Marshall JMP1930 Trem Problem

[Vaughan]

Hi Guys
I have in my workshop a JMP1930 (probably the worst amp Marshall ever made!!)
I've been working through a restoration job for my customer and so far the amp works OK.
But....... I cannot get the Tremelo circuit to work. It's a fairly standard Phase-Shift circuit
but it refuses to oscillate. I've changed all the components and changed the ECC83 for a
new one .... Nuttink!
Any Ideas please.

Vaughan

[Jazz P Bass]

Does the switch function properly?

[Vaughan]

Yeah, the switch is fine. Its a horribly noise amp with all the controls being straight after the inputs.
Both sets of inputs work fine. the only adjustments for the trem are speed and intensity(depth) on the front panel
and what seen to be an adjustable bias pre-set pot mounted on the tag board.
There are no voltages given on the drawing but they seem to be about right ..... Real puzzle!

[Jazz P Bass]

So what voltages do you have?
V1 3 & 6.
V1 8
What voltage is on either side of the 1 meg R?

[Vaughan]

B+ = 335V
V1b Plate = 124V
V1b cathode = 1.81V
Cathode voltage varies from 1.61 to about 2V and the V1b grid also varies from 0V to 2.9V when
turning the "intensity" control (lower othe two pots) from zero to full.

[Jazz P Bass]

So the oscillator appears to be oscillating.
Whether or not it is enough, as far as the amplitude , I do not know.
What is happening voltage wise (Vac & Vdc) on both sides of the 500pf coupling cap.

[Vaughan]

Sorry
Ido't think I described the voltage variation correctly.
The circuit is not oscillating but the voltage varies depending on the position of the 1K "intensity"(depth) control.
I have checked the circuit using a "scope" with and without input signal at all settings of the controls.
Starting to tear hair out now.....and I don't have too much more to lose.
Thanks for your suggestions, it gives me another perspective on the problem.
I'll have another look tomorrow when the headache has gone.

[Jazz P Bass]

One more question.
Is the 25uf cap new?
Is it installed correctly (polarity)
I can see why you are wanting to bang your head.
Maybe go back & physically check that the connections are indeed true.
Point to point.

[Enzo]

Well yes, of course the cathode voltage changes when you vary the cathode resistance.

Measure voltage from cathode to grid of the B section , NOT grid to ground. WHat voltage is biasing the grid?

There are the three feedback caps from plate B to grid B. Between pairs of them are resistive paths to ground. One is switched, the other variable for speed. Measure, with power off, resistance from those two cap junctions to ground. It is not enough to check the switch and speed pots, we must verify the whole circuit there is intact. A broken ground connection is the same as an open resistor to the oscillator. For that matter measure resistance from grid B to ground, should be about 1 meg.

If you replaced everything, then reverify the feedback caps are the proper value, and recheck the resistors. It is very easy to inadvertantly install a 470k where the 470 OHM went - we have all done that.

Try disconnecting that .68uf or whatever it is to the right of the speed control.

Tap on grid B with something metal - a meter probe perhaps with black lead ungrounded. This should inject some noise or pops into the triode of the oscillator. While doing that scope the plate - is the noise being amplified? If so, follow it through those feedback caps. Is the noise pulse getting through them?


Turn the intensity control to full - that ought to be at max resistance or 1k. Speed control on slowest. Now slowly run the other trim control through its range to vary the tube bias. Does the oscillator try to wake up anywhere along that?

[Vaughan]

Hi Enzo
Thanks for your response. I had tried all the things you suggested - except one!!
On removing the 0.68uF to the right of the speed control - hey presto, it all sprang(sprung?) into action.
Tested the cap on my tester all OK
Tried several different values (both larger and smaller) but with the cap in place it's dead!
Without the cap in and with a little adjustment of the 5K trimmer quite a nice trem is available.
Question - Why the *&^%$** is the cap there at all???

[Vaughan]

As a sort of follow on to the last post. I've found several photos on the internet
of other peoples JMP1930 and when you look at the board they've all got this cap in place.
So howcome mine doesn't work??

[Enzo]

DO you have the old part you replaced? Is it indeed the 0.68uf value? Or might it be 68pf instead and the schematic is a typo? 0.68uf sounds to me a bit large to hang on that feedback loop.

If removing it makes the amp work, stop fixing it and enjoy the working amp.

[Vaughan]

Thanks Enzo
I'll take your advice and stop right now.
The Cap was the correct value and tested OK on My bridge tester - so who knows!
Thanks to all the respondents for stimulating my overworked brain.