1987x Buzzing/humming - Filter Caps?

[Splitslim]

I'm not following how W3 is connected between the hum balance area on the PCB and the effects loop card. The schematic shows a closed loop for the hum balance.

 

[myersbw]

It's entirely possible...did you look under the board?
The signal is supposed to be away from the power supply...

So long as caps are drained, it should be easy enough to take a dental mirror and angle under to at least see what's going on with traces going to W2 & W3. I'm also wondering if you have some revision that doesn't have a schematic floating on the net yet...?

 

[myersbw]

I'm not following how W3 is connected between the hum balance area on the PCB and the effects loop card. The schematic shows a closed loop for the hum balance.

That is EXACTLY my point. Those resistors are R31 and R32...they USED to be in those pcb holes on the board....someone bridged them to turrets, W2 & W3. Not good!

But, again...I can't rule out a revision and W2 and W3 are now something entirely different like maybe the 6.3 V winding connection point. What would be helpful is to actually see the loop card to see if labels there are as Marshall indicated in the schematic i referenced.

And, here's something to complicate it a bit. Let's just say, W2 & W3 are correct. The resistors paralleled with R31 & R32 are 100 ohms in value. So, The resistance of that combination is now 50 ohms a leg...a bit low!

But, if those resistors were added to "repair" opened wire wounds...and as the wire wounds are left connected at the ends...you know how a coil-loaded antenna floating in that circuit...absolutely not good. Minimally, those resistors, if open, should've been removed. Again...it comes back to a lazy tech.

 

[Splitslim]

I dug up this old thread where this "fix" was discussed, connecting the resistors to W1 then W2 and W3. Is this the same as mine?

http://www.marshallforum.com/threads/what-is-this-hum-balance-pot-on-my-1987x.97032/

 

[Splitslim]

And here is a close up of the loop card;

 

[myersbw]

Ah, good to see. So, it's the same as the outline on the schematic I referenced. Now, my question is...where do those two wires terminate on the main pcb? The schematic shows W3 and W4? Is your main pcb labeling different?

If that board wiring is going to the correct spots, but differently labeled...it may be following schematics i found on the Prowess site which does use different labels for the mainboard-side connects. Given that may be the case...I'd still clean up that up sooner than later.

 

[Splitslim]

From the effects loop card - W4 connects to the middle terminal on the treble pot. W3 connects to BN4 on the PCB which has a trace to C11.

I'm not sure where W1(red) and W2(green) go at this point.

 

[myersbw]

Ah, ok. Yep, looks like Marshall did a revision that changed the labeling. Your correct schematic can be found on the Prowess website. Here...
http://www.prowessamplifiers.com/schematics/Marshall/

But, after looking at your board and both revision schematics, it seems you have a hybrid of the two! Of which...I really can't seem to find the "accurate schematic" if it indeed exists.

Still, it's not too hard. i think you're back to shielding and/or pot issues with that MV...but...I'd highly recommend fixing that hum balance issue (clean it up...get those defective wire wounds out of there if they're indeed open).

Funny...C11 is correct on the DrTube link I sent...but, not the same C11 on the Prowess-located schematics. Yet, the board silkscreen is correct on the Prowess regarding the loop...geez, Marshall... :O

 

[ampmadscientist]

And here is a close up of the loop card;

You can put it back to stock wiring and go from there.
Hard to tell what's under the circuit board...

 

[Splitslim]

Here's an update.

I pulled out the pre-PI master volume and brought the amp back to stock. Whisper quiet and no issues. I then installed a LarMar PPIMV and rebiased the tubes to 65% and voila, it is working great! Lack of presence control at low volumes, but that is a known tradeoff. The amp sounds great and the PPIMV is a definite improvement from the pre-PI configuration.

Thanks to everyone for the help!

 

[splatter]

so your issue was inducted noise from the pot being to close to the power switch and not being properly grounded ?

I'm asking cause I have the exact same noise in a 2203 jcm 800 reissue .I'ts driving me nuts trying to find it . I don't have an after market master like you did though .If I remember correctly taking the 2nd pre out of my amp kills the noise

 

[Splitslim]

so your issue was inducted noise from the pot being to close to the power switch and not being properly grounded ?

I'm asking cause I have the exact same noise in a 2203 jcm 800 reissue .I'ts driving me nuts trying to find it . I don't have an after market master like you did though .If I remember correctly taking the 2nd pre out of my amp kills the noise

My issue was that the original pre-PI master volume did not have shielded wire. I left the pot in the same location for the new PPIMV, beside the standby switch, and used shielded wire with a PEC dual gang pot (LarMar type). No noise whatsoever. With shielded wire, the pot is fine beside the standby switch.

 

[ampmadscientist]

My issue was that the original pre-PI master volume did not have shielded wire. I left the pot in the same location for the new PPIMV, beside the standby switch, and used shielded wire with a PEC dual gang pot (LarMar type). No noise whatsoever. With shielded wire, the pot is fine beside the standby switch.

What did you do to bypass the presence control?

 

[Splitslim]

What did you do to bypass the presence control?

I didn't do anything to the presence control. I understand it's function is impacted (full on) at low master volume due to the NFB being reduced in this setup, but it hasn't really bothered me at this point. Do you have a recommendation to address this? I have read about installing another pot to act as a cut control, but I want to limit the amount of modding I do to this amp.

 

[myersbw]

I agree...if that were mine, I wouldn't alter the NFB (switching for the higher tap) or altering the NF resistor by including a pot or switching in resistors.

I just installed a PPIMV (Type 3) in a Dr. Z Galaxie I own. The only reason I put it in was for some low volume home practice. I don't feel the lack of a Presence control nearly impacts the tone or response as much as the fact that I'm now moving very little air with the same speaker cab. So, it's a moot point in my book.

That said, it is MUCH MUCH easier to mod and make use of some custom circuit in my Z vs. these Marshalls I've been working on...lol, nuff said!