I have a recently acquired 1987 Marshall Jubilee (model 2550). I've recapped, retubed and rebiased it.
The amp is a "sort of" channel switching amp. There's no independent EQ but there is a "clean channel" and a "lead channel". The lead channel introduces an extra gain stage and a diode clipping circuit. The amp has one weird behavior -- when on the clean channel, there is distortion that fades in and out / level jumps but only when the lead master pot is up. With the lead channel volume pot on zero the clean is perfect.
Here's the relevant part of the preamp schematic. Circled in red (because I think they're relevant) is the lead master pot, and the switch (actually a relay) that selects the channel.
With the switch in the position shown, the amp is on the lead channel.
The input signal comes in via V1A. The clean route goes more or less directly (ignoring the "rhythm clip" circuit) to the grid of V2A. The lead signal goes to the grid of V1B, and the signal off V1B feeds the diode clipping circuit and passes (via the lead master pot) to the grid of V2A.
When the channel switch flips to clean (the other position, not as shown in the schematic) the effect is to ground the lead signal heading into the diode clipping circuit and also to ground both sides of the lead master pot.
My thinking is as follows -- if both sides of the lead master pot were actually, properly grounded, the position of the lead master pot could not make any difference to the signal downstream as there could be no signal at the wiper of the pot. Since the position of the lead master pot does make a difference to the signal downstream, the assumption that both sides of the pot are grounded must be incorrect.
Therefore that channel switch (which is actually a 36-year old relay) can not be properly closing and grounding the signal and that component is likely the cause of the problem.
I'm not an experienced amp tech by any means, so any comments / critique of my thought process so far would be very welcome!
The amp is a "sort of" channel switching amp. There's no independent EQ but there is a "clean channel" and a "lead channel". The lead channel introduces an extra gain stage and a diode clipping circuit. The amp has one weird behavior -- when on the clean channel, there is distortion that fades in and out / level jumps but only when the lead master pot is up. With the lead channel volume pot on zero the clean is perfect.
Here's the relevant part of the preamp schematic. Circled in red (because I think they're relevant) is the lead master pot, and the switch (actually a relay) that selects the channel.
With the switch in the position shown, the amp is on the lead channel.
The input signal comes in via V1A. The clean route goes more or less directly (ignoring the "rhythm clip" circuit) to the grid of V2A. The lead signal goes to the grid of V1B, and the signal off V1B feeds the diode clipping circuit and passes (via the lead master pot) to the grid of V2A.
When the channel switch flips to clean (the other position, not as shown in the schematic) the effect is to ground the lead signal heading into the diode clipping circuit and also to ground both sides of the lead master pot.
My thinking is as follows -- if both sides of the lead master pot were actually, properly grounded, the position of the lead master pot could not make any difference to the signal downstream as there could be no signal at the wiper of the pot. Since the position of the lead master pot does make a difference to the signal downstream, the assumption that both sides of the pot are grounded must be incorrect.
Therefore that channel switch (which is actually a 36-year old relay) can not be properly closing and grounding the signal and that component is likely the cause of the problem.
I'm not an experienced amp tech by any means, so any comments / critique of my thought process so far would be very welcome!
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Well, I've never used one, but the schematic looks to me like the relay is actually in the clean position as drawn, and from your description the amp is actually working properly as is.
The amp is definitely not working entirely properly, because the distortion mixed with the clean sound with the lead master turned up jumps around in volume - it sounds a bit like intermittent signal breakthrough. But can you help me understand how the channel switch works, if I don't have it right?
The above clipping diode network (LED2, LED3, D1, D2, D3) provides extra distortion for the lead channel as I understand it. With the switch in the other (not-as-drawn) position that network is shorted, isn't it (both sides connected to ground)? So if you're correct, I don't understand how the as-drawn position could be clean and the not-as-drawn position the lead channel one.
The above clipping diode network (LED2, LED3, D1, D2, D3) provides extra distortion for the lead channel as I understand it. With the switch in the other (not-as-drawn) position that network is shorted, isn't it (both sides connected to ground)? So if you're correct, I don't understand how the as-drawn position could be clean and the not-as-drawn position the lead channel one.
I looked over the board while I was recapping it and didn't see anything obviously broken -- and I was working in the vicinity of RL1 replacing electrolytic caps C22 and C23, but that's a good thought. I'll pull the chassis out again today and take a closer look (and see if I can get some Deoxit to the relay contacts). So far I've just been poring over the schematic trying to work out where I should look first.
And yeah, @leadbelly -- it is definitely confusing and the "channels" are interactive. The clean channel appears to be connected to the downstream circuit (effects loop, PI, power amp) at all times, and even when the amp is working normally you can hear whether or not the clean channel "rhythm clip" is on, even when the amp is nominally not on the clean channel.
So, I’m not sure how permanent a solution this is, but blasting the relay with Deoxit D5 and then exercising it a bunch of times did seem to do the trick!! Hopefully it lasts because the original relay appears to be unobtanium.
Is this a SPDT relay? What is the coil voltage? What is the part number?
I've made PCBs for APT Holman audio preamps to adapt excellent quality, current production small signal relays to the 40 year-old circuit boards. Unless there is no room above the existing relay, it should be quite possible to make a conversion board.
Do you have photos, plus the info above?
Not sure blasting the relay with D5 is a good idea, as it could saturate the coil.
Other thing to do is actually confirm it's the relay by desoldering and testing, but that would be best with an oscilloscope and/or distortion meter and a good signal generator.
Photo please.
I've made PCBs for APT Holman audio preamps to adapt excellent quality, current production small signal relays to the 40 year-old circuit boards. Unless there is no room above the existing relay, it should be quite possible to make a conversion board.
Do you have photos, plus the info above?
Not sure blasting the relay with D5 is a good idea, as it could saturate the coil.
Other thing to do is actually confirm it's the relay by desoldering and testing, but that would be best with an oscilloscope and/or distortion meter and a good signal generator.
Photo please.
Thanks for the reply. It's a SPDT 5V relay, p/n 211B D005-P. I think it's this (long discontinued) Fujitsu line:
https://www.fcl.fujitsu.com/downloads/MICRO/fcai/relays/fbr211.pdf
D005 means 5 volt, P indicates gold-overlay silver-palladium contacts.
Externally looks identical to this (different model but from the same line):
https://www.fcl.fujitsu.com/downloads/MICRO/fcai/relays/fbr211.pdf
D005 means 5 volt, P indicates gold-overlay silver-palladium contacts.
Externally looks identical to this (different model but from the same line):