Twin Vibrato Issue

[Mesotech]

I have a Silverface MV Twin Reverb that someone brought me to look at. The issue they wanted me to look into was a bad hum.

I turned the amp on and quickly determined the hum was only present when either the vibrato footswitch or the reverb footswitch were engaged, though neither vibrato or reverb were functioning.

After peeking in the back of the amp, I noticed they had swapped the footswitch and reverb send/recovery positions, sending the reverb signal out into the vibrato footswitch input, and the reverb recovery into the reverb footswitch. Once I put these jacks back into the proper places, the hum went away. The reverb started working (and works well). However the vibrato has an issue.

When I engage the vibrato with the footswitch, I can hear the oscillation start, but it fades away quickly. When I turn off the vibrato with the footswitch and then turn it back on again, it will begin to oscillate again, but fade quickly away.

I havn't opened the amp yet, but wanted to see if anyone had ever experienced this issue before. From all indications, the vibrato system starts, but doesn't keep going. Otherwise, this amp is working normally (as far as I can tell).

[NTBluesGuitar]

Bad caps in the trem?

[Mesotech]

I'm not sure yet. I've been trying to research exactly how the vibrato circuit works, looking for reasons to make it not work.

Basically what I've learned is that the bug is functioning, the tube is fine, and the speed pot has no effect regardless of position. The intensity knob does work as expected. Once the initial oscillations fade away, there is a noticable drop in amp volume.

My current theory is that a load resistor may be shorted, causing a connected capacitor to drain and not recharge until the switch position is changed. Then the cap will recharge until the switch is engaged again, causing the cap to once again drain. Again, this is only theory at this point.

I did manage to open and inspect the amp tonight, and I saw nothing out of the ordinary. None of the caps were in bad shape or leaking. All solder joints appear original (except for the filter caps in the doghouse, which have been replaced at some point). The inside of the amp was filthy with dust, and I brushed it with a paintbrush to knock the loose stuff away. I did test the speed control (since it appeared to have no effect on the symptoms or function as I expected) but it tested fine.

I'll have more time tomorrow to go through the rest of the circuit. I was obviously hoping for a quick solution, but this one might take some head scratching to figure out.

[Kap'n]

Just about every trem issue begins with the three series caps.

[Mesotech]

Technology is great (or should I say, it might be great - because I haven't yet proven the culprit). Using a circuit simulator software program, I was able to recreate the vibrato circuit according to the schematic. Then I simulated failures of various components until I could duplicate the symptoms I experienced. The result seems to be one or the other of the 1M Ohm resistors is open.

When this happens, it causes the associated capacitor (used to flip the phase by 60 degrees each) to charge and not discharge. This ceases the oscillation. There are three of these RC filters, used to rotate the phace 60 degrees each until the phase is completely 180 degrees out. If the first resistor opens, the oscillator never starts. If the second or third resistor opens, I get the same results as my symptoms, but the time from start to finish varies slightly. The farthest resistor down the line allows for the longest amount of oscillation before it fades away. The middle resistor allows for a shorter duration.

So basically, according to the circuit simulator, I should find one or the other of these resistors open, and replacing it should fix the problem. I'll see if this is all true tonight, as I already have a few of these resistors handy.

[Mesotech]

I finally got back to this amp today. It turned out the 1M's were fine, which left me scratching my head again. After poking around some more, I found the 100K which ties the speed control to ground was loose at the ground side. This resistor is tucked up under the front panel and visually seemed connected. When I touched it with my meter lead, it proved to be disconnected. After resoldering the loose end back to ground, the vibrato worked again.

So in essence, the amp is now 100 percent functional again, and I didn't need to replace a single part. Gotta love it when that happens.

[Kap'n]

Half of troubleshooting is just poking around.

[NTBluesGuitar]

I was troubleshooting a SF Twin once and was really frustrated that the trem didn't work. I spent 30 minutes testing out my RCA shorted jack and wondering if it was bad, etc.

I poked around inside next, and all the resistors and such tested out okay.

I put the amp aside for a couple of days, came back, and finally noticed the photo-resistor was missing.