Marshall TSL100 Reverb Diagnosis

[theguitartech]

Hi all. Im pretty new to amplifier repair and I have the opportunity to practice on a Marshall TSL100 whose reverb isnt working properly. Never worked on reverb before so Im hoping you guys can nudge me in the right direction. Here's where Im at now-

440Hz sine wave from my signal gernerator into the front of the amp, clean channel, reverb mix knob at 100%. You can hear the signal through the speaker.

If I unplug the reverb tank and send signal to the reverb return I can hear it through the speaker. Also if I bang on the tank I can hear it through the speaker.

This amp has a small circuit board specifically for the reverb circuit. When I check the input of this board (CON 1) with my scope I get a clean sine wave on hot and a flat line on ground.

When I check the reverb send jack (PH1) with my scope I get garbage. When I check hot and ground seperately (probe attached to chassis ground) I find there is a clean sine wave on both the hot and ground side of the reverb send jack. The sine wave on the hot side is about double the size of the sine wave on the ground side. Backtracking from the reverb send ground towards the input of the board the sine wave on ground gos away at the - side of C5.

Any advice on what to check next? Should I be seeing signal on the ground side of the reverb send jack?

[52 Bill]

The drive circuit places the reverb tank input coil in the feedback loop of the driver chip(s).

You've noted checking everything but the tank itself. Have you tested the input coil and the connecting wire?

[theguitartech]

I checked both coils for DC resistance a couple weeks ago, though I don't remember what the readings were. They weren't reading open or shorted. I'll check again tomorrow.

[Enzo]

That circuit will not be happy trying to operate without the pan connected. If you MUST work without the pan, or the pan is open, connect a resistor in place of the pan input.

[theguitartech]

Thanks for the input guys.

Ok, I reconnected the pan to the circuit and the signal is indeed a lot happier looking. I retested DC resistance on both coils of the pan - 28 ohms on the input and 220 on the output.

I also went ahead and plugged the pan into an known working amp. It works fine.

The sinewave Im getting at PH1 isnt perfect looking but it's much cleaner then the garbage I got there without the reverb pan plugged in. I get the same signal inside the reverb pan before going into the input coil.

From another amp I worked on I noticed that the springs in the reverb pan would vibrate when I fed signal into the amp. Is that normal? I'm not seeing that at all here.