Problem with 6100 - Hopefully a minor one.

[Zeegler]

My 6100 is currently my only Marshall. I Sold my EL34 4500 which I very much regret, and I sold my JVM410. I decided to keep the 6100 because it's sort of a middle ground between the two. It does everything I want, but not a bunch of stuff I'll never use.

Anyway, typically, now that I'm down to one Marshall, the 6100 has started acting up. Sometimes when I turn it on, there is no sound whatsoever. Nothing. Completely dead. Everything lights up as usual. Channels switch as usual, Everything appears to be fine, but there is no sound. Initially if I turned the amp off and back on, it would usually be back to normal, and work just fine. It started out just doing it once in a while, and now it's at the point where it hasn't worked the last 12 times I've tried it.

I'm assuming since when it works, it works perfectly, that the problem is a single component failure that should be a relatively easy fix. Yeah I'm an optimist.

Anyone experience a similar problem or have any idea what could be wrong with this thing?


FYI - I have ruled out tubes, cords, and fuses,

I just tried it again to see if it would work. At first nothing, and then after a few times turning it on and off, it is producing sound, but none of the LEDs light up on the front, and it's stuck on channel 1. There is also some nasty sizzling hum going on.

I think at this point it might be something serious, so maybe I had best take it in to a tech.

[surfnorthwest]

To me it sounds like a issue with the heaters, but that is just a guess. Hopefully member Antoli will chime in, he is our resident 6100 guy.

[Mike_LA]

Ok, if you are ok with discharging your unit, then I have had luck turning the lights off in the shop and observing the sparks that appear upon startup. Resolder those connctions, good to.go.
best of luck, M

[Anitoli]

Is this sizzling hum coming from the Speakers or inside the amp such as a transformer?

Are you ABSOLUTELY positive the output fail fuses are 500MA?

Remove ALL the tubes and turn on the amp. Does the led's light? does it channel switch? Is it still making that sizzle hum?

[Zeegler]

Is this sizzling hum coming from the Speakers or inside the amp such as a transformer?

Are you ABSOLUTELY positive the output fail fuses are 500MA?

Remove ALL the tubes and turn on the amp. Does the led's light? does it channel switch? Is it still making that sizzle hum?

The sizzling hum is coming from the speakers.

Fuses are okay. Definitely 500ma

I removed ALL the tubes, turned it on. No lights other than the main power switch.

[Anitoli]

The hum is coming from the speakers even with the tubes out?

[Zeegler]

The hum is coming from the speakers even with the tubes out?

No. With the tubes out there's no sound at all.

[Anitoli]

Ok. Never assume tubes are always good. You are going to need to check this head with a new set of power tubes, just to rule out the old set. Are you able to bias them your self?

Somethings got me a bit worried though, the MIDI should load upon power up and the channel switching and led's should work.
Man i really wish this was in front of me. There is a possibility that several things could be going on here and it might not be an easy fix.

Can you check voltages without getting killed?

[Zeegler]

Ok. Never assume tubes are always good. You are going to need to check this head with a new set of power tubes, just to rule out the old set. Are you able to bias them your self?

Somethings got me a bit worried though, the MIDI should load upon power up and the channel switching and led's should work.
Man i really wish this was in front of me. There is a possibility that several things could be going on here and it might not be an easy fix.

Can you check voltages without getting killed?

I have tried a different set of power tubes in the amp, tubes that I know for a fact are good. I have a bias probe, but didn't bother re-biasing the amp because the different tubes didn't change anything, so I put the others back in. I did notice that the tube sockets and therefore the power tubes themselves tend to be rather loosely mounted and the tubes are free to "wiggle" around quite a bit. This doesn't seem like it would be a good thing, but the sockets themselves are connected via flying leads rather than soldered directly to the board, so I guess it doesn't do any harm for them to move around a bit.

Yeah, I can check voltages safely as long as I know exactly what and where. I have some basic electronic knowledge, but not enough to read a schematic on something this involved. Are there any fuses on the interior that I should check?

By the way, I very much appreciate you taking the time to help me out with this. I have pretty much accepted the act that I'm going to have to take it to a professional, but in the meantime, I'll try anything you suggest.

[Anitoli]

Loose tube sockets CAN cause issues. If they are that loose you might want to replace them as the contacts could be disconnecting with the tube pins after the amp gets warm or vice-versa. The sockets are inexpensive, it's the work involved in changing them that is time consuming. But i will add if you decide to go this route i would do a re-cap on this board because the only way to recap it is it has to be out and separated from the tube sockets. Give this some thought, anyways.

Now you're going to need to measure the anode voltage, screen grid voltage, heater voltage, and the bias voltage on all 4 tube sockets with no tubes in the power amp, preamp doesnt matter right now.
Write down the readings for each tube and post here. With your meter set to the highest DC rating and the black lead on ground ( any clean chassis point will work)the amp needs to have the standby off ( play position) measuse the DC voltage at the anode ( red arrow)
screen grid (yellow) bias ( green). Now set you meter to read AC voltage in a range that is low enough to read below 10 volts. With the meter referenced to ground measure either blue arrow point.

Let me know what your measurements are.

Socket.jpg (171.56 KiB) Viewed 1957 times

[Zeegler]

Loose tube sockets CAN cause issues. If they are that loose you might want to replace them as the contacts could be disconnecting with the tube pins after the amp gets warm or vice-versa. The sockets are inexpensive, it's the work involved in changing them that is time consuming. But i will add if you decide to go this route i would do a re-cap on this board because the only way to recap it is it has to be out and separated from the tube sockets. Give this some thought, anyways.

Now you're going to need to measure the anode voltage, screen grid voltage, heater voltage, and the bias voltage on all 4 tube sockets with no tubes in the power amp, preamp doesnt matter right now.
Write down the readings for each tube and post here. With your meter set to the highest DC rating and the black lead on ground ( any clean chassis point will work)the amp needs to have the standby off ( play position) measuse the DC voltage at the anode ( red arrow)
screen grid (yellow) bias ( green). Now set you meter to read AC voltage in a range that is low enough to read below 10 volts. With the meter referenced to ground measure either blue arrow point.

Let me know what your measurements are.

Socket.jpg

Well. That was interesting for sure. I started checking the voltages as you instructed, but after the first three measurements on the first tube, a cap burst right in front of my eyes.

The little guy here bracketed by arrows began spewing smoke out of the top. Of course I unplugged immediately. Not sure if this is a cause or effect, but I figured there's not much point measuring the other tubes. In case you're wondering, the first tube read as follows:

ANODE 527
SCREEN GRID 525
BIAS -048

That was as far as I got.

[Anitoli]

Well, that must have been a surprise. That cap went south either by just being old or there is a voltage problem meaning excessive voltage but that doesnt seem likely as it is after reg402 which would be -15vdc. It is c404 on the schematic. I would recommend changing all the electrolytics in that head considering the age of these amps and you prolly dont know for sure how many hours are actually on it.
Just for contemplation, these caps used at the factory are rated at 1000 hours load life. Now say this amp was used an average of 6 hours a week for a year. That would be 312 hours. After 4 years that would be 1248 hours. Now assume this 6 hour usage rate continued for the amps life ( i'll guestimate it was made in 96 ) that would be 19 years equaling 5928 hours. That is WAY past the load life on those caps.
This also doesnt take into account how hard the amp was being used and how hot it was running. Playing the amp in a 50 degree basement doesnt get it as hot as cranking it at a gig with a room temp of 85 degrees.

The unloaded voltages that you measured seem right. Once tubes are in there drawing current the voltage will drop off.

Just a question but are you anywhere near New England?

[Zeegler]

Well, that must have been a surprise. That cap went south either by just being old or there is a voltage problem meaning excessive voltage but that doesnt seem likely as it is after reg402 which would be -15vdc. It is c404 on the schematic. I would recommend changing all the electrolytics in that head considering the age of these amps and you prolly dont know for sure how many hours are actually on it.
Just for contemplation, these caps used at the factory are rated at 1000 hours load life. Now say this amp was used an average of 6 hours a week for a year. That would be 312 hours. After 4 years that would be 1248 hours. Now assume this 6 hour usage rate continued for the amps life ( i'll guestimate it was made in 96 ) that would be 19 years equaling 5928 hours. That is WAY past the load life on those caps.
This also doesnt take into account how hard the amp was being used and how hot it was running. Playing the amp in a 50 degree basement doesnt get it as hot as cranking it at a gig with a room temp of 85 degrees.

The unloaded voltages that you measured seem right. Once tubes are in there drawing current the voltage will drop off.

Just a question but are you anywhere near New England?

I feel quite confident that I can replace all those caps with no problem, but I'm guessing it's probably not worth replacing the caps yet until I know what else is going on. The amp is actually a 92, so based on your estimate, that would be 23 years at 312 hours a year = 7176 hours. Is it possible to get better quality caps that are rated for higher hours?

I'm near Buffalo, so not really near New England.