Marshall JTM30 mystery

[scr@tchy]

Ok, after some poking about I've been told that I need to jumper some terminals on the transformer and I should be able to convert it to run on 120v. I have the schematics for the amp but would appreciate it if someone could point me in the direction of the procedure, or schematics of the procedure.

[Bruce / Mission Amps]

Ok, after some poking about I've been told that I need to jumper some terminals on the transformer and I should be able to convert it to run on 120v. I have the schematics for the amp but would appreciate it if someone could point me in the direction of the procedure, or schematics of the procedure.

Look at the way the transformer leads are wired in series.... you need to fold the primary back on itself.
The tranny has two 120v primary windings run in series.....for 220v-240v.
I don't have one here to look at but the schematic should show how to do this... if not take a picture of the circuit board where the power connector is located and maybe we can help you move the jumpers around to get the correct phase at 120vac.
I added a third picture close up of what I think you might have to do...

[felixq78]

JTM 30 problems

These are great amps but they are well known for transformer problems.
Find a good tech and replace the tranny with an improved/tricked up one.
The stock transformer is rubbish, I replaced mine under warranty twice but it kept blowing as they wouldn't install anything but the standard transformer which wasn't the right one for this amp.

[felixq78]

JTM 30

I don't believe the amp was OK when it left him. He's blown the tranny and decided to offload it. Report him.
These amps have a Design fault, it's common with JTM 30s and JTM60s as well.

[jtm310]

Hi guys! Sorry for reviving an old thread, but I'm trying to the do the opposite as the OP so I thought I'd ask here.. I've got a 2x10 jtm30 combo and I'm trying to rewire the transformer for 230v..how do I go about it? Here's a pic:



Thanks in advance!

[Jazz P Bass]

Need more pics.

Like where the Blue & Brown go. (power switch?)

You may have to use your meter to see what is what.

Normally, if a transformer is dual primary, there are two separate windings.
In a 220V setup, the windings would be wired in series.
In a 110V setup, they are wired in parallel.

So if this pic shows a 110V setup, the question becomes "where are they paralleling the windings?"

As a side note: The 220V unit needs a 220V switch & a smaller mains fuse.

[jtm310]

Hey jazz p bass thanks for the quick reply! That diagram you have alone is a lot of help already in understanding the pin outs , apologies as I forgot the switch side:



Also I know needing a fuse half the current requirements, but what do u mean by a 220v switch? Thanks!

[Jazz P Bass]

I think what you have there is a 110V transformer.
(It sure looks like it is wired for 220V though.)

On the power supply schematic there is a triangle with an exclamation mark.
Right next to it is two switch part numbers.
110V & 220V.

jtm30-120v-mod.pdf

[jtm310]

oh wow i had no idea a different switch had to be used...i thought all it took was wiring in series and moving some wires that go to the switch around..

[g1]

"where are they paralleling the windings?"

I think maybe at the switch (spade lug jumpers) ?

oh wow i had no idea a different switch had to be used...

A 120V switch is not rated to safely handle 240V.

More pictures would make this a lot easier. Need to see the labelling on the fuse board and need a close up of power switch.

[jtm310]

Hey thanks for the help but i think i've decided to leave it as is..i've got another problem though, a common one it seems with these amps..

the clean channel sounds great and loud, but the boost channel has very low output even on 10 its prolly equal volume to the clean on 4.. it didn't use to be like this and i have no idea what caused it..quick google search says a possible cause it the OT.. I checked the OT by unplugging the centre tap lead and measuring resistance across the outer plugs..that gave me twice as much as centre tap to either plug which indicates that the OT is fine..Or is it? It seems weird that the OT is blown yet gives such glorious cleans on the clean channel and such low output on the boost channel..Any help appreciated, thanks!

[g1]

If it works properly on clean, then it can not possibly be the OT.
It can also not possibly be anything else that is in use when in clean mode.
It must be something that is only part of the boost channel, or the channel switching circuit.

[jtm310]

thanks for clearing that up, that's kinda what i suspected too..none of the caps seem blown..perhaps reflowing the joints in the boost portion of the circuit might help..

[felixq78]

JTM30/60 Tranny problems.

These are great amps but they are well known for transformer problems.
Find a good tech and replace the tranny with an improved/tricked up one.
The stock transformer is rubbish, I replaced mine under warranty twice but it kept blowing as they wouldn't install anything but the standard transformer which wasn't the right one for this amp.

I have since discovered that the problem isn't the transformers although they do blow up as a result of the real design fault. These amps don't have a bias pot i.e. they have a fixed bias which is just plain dumb. This causes them to overheat and cook the tranny. I bought a JTM60 and went through 2 trannys before discovering that the amp had been repaired then returned to the Australian supplier (Yamaha Australia) who illegally resold it as new to the shop from which I purchased it, without their knowledge. All this was discovered by accident when I rang the official Marshall warranty techs to chase up my repair and the young feller there noticed that it had been in the shop a year earlier, he then informed me of all the backwards and forwards repairs that it had been through which prompted me to do more detective work which I confronted the retailer with and he gave me an immediate refund and I gave up on Marshall and bought a Vibrolux. Years later I still missed the great tone that the JTMs have so I hunted around for a cheap 2nd hand JTM 30 had it modded i.e. had a bias pot installed by Murray at Valve Heaven in Surry Hills and it's been sweet ever since. If the bias is off they overheat and cook the tranny.

[Audiotexan]

You've got some real leaps in here, and also a really bad experience. Try not to let the two affect each other too badly!

Also, "welcome" to MEF! (You 2008 lurker you!)

I have since discovered that the problem isn't the transformers although they do blow up as a result of the real design fault.

One incident is hardly enough to be anecdotal, much less a design fault. So this is quite a leap.

These amps don't have a bias pot i.e. they have a fixed bias which is just plain dumb.

This is not a design fault. Just something you disagree with, or a feature you wish it had.

"Design fault" implies that an engineer missed something that inherently negatively affects the proper/safe operation of the circuit.

A REAL design fault would be using a primary transformer that doesn't have a thermocoupled safety. Or in more extreme example, using a 1/4watt resistor where a 14watt was called for, and the 1/4w bursts into flames the instant the expected voltage/current was applied.

This causes them to overheat and cook the tranny.

FALSE.

Especially given that almost all major tube amp manufacturers (at least in this country) intentionally bias tubes COLD out of the gate. Which means LESS current draw (so the tubes last longer), which in turn means LESS heat generated (by the transformer working to supply the needed current), and LESS stress on the transformer/bias supply as a whole.


As for the remainder:

I bought a JTM60 and went through 2 trannys before discovering that the amp had been repaired then returned to the Australian supplier (Yamaha Australia) who illegally resold it as new to the shop from which I purchased it, without their knowledge. All this was discovered by accident when I rang the official Marshall warranty techs to chase up my repair and the young feller there noticed that it had been in the shop a year earlier, he then informed me of all the backwards and forwards repairs that it had been through which prompted me to do more detective work which I confronted the retailer with and he gave me an immediate refund and I gave up on Marshall and bought a Vibrolux. Years later I still missed the great tone that the JTMs have so I hunted around for a cheap 2nd hand JTM 30 had it modded i.e. had a bias pot installed by Murray at Valve Heaven in Surry Hills and it's been sweet ever since. If the bias is off they overheat and cook the tranny.

I'm really sorry about your bad experience. But try not to let it cause you to miss out on other potentially great things!