LA2A Clone Sudden Failure (high voltage at V3)

Hello people much smarter than I am:

I built an LA2a clone a few years back (essentially following the Cayocosta wiring with some mounting alterations). It's been a wonderful unit functioning pretty much as expected until the other day...

The compression side-chain stopped working. The unit continued to pass audio.

I should have stopped after I got to the following voltages:

AC:    122
C7b(+) 280

V1 1: 123
    3: 1.1
    6: 127
    8: 1.1
V2 1: 102
    3: 4.3
    6: 230
    7: 78
    8: 108
V3 1: 273
    3: .1
    6: 274
    8: .9
V4 2: 5.5
    5: 174
    6: 97

I tested the resistors and capacitors around the exceptionally high voltages (in red) and they were within tolerance. I was not able to find any shorts or ground faults.

I decided to swap the 12AX7 tubes (in retrospect, stupid). The sidechain appeared to work, the output circuit appeared to fail. OK. Bad tube.
I replaced the 12AX7 in the side chain today (and placed the confirmed-ok tube back in the audio circuit). Annnnnd... now both circuits are failing. The voltages remain out of whack.

The only odd symptom that I've had in the past is that the meter bulb burns out quickly - I've just stopped replacing it. Other than that this unit has been fairly quiet (around -55 db noise floor on the low end - clean up top).

I'm at a loss for what to test. I'm hoping someone else has seen these voltages or can tell me where in the circuit I should look for faulty parts or wiring.

Thanks for taking the time to read this. Your genius is much appreciated.

--Leo

Cathode is tied together on V3, that reading should be the same.

Paul

Pins 3 and 8 that is...

I notice a lot of people have read this but only one has commented. (You are correct - I checked the voltage again at pins 3/8 - they are the same - .1V). I'm hoping someone out there has some suggestions. I'm at a total loss.

I've tested the voltage again today hoping things would miraculously fix themselves. To the contrary, I now have completely new, far worse voltages.

AC - 123V
250-0-250 out (red) - 280V AC
6.3V (green) - 3.7V AC

C7b(+) 318 V
Neon: 60 V

V1 1: 312 V
    3: 0 V
    6: 312 V
    8: 0 V
V2 1: 0 V
    3: 0 V
    6: 320 V
    7: 0 V
    8: 0 V
V3 1: 314 V
    3: .1 V
    6: 315 V
    8: .1 V
V4 2: 6.5 V
    5: 197 V
    6: 111 V

Things look a lot different from yesterday... but far worse. It seems clear some part has failed or is grounding but I cannot identify anything in my testing. (The resistors are all within range, and the capacitors I can test are all within range - my multimeter cannot read the 30u/40u caps).

If anyone has any experience with something similar I'd be very grateful for some suggestions.

Thanks again.
--Leo

You need to be a bit more systematic. It is unlikely anyone's will be able to help you just from a few voltage readings.

So start from scratch. Take all the tubes out. Power up and measure transformer secondary voltages (ac). If it has a solid state rectifier then check the unloaded ht volts. If these are all OK last least you know the PSU is OK.

Then fit just the audio chain tubes one at a time and check again. You need to find the point where adding one tube breaks it. Then you can start checking individual components.

Hope that helps.

Cheers

Ian

Thanks, Ian, for your reply. I am asking for help with testing methodology as much as I am specific LA2a assistance, so your suggestion is useful.

I have done as you suggested, removed the tubes, and tested the AC wall voltage, the voltages from the power supply (AC) and the power coming from the rectifier. These are quite similar to the voltages I posted last time.

WALL - 124V AC

MAIN (power transformer) - 294V AC at each wire (should be 250)
SECONDARY (power transformer) - 3.7V AC at each wire (should be 3.15)

B+ Voltage (after the rectifier) - 405V DC (should be closer to 245)

Would a power transformer failure cause these kinds of voltage spikes? Is my high wall-voltage enough to drive the other values up to this extent or does this look like a failure in the circuit itself?

I just don't know how to translate these diagnostic voltages into actionable repairs. And I don't have excess parts to swap out for testing.

Thanks again, those who take the time to read. Any help is greatly appreciated.

--Leo