Dean Markley RM-150-DR No Reverb

[misterc57]

After fixing some power supply issues on this amp I am left with the No Reverb issue.

The reverb tank is a OC Electronics 757

Measured 21 ohms at input and 186 ohms at the output

I have signal both at the input and output of the tank as well as at the connector to the preamp board. Also have signal at the 3 legs of each channel reverb pot.

Not hearing any reverb effect on either channel.

I do hear the crash of the tank when banging it, both with or without the reverb footswitch connected. When the footswitch is connected I can toggle the crash of the tank.

The input ground wire is not connected to the circuit, pretty sure by design.

I have not found the correct schematic but I do have the RM-80-DR schematic attached.

Any ideas? Thank you.

DeanMarkley-80-DR-Preamp.pdf

DeanMarkley-80-DR-Poweramp.pdf



edit. another thing is I can see that channel 2 reverb pot is not original.

[Helmholtz]

Also have signal at the 3 legs of each channel reverb pot.

One end of each rev pot should be grounded, so you should have no signal there.

Does the signal at the rev pots contain reverb or dry only?

[misterc57]

One end of each rev pot should be grounded, so you should have no signal there.

Does the signal at the rev pots contain reverb or dry only?

It was an injected 1K signal so I cannot tell if it was wet or dry.

However, I did tighten up the nut on the reverb jack for the footswitch, and I had reverb working intermittently a few times.

I thought it was coming and going when wiggling the cable at the pan output . I snugged everything up but it has yet to work again.

All 3 legs of each reverb pot has under 1 ohm R to ground

[g1]

Either you do not have signal at the reverb pot or the lugs are not 1 ohm from ground each. Both things are not possible.
(I suppose if ground is not really ground you could have odd issues)

[misterc57]

Either you do not have signal at the reverb pot or the lugs are not 1 ohm from ground each. Both things are not possible.
(I suppose if ground is not really ground you could have odd issues)

You are right. I had a bad reference to ground. Only one tab on each pot is at ground.

[g1]

So go back and scope at the reverb pot again. There can't be signal on the lug that is grounded. Is there signal at the wiper?

[misterc57]

So go back and scope at the reverb pot again. There can't be signal on the lug that is grounded. Is there signal at the wiper?

I had to put this aside for now. I will circle back to it. Thank you.

[misterc57]

So go back and scope at the reverb pot again. There can't be signal on the lug that is grounded. Is there signal at the wiper?

Things have changed, I do not know why. I have no signal on the three pot legs. I hear audio at the reverb output, none at the input.

I know it is not the pan cables or connections. Trying to trace back to where the input signal disappears but this is a tough board (for me, as things are so small) and I do not have a good schematic.

In the first pic the third pin going left to right is the pan input, no signal. I stepped back a couple of points on the traces, there is no signal.

The first pin (pan output) has signal. I assume this is the dry signal.

[g1]

Are you sure about the in and out? Sounds reverse of what should be. Can't see how there would be dry signal getting to the tank output.
The schematic shows 2 small transistors driving the tank. 'Out' of the tank goes to the input pin of an op amp IC. There is no way for dry signal to get there.

[misterc57]

Are you sure about the in and out? Sounds reverse of what should be. Can't see how there would be dry signal getting to the tank output.
The schematic shows 2 small transistors driving the tank. 'Out' of the tank goes to the input pin of an op amp IC. There is no way for dry signal to get there.

You are probably right that I have them reversed. Connected the other way gives a loud 299 Hz hum that varies in volume with the reverb pot. So my initial thought was the it is connected backwards.

[misterc57]

So input is pin 1 and output is pin 3.

and that being the case, the input to the tank cable has no ground connected at the board. The output cable from the tank is grounded at the board.

I connect the shield grounds together and reverb is working fine now! Yay!

[g1]

No idea what happened there. Does the tank seem like a stock unit or might it have been replaced?

[misterc57]

Small stock tank to fit in a tight small space. The amp head is a two space rack unit that slides into the combo cab. Tank was loose inside the amp unit when I got it. Ground shield on input cable was not connected and at first I thought by design.

The reverb is working perfectly now after I connected the ground shield.

Thank you

[g1]

Glad you got it working. My concern was that the schematic shows the tank in and out 'grounds' going to different places.
Maybe the schematic is not correct for this model. But if it is, there may be issues with shorting the tank input (-) to ground rather than pin2 of IC9A in drive section (which gets grounded through 10 ohm). In effect this would short out the 10 ohm resistor there.

[Helmholtz]

Glad you got it working. My concern was that the schematic shows the tank in and out 'grounds' going to different places.
Maybe the schematic is not correct for this model. But if it is, there may be issues with shorting the tank input (-) to ground rather than pin2 of IC9A in drive section (which gets grounded through 10 ohm). In effect this would short out the 10 ohm resistor there.

Good point!

Reverb will work with drive transducer grounded but that would change current drive to voltage drive, changing reverb frequency response (less treble).

Quite a bit of misleading info here.
Wondering if using the scope instead of the signal tracer could have avoided confusion.

BTW, what happened to the Randall R-120 reverb issue?