Marshall JTM45 reissue very poor tremolo

[blindboybenton]

Good moring all

I had a local musician give me this amp for repair.He brought it used and didnt notice that the tremolo was not right.
1962_Bluesbreaker_combo.pdf

If you turn it up full it works , not to fast, If you back it off you get to about 7-8 and it's at its cut off point, Turn it back up again and it seems to start but only after about 2 seconds.. In short there is no tremolo operation until you get to about 7-8. Intensity pot seems to be working

I went for the tube first and then i replaced the Jfet. ( J174) like for like and all new . Checked and changed the 22k preset pot, when you adjust the pot, there seems to be a switch the trem off or on point, with my limited electronics background, I'm unsure if that is correct or not

checked the soldering and tested with another 1m pot ( speed ) All resistors test correct in circuit, Change C16/19 and 47nf across V6a. I can't change any others at present as i ain't got any

Anode 330v and 127v
Cathode Pin 3 = 0.7
Cathode Pin 8 = 130v
Grid pin 2 = 0.13v
Grid Pin 7 = 127v all tested with speed up full

Can anyone help ? It would be good if somebody could expain how the circuit works.

bassman 1965

[52 Bill]

Do you have the footswitch turned on?

If you turn it up full it works , not to fast, If you back it off you get to about 7-8 and it's at its cut off point, Turn it back up again and it seems to start but only after about 2 seconds.. In short there is no tremolo operation until you get to about 7-8. Intensity pot seems to be working

What you are describing sounds like the low frequency oscillator is stalling at lower speeds.

If you look at the schematic that you posted, The trem circuit is all part of tube V6. V6a is the LFO and V6b is the drive/audio interface circuit. So you want to work on the V6a part of the circuit.

If you look at the circuit between the plate and grid of V6a there are a string of caps and resistors that form a phase shift oscillator. Basically, test all of the caps and resistors that connect to V6a for values. Make sure that there are no loose parts, etc. The shotgun method would be to replace all of the caps in the feedback string.

Once the LFO is working correctly, you will need to go back and adjust the preset bias control for the jfet to get the smoothest and widest range of trem depth.