Troubleshooting Marshall VS100 without powering up?

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Hi all,

I'm troubleshooting a Marshall VS100 head (schematic attached) that makes a loud constant buzz when powered on. It makes this buzz with and without a guitar plugged into the input jack. I worry that something in the amp is passing DC to the speaker cab, so I don't want to leave the amp on for very long. The buzz gradually gets louder the longer it's on, but I haven't left it on for more than 5-10 seconds for fear of damaging my cab.

Things I've tried:

  • Checked the speaker cable with multimeter (tests fine)
  • Tried different amp with same speaker cabinet (cab works fine)
  • Swapped 12AX7 preamp tube with new one (no change, buzz still present)
  • Jumped FX loop (no change, buzz still present)

Normally in this situation I'd check power supply voltages and trace an input tone with a signal trace, but I can't do that without leaving the amp powered on. Unfortunately, I don't have another amp with an effects loop, so I can't run the preamp of the VS100 to the power amp of a different amp.
The filter caps may be old, so I'll replace them and see what happens. I haven't found anything obviously wrong when visually inspecting the boards, but I'll keep looking.

Is there a way to troubleshoot without powering up the amp? Can I connect a big resistor across the output instead of a speaker so I can check the power supply voltage?

Thanks for any and all help.


  • marshall_vs100r_100w.pdf
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Is there a way to troubleshoot without powering up the amp? Can I connect a big resistor across the output instead of a speaker so I can check the power supply voltage?
EDIT: Follow the advice above - a SS amp doesn't need to see a load. The dim bulb tester is very useful.
Yes, for a tube amp, a high wattage (depending on the power output of the biggest amp you are likely to test) resistor in the 5-15 ohm range is a pretty standard item for the workbench - especially if you work in the evening and don't live alone!:D
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As Fahey said:
No Load.
Measure at the output and see if there is a DC voltage higher then 1V Dc (either negative or positive)
Then come back.
The small tube is in the preamp. Your power amp is SS.
The chance that your output transistors are gone is very real.
But that's the following step.
This IS NOT a tube amp. DO NOT connect a load to it until you are SURE there is no DC on the speaker jacks. The one little preamp tube in it has NOTHING to do with the output stage.

Turn it on with NO LOAD, and see if DC is present on the output. I bet it is, and if so, the most likely failure is a shorted output transistor, though there are other possibilities.

If there is not DC there, you have lost a filter cap, or more likely the solder to one.
Built a bulb limiter and tested DC on the output by plugging in speaker cable to output jack and measuring across the plug. During power-up and idle, there is ~.015VDC across the plug. During power off, there is ~2.5VDC on the output, but it quickly decreases to ~0VDC.

Does this sound like a filter cap issue?
Hm, Wat about the bulb? Is it lit?
Can you measure the power supply voltages?
+ and - 45V DC or something. My memory is failing at the moment.
Just finished cooking a meal with fish, pasta and tomato sauce.
So, is the bulb lit?
Are the supply voltages present?

Let us know...
What bulb did you use?
It needs to be a filament (white hot wire inside) type


not any modern CFL/fluo/Led lamp ; preferrably between 40 and 100W ... "real" watts, not "equivalent" watts.

When you turn amp on, bulb will blink , because supply caps need a second to be charged, and then filament will barely light red or orange, meaning a little current is passing trough it.
If it does not blink, something is open in the mains path inside the amp, either a blown fuse or, God forbid, an open transformer primary , which we'll test later.

As you see, we need step by step input from you.

For some obscure reason I HATE, schematics no longer list main rail voltages (stupid ) , just refer to them as +V .-V , HV rail, and some other *names* , in this case +LT and -LT .

C'mon guys, we are TECHS, we speak NUMBERS here.
By the same token, transformer secondary voltages (which you might have to read during troubleshooting) are: Red-Black-Red around 30VAC-30VAC ... somewhat less with bulb limiter.
You are forgiven for not recognizing them :) , we expect around +/-40/45V there , what's needed for 100W into 4 ohms loads.
You will find somewhat less there, because bulb limiter is "eating" some of your mains voltage, so around +/-30V wouldn't worry me.

Now if amp is "overeating" , bulb shines brighter and rail voltages are down to, say, +/-10 V or less, then : "Houston, we have a problem",

Don't spec transformer AC voltages either, same stupidity.

I guess they try to stop exactly what you are doing now: repairing it yourself , asking for help in a Forum, instead of sending it to Authorized Service.

FWIW the Car Industry has published or allowed publication of millions of Car Servicing/Tuning manuals available to car owners or anybody who cares since forever ... don't think they went broke because of that !!!!!!

So, turn amp on, watch for bulb to blink (or not), measure +/-LT rails, again DC at speakers, also +/-15V rails, check that tube filament lights, that tube has some 100 to 200V DC at its plates, and post results.

Some of these are not needed yet, but speak about the power supply health.
i'm with Enzo on this one that it's likely bad solder joints on filter caps (or failed/open filter caps)
the other possibility is a shorted diode but rail voltages would be affected.

ECA photoflood lamps are usually rated at 250 watts far to high a current limit for this situation.
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Took some more readings and got the following results:

* +LT: ~ +40 VDC, measured at R33
* -LT: ~ -28 VDC, measured at R36
* ~ 0VDC on speaker jack during idle, ~ +2VDC during power off
* +VCC: ~ +11 VDC, measured at pin 8 of IC4(c)
* -VCC: ~ -12VDC, measured at pin 4 of IC4(c)
* Preamp tube lights up, ~ +138 VDC measured at one of the tube plates

Anything look out of order?
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