Peavey 450 Tour

[lowell]

So what would be the approach here since I cannot measure the DC on the output being that the triac is sinking it all to ground? BTW I measured the triac with no power to the amp and it isn't a dead short. I also still have a low reading on the +15v rail and the 7815 regulator is getting hot too. D19/20 across the PA rails read good in circuit as do R121/122, and therefore C30/31 too. So at this point it doesn't seem like a shorted output transistor either because the aforementioned readings would be off.

I just tried reading Vdc at Q3's base (R1 coil transistor - bottom of schem in protection circuitry) but there is only AC there as my meter was moving around a bit. I thought that this would tell me that the relay coil was active therefore disconnecting the speaker, and therefore that there was a DC-at-ouput fault.

So not sure how to proceed with this. I'd like to get a solid reading or two that will tell me what this issue is before I start pulling output transistors.

[Jazz P Bass]

If you are not attaching a load at this point, the triac can be removed for further testing.
Is there a voltage on the triac gate?
I guess it should be obvious that you must get the supplies in order.

[lowell]

I'm wondering if the two issues are related... could a low +15v rail cause the power amp to malfunction? I would imagine that it could skew the bias towards the negative rail a bit.

I will pull the triac... good point forgot that it's only there to protect the speaker.

[Jazz P Bass]

The front end of the output section utilizes the + & - 15 volt supply.
With one or the other malfunctioning, the output will never stabilize.

[lowell]

Right...that's what I'm sayin. Would a merely low reading cause such a huge DC output issue?

[Jazz P Bass]

You will not know until you get the supplies corrected.
Look at the schematic for the front end.
All of the opamps need the proper voltage.
Q25 is relying on the +15 to be there.
Q26, the -15.
The whole bias string is looking for both.

[lowell]

Yep pulled triac and 10.2 Vdc on speaker side of the output inductor, before the relay. Can I disconnected the Low V supplies (cut solder traces) to see if the supply itself is working? I don't THINK this will harm anything. The amp seems as though it would just be biased OFF.

[lowell]

So disconnected the low V supplies from rest of amp. The supplies themselves (with no load) are good. Reconnected them and now the -Low v supply is 800mv, the +Low v supply is 7v. I then tried disconnecting P1 and P3 connectors and the supplies remained at these figures.

If I leave the amp on even with bulb limiter in place I smell "hot semiconductor". I'm worried about damaging more components if I continue troubleshooting. How should I proceed with this?

thanks!!

[Jazz P Bass]

Follow the heat.
I hate to say it but without some clue, every circuit attached to the 15 volt supplies is suspect.

[lowell]

Okay can someone explain a few things for me please? Why are there 2 full-wave bridge rectifiers (U11/U12) across the main rails? I see they both are on different windings, and that Relay S2 shuts U11 on/off. Why does this happen?

Now after removing the triac both low v supplies are reading very low. +7vdc/-800mvdc. I'm taking a guess that the DC on the output is causing this somehow cause now there's also DC reaching U6 pin5. Which I believe causes pin7 to clamp up to that +46Vdc. This DC on the output also I believe turns the fan on and relay S1 then turns on and disconnects the DC from the speaker via the protection circuitry at the middle bottom of the PA schematic page.

So being that things are so skewed right now, I need to narrow down the culprit/s. What should/can I disconnect to begin narrowing down the issue? I'm afraid leaving the thing on and feeling for heat may damage more components.