Marshall 1987T tremolo issues

[JohnNYC]

Hi all,

I'm new to this board...thanks for accepting my membership!

I am doing a restore on a friend's 1969 Marshall 1987T and I have almost everything running smoothly and up to spec. It sounds amazing...all except for the tremolo.

The tremolo oscillates just fine and inserts itself into the circuit for a VERY noticeable effect. Max speed almost sounds like a "ray gun"!

However, as I add intensity, I notice that I also add some distortion to the signal. And, with the addition of the tremolo, the output decreases slightly.

Upon further inspection, tracing the signal through the oscillator circuit with a scope, it appears that only half of the sine wave is making its way through. I got this from inspecting the signal coming through the collector of the 2G374. Here's a picture:

[img:722:563]http://home.earthlink.net/~valleypoint/ ... lector.jpg[/img]

When I scope out the output, you can see the composite signal not delivering a full oscillating sine wave, compared to a Fender Vibrato. It's much "fuller" on the positive swing of the wave.

[img:722:582]http://home.earthlink.net/~valleypoint/ ... em_out.jpg[/img]

Is this normal for this circuit, or should I start checking components?

I do understand that Marshalls weren't best known for their tremolo circuit. I'm just trying to get the amp fully spec'd, even if my friend NEVER uses the tremolo!

Thanks in advance for any assistance you can offer.

FYI, if you can't find an original 2G374 transistor, the NTE 102A makes for a suitable cross-reference replacement.

Best regards,
JohnNYC

[martin manning]

Does the oscillator's signal look symmetric? If so perhaps the transistor is not biased correctly?

[Reeltarded]

John, I have had 8-10 good T models and they all sound tilted and .. shitty. Baup baup baup instead of wuuuwwuuuuuwwwuuuuuwwwuuu..

You very well may be finished. Maybe not. Never a reason to use the trem here!

[JohnNYC]

Taking your advice, Martin, I checked the biasing of the transistor.

Here's a snapshot of the oscillator circuit:

[img:473:361]http://home.earthlink.net/~valleypoint/ ... ircuit.jpg[/img]

It turns out that the 68k resistor at the base was actually 680k!! And it was originally what Marshall put in there.

I swapped it out and put the correct value and it seems to be a little better. It's still not a symmetrical signal, but it looks a little better on the scope.

BTW, when I said "ray gun" tremolo, I checked out the frequency at max speed, and it's about 14 Hz!!

Thanks for all the help...this is a great forum!!!

All the best,
JohnNYC

[martin manning]

I don't know how much you want to keep messing with this, but you could try putting a pot in place of the 4k3 bias resistor and dial in an optimum bias point. On the speed issue, you can try increasing one or both of the 0.01u caps to 0.02.