Marshall VS100 Combo Terrible Fizz Sound When Playing

[misterc57]

scope images

Here is my first shot at using the scope. This is a 440 hz signal. Not sure about the feed voltage but it was probably close to 100 mVAC.

The readings were on the output jack. Pictures show results and scope and amp settings.

Amp was setup with a dummy 8 ohm load, not a speaker.

[Jazz P Bass]

Need to know the amplitude of that scope shot.

That signal looks pretty bad.

[misterc57]

Need to know the amplitude of that scope shot.

Need help here. By amplitude you mean? ... scope volts/div of .2?

Yes I thought it looked bad also. Not a clean AC signal. Hopefully I did not setup something wrong on the scope to cause that.

[olddawg]

Need help here. By amplitude you mean? ... scope volts/div of .2?

Yes I thought it looked bad also. Not a clean AC signal. Hopefully I did not setup something wrong on the scope to cause that.

Where are you injecting the signal? If your preamp is clean, you should be inputting the signal to the effects return or main in jacks. That way you are just testing the power amp and don't have tone controls, etc involved. If it's a 2 channel scope you should be able to monitor the input and the output signal. Basically bring up the amplitude on your signal generator from zero until the signal across the load bank loses symmetry while also making sure the input sign wave is clean. What kind of probes are you using? 10:1?

[Jazz P Bass]

Need help here. By amplitude you mean? ... scope volts/div of .2?

Yes I thought it looked bad also. Not a clean AC signal. Hopefully I did not setup something wrong on the scope to cause that.

No, the scope is 'showing' the signal as it is.

The reason that I asked for the amplitude (voltage height) is that the top of the waveform is clipping.

[J M Fahey]

Even without knowing exact amplitude, although you could measure it separately with your multimeter set to 20VAC, you are missing the entire top half of the signal.
Thatīs bad enough to cause "terrible fizz" .

Ok, some good news among bad ones: now we have a definite problem to troubleshoot, instead of the vague generic "it sounds as sh*t".

believe or not, at least we are moving forward.

Will suggest some voltage and resistance readings

And no, itīs not a fault of the input transistor pair, funny signals there just mean they are *trying* to compensate for the output stage failure.
Stay tuned

EDIT 1:

a) with amp off, obviously no speaker. please put black meter probe on the speaker out terminal, which is also where D12 cathode and D13 anode join, set meter to lowest ohm scale (should be 200 ohms) and with red probe measure resistance to emitters of: T9, T10, T11, T12.
I expect very low indicated value, around 1 ohm or less.

b) set all knobs to 0, do not inject any signal anywhere, just in case use lamp limiter, it wonīt affect results much, turn amp on, wait a minute, leave black probe where it was, set multimeter to 200mV DC scale, measure voltage at emitters of T9, T10, T11, T12.
I expect from 10 to 50mV at each point.
You tell me the polarity you find, + or - .

Post all 8 values: 4 resistance, 4 mV

c) now with black still on speaker out, set meter to 200V DC scale and post voltage on collectors of T9, T10, T11, T12.

Post these results and again you tell me the polarity you find.

[misterc57]

Even without knowing exact amplitude, although you could measure it separately with your multimeter set to 20VAC, you are missing the entire top half of the signal.
Thatīs bad enough to cause "terrible fizz" .

Ok, some good news among bad ones: now we have a definite problem to troubleshoot, instead of the vague generic "it sounds as sh*t".

believe or not, at least we are moving forward.

Will suggest some voltage and resistance readings

And no, itīs not a fault of the input transistor pair, funny signals there just mean they are *trying* to compensate for the output stage failure.
Stay tuned

EDIT 1:

a) with amp off, obviously no speaker. please put black meter probe on the speaker out terminal, which is also where D12 cathode and D13 anode join, set meter to lowest ohm scale (should be 200 ohms) and with red probe measure resistance to emitters of: T9, T10, T11, T12.
I expect very low indicated value, around 1 ohm or less.

b) set all knobs to 0, do not inject any signal anywhere, just in case use lamp limiter, it wonīt affect results much, turn amp on, wait a minute, leave black probe where it was, set multimeter to 200mV DC scale, measure voltage at emitters of T9, T10, T11, T12.
I expect from 10 to 50mV at each point.
You tell me the polarity you find, + or - .

Post all 8 values: 4 resistance, 4 mV

c) now with black still on speaker out, set meter to 200V DC scale and post voltage on collectors of T9, T10, T11, T12.

Post these results and again you tell me the polarity you find.

Will do tonight. Am I doing b) and c) without a speaker (load) connected?

Also wondering if I can run the same test I did with no load at all to see if the clipping goes away. Thoughts?

Thank you. Mark

[misterc57]

Even without knowing exact amplitude, although you could measure it separately with your multimeter set to 20VAC, you are missing the entire top half of the signal.
Thatīs bad enough to cause "terrible fizz" .

Ok, some good news among bad ones: now we have a definite problem to troubleshoot, instead of the vague generic "it sounds as sh*t".

believe or not, at least we are moving forward.

Will suggest some voltage and resistance readings

And no, itīs not a fault of the input transistor pair, funny signals there just mean they are *trying* to compensate for the output stage failure.
Stay tuned

EDIT 1:

a) with amp off, obviously no speaker. please put black meter probe on the speaker out terminal, which is also where D12 cathode and D13 anode join, set meter to lowest ohm scale (should be 200 ohms) and with red probe measure resistance to emitters of: T9, T10, T11, T12.
I expect very low indicated value, around 1 ohm or less.

b) set all knobs to 0, do not inject any signal anywhere, just in case use lamp limiter, it wonīt affect results much, turn amp on, wait a minute, leave black probe where it was, set multimeter to 200mV DC scale, measure voltage at emitters of T9, T10, T11, T12.
I expect from 10 to 50mV at each point.
You tell me the polarity you find, + or - .

Post all 8 values: 4 resistance, 4 mV

c) now with black still on speaker out, set meter to 200V DC scale and post voltage on collectors of T9, T10, T11, T12.

Post these results and again you tell me the polarity you find.

All readings with no load connected.

a)
T9E 142 ohms
T10E 1.02 ohms
T11E .56 ohms
T12E .59 ohms

b)
T9E +150 mVDC (reading kept fluctuating, not stable)
T10E +12.53 mVDC (stable)
T11E -1.54 mVDC (stable)
T12E -1.4 mVDC (stable)


c)
T9C +36.6 VDC
T10C +36.6 VDC
T11C -36.6 VDC
T12C -36.6 VDC

[misterc57]

So these last results led me to snoop more around everything at T9. I found the big 5 watt resistor to be wobbly. After resoldering that one, and three other 5 watt wobblers the amp is sounding super fine and I am super happy!!! I should have caught these earlier but missed it.

Thank you all for pointing me to the right direction. I learned a lot from this one and my scope is now going to stay on my bench joining my other equipment!

Now I need to order some replacement pots!

Thanks again! Mark

[J M Fahey]

a)
T9E 142 ohms
T10E 1.02 ohms
T11E .56 ohms
T12E .59 ohms

b)
T9E +150 mVDC (reading kept fluctuating, not stable)
T10E +12.53 mVDC (stable)
T11E -1.54 mVDC (stable)
T12E -1.4 mVDC (stable)

T9 emitter ballast resistor open or cracked pad/track.

Obviously the resistor itself was fine,proof enough that only *some* failures are *parts* related.

Thatīs why shotgunning fails so often.

You can buy most any part over the counter ... except the PCB itself.

Thatīs why I am SO much against messing with the PCB, replacing parts without actual proof they are there, etc.