A-75 circuit board DC offset problem

[Zapped]

I am trying to get an A-75 running. I set the project aside years ago, after an unfortunate polarity reversal in power supply connections (Yikes!) to the output stages just before final bias adjustment. Now I'm trying to go back and fix any problems and fire it up.

On my circuit board I have one channel that passes all checks; the other has a problem: 17.8 volts of DC offset! The signal trace looks fine at this card output, but when I change the scope to show both AC and DC, the trace jumps up to the ceiling; multimeter check of the DC value confirms the problem and indicates the 17.8 volt offset.

Short of tearing into this board and replacing everything on the problemmatic channel, does anybody have any idea where to start? My check of the voltages across R9 and R10 went okay; the voltages across R7 and R8 behave roughly as expected, and I can adjust P1 and P2 to get a nice symmetrical sine wave. But I've got this exceedingly high offset problem.

Thanks for any suggestions or ideas anybody might have.

 

[Zapped]

No opinions?

BTW, this is one of Nelson's older designs, a collaborative effort with U.S. Astronaut Norm Thagard, and I recall jumping with joy when I saw the issue of Audio Amateur that first showed it to the world (#4, 1992).

 

[Zen Mod]

try just FE without output bank

 

[Rodeodave]

As always, a picture of the board and a schematic of the circuitry in discussion will be greatly appreciated and very helpful.
Are you talking about the frontend, the output stage or the whole channel?
And how do the Zener Diodes check out?

 

[Zapped]

Thanks for responding, guys! The offset problem I described is just with the circuit board, without the banks of output transistors attached yet. I just took my readings from the screw-tighten connectors on the edge of the circuit board.

I will look around and see if I can locate a legible schematic. I think there is one in the A-75 article on Nelson's web site, but I think that copy was scanned from a hard copy, and it's not exceedingly legible. It's here:

http://passdiy.com/pdf/a75p1.pdf

The schematic for the whole thing is on pages 9 and 10, with the output transistors included in the circuit, but the output devices sit on two huge heat sink banks, which are yet to be connected to my amp. (This was where I screwed up the first time around, so I want everything to be perfect before I connect up any output stage this time.)

Thanks.

 

[Nelson Pass]

It's legible enough. Time to measure all the spots on the circuit where
the schematic calls out a voltage and report back.

 

[Zapped]

Thank you, Mr. Pass! I should of thought of that. I've been through the article many, many times over the years, and it should have occurred to me that those voltage guides are right there in the schematic to aid in troubleshooting. Duh!

I have my TAA magazine copy open in front of me tonight to locate all the points where I need to measure, and I will get out the probe and check the values indicated on the schematic first thing tomorrow morning, and report back.

Thank you for taking the time to point me in the right direction.

 

[Zapped]

I went through the whole set-up routine for the A-75 circuit board again yesterday, and one more time time today, with the following results for my problem channel:

voltage across Z1 8.85vdc
Z2 8.79vdc (these look okay)

across R9 5.67vdc
R10 5.73vdc (these look okay)


I had to back off on P1 and P2 from a midpoint setting, because voltage across R7 and R8 at half AC power was way over the .5vdc starting point. This allowed me to get the R7 and R8 values to 99mv and 102mv respectively to begin with, and I then increased my variac to full AC power. Twiddling with P1 and P2, I got the R7 value to .79vdc and R8 to .78vdc.

voltage across R1 was then 4.63vdc
and R6 was then 4.81vdc (I assume this is okay too)

My regulated power supply readings are both about 48 volts (within range). The Drv+ to Drv- potential was a little low at 5.7vdc (should be 7 vdc).

Unfortunately, looking at the board output, today I got NO sine wave on my scope trace at all (!) at the expected scope time setting (.1millisec). At a setting of .05 microsec I got a trace of 7 or 8 sine wave cycles. Is that high speed oscillation? Unlike my earlier results, this time I got virtually no DC offset (around 2 mvdc by my voltmeter), but that's probably because the whole darn thing has given up the ghost. Yes? Out of the frying pan and into the fire.

Worse! I get similar dc voltage values on my previously working other channel, but now get no sine wave output on that one either. BOTH channels now flunk the test at the output.

Maybe it would be better to test (and match as required) and install all new active devices (I bought replacements for all the Qs almost a decade ago, plus caps) and start over on the board from scratch.