Peavey CX4080HZ blown channel repair

[Delta362]

Recently a Peavey CS4080HZ came in for repair. The owner had previously attempted some repairs (replaced 1 ohm limiter resistors R33 and R34 before as they were blown). He gave up and now I have it in for repair.

The same 1 ohm limiter resistors were cracked so those were replaced. I found most of the output transistors were shorted on the left channel and I replaced all of them (Q313-328). Q331 and Q332 were also blown and replaced with IKW75N60T, along with a shorted D309 (VS-16CTQ100STRL-M3).

With the power off, I checked the small transistors and diodes and could not find any other shorted components on either channel.

With the transformer's 6-pin connector unplugged (this removes the HIRAIL and LORAIL voltages), I then powered the amp on without the LBL. You can hear the output relays clicking on after a second or two. All low voltage readings are good and seem to match the schematic. Amp was then shut off.

I then added the LBL and plugged in the 6-pin connector. When powering the amp on, my bulb is glowing fairly bright. I've been looking for any shorts, but haven't been successful yet. Z106 and Z306 appear to be shorted when measuring voltages, but when powered off, they don't read shorted in either direction. Q331 and Q131 also look like a short from G to E when measuring voltages, but when checking with power off, they don't measure a short.

I've attached the schematic and included the voltages that I measured at Q331, Q332, D309, D310, plus Q131, Q132, D109 and D110. Any suggestions as to where to look next? Thanks!

[x-pro]

There's nothing wrong with these transistors. To turn on Q331, Q332,
it is necessary to input a signal to the amplifier input, set the amplifier output level so that the amplitude of the output signal is close to the LORAIL supply voltage level. Then increase the input signal slightly.
The cascade of including additional increased voltage on the keys Q331, Q332 works only in dynamic mode.
Why is the voltage at G the same as at the emitter? Because there is no dynamics, there is no reason for switching on. And this voltage comes through Z306, in its diode inclusion for this LORAIL. and additionally through R353 (4.99k). Since the G of the transistor is isolated from the emitter, there is no current.

[Delta362]

That part makes sense, but why would my LBL be fairly bright? It's not dim like I'm used to with other amps when things are working correctly. I'm hesitant to remove the LBL and power it up without it. Thoughts on the LBL?

[x-pro]

what is LBL?

[Delta362]

The incoming transformer voltages for the 6 pin connector (J1) has 120VAC for the two high voltage rail bridges, and the low rails have 20VAC on the transformer incoming leads. I would have thought I'd have something substantial for the hirail voltages, but it's definitely a lot less.

[Delta362]

LBL is light bulb limiter, or Dim Bulb Tester

[x-pro]

If no load is connected to the output of the amplifier, nothing should leak. At the same time, the amplifier should operate properly from the LORAIL power supply, also without a load.

[Delta362]

Shouldn't I be expecting to see a higher DC voltage for the HIRAIL voltages?

[x-pro]

The capacitors in the supply C3, C7, C2, C6 are at what voltage?
Also, what voltage are C1, C5, C4, C8 ?
In general, what are the supply voltages of Loreal and Hireal relative to the Common?
I don't think the voltages you mentioned are operating values.
At +/-10V Loreal and +/-20V Hireal, the amplifier will not work at all. The operating points of the transistors will be disturbed.

[Delta362]

The transformer voltage coming in to pins 1-3, and 4-6 are 120VAC, while pins 2-5 have 20VAC. So something is definitely pulling my hirail down still. I was unable to work on it yesterday, but I'm planning to remove Q131, Q132, Q331, Q332 and see if the hirail voltage comes back up, while the lorail also comes up.

[x-pro]

The capacitors in the supply C3, C7, C2, C6 are at what voltage?

what's on the label of the capacitors?

[x-pro]

I'm planning to remove Q131, Q132, Q331, Q332

They're off and out of the way. They're not the cause.

[Delta362]

The large filter caps voltages are the HIRAIL and LORAIL voltages (approximately ±22VDC for hi, and ±11VDC for low), both of which are really low from what I'd expect given the transformer's 120VAC and 20VAC inputs to the bridges (which I also confirmed are not shorted). I'll need to double check all of the output transistors that I replaced to make sure I didn't mistakenly swap locations of the NPN and PNP Q313-Q328.

[x-pro]

The PEAVEY CS 4080HZ is a two-channel power amplifier that delivers 2040 watts to each channel.
It cannot have a +/-22V supply to provide that much power.
I'll ask again: What does it say on the capacitor cases?

[x-pro]

post 11 ? I'm not asking you to measure the voltage here.