched into the thick metal lid of the PPG, as the cap had exploded, filling the machine with aluminum confetti. the metal can of the cap only weighed a couple of grams, but it flew up quick enough to punch a serious facsimile of it's own shape into a thick piece of metal.

[quote author="MartyMart"]I don't have any PPG problems ...... since I bought the PLUGIN !!!

:wink:[/quote]

Wait until the next OS upgrade comes out for your computer, and see how well your software works. A real PPG took 20 years to become vintage, a software PPG will be vintage in 2. :shock:

If the regulator isn't getting warm - and there are no HOT components anywhere - it has to be a faulty voltage regulator.

Can you power the board from a bench PSU to prove this?

JG

[quote author="AudioJohn"]If the regulator isn't getting warm - and there are no HOT components anywhere - it has to be a faulty voltage regulator.

Can you power the board from a bench PSU to prove this?

JG[/quote]

I have swapped out the regulator, it was expensive too. The few chips that have sockets have inbuilt decoupling caps, so if one of them is getting hot I can't tell as it is under the IC, so tomorrow I will pull all the socketed IC's and check the socket caps.

[quote author="Steve Jones"][quote author="AudioJohn"]If the regulator isn't getting warm - and there are no HOT components anywhere - it has to be a faulty voltage regulator.

Can you power the board from a bench PSU to prove this?

JG[/quote]

I have swapped out the regulator, it was expensive too. The few chips that have sockets have inbuilt decoupling caps, so if one of them is getting hot I can't tell as it is under the IC, so tomorrow I will pull all the socketed IC's and check the socket caps.[/quote]
Is the regulator fixed voltage or adjustable? Had similar faults with 317-style regs with not obvious faulty 10uF decoupling capacitor between adj.terminal and 0V for improved ripple rejection. Same circuit running faultless without this cap.