Seeking troubleshooting tips on my Marshall JTM30

[maddtapper]

I just recently acquired a DOA Marshall JTM30 210 combo. I have done a few things so far and seem to have hit a wall...

When I got it it powered on, with no sound.
I took the chassis out and found a blown resistor... R126 on the linked schematic... I also changed the tube sockets and output transformer while it was apart... As they were inexpensive and I don't know what this poor amp has been through... I also ordered all the radial caps to change while it's apart as well...

The issue is one wire coming from the output transformer to a pin on the power tube socket instantly starts to arc when I take it off standby with no tubes in... This wire is right beside the resistor that was blown...The other wire feeding the other power tube socket seems fine... All resistors in this area of the board test fine...

Also there are to caps suspiciously missing at c122& c123 respectively by the preamp tubes... Not sure why these would have been removed?

I've attached pictures of both spots and a link to the schematic... I'm at a loss as to what to do next... Again, I got this in this condition, so the rough looking solder joints were already there... You can see where the arcing has been happening on the under side of the board pictured... I'm fairly competent at servicing and maintaining my gear, but I'm a little beyond my level of knowledge...

Thanks in advance!


http://www.drtube.com/schematics/marsha ... -61-02.pdf


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Btw I did add the jumper seen on the bottom of the tube socket when I replaced the resistor because I suspected the trace may have burned through... And the open hole visible from both the top and bottom of the board, is where the wire from the output transformer goes...

[xtian]

When the board burns, the carbon it creates is conductive. You've got to clean up that mess. Scrape , drill, or cut out all the burnt bits. Reroute what needs it.

Sounds like you're in over your head ?

[maddtapper]

Maybe a little... Lol! But I'm never gonna learn without trying! I'll do that. Never even occurred to me.

[Stevem]

You need to grind out all of that char from the board until clean Fiberglas shows up!
I use a battery type Dremel tool from Sears and a loaded with a small carbide bit.
Be sure to not breath in any of the dust that will be made as the fiber dust is nasty stuff in it gets in your throat and lungs!
While you are working on the amp derate the fuse by 1/3 rd so that if things still arc over you will not lose your new PT!
That resistor smoked due to a bad output tube which was the likely start of the whole issue along with its former owner just sticking in new fuses until the smoke really started to billow out, so you will need to add a matched pair to the replacement parts list also, and 6L6GC type output tubes work out better in these amps.

[maddtapper]

Ok, got everything cleaned up... Stuck in known good tubes... To reiterate:
New caps (all axial caps)
New ceramic tube sockets
New power tranny from Hammond, lugs soldered in place.
No more Carbon or arcing on the board...


Now... About the 2 caps that where gone.... Any reason those would've been removed? Didn't know if that may have been a factory mod? The replacement caps where too tall, so I had to mount them laying over... But everything is connected and soldered neatly...
I put new caps in per the schematic, (c122&c123) and it powered on. But Tubes didn't glow and blew a fuse...f101 on the schematic above... Beside the power tubes...


I'm gonna run to radio shack and grab some fuses... These are 6.3 amp and I don't have any laying around... Maybe it was just a fluke? Gettin pretty bummed here... Ideas?

[martin manning]

F101 is the filament circuit fuse. I'd guess the diode bridge DB1 is shorted, or perhaps the filament circuit electrolytics C122 and C123 are or were installed with the wrong polarity.

[xtian]

Now... About the 2 caps that where gone...

I don't know about your amp, but the Fender Hot Rod series of amps has interchangeable PCBs, and in the Deluxe model, there are empty spaces where additional filtering can be placed for other amp models.

[maddtapper]

D108 & D109 both test open both ways... So it turned into another day... Dammit...


Well, hopefully after replacing those, I'll be in business! Hell, I've kinda rebuilt the whole thing, so it should be dependable from here out! Lol!


Btw, thank you guys so much! This an amazing group and I'm glad I finally found it!

[Stevem]

You still have a radio shack open near you, how's that?
Leave the heater fuse out and confirm that you have B+ voltage and then at least you will know you are dealing with just a heater circuit issue!
I would also confirm that the heater fuse blows with all the tubes out or not and when you do this to make life easyer for the power transfomer derate that fuse to 2 amps.

[rp]

Check out the TAG Dim Bulb Limiter sticky. With the few bucks you save on fuses it will pay for itself. Or just wire a socket in series with an old extension cord if you are in hurry.

[maddtapper]

Well... Still not working... Dammit!
I was hoping to gain experience WITHOUT ripping my hair out! Lol!

So to recap... No pun intended...

Changed output transformer,
Replaced all axial caps on power board
Replaced tube sockets with ceramic sockets
Replaced defective diodes,resistors....
Replaced and removed two smoothing caps...
Cleaned carbon from board...


On diodes, I replaced d108 & d109... Test fine when removed but open when soldered in place...

Replaced r128 and r108... As they both tested as shorted...

Now, I installed c122 & c123 per schematic, but they where not there at all to begin with. I idiot checked polarity several times... They where installed correctly... But installed, it blows the f101 6.3 amp fuse... So I pulled em back off for now...

Now as for the diode Bridge shown in the schematic, I don't see that part at all! I see a couple of wires running where something like that would go... But no actual diode on it... Just like the same kinda wire you'd see as part of a cap, resistor, etc... But again, it looks factory...

Last thing, I talked to the guy who owned it previously, and this is what he said:


"That sucks, i was hoping you had a quick fix for it. Yep, played fine right up untill the last 4 or five time it wouldnt sound out on me. Developed a hum after i would play it for several minutes and then nothing at all over the course of a week or so. I dont know what it means but the furtherest tube to the right wasnt heating up at all."



Now that said, I attached my bias probe... No plate voltage or anything...
But the tubes all glow... And of course no sound...

[drew]

Post some photos of the whole unit, not just little bits of it.

[maddtapper]

Here are some pics... Hope this will make it easier to visualize...


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[Stevem]

Do you have another amp so that you pull the signal off of the effects send Jack and at least see if the preamp section is working?
What voltages do you have on pins 3 ,4 and 5 on the output tube sockets?

[maddtapper]

Well I'm not sure on pin voltages as I have the board out right now...

Before I put it back, I was hoping for some opinions on the 2 caps missing in the circuit... Why would they be removed if they were?

And the diode bridge ( db-1 on the schematic) where is it? I see br101 on the board between the last preamp socket and first power tube socket... That's the only place I could see it going...


I do know for a fact, that there is no plate voltage right now, if that helps...


Also in the last couple of pics you can see the sockets after I cleaned them up... (Compared to the first pics earlier in this thread...)