JCM 900 4100model - caps and rebuild advice

[mikroman]

1997 amp. There are no hotspots anywhere on any pcbs. All of the caps look original and new. No swelling or leaking. I tested all of the tubes with a tube tester and are in great shape. The mains fuse will blow after 5-6 seconds of power on. I think I should recap it. Check resistors. What would any of you suggest. Thanks

 

[Pete Farrington]

Use a light bulb limiter rather than repeatedly subjecting an amp to fault current.
Use a low power bulb, eg 40W, 60W.
Take the valves out.
Power up in standby mode and note how the light bulb brightness responds over time, eg 30 seconds.
Switch standby into play mode and note how the light bulb brightness responds over time.
https://www.drtube.nl/schematics/marshall/4100-60-02-1-iss11.gif
https://www.drtube.nl/schematics/marshall/4100-60-02-2-iss11.gif
All but the most expensive valve testers don’t tend to subject the valve under test to anywhere near the voltage level found in a guitar amp. They are fine for weeding out badly failed valves, but no good for confirming the remainder really are fit for service.

 

[tschrama]

No indication yet to replace capacitors.
Try to isolate the problem
Pull output tubes, still blowing fuse? If not, output stage is the problem.

Put the output tubes back one by one and test in between. Etc etc etc

 

[DreamerDeceiver]

1997 amp. There are no hotspots anywhere on any pcbs. All of the caps look original and new. No swelling or leaking. I tested all of the tubes with a tube tester and are in great shape. The mains fuse will blow after 5-6 seconds of power on. I think I should recap it. Check resistors. What would any of you suggest. Thanks

Thanks for having the same year and model.

This info is priceless.

 

[mikroman]

Use a light bulb limiter rather than repeatedly subjecting an amp to fault current.
Use a low power bulb, eg 40W, 60W.
Take the valves out.
Power up in standby mode and note how the light bulb brightness responds over time, eg 30 seconds.
Switch standby into play mode and note how the light bulb brightness responds over time.

Valve testers don’t tend to subject the valve under test to anywhere near the voltage level found in a guitar amp. They are fine for weeding out badly failed valves, but no good for confirming the remainder really are fit for service.

I have seen the light bulb used in radio circuits. Great idea.

 

[Tatzmann]

Mains Fuse, how hard can it be.
You are not fit to work on an amplifier.
Turn it in.

 

[mikroman]

Mains Fuse, how hard can it be.
You are not fit to work on an amplifier.
Turn it in.

Thanks for the advice. There's always "one of you" around forums

 

[mikroman]

I built light bulb limiter today and tested the amp without any output tubes. Fuse OK. Now to the nervous part; move on and put back one tube at a time. I'll post news after I dig up some bravery.

 

[mikroman]

I built light bulb limiter today and tested the amp without any output tubes. Fuse OK. Now to the nervous part; move on and put back one tube at a time. I'll post news after I dig up some bravery.

Fails after first tube goes in, within 8 seconds. Light bulb starts to glow brighter, then....loud buzz as before and fuse blows.

 

[Pete Farrington]

What bulb wattage?
With no valves in, note the VDC at pins 3,4,5 of the output valve sockets.

 

[mikroman]

What bulb wattage?
With no valves in, note the VDC at pins 3,4,5 of the output valve sockets.

Thank you sir. 40 Watt bulb. I will check the voltages.

 

[Pete Farrington]

I wonder how a 3A mains fuse could blow, with the current through it limited by a 40W bulb?

 

[neikeel]

You’ve had good advice thus far.
Let’s break it down.
3A mains fuse is there to protect/detect mains transformer, rectifier and first filter cap problems. So if it fails with mains on and HT off (ie in standby) the issue sits there somewhere.
It is more common for a shorted out tube (or screen resistors) to take out the 1A HT fuse.
I presume that your are using T rated (ie SLO blo) fuses?
A 40w bulb at normal brightness will be 40/240 ie 1/6 of an amp. When it goes brighter- switch it off.

 

[Spanngitter]

On a side note:
For 230/240Vac a T1.5A or max T2A is sufficient, for 110/120Vac Input you should be able to run on a T2A or max T3.15A

 

[mikroman]

You’ve had good advice thus far.
Let’s break it down.
3A mains fuse is there to protect/detect mains transformer, rectifier and first filter cap problems. So if it fails with mains on and HT off (ie in standby) the issue sits there somewhere.
It is more common for a shorted out tube (or screen resistors) to take out the 1A HT fuse.
I presume that your are using T rated (ie SLO blo) fuses?
A 40w bulb at normal brightness will be 40/240 ie 1/6 of an amp. When it goes brighter- switch it off.

This is a NA model with a 4A slo blo fuse. It won't blow in stand-by; but, about 8 seconds after switch on it fails. My thinking was the filter caps. And yes, excellent advice and thanks for steering me there without solving the problem for me.

 

[Pete Farrington]

I’m struggling to envisage how a 40W bulb could pass enough current to blow a 4A fuse
Was the light bulb limiter in circuit?

 

[Trouble]

I agree, the fuse shouldn't blow if dim bulb tester is working correctly.