Amplifier service

Thatbastarddon said:
That big resistor(ceramic) in that block that you drew out is where a choke could be, if I’ve read correctly. Just speculation from a little light reading...maybe a problem spot in the tail end of the power supply section...thermal/current related.(they used a big ceramic resister there, instead of a choke-cheap way to an end)

Ray, this is the JVM 50 watt head, right? The board says 410, but it’s actually the 205?(I don’t mean to be a pain/slow on the uptake)
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Yes... That big resistor is there in place of a choke.

The amp is a 50 watter. There is no power tube soldered to V1 and V4 on the main pcb.
 
syscokid said:
Yes... That big resistor is there in place of a choke.

The amp is a 50 watter. There is no power tube soldered to V1 and V4 on the main pcb.
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Sorry, I was distracted by all the TI chips...these things frighten me intensely.
:wink:
This amp is not for me.
My speculation would be power supply...somewhere. Much like the Bugera I seem to have inherited.
Not output transformer.
I like the outlined shape in the pic for the culprit...
 
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syscokid said:
When you mentioned a few posts ago that your amp would clear up after you quickly flipped the standby switch, that to me makes me think that it's somewhere in the power supply. This is the area where I outlined over your photo that I reposted. When the good people of Mercury Magnetics suggested "power supply", did you tell them what happened when you flipped the standby switch?
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Yes. They suggested a diagnosis by a qualified tech, not to but a power supply.
 
Thatbastarddon said:
That big resistor(ceramic) in that block that you drew out is where a choke could be, if I’ve read correctly. Just speculation from a little light reading...maybe a problem spot in the tail end of the power supply section...thermal/current related.(they used a big ceramic resister there, instead of a choke-cheap way to an end)

Ray, this is the JVM 50 watt head, right? The board says 410, but it’s actually the 205?(I don’t mean to be a pain/slow on the uptake)
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Yes, it is the 205c. I did not even see that on the board, so I am the slow one!
 
eSGEe said:
Have we reached the "F" Marshall level?
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Maybe 'F - Tubes' in general perhaps. I've got my eye on a Roland JC-40. Smaller than the JC-120 and will easily fit in the trunk of my Mustang (which my DSL40C does nicely) but just a more reliable, less temperamental platform...
 
eSGEe said:
Have we reached the "F" Marshall level?
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Robert Herndon said:
Maybe 'F - Tubes' in general perhaps. I've got my eye on a Roland JC-40. Smaller than the JC-120 and will easily fit in the trunk of my Mustang (which my DSL40C does nicely) but just a more reliable, less temperamental platform...
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Yes, more about tubes than Marshall. That Amp has given me 9 years of good service. The amp tech said I could not possibly been running the bias at double. There were no obvious issues. I have nothing to complain about with that amp.
 
RVA said:
Yes, more about tubes than Marshall. That Amp has given me 9 years of good service. The amp tech said I could not possibly been running the bias at double. There were no obvious issues. I have nothing to complain about with that amp.
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One thing I noticed about the Marshall JVM series of amps while trying to research for any common problems is that they don't have any common problems. They are pretty solid.

Is the tech that you've been in contact with, reputable?

I'm really obsessed in finding out what is going on with your JVM... :BH:
 
syscokid said:
One thing I noticed about the Marshall JVM series of amps while trying to research for any common problems is that they don't have any common problems. They are pretty solid.

Is the tech that you've been in contact with, reputable?

I'm really obsessed in finding out what is going on with your JVM... :BH:
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I really appreciate all your help Sysco. The shop, Davidson Electronics, has been around a long time and has an impeccable reputation. We will have our answer in 10-15 business days!
 
Don O said:
Plexi, I don’t thats the Evil Twin. The 1994+ Evil Twin version should look like this.

View attachment 23259
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Don, It's a strange duck on Ampwares it shows it as an Evil Twin but pre dates the Evil Twin by 4 years and it has tone master transformers.
As soon as the Power transformer heats up it cuts out super low volume I got a pair of USA made Jensen's and a flight case out of it.
 
syscokid said:
One thing I noticed about the Marshall JVM series of amps while trying to research for any common problems is that they don't have any common problems. They are pretty solid.

Is the tech that you've been in contact with, reputable?

I'm really obsessed in finding out what is going on with your JVM... :BH:
Click to expand...

My owned-since-new JVM410C has been a good amp. Ate a lot of tubes though over the years though....its a dedicated studio mule.
 
Ok, I just quickly looked for a JVM205c schematic without any luck (I'll look properly later), but after reading right through this thread, here's my first thought on where RVA's problem "may" be.
So when it does "cut out", wiggling the standby switch restores it to working condition, right?
Looking at the gutshots on my 'phone screen it appears that the standby switch may be after the rectifier (it may not be) which would mean that the switch is carrying DC voltage. This could be where the problem lies. Switches do not like to carry high DC voltages. Each time the switch is switched there is arking on the switch contacts. Over a period of time this leads to poor contact within the switch when switched "on". Wiggling the switch can sometimes restore decent contact, until it gets really bad. Just my first thought. Cheers
 
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ivan H said:
Ok, I just quickly looked for a JVM205c schematic without any luck (I'll look properly later), but after reading right through this thread, here's my first thought on where RVA's problem "may" be.
So when it does "cut out", wiggling the standby switch restores it to working condition, right?
Looking at the gutshots on my 'phone screen it appears that the standby switch may be after the rectifier (it may not be) which would mean that the switch is carrying DC voltage. This could be where the problem lies. Switched do not like to carry high DC voltages. Each time the switch is switched there is arking on the switch contacts. Over a period of time this leads to poor contact within the switch when switched "on". Wiggling the switch can sometimes restore decent contact, until it gets really bad. Just my first thought. Cheers
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Hahahahaha! Leave it to Ivan H to suggest something sensible, fairly cheap, and easy!
:cheers:
 
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