Low V supply issue Firebass 700

[lowell]

This amp came in w/ both of the 1a +16v/-16v supply fuses blown. I metered all components in the supply and found all diodes and caps seem good w/ the meter, however there are some suspicious ohm readings on the supply rails.

Q119 Pin1 to ground reads OF
Q125 Pin2 to ground reads 30Meg ohms
This seems odd that each supply has a different resistance reading to ground. Seems even more odd that all 3 +supply caps are either open or disconnected... (I pulled them and I do get a rising ohm reading w/ the meter, then reinstalled them)

I'm not sure how to test the regulators, as I cannot read voltage when the fuses are blowing. (FYI the fuses sill blew w/ a series bulb)

Q119 pin3 to ground reads 6k
Q125 pin3 to ground reads 4meg

I guess I could disconnect Q119 to see if fuses still blow. But would both fuses blow if only the +regulator was at fault? That seems the only suspect at this point. I'll try it, in mean time if someone thinks that's not it or has an idea I'm all ears. Otherwise I'll report back.

[booj]

I'd be suspecting the regulators are bad, but what the cause is for that I couldn't tell lya

[Enzo]

Isolate the problem. +/-16v goes to the preamp. Do fuses still blow when the preamp cable is disconnected?

You may have checked individual parts, but what is +16 is shorted to -16? A dead short op amp can do that. There are three ICs on this board that use that power supply.

Try this, remove Q119 and 125. Stil blow fuses?

Not sure what you are talking about with the two polarity caps and opens. Keep in mind your meter applies a small DC current across its probes in resistance scales. If you want similar readings on both polarity power supploes, you will need to reverse your leads. Not only that, any residual voltage stored in a cap wil interfere with the readings.

As far as an ohm meter goes, I'd say the caps either read shorted or not. ANything further is asking more of the reading than is really useful.

Both fuses blow because both sides of the AC are involved in full wave rectification.

[lowell]

Enzo,
Yeah I had the probes oriented correctly when checking DC resistance of the +/- supply rails. I disconnected the preamp connectors and the fuses still blew. Will try the regulators.

[lowell]

the fuses do not blow w/ the regulators pulled. I'm guessing your next move'd be the opamps on the power amp board? I'll test them for shorts.

[lowell]

none of the opamps on the poweramp board are shorted. Must be a regulator?

[Enzo]

OK, so the three ICs are not the short. And the short is after the regulators or is the regulator themselves.

Your meter should tell you if the Vreg is shorted. If the fuses pop right now, this is not some subtle inside short.

But since we know the problem is on the power amp and after the Vregs, and not the ICs, then what else is connected across?
CR140, CR144 right after the Vregs.
CR102, CR103 clamping the input line.
CR100, CR130 protecting against reverse V at the PA's own 15v zeners. (Top and bottom near center)
C101,102,123,126 power rail bypass caps right at the op amps
CR101,C100, CR131,C127 these are hte PA 15v zener regs, and if one shorts...

Measure resistance while the Vregs are out between +16 and -16 traces. Are they shorted together? Either shorted to ground? Then find the 15v rails. Either of them shortede to ground or to each other?

[lowell]

Nothing is a dead short concerning the forementioned components. I tested the Vregs for dead shorts and there aren't any. Is that how you test regulators? Or should I diode test, or ohm them?

[Enzo]

The three leg Vregs are entire circuits inside, not transistors, they are integrated circuits after all. I check them for shorts with an ohm meter. If they are shorted I replace them. If they do not show as shorted, they can still be bad. There is only so much I can test of its innards rom the outside.

Mouser sells TO220 7815 Vregs for 26 cents up to something under a dollar. If I have any reason to suspect one is bad, I just replace it. I sell my labor at a dollar a minute. How much time should I put into testing a 26 cent part?


I neglected to mention C133,C137 across the Vreg outputs, but if you checked the diodes there, then the caps are probably not shorted either. Swap out the Vregs and see what happens. If fuses still blow, at least you know it isn;t the Vregs.

You are using slow blow fuses, I hope.

[chipprogr]

Im a factory authorized service tech for Peavey.. maybe i can be of some help here. Your testing of the voltage regulators to ground will be fruitless because there is a small diode attached to the ground side of the regulators. This is there to push a 15 volt regulator up to almost 16 volts. The diode is acting like a .6 volt increase in the regulators ouput. I would highly highly suspect the 70413080 transconductance opamp. these are a pain in the arse. They act like a compressor limiter /clip eliminator circuit.

The fact that you have no fuse blowing without the reg's indicates that your primary supply is good and functional. It would be unlikely for both fuses to blow if you had a part that was affecting only the +16, or -16 line, you have a problem affecting both at the same time. ALso stating that you find no shorts with power off tells me is a part thats failing under voltage. Id pull the 3- 8pin IC's and see if you still blow, if they dont, replace the 3080 1st. These are a CA3080e IC, and they arene thard to find, look on ebay or order directly from peavey part# 70413080 phone# 800-821-2279

Thanks -Eric

[Enzo]

Welcome to the forum, Eric.

Actually, the fuses don;t care which polarity is loaded down. It is a full wave rectification, so each fuse supplies current one way each half cycle, then the othr fuse for the rest of the cycle. And of course the opposite polarity then also shares both fuses the same way. SO if there is a dead short across either rail, then both fuses will open.

And you are right, there is a diode in the ground leg, but that doesn't affect checking the part for shorts between terminals.

[chipprogr]

yea, posting at 2am when your tired can lead to answers like mine, enzo is absolutely correct about the fuse comment. Checking the vregs for shorts is right too, but the poster had checked them to ground on the pins that say gnd in the diagram and the diodes are blocking that reading. Not sure why peavey would label these as Qxxx instead of a U or IC because they are definately not single transistors.

Sometimes i will have a circuit that shorts under power like this with absolutely no visible damage and i have to force a part to show itself. This can be very frustrating if you have a board that has several opamps that are all soldered in , but this allows me to quickly identify the failed part. The fuses are low rated and will blow early, but a beefy AC/DC adapter (over 1000ma) or a 12 volt lead acid type batter can be handy to momentarily touch to the power supply ( bat neg to -16 rail, momentarily touch batt+ to +16 rail) JUST LONG ENOUGH to get the failed part to either warm up or pop. In all the years ive used this , ive never burned up a circuit trace.

Thanks for the welcome enzo -

Keep us posted -Eric

[pecorporation]

If the regulators are branded KIA, replace both with JRC components. KIA are subject to fail on their own.

[lowell]

Replaced the Vregs. They get quite warm immediately after powering on. I have only kept it on for about 30 seconds, fuses have not blown yet. Is it normal for regulators to get hot like that, and so quickly?

[chipprogr]

they maybe into thermal curent protect. Take a volt meter, get your leads to where you can check the output of them within that 30 sec time window and see if you have respective + and - 15.5-16 volts dc comming out of them. If so, your golden, if not, pull the 3080 ic 1st then try again. My bet s still on that IC being the issue.