Troubleshooting amp Reverb

[sonolink]

Hi guys,

This friend of me brought me an Engl Screamer combo that has lost its reverb!
He tells me one day he switched the amp on and it began sounding with tons of reverb so he turned the reverb down but could never turn any back on. It's dry since then.

Any ideas where to begin looking?

Thanks in advance for your help and time!
Cheers
Sono

 

[rock soderstrom]

First idea is to check if the output transducer and recovery amp are still functional? Kick the amp or beat the reverb pan . Can you still hear the spring reverb earthquake sound?

If yes, than your problem is on the driver side. Check the wiring and the driver tube.
Measure the resistance of the input and output coil of the reverb pan with your DMM.

 

[CJ]

Spray the cables and rca jacks if any.

 

[sonolink]

Thanks for your input Rock Soderstrom and CJ

If yes, than your problem is on the driver side. Check the wiring and the driver tube.
Measure the resistance of the input and output coil of the reverb pan with your DMM.

I might be wrong but I think the reverb is not driven by a tube in this amp.
When you say to measure the resistance of the input and output coil of the reverb, you mean between the signal pin and the ground pin of the RCA connectors on the input and same thing on the output on the reverb itself?

Cheers
Sono

 

[rock soderstrom]

When you say to measure the resistance of the input and output coil of the reverb, you mean between the signal pin and the ground pin of the RCA connectors on the input and same thing on the output on the reverb itself?

Yes, exactly.
Please see if you can measure anything at all or if one of the coils is open.

Check also which code is on the Reverb Tank, then we can find out which impedances the windings have.

 

[rock soderstrom]

I might be wrong but I think the reverb is not driven by a tube in this amp.

You are right, the reverb was realized in solid state technique, the tank is nevertheless an 8Ohm type, as in a classic tube driven reverb but without a transformer.

# Engl Screamer 50 schematic

 

[mad.ax]

There's a dedicated DC rail for the reverb, so check that you get DC supply at pin 6 of U2 and pin 7 of U3.
If yes, then check that the reverb relay works.
If yes, yank the tank to validate the recovery stage.
If yes, focus on U2.

 

[mad.ax]

Of course all the above is assuming the tank's coils are not open...

 

[sonolink]

Yes, exactly.
Please see if you can measure anything at all or if one of the coils is open.

I get 1R on the red rca and 1k on the input. Trying to get the codes of the tank now

 

[sonolink]

The tricky but now is finding U3 and U8....

 

[mad.ax]

Finding U8 might be impossible indeed... As there's not that many ICs in this amp!

There are only 3 ICs, and one of them has 14 pins so forget it. It's on a separate board anyways.
The two others are 8 pins, should be easy to spot. One of them is a TBA820, that's U2.
The other is a TL071, that's U3.
If you have trouble spotting them, follow the cables that comes from the reverb tank...

I tried to look at pcb pictures of this amp, and it looks like you will need to unscrew the main board in order to see the component side...

 

[rock soderstrom]

I get 1R on the red rca and 1k on the input. Trying to get the codes of the tank now

The schematic says it should be a 9AB2A1B with about 1ohm resistance at the input and around 200 ohm at the output of the pan.

The 1k at the output is actually too high...

 

[mad.ax]

"The 1k at the output is actually too high..."

Strange, but I'm not sure that is the culprit because the amp was working with this tank...
Looks like this amp would be fited with a 9aCxxxx instead of a 9aBxxxx. Could be a change in the production line...
If Sonolink gets to the sticker, we will know...

At any rate, I cannot imagine how a 2K coil could fail and become a 12K one?? Either it breaks and become open, either it shorts and its resistance gets LOWER.

 

[CJ]

Sweaty hands can read 12 k
I have seen transformers with coils that go high, the wire breaks and the ends stay butted up against each other enough to create a partial connection that then reads much higher than the original DCR.

 

[sonolink]

Ok guys I think I got to the source of the problem: the female connector on the board that connects the RCA cable to the tank is a bit dodgy and since the male connector "hangs" from the amp, when this one is moved around it probably breaks the connection.

Since getting a new female connector to replace the dodgy one on the board, having to disassemble half amp to desolder the female connector, etc, is kind of a nightmare I've used hot glue to keep it in position. The reverb works fine...

Thanks a lot for taking the time to help me out on this one
Cheers
Sono