JCM2000 TSL Power Transformer

[mikeboone]

Looking at this amp for a friend. Blowing mains fuse on power up, accompanied by loud and vibrating hum. I disconnected everything thing from the secondary and problem remains.

From the schematic it looks like it has dual primaries for 120/240 wiring. In the 120 config it seems like they're in parallel.

The resistance (in order down the line), 1-2 is about 2.5 ohms, 3-4 about 2.5 ohms. I presume these are the 2 sets of windings. No continuity between those sets of windings, if indeed I am seeing it correctly.

If I pull the red wires off (top of pic) the Fuse doesn't blow.

Any other way to test the PT?

[Enzo]

Yes, the two primary windings are in parallel for 120v operation, but it is CRITICAL that they are in phase.

Leave the secondaries all disconnected. The top in the photo appears to be the primary. I see two red wires and a dark blue, is there a brown hidden under the wire harness upper right?

The two red wires appear to join in the center on a pair of posts, To me it looks like this is set up for 240v. On the AC board I see two groups of three posts, one just above the red wires and one just below. The lower group has two brown wires and the upper group two blue wires, yes?

Look at your schematic, see group of three posts W1-2-3, and the other group W9-6-8. I think those are the same groups. Note fdor 120v all three posts would be ocupied in each group: the two wires from one end of each primary and the wire from the powr switch. In each case.


Now look at the drawing, and in 120v mode note that the two windings are parallel, but more important, the two bottom ends are wired together and the two top ends are wired together.

I am thinking the two red wires on the posts, the lower one moves down to the empty post by the brown wires, and the upper red wire moves to the post above the blue wires at top.


But here is the first test: If i read you, left to right across the top is blue and red wires with a couple ohms between them. And they have no continuity to anything else, right? So connect ONLY that pair of wires to the 120v and see if it blows fuses. I bet it doesn't, but if it does, your tranny is history. Now disconnect those and connect the red and (brown) wires from the third and fourth tranny terminals ONLY to the 120v. If the two windings can individually function, then they will also together, IF THEY ARE IN PHASE.


Before you blow any more fuses, and the stress that comes with that, look up "light bulb limiter" and make one and use it. It is extremely simple, but will save fuses and perhaps your amp.

[mikeboone]

Thanks for the reply. The amp came to me wired this way, outside the chance that perhaps my buddy got brave and started moving wires around, I "think" its wired correctly. But instead of getting bogged down in that, I tried your suggestions (pictured) on what I believe to be each of the primary windings and the problem remains. I don't need to leave it on waiting for the fuse to blow, it honks rights away.

Just to make sure there wasn't anything funky going on with the amps internal wiring, I wired a power plug through a fuse block and clipped it directly to the same lugs and it honks right away. Honks is a technical term for a tranny that vibrates on the bench and sounds like a goose with bronchitis...

I am relatively certain at this point that the PT is a gonner, but I am still no pro and hope you can chime in again... Thanks for taking the time to help me.

[Enzo]

And ALL the secondary wires are now connected to NOTHING, right?

If so, you are likely correct, its a goner.


Go over to RF Keen's Geofex web site and find his simple transformer tester. You can make it in a few minutes and it will tell you the tale.

[mikeboone]

Also, the picture I posted it the original post is the WRONG one! Sorry. I was looking for info online and saved that one on my phone next to the one I took of the amp. That other one is indeed wired for 220VAC. I tried attaching the correct picture but its not cooperating.

And, yes sir, I disconnected all secondary for this test.

I will go check the GeoFX page and check that out. Also looked earlier at the light bulb current limiter, very cool.

You guys will make an amp tech of me yet!

[Enzo]

Geofex has a lot of useful and interesting material.



Oh well, at least I correctly analyzed the wrong photo.

[JMMP]

Have a PT from a JC900 that the guy wanted swapped out for Mercury avail, if your interested (and in need). In central NJ.

[mikeboone]

Thanks but I already ordered one from Classic Tone. I use their iron in my builds and have been happy with them so far.

[mikeboone]

So I got the new PT in. Hooked up the PRI (I made a light bulb limiter, BTW) and fired right up, no issues. Wired in the secondary's and again, no issues. No tubes in it yet, they'll be here tomorrow.

QUESTION: The original PT didn't "appear" to have a center tap for the heaters, but there was an unused lug on the original PT right next to the heater taps. Only 2 wires leading away from the PT lugs. The new PT has center tap for the heaters... Do I ground that?

Link to PT spec: http://www.classictone.net/40-18084.pdf

[Enzo]

The original used a virtual center tap, like fender did for years. Look on your schematic. See where the heater winding on the transformer conects to W9 and W10, follow to the left, see the two 100 ohm resistors R8 and R78? Those form the virtual center tap. If you ground the center tap, remove those resistors, or just leave the center tap unused and leave the resistors in place, they are doing the job.

[g1]

The TSL has resistors providing "virtual" center tap for the heaters. (R8 & R78 100R resistors). So you would use one method or the other for the center tap. If you want to use the heater center tap you would need to remove those resistors. I would suggest you stick with the stock arrangement, keep the resistors and tape off the extra heater center tap lead.

[mikeboone]

Thanks guys... I was little unclear on this. I am going to include it in my notes. Once I write it down, I usually remember.

Now the real fun starts... Trying to determine if this guy just crapped out or if something downstream killed it.........

[mikeboone]

Hello guys...

I got the new PT installed but am having a little trouble. The plate voltage to the power tubes is very low, 285VDC. I pulled the tubes out and the voltage is still low at 425VDC. The next step was to re-check my wiring to ensure the transformer was wired in correctly. Enzo mentioned (vehemently) that phase was critical since the primaries are parallel.

I checked and double checked. Diagram attached. The black & brown are on W9 and W6 respectively. The Black/White and Brown/White are on W2 and W1 respectively. I pulled the HT taps from the circuit and it measures 355VAC across the 2 red leads, which after the rectifier should give me 501VDC? Yes, No?

I did observe that at power-up the plate voltage on the power tubes starts out about 365VDC and dwindles down to 285 over a few seconds.

In my little skull, something in the filtering/distribution is loading it down but not shorting it out. By the way, the handy-dandy limiter bulb has a low amber glow after a bright flash initially.

Things I have checked...

1. Several posts on these amps suggest C46 is a cause of issues. I looked at where it is in the circuit and decided the circuit should work without it? Yes, No? At any rate I removed it, no change. I can replace it if the circuit cant live without it.

2. I checked all the rectifier diodes on my meter. OL (infinite) reverse bias, then about .5V forward voltage drop.

3. Remove the OT from the circuit in case it was flaky and loading down the circuit. No change.

4. Checked screen resistors, all are 1K.

The only other thing I can think of is to change the filter caps in case one (or more) are failing. None are swollen. I just hate shotgun repairing. I'd like to know if there is a way I can test and tell for certain if they are bad.

Any suggestions (including putting it in the dumpster) are welcome. There are no amp repair guys in the area beside me and I am not even really one. I got flagged as such when people found out I build my own amps, but even then, they are just clones of stuff I like. I'm no expert. I did one kit, then scratch built the others just as a challenge.

[52 Bill]

I think that the light bulb limiter is causing your B+ voltages to measure low. Measuring the unloaded transformer with your voltmeter draws no current at all, so there is no voltage drop and the value seems normal. Once the circuit is drawing current, the voltage drop becomes noticeable.

Read the ac voltages on the heaters, if the limiter is causing the voltage drop, it will also show up there.

[mikeboone]

We live and we learn... Thanks! So, the bulb limiter is for troubleshooting only? Then bypass it once I am sure there are no major shorts or power related issues?

Its working much better plugged straight in, 475VDC steady with tubes in and bias set.

Still has low output, but the guy dropped it off to my house in pieces. Just putting her back together one step at a time. Havent started looking at other issues yet. Getting past the shorted PT and now this little obstacle were huge.