Marshall VS100 Combo Terrible Fizz Sound When Playing

[misterc57]

Welcome back everyone!

Traced the problem to T1 (2SA872) on the output board. Clean signal at the emitter, fizz at the base.

Not sure if I should check anything else before trying to find a replacement. Any suggestions?

BTW, I know very little about transistors.

Thank you! MC

[g1]

I think any problem at the output may end up fed back to earlier stages.
It may be better to start off with some DC voltage levels. How about DC volts for E,B, and C of T9 & T12. The symptoms sound like what could be caused by excessive crossover distortion.

[olddawg]

I think any problem at the output may end up fed back to earlier stages.
It may be better to start off with some DC voltage levels. How about DC volts for E,B, and C of T9 & T12. The symptoms sound like what could be caused by excessive crossover distortion.

It does indeed sound like crossover distortion or it could be an artifact or an asymmetric wave form. Do you have a load bank? Can you look at the output waveform? Btw, freeze mist is your friend. Tickle that transistor (and parts in that vicinity) and see of it gets bad or worse.

[misterc57]

I think any problem at the output may end up fed back to earlier stages.
It may be better to start off with some DC voltage levels. How about DC volts for E,B, and C of T9 & T12. The symptoms sound like what could be caused by excessive crossover distortion.

I can't get to those legs, but I can take readings tonight at resistors or diodes the connect straight to them. Is that ok for these readings?

[misterc57]

It does indeed sound like crossover distortion or it could be an artifact or an asymmetric wave form. Do you have a load bank? Can you look at the output waveform? Btw, freeze mist is your friend. Tickle that transistor (and parts in that vicinity) and see of it gets bad or worse.

I do not have a load bank? I have a signal generator but found that just plucking a guitar was giving me a better signal to listen for this noise/distortion.

I have two scopes available, but no experience using it (but I can give it a go). I have no freeze mist but will see if I can get some.

[Jazz P Bass]

Have you tried 'lifting' T16 jfet? (ie: removing it to see if that is the source of the 'fizz')

[misterc57]

Have you tried 'lifting' T16 jfet? (ie: removing it to see if that is the source of the 'fizz')

Not needed in circuit to pass audio? Will give that a shot also.

[Jazz P Bass]

Nope.
It's part of the 'mute' circuit.

[misterc57]

More readings and tests.

T9 B 1.24 VDC taken at D6
T9 C 38.9 VDC taken at D13
T9 E -4.6 to -2.9 mVDC taken at both sides of R22

T12 B -1.1 VDC taken at D7
T12 C -39.1 VDC taken at D12
T12 E -7.5 to -8.2 mVDC taken at both sides of R17

After I took these readings the FIZZ was gone. But only for a while, it came back. Only thing I did was push T16 back a tad, thought that night have done it.

Next thing I did was remove T16. Bottom of board looks good, clean solder joints. Still have FIZZ.

I have a can of compressed air, sprayed upside down I am able to get the components chilled. I did this on T1 and T2 but it made no difference.

This is going to be very satisfying when resolved!

Thanks! MC

[olddawg]

T9 B 1.24 VDC taken at D6
T9 C 38.9 VDC taken at D13
T9 E -4.6 to -2.9 mVDC taken at both sides of R22

T12 B -1.1 VDC taken at D7
T12 C -39.1 VDC taken at D12
T12 E -7.5 to -8.2 mVDC taken at both sides of R17

After I took these readings the FIZZ was gone. But only for a while, it came back. Only thing I did was push T16 back a tad, thought that night have done it.

Next thing I did was remove T16. Bottom of board looks good, clean solder joints. Still have FIZZ.

I have a can of compressed air, sprayed upside down I am able to get the components chilled. I did this on T1 and T2 but it made no difference.

This is going to be very satisfying when resolved!

Thanks! MC

Have you simply tried resoldering T16 and other components in that area. Sometimes the "resolder the everything" approach is valid.

[misterc57]

Have you simply tried resoldering T16 and other components in that area. Sometimes the "resolder the everything" approach is valid.

Will do. I was also thinking of swapping T1 and T2 to see if the fizz location changes (to rule out the transistor currently in T1). Does that make sense?

[olddawg]

Will do. I was also thinking of swapping T1 and T2 to see if the fizz location changes (to rule out the transistor currently in T1). Does that make sense?

Sure if they are the same transistors and you have the skills not to tear up the board. Muting transistors fail in a lot of audio amps. Those transistors are cheap. If it were me I would just order a couple and swap them out with new since you are taking them out anyway, unless you are in a hurry. I would try resoldering the general area first since you said it was intermittent.

[J M Fahey]

NO .
Sorry for the scream but Transistors are not tubes, even worse on output transistors which are opposite polarity.

The crude "swap tubes to see if problem follows tube" trick does NOT apply in the SS World.
Sorry.

Traced the problem to T1 (2SA872) on the output board. Clean signal at the emitter, fizz at the base.

Weird, if anything the opposite should be true, since "emitter follows what base does".

I rather suspect your scope probe will not affect emitter but injected hum or created instability when touching the sensitive base ... which of course does not mean the transistor is bad but simply reacting to something you did.

[olddawg]

NO .
Sorry for the scream but Transistors are not tubes, even worse on output transistors which are opposite polarity.

The crude "swap tubes to see if problem follows tube" trick does NOT apply in the SS World.
Sorry.


Weird, if anything the opposite should be true, since "emitter follows what base does".

I rather suspect your scope probe will not affect emitter but injected hum or created instability when touching the sensitive base ... which of course does not mean the transistor is bad but simply reacting to something you did.

I'm sorry if I gave bad advice Juan, but I see no problem swapping transistors of the "same designation". (Provided they aren't shorted or in some other catastrophic failure mode of course.) I've done it many times troubleshooting weird problems, especially if it was a stereo amp with one good channel. He said the problem went away when he was messing around in that area. That usually means a bad connection or some marginally functioning component. Usually a cap, but I've seen transistors fail in some rather interesting and mysterious ways over the years.

[J M Fahey]

Dear olddawg , I wasnīt talking to you, in fact I didnīt even *see* , let alone read your post but to misterc57 .

Still maintain that it is nonsense and only justified for, say, Tom Hanks stranded in an island and able only to work with recycled parts from the crashed plane.

The same way that Enzo said that the proper way to recharge a standard battery is by replacing it, the proper way to shotgun a transistor is by replacing it by a fresh one.

Tubes are expensive, scarce and plug-in, and probably you donīt have fresh ones available anyway, so swapping position is a useful test option, but for a dozen a dollar transistors, which to boot are soldered and even worse, to fragile easy to lift pads, so needless swapping can easily lead to a damaged or destroyed board, itīs a shot in own foot.
I can not suggest it even if for a test.
In any case, transistors are simple devices, compared to tubes which have dozens of failure or poor performance problems, so in general simple voltage measurements tell us whether they are either fine, open or shorted.

No worn / gassy / shorted screen / microphonic / shorted cathode to filament / hummy /whatever but honest open or shorted with the very very inusual leaky/hissy one because of some static discherge which damaged but not killed the crystal.

Following Enzo again: why waste $30 of bench time to test a doubtful part which costs cents and you have nearby?

Nothing personal, quite the contrary, wishing a relatively simple repair does not become a time guzzling nightmare.