Marshall JTM30 mystery

[scr@tchy]

Hi all,
I bought a JTM30 2x10 recently from eBay and though the seller seems reliable the amp came DOA. My repair person says it's the transformer but the seller maintains that it was working before it left his house.

Here is what it is doing: Power lights up on front, small glow in the power tubes, no tubes will get warm at all, no sound whatsoever.

The question is, could it be something other than the transformer causing these symptoms and could it have happened in transit like the seller states?

If it turns out this way, can anyone recommend where I can buy a reasonable priced replacement transformer for the JTM30?

Thanks,
scr@tchy
URL="http://www.daughtersandsons.net"]www.daughtersandsons.net[/URL] -hear my band!

[52 Bill]

Sounds like a B+ problem? Filament supply is ok , but no high voltage.

It could be any number of things, including the transformer. Something, (a wire or resisitor in the power supply) could have come loose, the high tension fuse may be blown, the standby switch may be broken, etc. Your tech should be able to tell the difference between the problems. If the power transformer's high voltage secondary has opened up, it could be a repairable connection where the winding attaches to the terminal lugs.

And yes, it could have happened in shipping.

[scr@tchy]

Hi,
Thanks so much for the reply, I don't think my guy really went through it and just did some assuming. I'll have somone check those things you mentioned.

[DirtyGeorge]

You say the power tubes heaters glow. Do the preamp tube heaters also glow? I believe the power tubes heaters are AC but the preamp heaters are powered by a DC supply. If they are not glowing then your tech should check the heater rectifier circuit.

DG

[scr@tchy]

this just in...

...my tech has also said that the power tubes were getting proper current, I didn't see any glowing in the preamp tubes. Thanks for the suggestion, a friend and I will be going through it with all of your suggestions in mind. And on a sheet of paper next to us.

[52 Bill]

Here's how to test your amp:
1-Set up amp and turn on power switch. Does the pilot light, light up? If yes, go to #2. If no, then check the fuse located on the back panel.

2-Look in the back at the tubes. Each tube should should show signs of glowing heaters. These will be small (maybe 1/16" diameter) points of light. Some tubes will have one some may have two points, but all tubes should be glowing. Sometimes you will have to look down into the top of the tube to see the glow. Your amp has different voltage supplies for the output tubes and the pre-amp tubes, so even if you see the heaters in the power tubes, make sure that the small pre-amp tubes are also lit. Do you see heater glow in all of the tubes? If yes go to #3. If no, then if you are qualified, you will need to open up the amp. Turn off power switch, unplug speakers and reverb cables, pull the ac cord and carefully remove the chassis from the case. Remember, you just had this amp plugged in, and there may still be high voltages stored inside the amp. Look at the circuit board and you'll find a fuse marked f101. This is the fuse for the heater supply. Remove it from the board and test it for continuity with an ohmmeter. If it is open, then replace it and re-test amp. If it is good, then check the wiring that connects the transformer to the circuit board. Also check the circuit board for burnt traces etc.

3-If all of the tubes light up, then turn on the standby switch. Plug in a guitar and see if any sound can be heard, including hiss, hum or other noises. If you shake the amp with the reverb control turned up, do you hear the tank rattle? If you hear no sound, go to #4. If you hear sounds, but no guitar signal, try plugging a guitar cable into the FX loop jacks.

4-Check the speakers and the speaker wiring. Use a meter to check for bad connections and open voice coils. If these are ok, then check to see if the tubes feel hot (or warm)? Be careful here as they can get very hot! If the tubes feel cold or barely warm, your amp may have a high voltage problem. If you are qualified, you will need to open up the amp. Turn off power switch, unplug speakers and reverb cables, pull the ac cord and carefully remove the chassis from the case. Remember, you just had this amp plugged in, and there may still be high voltages stored inside the amp. Look at the circuit board and you'll find a fuse marked f102. This is the fuse for the high voltage supply. Remove it from the board and test it for continuity with an ohmmeter. If it is open, then replace it and re-test amp. If it is good, then check the wiring that connects the transformer to the circuit board. Also check the circuit board for burnt traces etc.

Try this and let us know what you find out.

[DirtyGeorge]

I just want to add one thing to 52 Bill's excellent procedure. Be sure to discharge the power supply filter caps before doing anything else inside the chassis. I use a 1K 5W resistor for this.

DG

[scr@tchy]

much thanks!

Thanks, I will print this page and bring it with me when my friend and I look at it, have no fear I will discharge the caps. I will also report back to let you all know how it turned out.

[scr@tchy]

also...

Haven't gotten together with my friend yet, but I had another thought. I bought this amp from a fellow in Greece and was wondering could it be the differences in our voltages (I am in the U.S.) that's causing all my troubles?

[Bruce / Mission Amps]

Haven't gotten together with my friend yet, but I had another thought. I bought this amp from a fellow in Greece and was wondering could it be the differences in our voltages (I am in the U.S.) that's causing all my troubles?

Ha ha.. yes it sure could be... Greece is on a 220v mains system.
Look at the back panel where the power cord plugs in and read the part where it says 230v at 50Hz.

Make sure it wired for 120v.

[scr@tchy]

It sure does say 230V 50Hz. So, how do I wire it for 120V? I'm good with schematics if need be.

[52 Bill]

So maybe your tech's original diagnosis was right after all...transformer.

[scr@tchy]

Alright, now I'm confused, knew it would happen soon! If the problem is that it needs to be re-wired for 120V, how would it be that the tech's bad transormer theory is correct?

[52 Bill]

Maybe he saw the voltage rating sticker on the back of the amp!

At half voltage, nothing on your amp will work correctly. It may be that your transformer is only made to work on 240vac and cannot be rewired to work on 120vac. Some are and some aren't. Check your schematic and your transformer to see if yours can be.

[scr@tchy]

Ahhh, the smoke is clearing. Thanks!