#1
Hi,
I am desperately in need of some advice. I have a Marshall VS100 - its a hybrid having tube preamp and SS power, however since I turned it on without the loudspeaker connected it has started making crackling noises that have got progressively worse so that it really horrible. I tried changing the tube, thinking this was the problem however the problem persists, what other problems may be with it and how can I fix them. Please help me!!!

Thanks
#2
Hopefully, you didn't blow a transformer. Look for fuses or some sort of reset switch, maybe?
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#3
Playing an amplifier without the speaker(s) connected can cause damage, but in my experience that is rare. It is however quite possible to damage any amplifier if the wires to the speaker(s) were shorted together for a long time while the amplifier was trying to output significant power. Usually, the output transistors (often mounted on large metal finned heatsinks) overheat when this happens, and they fail like you are describing.Most reputable amplifier designs have thermal protection for this, however.

Check the connections to the speaker. It may be that one or both is not really secure. If the speaker uses slide-on type tabs, make sure the terminals on the wires to the speaker are tight mechanically on the speaker tabs. Also, the wires may not be making a solid connection into the terminals.
#4
Thanks guys but the weirdest thing just happened...I decided Id try the amp again several hours after Id fitted the new tube. Well I'm not sure f you are supposed to wait a while or it was just a miracle but the amp now is exactly fine. It is very odd as I have not touched it since! Thank you very much though and I still will check it over to make sure!
#5
just so you helpful guys giving advice know, that amp does not have a PT or an OT it is a SS amp with a single 12ax7 and that is it.
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#6
That's what I thought but wasn't sure, dkcol already covered the only thing I could think of. I'm wondering if the tube socket is dirty and had a bad connection now that I think about it a little more...

If this happens again try that. Squirt some contact cleaner into the tube socket, with the tube removed, then plug it in and remove it a few times. Let it sit about a half hour to be sure all the contact cleaner can evaporate before turning it on. I doubt that it would hurt the amp, but it's always best to be on the safe side.

If you need to you can unplug the amp and use a pipe cleaner to clean the pin holes in the tube socket using a little contact cleaner in each pin hole. Not very likely you will hit anything inside, usually the tube sockets are open on top and only have a couple of wires connected to the outside, the connectors are usually bent away from the socket itself.
Hmmm...I wonder what this button does...
Last edited by Paleo Pete at Apr 23, 2015,
#7
Great, I've turned it back on and it's back to how it was before!!! Not so much a "crackling noise" as its like a constant crackly horrible sound. This morning it was fine, all thats happened is that it's been off for several hours!!!!! Help

edit: I just opened it up again and took the tube out and had a look at the connections - I'm now pretty certain that this is fine. By accident I touched the metal casing with my hand and the problem stopped, I assume then that it is an issue with grounding. I then tried putting a guitar lead into the socket and that seemed to make the sound louder and putting my hand on the metal case only slightly lowered its volume. It is still able to play sound through it, but it has the annoying noise. I also checked the connections and they do seem to be fine, however is there a way to make it ground properly? Or is this even what it wrong with it?

edit: I think I've really screwed it up now, I think I connected the wrong parts together on the transformer and it made a bang and stopped, it blew the fuse so hopefully that's the only damage

So now the question is what type of fuse do I need and still how to resolve my problem
Last edited by EpicGuitarGuy13 at Apr 23, 2015,
#8
Sorry I couldn't get back to you yesterday.

BE CAREFUL touching anything inside the amp with power on or the AC plugged in! There are LETHAL voltages/currents inside.

A fuse blows for many reasons. The needed type should be marked somewhere on the amp, usually with a number with an A after it. I am speaking about a replaceable fuse in a "holder". If there isn't one visible, you need to take the amp to a tech to have him/her look into the problem(s).

I have repaired several amps after someone plugged theirs into 230 Volts with the switch set for 120 Volts. That can cause severe damage. The crackling you heard may have been a symptom of that, or perhaps something getting ready to fail completely (like a filter capacitor in the power supply, etc.). It is nearly impossible to diagnose it without an electronics bench test.
#9
Yeah I've realised that now! You said earlier about an output transistor, where might I find it? I have a multimeter and is there any tests I can do to see if components have fried? I can send pictures if that may help

edit: I've just checked over all the components and "REG1" seems to be very lose - im not entirely sure what that stands for. Also one of the capacitors seems to rock slightly more than the other ones.
Last edited by EpicGuitarGuy13 at Apr 25, 2015,
#10
Quote by EpicGuitarGuy13
Yeah I've realised that now! You said earlier about an output transistor, where might I find it? I have a multimeter and is there any tests I can do to see if components have fried? I can send pictures if that may help

edit: I've just checked over all the components and "REG1" seems to be very lose - im not entirely sure what that stands for. Also one of the capacitors seems to rock slightly more than the other ones.

There is no OT(output transformer) or PT(power transformer) in your amp. almost no SS amps have these.
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Morley Bad Horsie 2
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#11
Quote by dkcol
Playing an amplifier without the speaker(s) connected can cause damage, but in my experience that is rare. It is however quite possible to damage any amplifier if the wires to the speaker(s) were shorted together for a long time while the amplifier was trying to output significant power. Usually, the output transistors (often mounted on large metal finned heatsinks) overheat when this happens, and they fail like you are describing.Most reputable amplifier designs have thermal protection for this, however.

Check the connections to the speaker. It may be that one or both is not really secure. If the speaker uses slide-on type tabs, make sure the terminals on the wires to the speaker are tight mechanically on the speaker tabs. Also, the wires may not be making a solid connection into the terminals.


I didn't say output transformer, this person seems to think theres an output transistor?
#12
Quote by EpicGuitarGuy13
I didn't say output transformer, this person seems to think theres an output transistor?

Well that I'm not sure of
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
#13
Well if this "REG 1" is and one of the big capacitors are loose could this be the problem? If it is Id have to take the board completely out and resolder it which would be a pain. But if t the solution then I will
#14
Hi again, I managed to take the PCB out the chassis and inspected it. It looks fine, now Im wondering how do I test FETS (field-effect transistors - different to normal ones) because these may be the problem. I've done some research into them but it would be great if someone could give me rough instruction on how to do it and what kind of value the test should show if it is/ is not working