Jumpering W3 and W4 will only bypass the FX loop. The master is connected between W3 and BN4 on the PCB which is after the FX loop.
1987x Buzzing/humming - Filter Caps?
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Jumpering W3 and W4 will only bypass the FX loop. The master is connected between W3 and BN4 on the PCB which is after the FX loop.
Since I bypassed the FX and am still getting the noise, doesn't that eliminated the FX loop jacks as the culprit? My understanding is the FX loop jacks are only in the signal chain when the FX loop is activated.
I've decided to pull this master volume out and replace it with a Lar/Mar PPIMV using a dual gang PEC pot in the same location. I'll reconnect W3 to BN4 using the stock brown 22awg wire and be sure to use sheilded wire on the PPIMV.
ampmadscientist
Well-Known Member
I would eliminate the MV completely, wire it stock and re-test.
I would not modify a broken amp.
Verify that stock works correctly first.
ampmadscientist
Well-Known Member
I sounds like an open connection to the input of the PI. Like the PI input is not connected to anything.
maybe the pot is defective...maybe a wire is accidentally broken under the insulation.
1. make the amp work stock first.
2. make sure that all the bugs are fixed first.
3.Then modify - last.
ampmadscientist
Well-Known Member
There is a problem w/ PPIMV kits.
There should be a pot which has a known voltage insulation rating.
The insulation could fail, and short the bias voltage to the chassis.
If the pot has no documented DC voltage rating- I would definitely not use it.
Many cheap pots - are rated 35 Volts or lower.
This is why the vendor may be hiding the specifications of the part.
I would not use a pot rated lower than 250 VDC.
I would never use a pot - unless it has a real specification sheet, from the actual manufacturer.
ALL legitimate parts - have a specifications sheet.
If there is no spec sheet - be suspicious.
Say anything you like!
you do not install "unrated" capacitors in your B+ rail...do you?
Then why would you install an un-rated DCV pot in the bias circuit?
There should be a pot which has a known voltage insulation rating.
The insulation could fail, and short the bias voltage to the chassis.
If the pot has no documented DC voltage rating- I would definitely not use it.
Many cheap pots - are rated 35 Volts or lower.
This is why the vendor may be hiding the specifications of the part.
I would not use a pot rated lower than 250 VDC.
I would never use a pot - unless it has a real specification sheet, from the actual manufacturer.
ALL legitimate parts - have a specifications sheet.
If there is no spec sheet - be suspicious.
Say anything you like!
you do not install "unrated" capacitors in your B+ rail...do you?
Then why would you install an un-rated DCV pot in the bias circuit?
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ampmadscientist
Well-Known Member
Except
disassemble the pot, lube the track with non-conductive lubricant such as deoxit G 100, deoxit D100, or LPS-1.
The factory lubricant will not last.
Then, reassemble and install it.
Yes, PEC is rated 500 VDC.
Although PEC is the highest rated, there is an issue w/ the lubricant.
After re-lubing the pot with a better treatment, then it's pretty good.
(use only a non conductive lubricant which is highly rated)
(do not use non-lubricated cleaners)
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Haven't seen a schematic, but a physical switch would infer that if designed the way I'd expect it. I'd agree with the rest, reverting it to stock is the place to start.
ampmadscientist
Well-Known Member
I'm just wondering if it's really wired right...or if there is a break in the signal path (that's what it sounds like).
Guitar-Rocker
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Correct. Now you know the problem lies in the (inserted non factory Master), whether it is a bad connection or any one of the previously mentioned items to check.
I think there's more to it than that. I printed out the photos and the 1987x schematic. Look really close at W2 & W3 and what they did when they modified the hum balance circuit. They eliminated the pot (VR2) and directly pushed those resistors to W2 (which, if the EFFECTS card board label is right is (zero volts) and presumably ground)...but...R31 is now going to W3 which, also if the EFFECTS card layout is correct...is shoving half of the 6.3V lamp circuit winding right to the Loop/PI input.
I don't know if Marshall changed any post designators in 1987x revisions (?), but I'll bet that's your problem. Basically...you're getting your filament circuit fed straight into the PI...isn't AC lovely? :O
I don't know if Marshall changed any post designators in 1987x revisions (?), but I'll bet that's your problem. Basically...you're getting your filament circuit fed straight into the PI...isn't AC lovely? :O
This is based on your assumption that W3 on the effects card is the same as W3 on the PCB, correct? I don't think they are related as BN4 from the PCB connects to the W3 post on the effects card. I could be wrong. Additionally, R31's trace underneath the PCB goes to W3 with the other end going through VR2 to W1 and R32's trace goes from W2 through VR2 to W1 as well. Leading me to believe they are connected properly.
Thanks for your help!
it should be. Look at the schematic. W3 and W4 are there to connect to the FX loop...the schematic labels that and the points of W3 and W4 are the same place you would park the FX loop.
My only assumption is that THIS is the schematic that relates to your amp...
http://drtube.com/schematics/marshall/1987-01-60-02_iss4.pdf
You also need to closely inspect that board as that part of the circuit was modded. VR2 was eliminated...the wiring from those resistors was jumped across to those posts.
I'm not seeing your FX loop board in your photos...?...is it the Marshall board you have or was it something like a Metro board added?
My only assumption is that THIS is the schematic that relates to your amp...
http://drtube.com/schematics/marshall/1987-01-60-02_iss4.pdf
You also need to closely inspect that board as that part of the circuit was modded. VR2 was eliminated...the wiring from those resistors was jumped across to those posts.
I'm not seeing your FX loop board in your photos...?...is it the Marshall board you have or was it something like a Metro board added?
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ampmadscientist
Well-Known Member
It's entirely possible...did you look under the board?
The signal is supposed to be away from the power supply...
Well...I'd say first affirm that the link I sent for schematic above is indeed the same as your 1987x (revisions change a lot of things sometime so I can't really be adamant about what should be unless the diagram matches the chassis).
I've only found that one schematic and the inference is W2 is a ground reference...W1 would be the 30V to the FX card for power...W3 & W4 "should be" the signal feed for the FX loop. No power involved. They could tie to W2 for a ground (again, if the layout is the same as the schematic), but W3 should be a no-no as it would seem it should be part of the loop signal along with W4.
The ONLY reason those resistors were extended across to those turrets are that someone was too plain lazy to disconnect the pots and inputs...tilt the board up and do it right. VR2 probably would be connected to W2 for the ground reference.
Again, I'd disconnect the front panel pots & jacks...tilt the board (caps drained) ...and straighten out that balance-hum circuit the right way. And, while at it...verify if W4 and W4 are the correct connect points to the FX loop card.
I've only found that one schematic and the inference is W2 is a ground reference...W1 would be the 30V to the FX card for power...W3 & W4 "should be" the signal feed for the FX loop. No power involved. They could tie to W2 for a ground (again, if the layout is the same as the schematic), but W3 should be a no-no as it would seem it should be part of the loop signal along with W4.
The ONLY reason those resistors were extended across to those turrets are that someone was too plain lazy to disconnect the pots and inputs...tilt the board up and do it right. VR2 probably would be connected to W2 for the ground reference.
Again, I'd disconnect the front panel pots & jacks...tilt the board (caps drained) ...and straighten out that balance-hum circuit the right way. And, while at it...verify if W4 and W4 are the correct connect points to the FX loop card.
