Crate GX-1200 - Problems persist after new parts

[g1]

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

Both of us said to float the emitters, in agreement with your earlier question. It's like you are not reading the posts here.
I assumed you had the emitters floating, had the variac at full line voltage, and have + and - 1.4V at IC3 outputs.
But you do not respond to confirm.
If we do not know the operating conditions it is impossible to advise.

[skidmark]

Ok, all 4 emitter resistors are clipped and I can run it up to full AC voltage. The amp is now drawing about 180 mils of AC current idle.
I'll start some DC measurements soon. Gotta entertain some grandchildren for a bit.
I'll post the all the readings in the next post.
Al

[skidmark]

Just found broken pc run between R61 and IC3 pin 8.
Standby ...

[skidmark]

First I guess I better appologize for not properly responding to you instructions on floating the emitter resistors.
I've been putting in a lot of hours on this amp, determined to get it fixed. I really need your help. So sorry.

at very first power up pin 1 of IC3 = 1.305, and within seconds settles down to 12.70 after maybe 30 seconds.
Pin 2 = .368 stays steady
Pin 3 = .386 stays steady

I've got my limiter built, but can't find a 40 watt light bulb anywhere around the house. Went to Lowes,
they have purged their shelves of almost all incandescent bulbs. No 40 watt bulbs there. Got wife stopping
by Dollar General to see if they have some.

Here is a fresh reading of IC3 pins while adjust AP1 pot. All min to max.

2 = .327 - .366
3 = .327 - .366
1 = 1.567 - 1.241

5 = .294 - .335
6 = .294 - .335
7 = 1.063 - 1.386

[DrGonz78]

They sell 40 watt utility bulbs for your fridge and oven. Also if you see a 60-100 watt bulb should do just fine really. I see them all the time at goodwill because they sat in boxes for years.

[g1]

5 = .294 - .335
6 = .294 - .335
7 = 1.063 - 1.386

Are these positive voltages and not negative? If so, do you have the -16V going into R55 and R56?

[skidmark]

Important update. I just found IC5 was bad. Pin 1 appears to be somewhat internally shorted to the -16 volt rail. It was reading 15.x. Even when I totally isolated the pin nothing changed.
When where and how I do not know. I must have done it while troubleshooting.
I replaced it so now I'll need to redo all readings.
I got my limiter jig built.
With the new IC5 installed nothing much changed. While taking the new set of voltage readings I accidentally shorted
IC3 pins 2 and 3 together. The limiter light went out and stays out. Both pins 2 and 3 read about .343 volts.
I can adjust AP1 now. As I slowly turn the pot the voltage drops to about .322 then the limiter lights up and all the
voltages crater to approximately what they were before. +-8. If I turn the pot back to where it was before the limiter
lit up, it stays lit up until I short pins 2 and 3 of IC3 again. I gotta go away and think about this. This is the screwiest amp I've ever worked on.

[skidmark]

GX-1200H PCB layout in case anyone needs it.

[skidmark]

Does anyone have a GX1200H on the bench that could please get me an exact voltage reading of the + and - 16 volt DC rails.

[Jon Snell]

The +-16volt rails are as described; +-16volts or very nearly (+-10%).
IC3 (5532) is a dual op amp that acts as a level shifter and bias adjustment. The voltage between pins 4 & 8 is 39 volts. It is not recomended to reference the voltage reading to ground (0v) as it will be a ball park figure and meaningless.
IC5 drives the insert out socket and the input to the power amplifier.
You seem to be throwing parts at it!
If you float the final emitters, you will skew the reading evenmore as the DC feedback will be lost.

If C27 is leaky, the voltages will be all over the place, much as you are experiencing.
If you lift C25, the power amplifier is separated from the driver stage and that will allow you to work without possible DC leaks from other bits.

Don't pannic, it's only a simple amplifier. Take each step slowly and note your findings, otherwise components and ICs will get damaged allong with the expensive bits.

[skidmark]

Thanks much Jon for the info and the reply.
I have not been wholesale swapping parts, this board can't tolerate much of that.
So far all the parts I have replace have been bad with the exception of one. IC4, It was probably ok.
I had 2 bad Zeners, D30 and D31. they were leaky and had a minimal effect on the +-18 volt rails. I replaced them anyway and that cleaned up that.
However, as you have read I am having a rough time with the +- 16 volt rails. I'm goin to refrain from getting into a big story, so here's a summary.
Something is going on in the feedback loop. +- 18 start to come up then all of the sudden both sides turn on forward biasing the finals and my limiter bulb ( 40 watts ) lights up.
For the longest time I tried troubleshooting with my variac and still do, but I decided to switch to the limiter.
Interesting side note, after power up and limiter lights I only have +- 8 volts DC for the +- 16.
While probing with the DCM I accidently shorted pins 5 and 6 of IC5 and the amp came on up. +-16vdc and +-40 vdc. Very little DC on speaker, maybe .03vdc.
I have already pulled C27 and tested with digital, d'Arsonval meter (Triplett), and the Peak capacitance checker. It checked good. However, I will concede
I'm human and have made mistakes in the past so I will retest C27. I've also pull C25 and done a lot of testing with it out. I was also leaky in the
beginning so it was replace. Major change in all voltage readings after doing so.

Tomorrow's plan is to retest C27 per your input. Also I have a very nice Siglent 4 channel digital storage scope which I'm going to record the entire +- 16v
startup picture and study it to see what's going on. This is assuming C27 still proves out to be good.

If you have any further specific questions on a detail not covered here just ask.

Al

[DrGonz78]

If you lift C25, the power amplifier is separated from the driver stage and that will allow you to work without possible DC leaks from other bits..

I like this idea! If you have done it then please post voltage readings. We are here flying blind.

[skidmark]

I like this idea! If you have done it then please post voltage readings. We are here flying blind.

Wiil Do.
Al

[skidmark]

My day got away from me so I did not get in until late. Too late to get the storage scope out and record a picture of the rails for ya. But, I did pull C27 again and rechecked.
It's not leaky and value is good.
Gonna try for the storage scope again tomorrow after work.

Al