Marshall DSL 100w (Bad PT??)

[hesamadman]

I have a Marshall DSL2000. Keeps blowing fuse. Suspected an issue with the PT so I replaced the fuse, then disconnected the 2 heater taps from PT. (This PT has all connections with a female connector)
Blew the fuse again. So i disconnected the 2 300v windings. Blew the fuse. Replaced fuse, disconnected the bias supply??? Im assuming its bias supply according to schematic I found here in the library. Blew the fuse. At this point I ONLY have a 120v line on the PT. Nothing at all is hooked up. I should also add that the 120v supply is alligator clipped directly to transformer with an external fuse holder. And fuse still blows. This way I knew there was literally nothing from the amp hooked up to this PT. Im assuming its bad? Any takes on this?

[sluckey]

Look at page 4 of this pdf...

http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm2000_100w_dsl100.pdf

Notice C1 is connected across the PT primary. Remove that cap (or just unsolder one end). Does the fuse still blow?

EDIT... Also verify the jumpers are correct for 120V operation.

[hesamadman]

Notice C1 is connected across the PT primary. Remove that cap (or just unsolder one end). Does the fuse still blow?

At this point the PT isnt even connected to the amp at all. I have a temporary 120v source on the PT coming from outside the amp. I have a fuse in line. Nothing else connected on the PT. Only 120v primary. And it still blows the fuse. Kinda crazy.


-I just tried a different slow blow fuse. Same result. Tried different configurations on the primary leads even. I have a second DSL here im using for reference when hooking these up so I dont get anything crossed. I think these are very similar to the JCM900 replacement tranny by hammond but Im not 100%. At least the voltage readings are close.


A friend of mine brought me two of these. Though one is a TSL and one is a DSL. Power side is the same that I can tell. Same PT. Both amps are having problems. One i think just needs re tube. The other he said was burning and smoking. No sign of anything on the board or on the PT even. But it keeps blowing a fuse. The PT in the other amp is fine. I could swap it but I need to see if there is a cause for it before I change it and blow up another one.

[sluckey]

At this point the PT isnt even connected to the amp at all. I have a temporary 120v source on the PT coming from outside the amp. I have a fuse in line. Nothing else connected on the PT. Only 120v primary. And it still blows the fuse. Kinda crazy.

OK, I didn't realize you had all four of the primary wires disconnected. That PT has dual primaries for operation at 115 or 230. The two primaries must be jumpered together such that they are connected in parallel for 115 operation. Exactly how are you doing that?

[hesamadman]

OK, I didn't realize you had all four of the primary wires disconnected. That PT has dual primaries for operation at 115 or 230. The two primaries must be jumpered together such that they are connected in parallel for 115 operation. Exactly how are you doing that?

I have alligator clips from 120 tap to 120 tap and an alligator clip from neutral tap to neutral tap. Ive checked resistance from each tap to chassis. From each pin to each pin. No shorts.

[sluckey]

Just trying every little thing before saying the expensive PT is bad.

Try one more little thing. Plug the amp into the wall with the PT primary wires completely disconnected. Turn on the mains switch. Does the amp mains fuse blow? If the fuse holds, then pull the PT from both amps and put the known good one in the DSL amp. Substitution is a valid method of troubleshooting.

[darryl]

An invaluable item for troubleshooting overload conditions in amplifiers without blowing fuses is a series limiter.

sluckey has the details on his website:  http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

[hesamadman]

Try one more little thing.

I will try this tomorrow. Thats a good idea.

An invaluable item for troubleshooting overload conditions in amplifiers without blowing fuses is a series limiter.

I have one of these on my bench. I'll have to use it tomorrow. Im running out of fuses ha ha. I was actually wanting the fuse to blow each change I made so I could see what would stop it. Well....i didnt WANT the fuse to blow.    I was just using the fuse blowing as a tool rather.

[Ritchie200]

I thought you had a separate dedicated 120v supply hooked up to the tranny now?  If so, that would take the switch out of the equation.  If I misunderstood and you are still running through the IEC connector, check the cord and the connector too.

Jim

[sluckey]

Rather than blow all your fuses consider getting several different size circuit breakers to use on the bench. Put pig tail gator clips on them and just clip across an empty fuse holder. You could have already paid for one breaker. Weber has'em cheap...

http://www.tedweber.com/amps/fuses-circuit-breakers

[hesamadman]

This might not be the prime way to handle things but I did just hook the PT up to a power source with a 15 amp breaker and.....well it sizzled. I only flipped the switched for a moment at a time so no fire occurred. I am going to try a known good PT next.

[PRR]

It is possible (easy) to jumper two 120V windings *backward* so the PT acts as a near-short.

I would try it with *one* 120V winding at a time. One winding is lame for loud playing, but OK for short bench-test.

> I was just using the fuse blowing as a tool rather.

The lamp-limiter is the right tool. That's a big amp and when happy will make a 100W incandescent lamp glow dim. But when UN-happy it makes the lamp glow FULL BRIGHT. And the lamp won't die of that.

Now that 100W incandescents are scarce, grub in a good lamp store for a few 150W or 200W incandescent lamps. That will cover the big amps. 150-200 inky is still pretty legal and popular for garages (though CFL and LEDs will soon displace them).