Crate GX-1200 - Problems persist after new parts

[skidmark]

Do not connect the speaker or any load and solve the issue of DC at the output. Once that is solved you will not have high current when speaker connected.
You will not be able to get proper DC conditions til the supplies are balanced.
If you have a variac, you do not need the 15W limiter bulb, are you still using it?

Yes, I have a 465 Tek scope. And no there's no oscilation.
Al

[skidmark]

Do not connect the speaker or any load and solve the issue of DC at the output. Once that is solved you will not have high current when speaker connected.
You will not be able to get proper DC conditions til the supplies are balanced.
If you have a variac, you do not need the 15W limiter bulb, are you still using it?

So we're in agreement, the unbalanced +- 16 is the immediate culprit?
Currently it's at +3.01 and -3.5x
The 500 millivolts is enough to throw it off? Agreed?
Al

[skidmark]

Do either of you have a 1200 to take some reading on, especially around the Zener diodes and R89 and R90 for the +-16 rail supply voltages?
What voltage drops do you measure across R89 and R90 with 120VAC input and 33VAC input.
Having some idea of what the normal current is through R89 and R90 will help me decide what to do.
I've already more soldering on this PCB than I originally planned. I gotta figure out which current node to chase on the +16 rail.
That would a lot, I hope.
Thanks Al

[The Dude]

So we're in agreement, the unbalanced +- 16 is the immediate culprit?
Currently it's at +3.01 and -3.5x
The 500 millivolts is enough to throw it off? Agreed?
Al

Not necessarily. Many times, one side of the supply drives more circuits, draws more current, and will come up slower on a variac. The two supplies won't start matching until you get close to full supply. Have you been able to power the unit up to full voltage without a load so that you can measure the supplies at full line voltage?

[skidmark]

Well I'm inclined to agree with you, however I'm facing a quandary. At 30% variac I got over 1 amp line current. The amp is fused at 2 amps, and if I leave it there very long
the heat sink get scalding hot. So I can't hit it with full line voltage.
All 4 finals are getting turned on bucking each other.

Can the design of this amp handle cranking line voltage up via a variac?

One thought I had was to lift one end of all 4 emitter resistors on the finals, then crank it up and try to troubleshoot.
This is why I was hoping one of you had a 1200 to get some readings for me.

Al

[g1]

1 amp at 30% variac is too much. Anything making the heatsink hot with no load connected it too much.
Yes, you can try disconnecting the 4 emitter resistors at the output devices. Bring it up slow and see what you get. IC3 outputs are what is probably turning on outputs too hard.

[skidmark]

IC3 pins 1 and 7 are at 1.3x. So I would say they are turned on.
Do you have an amp there to get fresh reading on the pins of IC5 and IC3 with amp idle.
Remember, I have C25 removed to completely isolate the finals.

Al

[g1]

No I don't have an amp here, I doubt anyone will. Where are you at with variac and power supply voltages? 1.3V at IC is already too much, assuming one is plus and one minus.

[skidmark]

Yes I know it's too much. I have meticulously tested all the components in and around that area and could not find any bad.
However, there is always the possibility I overlooked something.
Al

[g1]

What kind of variation can you get from those IC3 voltages by adjusting the bias trimmer all the way to each end?

[The Dude]

If you can remove the output transistors, unsolder the emitter resistors or some other method of disabling the outputs and get the amp up to full line voltage, recheck the voltages coming from IC3. See if it still puts out enough voltage to turn the outputs full on. Also, have you tried turning the bias pot all the way off to see if that allows you to bring the amp up without overcurrent?

[skidmark]

What kind of variation can you get from those IC3 voltages by adjusting the bias trimmer all the way to each end?

.229 - .215 mv
Al

[skidmark]

Dude and G1. Lets do this the diplomatic way.
Do I replace IC3 again ( may have blown it but no supporting evidence). or float the final emitters.

Al

[DrGonz78]

I like the idea already suggested about making sure bias pot is at its coolest setting. Also make sure d13 d14 and possibly d29 are good. Removing IC’s is hard on the board too. Is the IC getting warm?

Edit: what about D11 & D12? We’re they tested?

[skidmark]

I like the idea already suggested about making sure bias pot is at its coolest setting. Also make sure d13 d14 and possibly d29 are good. Removing IC’s is hard on the board too. Is the IC getting warm?

Edit: what about D11 & D12? We’re they tested?

I did the bias pot reading for you. Here it is again. From end stop to end stop
IC3 pin 2 = 229 - .215 mv
IC3 pin 1 = +1.39. No change from end stop to end stop