Marshall Mercury Tremolo problem

[Ewan Penkey]

Hi, I hope someone here can help.

I have this old Marshall Mercury I'm fixing up for a friend.
I've completed all the usual servicing bits, and was making good progress, but the tremolo has me a bit stumped.
The tremolo works, and the intensity control works well, but the speed control does so little it's virtually imperceptible.
I've replaced the capacitors, and some resistors which had gone a little high.
I've tried changing the pot to lower and higher values, and I've tried altering the value of R19 slightly in either direction.
Nothing seems to make the speed control have much effect at all.
Any ideas please?



Many thanks,

Ewan

[pdf64]

That seems nuts, dunno how it could have any useful range
I suggest to try halving R19 to 100k, and using a 1M or 2M pot for VR3.

I remember a friend having an orangy red Mercury combo as his first amp - happy days

[Ewan Penkey]

Thanks - I'll give that a try.

Ewan

[Ewan Penkey]

That seems nuts, dunno how it could have any useful range
I suggest to try halving R19 to 100k, and using a 1M or 2M pot for VR3.

I remember a friend having an orangy red Mercury combo as his first amp - happy days

Many thanks PDF64 - I halved R19 to 100k and after a bit of experimentation, settled on a 500K Linear pot. The speed control has a sensible range now. :-)

Ewan

[Chuck H]

Could be a schematic typo? Maybe VR3 was supposed to be 1M all along. Time passes and pretty soon things get funny even at the manufacturing level. There are a few cases like that I've read about here.

[g1]

Could be a schematic typo? Maybe VR3 was supposed to be 1M all along. Time passes and pretty soon things get funny even at the manufacturing level. There are a few cases like that I've read about here.

I'll wager that is not a Marshall factory drawing from the 70s. So it's questionable.
If Ewan drew that up based on what he found in the amp, no guarantee it's factory either.

[Chuck H]

Revisiting the visual that seems very likely.