Marshall VS100 Combo Terrible Fizz Sound When Playing

[misterc57]

My latest challenge. I need help trying to repair my VS100. When playing guitar every note sounds very FIZZY. Problem is on both clean and gain channels.

At first I thought it was a bad speaker, but the speaker is fine (sounds great connected to other amp). I have ruled out speaker and speaker cable.

Ran a 440 hz signal into the amp and tried to trace where it goes wrong, I was listening at all the op amp outputs. Could not detect it because a constant 440 signal does not generate the fizz sound, but guitar notes do.

The schematic says there should be -15 & +15 VDC at the op amps pins 4 & 8. I am only seeing around 11.9 VDC - and + at all the op amps.

Other things to mention; The FX mix pot and OD2 volume pot have the stem snapped off and will need to be replaced.

I would like to spray/clean the pots. They look like sealed units. Can these be cleaned?

Schematic attached.

Thank you! MC

[g1]

I think this is the one with the typo on the schematic. It shows +/- 15V coming out of the 12V regulators.
Can you plug your guitar into the FX return and see if you still have the problem? I think on this model you will still need to have a dummy plug (or spare cord) in the main input to enable the return jack.

[misterc57]

I think this is the one with the typo on the schematic. It shows +/- 15V coming out of the 12V regulators.
Can you plug your guitar into the FX return and see if you still have the problem? I think on this model you will still need to have a dummy plug (or spare cord) in the main input to enable the return jack.

Trying that configuration I still have the FIZZ and also have a high pitch motor-boating noise. If I rotate the FX Mix knob towards 10 the motor-boating gets quieter and the amount of fizz seems to reduce, but it is still present at FX Mix knob at 10.

[Jazz P Bass]

My suggestion would be to check the supply voltages.

Vdc & Vac.

The main rails & the opamp rails.

Note: that schematic link is dead.

[olddawg]

I would run program music off an iPad or anything with a variable line our into the main amp in or effects return to determine if the distortion is in the main amp or the preamp. Also run the pre out or effects send into any clean amp.

[misterc57]

My suggestion would be to check the supply voltages.

Vdc & Vac.

The main rails & the opamp rails.

Note: that schematic link is dead.

Trying to attach schematic again...

I am not clear on where to measure the AC and DC rails on the board itself.

Here is what I did measure on the power supply board;

2 red wires (CON11 and CCON13) had 59.5 VAC across them.

C10 +39 VDC
C13 -39 VDC

[misterc57]

schematic

Schematic attached (hopefully)

[misterc57]

Don't know why the schematic will not appear.

[J M Fahey]

Crude non Tech explanation: there was a server crash a couple years ago, actual schematics were lost but "the list of names" is still there.
When you upload vs100r_100w.pdf the system thinks: "hey!!!, I already have that one, even name and bit count match!!!" and when asked, downloads the "ghost" of the lost one .... which is a "clear gif" , meaning nothing.

RE-upload it but either under another name, or zip it or "print" it using Cute PDF or something , which will create a new .pdf with the same image but varying a few bits.

[misterc57]

Schematic renamed, try again...

Rename did not work. Now trying CUTEPDF output...

Or to make things easier, how about this link to the full schematic... http://www.amparchives.com/album/Mar...mbo/index.html

[Jazz P Bass]

The idea is to check the high voltage rails with your meter set to read Volts dc.
They should be close (but opposite polarity).

Then, on the same two high voltage points (one at a time), you set the meter to read Volts ac.
This will tell you the general health of the supply capacitors.
A good cap will have a low Vac reading.
This is refered to as 'ripple'.
A cap that is causing grief will have a reading (Vac) in many volts.

The + & -12Vdc opamp supplies should (ideally) have zero volts ac ripple.

[g1]

Here's a working copy of that full schem (resized file):

[J M Fahey]

The idea is to check the high voltage rails with your meter set to read Volts dc.
They should be close (but opposite polarity).

Then, on the same two high voltage points (one at a time), you set the meter to read Volts ac.
This will tell you the general health of the supply capacitors.
A good cap will have a low Vac reading.
This is refered to as 'ripple'.
A cap that is causing grief will have a reading (Vac) in many volts.

The + & -12Vdc opamp supplies should (ideally) have zero volts ac ripple.

Beware that cheap multimeters, typically the ones with just 2 AC voltage ranges, 200VAC and 500 or 750 or 1000VAC can NOT measure VAC MIXED WITH VDC , which is the case when measuring supply rail ripple and will give you ridiculously high values.

The ones which DO work are those which have sensitive VAC scales, say 200 - 400 mV AC - 2V (2000mV) AC , etc.

[misterc57]

All measurements against chassis ground;

+LT at C10 +39.5 VDC, 99 mVAC
-LT at C13 -39.5 VDC, 73 mVAC

at IC4 (first op amp);

pin 4 -11.9 VDC, .22 mVAC
pin 8 +11.9 VDC, .21 mVAC

Next test: Ran a cable from the effects SEND to another amp, no FIZZ when playing guitar on all channels!

BTW, what does LT mean on the schematic?

Thank you

[olddawg]

All measurements against chassis ground;

+LT at C10 +39.5 VDC, 99 mVAC
-LT at C13 -39.5 VDC, 73 mVAC

at IC4 (first op amp);

pin 4 -11.9 VDC, .22 mVAC
pin 8 +11.9 VDC, .21 mVAC

Next test: Ran a cable from the effects SEND to another amp, no FIZZ when playing guitar on all channels!

BTW, what does LT mean on the schematic?

Thank you

So it seems the preamp is good. Try running program music into the effects return. See if that's clean. If it is, you have a problem with your breakin jacks or thereabouts.....If it's dirty the problem is with the output amp section.