Dead tremolo in 50's/60's Danelectro "Noble High Fidelity Amplifier" for Ampeg

[TASBrendan]

Hi all,
Looking for a little help with a dead trem in this amp. I've never seen a trem like this before and can't find a schematic for the amp, so I traced one out myself-
What I would like to know is, how the is oscillator supposed to modulate the signal when operating correctly and how would I see that on the scope? When I first powered up the amp, the trem worked but only if the rate and intensity were at the max.
The amp looks to be mostly stock except the pots were changed at some point. The ground connections on the pots are soldered directly to the chassis.

Thanks

[TASBrendan]

I should add that when I made the schematic, I was able to see DC modulation at the grid and cathode of the 3rd triode stage but didn't hear any modulation. I don't see it now and I've swapped out tubes with no change. The tubes are 2x12AX7s and a 6V6. The first tube is pretty standard 2 gain stage with volume and tone feeding 1/2 of the next 12AX7 with the other 1/2 as the oscillator. The filter network after the 3rd stage feeding back into the cathode has what I think are two resistors, but they're unmarked and read 7Meg on my meter.

[g1]

I wonder if it's similar to the Magnatone vibrato circuits, and you have some varistors in there. See attached

[The Dude]

Have you checked the footswitch switching jack? Maybe the contacts are dirty and not making a good connection.

[TASBrendan]

Very interestingly similar to the Magnatone! Varistor meaning the 3rd stage is acting like a varistor or there are varistors in the circuit? The foot switch jack appears to be ok, but I haven't taken it off and made sure the grounding at the chassis is clean. Oddly, the speaker and something else use the jack as the ground point, too. Using a pedal doesn't change anything, either.
Thanks for the help!

[Helmholtz]

Your "7M" resistors should be varistors.

Here's a great paper by member trobbins :
https://www.dalmura.com.au/static/Ma...o%20design.pdf

[TASBrendan]

Thanks Helmholtz! That paper is what I need! I was going to dive into Magnatone research today and that saves a ton of time.

[TASBrendan]

Well, something "simple" after all. In my drawing I forgot a .02 coupling cap at the wiper of the Intensity pot. Both .02 caps on either side of the pot were way leaky, about 8VDC across the pot. The pot is also way out of spec, about 2Meg at full range and never quite hitting ground at the bottom. So with those replaced, we have functioning trem. I should have suspected this first since the trem didn't work without the pot being at max. Ah, well it's a life of learning. Thanks all for the help and the Magnatone paper is going to come in handy in the future!