Signal related crackle

[EstynEstyn]

Hey,

I have 2 heads at the minute, a Marshall VS100 and a Sunn 1200s, both suffering from the same problem.

The issue is when signal is appllied there's an awful distortion/crackle that covers it. It's not audible otherwise, even with master maxed out.

So I'm led to believe it's a pre amp problem. Where to I go from here?

Thanks.

[Jazz P Bass]

Isolate the problem.
Go in the FX Return with the signal.(Marshall VS100)
That will bypass the preamp.
Your going to need a scope to nail this one.

[olddawg]

Isolate the problem.
Go in the FX Return with the signal.(Marshall VS100)
That will bypass the preamp.
Your going to need a scope to nail this one.

Exactly...run a signal source (like an ipod or cd player) into the effects return or main amp in jack. If it sounds good the problem is with the preamp. Take the effects send or pre-out and run that into any line level (power amp, aux or tape monitor in on a stereo) amp, if it is good the problem is with the power amp. If both are bad the problem is probably power supply related.

[J M Fahey]

You talk about "heads".
If you test them with the same speaker there is a strong suspicion of cracked flexible wires there, the ones who vibrate *a lot* because they go from voice coil to external terminals.
This defect causes that exact symptom.
Now, if it happens on different speakers, forget it.
You might also have a very poor power line connection.

[EstynEstyn]

Thanks for the responses guys.

I tested the VS100 today. The effects return still crackled, the effects send was fine. So it's a power amp problem.

http://www.drtube.com/schematics/marshall/pc0689p.pdf

This is a link to the power amp section. How should I go about tracking down the problem from here? Thanks for your help, its much appreciated.

[Jazz P Bass]

Is that the correct schematic?
It looks like the VS100R. It uses two output transistors.
I thought the VS100 used (4).
Why the discrepency I do not know.
Pull the chassis.
Look at the transistors on the heat sink.
How many are there (the large ones).
I did notice the input to the power amp is labeled "input from preamp VR13 wiper"
I would clean that potentiometer (VR13) before doing anything.

[EstynEstyn]

My Mistake. Your right. This is the right schematic.

http://www.drtube.com/schematics/mar...v100-62-02.pdf

[EstynEstyn]

So, I've measured all the caps and they check out ok. I used the diode function to measure the transistors and got a couple of odd reading. T7 and T8 read around 0.600v from Base to Emitter but only 0.030v from Base to Collector. Could this be my problem?

[Jazz P Bass]

Are you sure that you are reading the correct pins?
T7 & T8 both have a 100 ohm resistor from base to emitter.
That would look low on diode check.
Besides, that is the limiting part of the circuit.
I do not think that you are going to find a crackle problem with a static check.
It is good that it was done.
You need a scope.

[EstynEstyn]

Hmm, ok. I do have a scope, but it's not great. Is there any particular way to approach it with a scope, just follow the signal till I hit the crackle?

[Jazz P Bass]

Yup.

[J M Fahey]

Send a nice clean 300 to 400 Hz sinewave into the power amp, hook speakers to make it "work" and rise volume slowly until you hear crackling.
Do not confuse it with buzzy SS clipping which will appear even on a perfectly working amp, but only approaching 20V RMS.
Once you hear it, you will perfectly see and recognize it on screen.
Take a pictutre and post it if you wish.