Plexi Rehab

Marshall Amp Discussion

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BillyBob
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Plexi Rehab

Post by BillyBob »

Hello,
I need some opinions on repair work..
I've got a 68' Marshall Plexi 50w. I've owned it since 1988. Its sounds pretty good. It used to sound better back in the day. Its been in storage for the last 20 years. Its got some non original features; PPIMV, two extra blank holes in the rear chassis, missing rear panel, and replacement front logo. Otherwise nearly every component is original, and almost every solder joint still has the red dye on them.

Tonewise I'd like to form a "tune-up" game plan for this amp..........or maybe decide to leave it alone.
So far I have replaced the filter caps and re-tubed (NOS).

Some friends and players tell me dont touch anything else inside....that theres magic voo-doo in there, and guys are fanatical for that kind of tone (vintage components).
Other friends and players tell me even if you replaced all the components it would just sound like when it was new...and that Jimi Hendrix (or whoever) wasnt playing a 40-50 year old amp at their time.

I dont think I would ever sell this amp. Had it since I played bars years ago.
But I find myself not playing it now because it doesnt sound as good as it used to (or as good as I think it could).

What doyou reckon..?


My other amps are two Trainwreck Express clones, JCM800, and Blacface Fenders; Bassmans, Twin.
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martin manning
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Re: Plexi Rehab

Post by martin manning »

You are on the right path, I think, replacing the filters first, then looking carefully for other problems. Resistors and capacitors can fail by varying degree and degrade performance. If it it going to be played, fix what is not functioning properly. IMO it's silly to keep old failing parts just for cosmetics.
Do you know what model it is, so the correct schematic can be identified? Is the quality check sticker still on the top of the chassis?
Some photos of the interior would be good so we can see (and get all worked-up over) what you have.
Voltage measurements at the AC inlet (your local line voltage), each power supply node (filter cap positive terminals), and all tube pins would give a good indication of the current state.
What kind of PPIMV do you have? Was it sounding good after that was installed?
BillyBob
Posts: 54
Joined: Mon Dec 05, 2016 2:39 am
Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hey Martin,
Thanks for the message.
I've got good technical experience with Fender and other tube amp designs, but not Marshall stuff.
I'll take it out of the box this morning and get some photos and additional background notes up here shortly.
BillyBob
Posts: 54
Joined: Mon Dec 05, 2016 2:39 am
Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hello,
Here are some photos of my amp.
On the front panel, Volume II is the master. I was instructed to always plug the guitar into the upper right jack, although the other jacks still work but do sound bad or more quiet. I'm not sure what kind of PPIMV it is.. Since the start it always sounded buzzy (loose buzzy distortion) at lower volumes, but after the amp opens up a bit theres a sweet spot between the two volume knob stettings where it sounds real nice. Im not sure what schematic it is, 1987 is what I thought.

I replaced the three pre-amp tube sockets, the old ones, especialy V1 were unreliable (loose wobbled out from tube rolling..?)
I'm saving all the old removed components.
Ill post some more gut shots.
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BillyBob
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Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hello,
Some more gut shots:
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martin manning
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Re: Plexi Rehab

Post by martin manning »

Yes I think so, JMP 1987. There is some good documentation on this amp in the "SDM" layouts; see below. Looks like extra caps have been installed for the PPIMV to isolate the MV pot. That's one way to do it, but they are not necessary. You could restore the front panel inputs and controls to the stock arrangement and put the MV on the back, no harm since you already have a hole for it. All of the electrolytics have been replaced? Are the old can caps completely out of the circuit now?
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BillyBob
Posts: 54
Joined: Mon Dec 05, 2016 2:39 am
Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hey Martin,
Thanks for the message and layout drawing.
There are some differences between my amp and that drawing.
As a player, I was thinking to probably leave the MV on the front just because its easier to see and than reaching around the back.

So far Ive replaced the two blue can caps on the chassis top, and the other two large caps inside the chassis, the one side (clamp) mounted, and the large zip tied one. I used same values as I removed, and the original ones I removed looked very old, except for the zip tied one. It looked more recent (1980s-90s) So I'm guessing the one with the plastic zip tie on it might be non-original.

Many years have passed, but I believe its possible the same guy who added the MV may have added and zip tied in that cap..
Dont know if that matters, its always been that way for me, and sounded good.

When I replaced all the filter caps the bass response improved.

Im starting to look towards the other caps on the board now.
BillyBob
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Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hello,
I forgot to say, but I believe all filter caps are in circuit.
Two 32+32uf blue cans on top
One 33+33uf side mount
One 100uf zip tied
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martin manning
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Re: Plexi Rehab

Post by martin manning »

BillyBob wrote: Sun Dec 17, 2017 7:52 pmWhen I replaced all the filter caps the bass response improved.
That's the expected result.
BillyBob wrote: Sun Dec 17, 2017 7:52 pmIm starting to look towards the other caps on the board now.
Any electrolytics should be replaced, then you can start checking the others for leakage.
BillyBob
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Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hey Martin,
Thanks for the message and for getting me moving, making headway.
I got some parts on the way.

In the process I noticed a couple things that don't agree with the layouts or 1987 schematic (copy attached below)
In the preamp it looks like the two .68uf caps have been removed. There's no other cap value in their place, just the resistors with non-factory looking solder joints.

I vaguely remember from working on my other (high gain type) amps in the past that thats an important cap..
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BillyBob
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Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hmm..
I'll see if the new caps give me a good tone-lift. That may put a smile on my face and I'll call it good.
Otherwise I'll look deeper into ironing out (restoring) any of the circuit mods.
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martin manning
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Re: Plexi Rehab

Post by martin manning »

Reinstalling those 0.68u caps will give you a 5-6dB boost from the low-mid's on up.
BillyBob
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Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hey Martin,
Thanks for the help and message. Much appreciated.
From internet searching I've seen a couple different 50w plexi circuits and layouts by now.

I found a same model as mine on the internet: Both the Marshall gut shot and coresponding aftermarket layout drawing attached below. Note there are no .68 pre-amp caps, like mine.

I more easily found a different model than mine on the internet: both the Marshall gut shot and coresponding aftermarket layout drawing attached below. This one has the .68 caps, and other notable diferences; power transformer mount, and location of rectifier to name a couple.

All 1987 schematics I've seen have the .68 pre-amp caps.
And I'm unaware of any other circuits (schematics) besides the 1987.

The one with .68 caps also has only two toggle switches on the front.
Mine has the three toggle switches (US export model.?) and no .68 caps etc.

Am I barking up the right tree...both can be 1987..?
I imagine they would sound different.?
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martin manning
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Re: Plexi Rehab

Post by martin manning »

Sorry for not looking a little closer. I think you may have a model 1986, which is the bass version. That one has a shared 820 ohm cathode resistor for V1a and V1b, bypassed with an electrolytic. This configuration is like the Fender Bassman, as is the polarity switch (which is not shown on the 1986 schematic below). If the bypass cap is failing you will lose some gain and the channels will not be well separated, so it'd be a good idea to replace that cap. Also note there is no 0.68u bypass cap on V2a's 820 ohm cathode resistor (like yours), no 0.005u bright cap on volume 2 (which you do have), the treble cap is 250p vs. 500p, and that the power tube coupling caps are 0.1u vs. 0.022u.
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BillyBob
Posts: 54
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Location: North Carolina

Re: Plexi Rehab

Post by BillyBob »

Hello,
Thanks for the message and info.
After all these years I'm really learning something about this amp.!
That's great. Thanks for the guidance.

When the parts get here I'll see what tone-lift I get.
My idea at the moment is to check over and revive the
Tone, but not change the circuit.
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