Late MK2 Preamp with Tremolo problems

[Dote]

Hi gang,

as known, I was able to fix the clicking sound in a late MK2 preamp (with knobs, no sliders for the EQ). That was one of the LDRs not working. Oddly enough the Tremolo stops working when setting the Rate very low (say below 4 on the dial).

I swapped almost all the resistors and ICs around the Oscillator section (U1A and U1B), R15 is pending to be changed to new 47k.

Could the MPS14 be the culprit? A bad 100k Rev Audio pot of the Speed control? Any help? Attached you will find the schemo.

One sidenote:

I tried the MK2 preamp next to my MK1 slider preamp on my beloved MK1 piano. Interesting to note how different the two preamps sound. Where the MK1 preamp has a very organic depth and a lot of treble, the MK2 is a bit more silky. The overall volume of the MK2 is extremely dependent on the Treble knob setting. The bass and Treble dials do not at all interact. Very independent to my ears. Sounds a bit like a MK2 even with the MK1 (1974) Piano. Maybe thats psychoacoustics?

Thanks in advance,
Dote

[vanceinatlance]

I have something similar happening on my MarkII Suitcase with sliders. I believe in my case the potentiometer is probably the culprit. I too replaced the LDR's on mine to stop the insane popping and clicking. I have a situation where one channel disappears when the intensity is above four. If I turn the dial back and fourth, I can hear it come in and out a bit. Unfortunately, I have not yet replaced it to verify for sure. It's nice and quiet now with the new LDRs and I have been happy enough to put off the potentiometer for a while. 
Maybe if you turn the dial back and forth a bit you could see if it intermittently works?

[Max Brink]

It might be important to point out that the MK2 preamp with knobs is for all practical purposes the same amplifier as the slider "Janus" model. They just changed the slider pots to rotary pots (and I believe this will only be on black rail MK2's).

It's really hard to troubleshoot an amp remotely, and I would always recommend seeking an amplifier expert if you are unfamiliar of the risks associated with working with any vintage electronics. It's important to note that the power amplifier is a dangerous place to be working on if you are unfamiliar with the proper safety procedures required to work on this circuit.

The good news is that it could be a really simple fix. If the vibrato is turned to a slow speed and it causes the amp to "shut down" it may likely be a bad connection between the preamp, and the power supply (board closest to the transformer in the power amp section). Could be the five pin connector but even more likely it's the pin connections on the board itself. Give them a good (and safe) shake and see if you can get it to come reliably back on.

[Dote]

Thanks for all the tips!

vanceinatlance was right, it was the speed pot. I opened up the pot and the conductive trace was in a really bad shape in the low speed region. Like almost not there anymore. Needless to say the remainings of the conductive material could be found inside the pot casing. Fortunately I had a similar CTS 100k (although lin) in my basket that I used the inner parts to repair the pot. So only the outer casing and the shaft are from the original pot.

The pot range is ok in my book, although the pot is lin now, there is a bit more fine tuning for the slow tremolo settings, but that suits me fine.

So look at the Speed pot if you have a tremolo that quits working in a specific range of the pot.

Thanks,

Dote

[vanceinatlance]

Hey, great to hear you fixed it! I guess I will have to get around to taking mine apart and looking at it. I got kind of lazy after replacing the LDRs.