Sunn SL260 reverb stopped working

[QdB]

I have a Sunn SL260 2x12 60w solid state amp. Reverb stopped working.
The reverb tank is horizontal mount, open end up. OC Electronics, 2 spring with no Identifying marks other than a patent application number.
The schematic shows 250ohm input and 2250ohm output.

What I have done.
1. disconnected tank output and ran the tank output to a tiny ref amp that I have, to check for any signal. No signal.
2. disconnected the wire that leads to the tank input and connected it to the ref amp. There is signal but it sound like it is being run through a faulty overdrive pedal. distorted and cuts intermittently.
3. Ran a headphone signal through the reverb tank to the ref amp. It gives a signal with reverb though faintly, Not sure that the headphone signal was giving enough juice, but something at least was going all the way through the tank.
4. Did a DC resistance test across both the input and output, though I am not entirely sure what values I should be getting. I read somewhere that DC resistance readings will not be the same as AC impedance rating.

Considering the distorted send tone, I suspect something in the send circuitry (maybe a bad cap?) but this is just a hunch founded on basically very little knowledge and experience.

Thank you in advance for any guidance.

[52 Bill]

The dc resistances will be something like 60 ohms for the input side and 200 ohms for the output of the tank.

The drive circuit of the amp places the reverb tank input coil into the feed back loop of the driving opamp. This means that neither end of the input coil is grounded and trying to listen to a signal from the reverb drive cable will not give normal results.

If the tank checks ok and the connections to and from the tank are all good, then check the return circuit of the amp either by touching the tip of the return RCA plug or by shaking the tank. If that checks out, then replace the drive opamp.

[QdB]

Thank you for the information. Particularly -

The drive circuit of the amp places the reverb tank input coil into the feed back loop of the driving opamp. This means that neither end of the input coil is grounded and trying to listen to a signal from the reverb drive cable will not give normal results.

This is good to know and something that I would have never guessed.

I checked the DC resistance again:
36 ohms across the input
186 ohms across the output

That input rating would seem out of spec by any standard (to me anyway).

I also checked the return circuit, I get a nice solid buzz through the speakers when I touch the tip with the reverb volume turned up a little bit.

Sooo, possibly bad input transducer?
I have neither the knowledge nor skill to wrap a coil that fine which I assume would be the only course as far as repair goes. So maybe track down a new tank I think.
With that, I would guess that the input impedance has to be at least the same or higher, what would be a safe range?

[Enzo]

The resistances are unlikely to be "wrong", you will mostly find either OPEN which is bad, or some lowish resistance, which is good.

Agree with Bill. Pull cables from pan. Turn on amp with reverb up. Now touch the tip of each reverb cable. One should hum, that is the return and plugs into the OUTPUT of the pan. Neither hums? Return circuit has problem. Make sure cables were into proper jacks. testing drive side - cannot plug into a jack with grounded sleeve.

Easy enough to sub in a new IC. They used 5532, but even a plain old 4558 would work.

[Helmholtz]

That input rating would seem out of spec by any standard (to me anyway).

According to Accutronics specs the 250 Ohm input transducer has a nominal DCR of 35R (Belton: 36R). Don't know about other brands.

[Enzo]

Your continuity readings tell us the pan is likely OK.

If you did need a pan, the very common 4EB2C1B pans would work fine.

Look at schematic, note low side of drive transducer does not go to ground, it goes to a resistor.

At this point I'd be voting on the IC.

[QdB]

I will look at getting some replacement ICs. Swapping it will be a fun challenge, it is not socketed, just soldered right to the board.

Thanks everyone. This is hands down, the best forum for these sorts of issues and fixes.

[Helmholtz]

Swapping it will be a fun challenge, it is not socketed, just soldered right to the board.

Use solder wick to unsolder.

[vintagekiki]

You're bad luck with your Sunn
https://music-electronics-forum.com/showthread.php?t=45400

Disconnect red and black cable from reverb tank.
1. Check that connectors on cables and reverb tank. If they oxidized, clean them.
2. Check that connectors on red and black cable properly soldered. Typically, cable shield's in interrupt.
3. With Ohmmeter measure resistance on reverb tank connectors.
The red (left) connector should have 250 Ohm, black (right) connector should have 2250 Ohm. If any of coils interrupt, resistance is infinite.
Remove reverb tank and check that some reverb spring are break. Check wires from coils, if interrupt, continue it and solder to connector.

_{Sunn SL260}

1) Reverb Tank - 4DB2C1D
https://www.amplifiedparts.com/products/reverb-tank-mod-4db2c1d-medium-decay-2-spring

[Randall]

I would recommend getting a solder flux pen applicator to desolder your op amp, assuming you don't already have one.

[g1]

The red (left) connector should have 250 Ohm, black (right) connector should have 2250 Ohm. If any of coils interrupt, resistance is infinite.

Those are impedance specs.
Corresponding DC resistance numbers would be around 35ohms at the input, and around 200ohms at the output.
So the numbers he gave in post #3 indicate that the transducers in the tank are both good.