Marshall VS65R problem

[Smoke 'n sparks]

I went over to play my VS65r that WAS playing normally last week down in the studio,but now, the sound coming out is like a bad tube.The quality got bad,then worse,then only sporadic bursts that could wreck the Celestion.It actually sounded like a heater was shorting against the cathode.On both channels.

Being a lazy guy- I went and bought a new TungSol 12AX7 for $19 at GC.Of course ,that wasn't the problem.

So I searched the net for some suggestions.I'm lazy and yet too busy to waste a lot of time on diagnosis.I've already been through so much bench time on both of my VS65's that I could write my own tips.Lucky for us,there's a lot of tips on this and similar Marshalls.

I will go thru some steps

1.Cleaned all input/output jacks.This is hard to do with success because the boards must come out.The contact point on all of them is a very tiny area under the arm that lifts up when you insert a jack.And,with any twenty year old Marshall you're gonna have jack problems.They are PCB mounted,and we all know what great wave soldering techniques Marshall has.One must put real lead into almost every thing mounted on the board to make it reliable,not counting semiconductor or cap,resistor or fuse problems.There are three fuses on this amp-one is glass and two are semiconductors.

2.For a steady state input I used a 1/4 mono jack from a cassette deck into the front panel.Amazing the sound comes out the speaker with bursts of power,(DANGER) and the headphone jack.I guess I'm trying to determine whether the pre or main amp (or both) are malfunctioning.

but this proves nothing.I will have to check voltages in the method that was shown in that 24 page novel with the guy from Italy.
I have read elsewhere that if the effect return will amplify,then it's in the circuit after the input jack that has problems,but that can't be logical-the return is mixed with the main signal and mix level with the effect vol control.I have read that the tube only amplifies the distortion channel, but there is a signal path from the SS clean section to the tube.Somebody will tell the truth-maybe I'll contact Blechley Park

Now this week I must add my Fender HRD 1 is also flummoxed.I am wondering if someone in the studio is a complete jackass and plugging in effects pedals (AGAIN) with the wrong wall rats.Man is this ever tough on an amp to get AC and hum injected into it.But I have seen this before,and it requires thorough analysis to discover that one or two components has failed because of it.

Signal tracing is going to happen next,first I have to find my probe or build a new one.

And I will have to add, the power switch doesn't always turn this VS65R amp on, or maybe it's on-but the LED is going bad-so that's a possibility that some components aren't really getting the line volts to rectify.I noticed the tube wasn't really getting the heater red......

It's gonna take more than a half hour on this one.I'm open to suggestions from more experienced techs.I'm lazy.And busy.



Thank you all in advance.

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nickb:




PS:

I'm lazy and yet too busy to waste a lot of time on diagnosis.



"You might as well give up now. "

Last edited by nickb; Yesterday at 09:25 PM.



Experience is something you get, just after you really needed it.

[Smoke 'n sparks]

NIckb

I'm no quitter,just printed all 24 pages of the back & forth with some guy with the same amp, from Italy- only to find that he never replied about a finish.We may never know whether the amp stayed fixed.I'm guessing he gave up.You did your best and I applaud that.

Suspense,drama,diagnosis-what more could an electronics blog have?

You have real patience,a virtue.We all do not have that attribute.Shops around here are gone,victim of high rents and Chinese landfill products.Those that are open,and distant- are charging $125/hr UP FRONT.That's also what the boss charges to fix your car.

Like I posted,I will have to go methodically through the exact sequences you and other folks have posted.It's the best set of tips I have come across on the web so far.

Parts are only 6 miles away.

The signal path from the input jack to the headphone jack is complete and clean.Just no amplification whatsoever.What's coming out of the speaker is clear also,with no amplification,but random powerful bursts of amplified signal enough to damage it.

Regards

[J M Fahey]

Ok, *where* are you?
I might suggest a good Tech in your area.

[nickb]

Thx for the new thread. The other one was overlong and would have been too confusing.

I think the first thing I like to be certain of is that the power switch is making reliable contact as that will cause all kind of unpleasantness. Bypass it (carefully!) if need be.

We don't want to wreck your speaker or the amp so I suggest you add a 100 ohm 5 watt resistor in series with the speaker, so forming a simple attenuator.

With that sorted and no input does it still make loud noises? Is this bursts of hum, or normal sound or what?


Now plug an unconnected jack lead into the fx return jack. If this kills the noise then the problem is in the preamp.

[Smoke 'n sparks]

Nickb

The powerful amplified bursts were a surprise ,they were microbursts so I had to lower the headphone level on the cassette deck.Far too high a level.
You're right,I must establish a full 120 volts is getting out of that power switch.....

I can do this Sunday.

thank you for the tips