Polytone 104 reverb not working

[Delta362]

Thank you for the help previously on the DC offset for the Polytone. I have everything working now, except for the reverb.

I measured the tank and have roughly 33 ohms on the input and 187 ohms on the output of the tank. I confirmed the wiring between the tank and the circuit board. I also confirmed the pot is 10k and has no issues as well as verified 10k from the pot wiper to the cap input of Mixer U4 of the main board.

For U2A, the schematic shows the 0.039 cap tied to ground with the 6.8k tied to pin 6. However, my components are swapped on the board. I don't see that as being anything of concern, but wanted to point that out.

When turning up the reverb pot, I get some nasty background noise with no reverb effect. The noise does get louder when turning up the reverb pot. I confirmed I have proper +/- 16V on the U2A rails.

I replaced the caps (1.0, 33pf, 0.039 and 0.02) to make sure I had no leaky signals coming in. When powered on with no signal, I can watch pin 1 and pin 6 both climb rather quickly from near 0v to +16V.

So then I suspected I had a bad RC4739. From digging thru old threads, I found the modern replacement drop in board on eBay from Fantasia Audio (yes I know to avoid electronic parts from eBay in general). That arrived today. I plugged that in and I'm still seeing the same high DC voltage climb on pins 1 and 6.

During my troubleshooting efforts, I inserted a signal into the reverb return cable. However, it was most likely too strong (200mV) as I just read that reverb tank return voltages are roughly 10mV and I was significantly larger than that with my test signal.. Could running that large of a signal into the opamp cause any damage to it?

I've been trying to rationally think of what I'm overlooking in this, but have come up empty. Any thoughts on how to proceed would be greatly appreciated.

Thanks!

Edit to clarify U2A is the reverb chip being discussed

[g1]

The order of the .039 and 6K8 does not matter, but there must be a good path to ground. Check solders. Also check that pin5 measures 10K to ground.

[Delta362]

Pin 5 is 10k to ground. Vibrato circuit works well.

Ground path measured good earlier, but I'll do another check on it to make sure I didn't break any wires on the board interconnections.

There's not much to go wrong here which is what is baffling me.

[Helmholtz]

Do you have -16V on pin 7?

[Delta362]

Do you have -16V on pin 7?

Yes, -16V on pin 7 and +16V on pin 14. Also to note is that the vibrato circuit works so at least U2B is functioning properly.

[Helmholtz]

It is essential that positive input pin 5 is at ground potential (0VDC).

[Delta362]

Confirmed that I have 0VDC on pin 5. When I injected my test signal (though way too large at 200mV), I was able to use my signal tracer and hear the signal on pin 5 as well. I did not hear it on pin 6 or pin 1.

[Helmholtz]

Looks like a bad opamp section, though not likely caused by the 200mV signal.

[g1]

So the DC at pins 1 & 6 was there with both old IC and replacement?
You said you plugged the new IC in, so I guess it is socketed. When you check resistance, are you checking at the actual IC legs rather than the socket solder pads?

[Delta362]

I jumpered from the board IC socket pins to my breadboard with a DIP-8 NJM4580D op amp and tested it. The reverb then worked as did the vibrato (which had been working fine). Next I replaced the SMT NE5532 chip on the adapter board with an SMT NJM4580V-TE1 op amp that I had and the amp is finally working 100% now. Thank you for all the help!