Marshall VS100 Combo Terrible Fizz Sound When Playing

[olddawg]

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.

No worries.. I pretty much said the same thing in my post as well if you read it. If your going to the trouble to take remove it... Replace it with a fresh 25 cent part by all means.

[misterc57]

T1 and T2 are the same transistor. I have no transistors sitting around that could replace the 2SA872. If you can point me to a USA source please advise.

I will have to order them and they are difficult to locate. I have only found some in China. That will take weeks to get. I am looking at some substitutes in USA, specifically 2SA1038, running $1 each.

BTW, I take it that the voltages I reported are ok? I see no designation for voltage readings on the schematic.

Thank you!

[misterc57]

NO .
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 think I had that backwards. Clean at base, fizz at emitter.

[J M Fahey]

Ok.
Emitter follows whatever is fed to base keeping a fixed "distance" of about 0.6V so if it does not, I strongly suspect an open transistor, or an open emitter resistor or it not receiving voltage at its collector.

So in the suspect transistor measure Vce (should be around 0.65V DC) , base DC voltage, plus emitter and collector to ground voltages.

Lost track, what transistor are we talking about?

[J M Fahey]

Just opened schematic and saw T1 is one of the input differential pair.
In that particular case, what you see on its emitter is useless, a weird waveform there means nothing, in fact what you are reading is the error voltage so it *should* be weird most of the time, go figure.

Not sure you have a problem there.

The real point is that you have clean signal at base of T1 which is the power amp input, and *apparently* bad sound at speaker out , is that so?

Have you scoped that output?
Please post a picture.

In case you can not get 2SA872 and I had a VS100 on my bench, I would order nothing and just use my general purpose PNP transistor: BC556 or BC560 which would work *perfectly* there.

2SA is a 100/120V part but you have only a +39Vvrail to deal with, so the ones I mentioned work fine, only pinout differs, check datasheets and insert proper legs in proper holes, bo big deal.Iīm certain our USA based friends will suggest an equivalent American or Japanese replacement.

TO92 - >40Vce - Hfe (gain) >150 - >300mW dissipation - >100mA current capability. common as dirt.
Although I do NOT suspect them yet, will wait for scope screen, driving output to, say, 10/15V RMS into 8 or 4 ohms, speakers acceptable (to us, not your Family or Neighbours of course).

[misterc57]

Here is where I am at.

T16 is back in. T1 and T2 swapped. No change in problem. Good signal at T1 Base and everything in signal path before that point. Fizz at T1 emitter and lots of spots after that point including all the way to output jacks.

Voltages at T1;

E = .67 VDC
C = -38 VDC
B = 56 mVDC

I think I have the legs correct. Flat side of TO-92 facing me, left to right is E, C, B.

[misterc57]

Just opened schematic and saw T1 is one of the input differential pair.
In that particular case, what you see on its emitter is useless, a weird waveform there means nothing, in fact what you are reading is the error voltage so it *should* be weird most of the time, go figure.

Not sure you have a problem there.

The real point is that you have clean signal at base of T1 which is the power amp input, and *apparently* bad sound at speaker out , is that so?

Have you scoped that output?
Please post a picture.

In case you can not get 2SA872 and I had a VS100 on my bench, I would order nothing and just use my general purpose PNP transistor: BC556 or BC560 which would work *perfectly* there.

2SA is a 100/120V part but you have only a +39Vvrail to deal with, so the ones I mentioned work fine, only pinout differs, check datasheets and insert proper legs in proper holes, bo big deal.Iīm certain our USA based friends will suggest an equivalent American or Japanese replacement.

TO92 - >40Vce - Hfe (gain) >150 - >300mW dissipation - >100mA current capability. common as dirt.
Although I do NOT suspect them yet, will wait for scope screen, driving output to, say, 10/15V RMS into 8 or 4 ohms, speakers acceptable (to us, not your Family or Neighbours of course).

I nave not scoped the output because I have zero scope experience. But now would be a good time to learn and I actually look forward to this. Can I just scope a 440 hz signal for this?

Is there any point in replacing T1 and T2 at this point, or shall we bench that for now?

The real point is that you have clean signal at base of T1 which is the power amp input, and *apparently* bad sound at speaker out , is that so?

Yes that is correct.

[g1]

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.

It still sounds like T12 may not be turning on sufficiently resulting in crossover distortion. The voltage readings referenced to ground are muddying things a bit. Can you measure TR9 & 12 from E to B on each (one probe to E, the other to B) ?

[J M Fahey]

Quote Originally Posted by misterc57 View Post
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.

It still sounds like T12 may not be turning on sufficiently resulting in crossover distortion. The voltage readings referenced to ground are muddying things a bit. Can you measure TR9 & 12 from E to B on each (one probe to E, the other to B) ?

True in general but in this particular case emitters are within 8mV of earth, so I can take T9b and T12b as true Vbe values.

And they look good: 1100 to 1240mV BE for Darlingtons, are very reasonable "2 diode" forward biasing.

Won`t worry about the slight discrepancy, because output rail DC must be , say, 40 or 50 mV offset; so in fact we might be looking at (properly read), say, 1240-50 and 1100+50 or something very close.

This tells me that output transistors look good and their emitter ballast resistors too.

Which makes me suspect we might have an oscillation/instability problem here, maybe an open Zobel or something, but in that case DC readings or shotgunning wonīt help, we need a scope screen.

Basic JMF theorem: IF you can hear something bad, you MUST see something bad on scope screen

Unless itīs a broken speaker, but then itīs not an Electronics Repair problem but a speaker replacement one.

Crossover distortion is ugly, but wouldnīt be described as unbearable fizz by a Rock guitar player.
A Piano player might, and with good reason, it is VERY prominent on complex clean chords, but guitar players are somewhat desensitized to it, by their constant use of distortion, also by naturally playing chords that donīt sound bad even when clipped.

[g1]

Thanks JM, I just needed a little reassurance about those numbers.
The oscillation you mentioned might also explain why the fizz disappeared for a bit when he was probing around the output devices.

[J M Fahey]

Please post a front panel picture of both your scopes, we might suggest some "paint by the numbers" settings, intended to show a 440Hz 10V RMS signal at the output, when you feed, say, 100mV at the input.

[misterc57]

My Scopes

The T935 I tuned up about a year ago. The 9020A I have yet to even turn on.

Thank you! Mark

[misterc57]

The T935 I tuned up about a year ago. The 9020A I have yet to even turn on.

Thank you! Mark

[ATTACH=CONFIG]40279[/ATTACH]

[ATTACH=CONFIG]40280[/ATTACH]

Will give the T935 a shot tonight.

[olddawg]

I was looking at this on my phone and didn't look at the schematic as well. I just assumed the previous statement that the transistors described were in a muting circuit when I said it was ok to swap them. If the OP doesn't understand how a diff amp works, be careful. The circuit either works or it doesn't. If the hfe is mismatched between the two transistor you will get a massive DC offset. If it is intermittent you will get gunshots. This is not fun if speakers are attached. Diff pairs are also sensitive to direct heat and cold. But if you have an audio output without a DC offset or crackles and pops. It doubt if the problem is the diff pair. If you do shotgun them. Replace both at the same time.

[misterc57]

Just to clear things;

The amp makes no noise when idle, aside from normal hiss. When plucking guitar strings, every note is fizzy sounding (sounds like a defective speaker). This is heard at and after T1 on the output board. Before T1 it sounds normal.

Preamp out to another amp sounds fine. A different amplifiers signal sent to this amp sounds fizzy. Different speakers and speaker cables have been tried.

There were two occasions during all my testing and probing where the problem went away, but not for long. Not sure what caused it to go away.

Hope to have some scope images tonight!

Appreciate all the help and input! Learning a lot here. Thank you, Mark