Need advice on a “new to me” project

[coreythompsonhm]

Hi there!


After building one small 5 watt amp kit about ten years ago, I unfortunately never got around to building anything else as life got in the way. I was recently given an jcm900 4502 combo that had gotten trimmed down into being just a head, “popped a fuse and I bought something else, here it’s yours.”


First things first, I pulled fuses. Mains fuse is correct value, blown. Output fuse intact and correct value.


I removed the chassis from the cabinet and discharged the caps, verifying with a multimeter.


Upon inspection, looks like the PT got real hot at one point (pic included).


Also, a resistor connecting two pins on an output tube burned up. Pic included.


Last thing I noticed is on the preamp board, two resistors have broken free of their solder on the pcb board.


With these issues, is it worth digging in and attempting a repair? Or is a burnt up PT a huge sign of more problems? I am very rusty on my electrical theory and am starting to brush back up on these topics.

[Bieworm]

Looks like someone already soldered inside the amp. The red wire above the burnt resistor on the power tube has soldering iron marks.

[coreythompsonhm]

That was my thought as well. I am thinking it’s good for a few parts, nothing more. Either way, it’s got me itching to build another amp!

[2deaf]

Also, a resistor connecting two pins on an output tube burned up. Pic included.

That is most likely a diode going from the plate to the cathode that has shorted.  I would snip out both of them and see if the power supply works without the power tubes installed.

I don't recall ever seeing diodes like this in Marshalls of this era. 

[coreythompsonhm]

You are right, they are diodes, not resistors. Looking at schematics, I am not seeing diodes at all in the power amp circuit. I will do as you suggested and clip the burnt diodes off, remove the tubes, and see if it powers on at least.

[coreythompsonhm]

Well, no luck. Removed the tubes, clipped off the offending diodes from the tube socket. Replaced the fuse. Fused popped on power switch being thrown to ON.


I’m reading up right now on how to diagnose the PT.


Including a closer pic of the shorted diodes.


EDIT: Tested the resistance of the PT. It’s toast 

[DummyLoad]

hmmm... for amps running around 400V B+ i thought that there should be 3 of those flyback diodes in series, rated at 1000V PiV, and they should have be connected to ground, not the cathode. one string from each plate phase.


--pete

[2deaf]

I thought there were three diodes in series on each tube, but I couldn't tell for sure from the picture.  Cathode is ground, so lots of amps have the flyback diode(s) right on the socket.

The extent of the diode and PT burning makes it look as though there was some time involved as opposed to a flyback surge of very short duration.  So why didn't the 500mA cathode fuse pop?  Sure it's slo-blo, but I would think that kind of overload would have done it.  And what about the other in-line fuse, the OT primary?   
 

[coreythompsonhm]

Yes, it looks like there are three diodes in series. I checked the continuity of all fuses, the only one popping is the mains fuse. I have not verified if the two fuses on the PCB board are the correct value or not.


EDIT: I called the guy up and asked some questions. His uncle was the one who cut it down from a combo to just a head. Before my friend got it, the back was completely enclosed with just a slot for access to speaker jacks, etc. His uncle was having issues with overheating so he just put bigger fuses in. That probably explains the extended overheating and burning the diodes.