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**Enzo** · 10-30-2008, 03:04 AM

First off, the SAC187 triac is not necessary for the amp to function, it is a protective device. That is called a crowbar circuit. When the thing detects DC on your output, it turns on the triac to short across the output. The triac often gives up its life doing thins, but it helps prevent your speaker cones from catching fire. Seriously.

When a PV SS amp blows up, the triac is usually bad too. Leave the SAC187 out of the amp until the last step.

ANy chance you have knocked the power amp assign button out of position?

I would:
Check with an ohm meter at both output jacks for shorts across the output.

REmove the SAC187s from both channels during tests

If you had the power transistor board out, I would verify all the individual wire connectors are on the right posts - This includes the transformer wires, output wires, what else?

Run down the rows and verify no shorted power transistors. Also run down the rows and verify all of those 0.33 ohm 10w cement power resistors are intact - none are open.

And also on the power transistor board right next to the ribbbon connectors on each end is R101 and R201 which are 180 ohms. Pull the ribbon cables off and measure resistance there checking for opens.

Then on the driver board, I would check the driver transistors in the little heatsinks.

Set my meter to diode test and go down the driver board and check every diode on it. None shorted we hope.

APply power. What happens? Not sure what you meant when you say it shuts down.

Looking for power rails on both channels, looking for low voltage rails for the op amps - +/-15VDC

**ChancesR** · 10-30-2008, 04:05 AM

Enzo -
THANK you for this. I will print this out in the morning and go through each one again.

By "shuts down" I mean this: Apply power to unit; Fan initially starts, loud pop through speaker, then no power (at all). I can duplicate by shutting off the switch and turning it back on.

I should know better - I've dealt with troubleshooting issues before - and I know that the more info given, the better the diagnostics. My bad. It's called 'panic' I suppose.

I am usually very confident in my board markings - being extremely careful - but now I'm doubting myself. There are four wires in particular that I'm now questioning. Each colored pair goes to each output from the transformer, and are black/blk-wht; and blue/blu-wht striped. These connect to a daughter board on the back panel.

Now that I've been away from it for a couple of hours - I also realize I was 'stuck' on this one section. Truth is.....I had also replaced one XLR jack. So the front panel was also removed. The gasp I did was probably heard in Michigan. I'm now wondering if I have that foam rubber inserted correctly - and if not, then could it be shorting something from there? I'm also going to disconnect the front panel and try applying power, I might get lucky.

I checked all of the power transistors this evening as well. Also the 10W cements. I compared everything between the two outputs - nothing *seems* amiss.

Again, thank you! I will follow through tomorrow with a full check.

Warm regards,
C

**Enzo** · 10-30-2008, 07:01 AM

So leave the wires from the power transistor card to the output jacks disconected for now. If they were wrong that will eliminate that. You can tell if the thing will work or not without them. The whole popint of troubeshooting is to isolate the problem. ANy area you can eliminate helps narrow it down.

Does your 304S schematic match your boards? SO far I am assuming they match mine.

Your description is troubling. What you describe sounds like a switching power supply starting then shutting off from cirrent overload detection. But the 304S should be operating from conventional power transformers, or so I'd think. My drawings don't cover the mains wiring or fan circuit. Linear supplies don;t usually have "startups." If you have the 304D on there it is a whole different ball game.

From your description after initial power up, leaving it turned on, there is no power supply voltages at all? Or just some missing? Zero function through the entire unit?

I am happy to help, but you will also find Gene Ford at the factory to be a very helpful guy. That is what he is there for.

**ChancesR** · 11-01-2008, 02:55 PM

Since my last post, I have pretty much disassembled this whole unit. I've checked all boards; driver, power and output. I removed the BTA16's that were the last parts installed before this problem. No changes in the power. I have the schematics for wallpaper now- and have been unsuccessful in finding a bad part or point on any of the boards. I called Gene - he mentioned the 6530/6531's - but not much else that he could think of. Those were also ok.

I was hoping someone could tell me about the mains on this. I DID find a square part (about one and a half inch) riveted on the inside back above the power switch. It *looks* like a bridge rectifier. Four spades on it, (wiring is attached to these spades) with all spades showing a shorted condition. If this IS a BR, then perhaps I've located my fault. I can't believe they would rivet this to the frame though.

Thank you again for the guidance -

**Enzo** · 11-01-2008, 09:51 PM

4 legs? Or just 3? Is it a Q4025 or Q4040 maybe? On a lot of their more powerful amps, PV uses a power tiac onstead of a switch directly. The main power swutch controls the triac, which then controls power into the system. Cuts down on switch failuer. If that triac shorts, then the amp is simply left ON. The switch cannot turn it off then. Since you switch still works, it either is an OK power triac, or that is not what we are looking at. And yes, those were rivetted to the chassis.

Another possivility is a thermal breaker, but those usually have two terminals.

SO when you switch on, it pulses the fan and then lies dormant - no sign of power at all, correct? LEAVE IT IN THIS CONDITION. Now is there mains voltage at the transformer primaries? if so is there AC on the transformer secondaries? No mains at the primaries? Then trace it around and see where the circuit is open. You power triac could be failing open. Anything is possible.

**ChancesR** · 11-01-2008, 10:34 PM

I'm not at the shop right now - and therefore not 100% sure about the 4 legs or 3. I *was* thinking 4 - I'll check tomorrow (Sunday). We're in the correct spot though. What you describe fits. As for the amp staying in the ON position regardless of the switch position.....I'll have to flip the switch off to see if this fails. I've been running this through my Sencore - so I've been switching it off/on with it. Amp switch left on.

I removed all of the thermal breakers yesterday and tested - all good. They needed fresh heatsink compound anyway.

I'll reassemble tomorrow also and check voltages after the power drops off. I'll also check that switch -.

Thank you!
Regards,
C

**ChancesR** · 11-03-2008, 11:24 PM

That bridge rectifier is indeed a 3-leg. I checked it more thoroughly - and it seems good.

I found a shorted zener on the power board. CR12. Part of the clipping circuit I believe. I also noticed (and changed) R13 and R14, both are 1.5Kohm. They were very dark.

I have checked all of the caps on this board, all are ok. Also went over the diodes again.

Guess I'm looking for more than just the zener - this has been a cascading effect with the problems. I'll be very happy to get this back to the owner. ha

Regards,
C

**ChancesR** · 11-07-2008, 09:01 PM

Received the zener this afternoon - now I simply have a good part, and no power (still). I have pulled all of the electrolytics, tested, all good. I have checked *seemingly* every part on all boards. What could I possibly be missing here?!? -- other than voltage.

Initial power on - unit makes an attempt, but voltages (and current) drop off. To zero. Checked for voltages - none. I have 110 to the unit, but nothing after.

I remember seeing somewhere a way to check the transformer. Would anyone know this trick? I sure hope it's NOT the transformer - doubt PV has one. yikes.

Thanks all -
C

**ChancesR** · 12-05-2008, 12:02 AM

Updates and more trouble on this. It has become the 2008 project, maybe 2009 as well. Many other amps have come and gone, but this one still lingers.

Status: Main power is functioning again. Fan spins, all is well. But I do not have front power led light, nor ANY power anywhere else on this beast. I have pulled the output board, and every part on it - no defective parts.

On the Drive Board (304), I have found that if I connect the ribbon cables (2), then all of the output trans. are shorted. I took readings on every device on the drive board - removed all the caps, tested with the LC103, removed diodes, IC's, and transistors. I thought I had found issues with Q200/204, Q100/104 - but even that turned out ok. The reason (I believe) that both of these channels *appear* to be shorted is due to the switch J174. So this leads me back to an original issue -
Would someone know how the Blue/Bl-Wht and Black/Blk-Wht wiring connects? These come direct from the transformer to a lug block mounted on the back of the unit. I would think the only difference in how this was connected would be monitor/main would be backwards.

I have talked with Gene - he suggested the 6534/6531's. They test fine also. I can give a rundown of the resistors/caps and diodes that read "short" on the drive board if this will help - or not.

I'm going to connect the scope again tomorrow to be sure about the voltage readings - but so far, only the line (110V) is present.

Regards,

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