Acoustic 470 PA Issue

[gbono]

I'm starting to believe this is the amp from hell. I spent way too much time trouble shooting and replacing the A/B channel lamps. During that process, a bit of lead triming accidentally shorted the 90V rail to the zener in the BJT regulator on the PA board (see attached). I had to replace the zener in the regulator and all those voltages are now passing to the other PCBs just fine.

The current problem is no output on the PA board - signal from the other two PCBs are loud and clear but right after the 47K reaistor on the PA input I get 12Vdc on top of the input signal. By the time the signal reaches the base of Q401 (differential pair) it's almost non-existent and noisey. Checked all the active devices in the circuit and they are passing.

The sad thing is that this board was working fine before the regulator ckt was damaged. Maybe someone else can see something I've missed.

BTW I've modeled this board in TINA and I get agreement on all the posted factory diagnostic nodal voltages.

[Enzo]

OK, there is 12VDC on the 47k resistor. Is there 12v on the other end of the cap C401? If not, then it is coming from the other direction. Does the 12v disappear when you plug something into the accessory jack? Or in other words, is there 12v on the line out jack? If so, it is coming from off screen to the left.

If there is 12v on the other side of C401, then it is leaky at the least.

[gbono]

470 PA

I doubled checked the voltages and there is only 12 Vdc on the inborad side of C401 (i.e. the node where R403/C401 and R404 connect). The voltage on the base of Q401 is still around 11 V and there is no change when I connect an unterminated connector into the accessory jack.

It has to be something really simple that I'm missing here???

[Enzo]

SO there is no 12VDC on R401? Then none of the stuff I mentioned about the jacks matters.

I might expect more like 24v than 12v there.

But assume nothing - start as if the amp showed up this way.

Check all the DCs. You still get +92v B+? Does the output bus sit at 46 more or less?

Q401,402 have DC voltage specs on all their legs - how close are they? You got the +44v on CR405? etc.

[gbono]

470 PA

Enzo,

the voltages are

VB+=90V

Voltage at CR405 = 35V
Q401
VB=12
VE=5
VC=0

Voltage at Q407
VE=20

I had replaced all the output transistors and they still test out okay.
I could swear I hear a high frequency tone coming off this board. I'm stumped is it oscillating?

[Enzo]

You don;t show Q402, but 401 is all wrong. Is Q402 shorted? IS 401 open?

Get that squared away, and I'd bet the rest falls into place.

I couldn;t tell you if it is oscillating without a scope on it or something like that

[gbono]

470 PA

I checked Q401 wih a DMM and the junctions appear to be okay. Same with Q402..........I'm wondering why there is virtually no signal at the base of Q401 - with .26 Vrms input to the boartd - the scope trace shows lots of noise...

[Enzo]

Worrying about signal at that base now is like worrying about signal at the grid of a 12AX7 with 40 volts on its plate. Get the DC voltages house in order.

You have no volts on the collector of Q401, that means no current flowing through R410,411. So even if 401 seems OK, it doesn;t seem to conduct. But with its emitter 7 volts positive to its base it wouldn't.\

What is on the base of Q402? 4 volts or something?