Thanks, g1; I'm going to try this tomorrow. It's driving me a little crazy!

I've measured the resistance between the 7th tab and the tabs on the primary side, and this is what I get:

tab 1 = .7
tab 2 = 5.7
tab 3 = 6.8
tab 4 = 6.8
tab 5 = 12.5
tab 6 = 13.6

It had occurred to me that the transformer might simply be rewired for 120 use. (That would mean connecting the hot wire to one of the "120" tabs and the neutral to the "0" tab on the same coil, right? Plus removing the jumper between coils?) So the theory here would be that there is a short in one of the primary coils but not the other, correct?

I think I'm beginning to understand. Here is an image of Ohm values by all possible connections between primary tabs. Connections were cut above each tab, including the jumper between tab 3 and tab 4.



TYPO: Column 5, row 6 should be 1.8, not 18.

This is the same thing, but with jumper attached between tabs 3 and 4:

Hmm, I'm not sure I understand. If there's a correct level of electricity on all of the primary tabs, then the primary must be ok, no? And if there's no electricity on the secondary tabs, all one can do is measure resistance in order to determine continuity. Since there's proper continuity on the secondary and proper voltage on the primary, but no voltage on the secondary, it appears that my tranny has exited the spacetime continuum. Or I'm missing something.

Not disagreeing -- just genuinely .

So, I unsoldered the tranny secondary leads and measured resistance: it's reading 1.2 Ohms across both secondaries and .9 Ohms from the center tap to each side. All of the primary tabs read 227 volts. How is this possible? If the primary is fully energized and the secondary has continuity, how can it not transform power? Have the laws of physics been locally violated on my dining room table? Or am I  :loco

Roly, I think we may be circling an answer here! Also, I just heard back from Roland (where's the shocked" emoticon??), and they say I can order a new tranny from their support office in downtown Los Angeles?!? I'll phone them today and see what happens.

You're right about the Limiting Lamp. I'll get on that as well. Also, I'm going to un-solder the secondaries today and test for continuity. It occurred to me that the thermal fuse in the primary might be blown. This occurred to me right after I discovered that there's a thing called a "thermal fuse."

If that's the case, and if the fuse blew for some reason that can be determined and corrected, then maybe the fuse could be bypassed and the transformer saved from the bin. That's two big "ifs" though.

Anyway, I'll post the results my findings as soon as I have them. Thanks to all for your ongoing help and support!

Thanks Roly. It's a real head-scratcher for sure! I tried emailing Roland tech support, but no luck.

Is it alright to use a tranny with (maybe) four volts lower than (might be) required? In tube amps, voltages have to be pretty close to correct or things start to blow up -- at least, in my limited experience. I don't mind lower volume, though, if that's the only side effect.

And I thought tube amps were complicated!!

Roly, the purchase was a complete punt, if I understand football metaphors. I bought it at a local Sally Ann for $50. There was no way to test it, but with a $50 tag for an amp I've wanted for about 25 years, my drool would have posed a shock hazard anyway.

The money's for charity, so I'm not complaining. Plus it'll still be a deal even if it costs $100 to fix, and maybe I'll learn something, too.

Here are some pictures of the actual transformer, if that helps. I also have links to PDF files of the JC-55 schematic and Service Manual.

Roland JC-55 Service Manual
http://www8.zippyshare.com/v/41207516/file.html

Roland JC-55 Schematic
http://www36.zippyshare.com/v/99326843/file.html

Thanks, Dr Gonz!

If the transformer is 24-0-24, that means it's a 48v volt transformer with two 24v in parallel, correct? And 1.5 amps total means .75 amps per secondary coil?

Does it matter if the replacement transformer is larger than required (the circuit just draws what it needs, right)? The closest match I can find from Hammond is model 266L48 (4 amps max) or 186F48 (2 amps max). I'll check out the rectifier first to see if it's blown.

Thank you for your help. I've been researching these these things all day, but there's no substitute for experience.

Thank you, Roly. This is great information.

My schematic-reading skills are very mediocre, so I'm not at all certain that the entire kit runs on those two 24v leads. I will post the circuit diagram below, followed by a blow-up of the power section:






What do you think?

I just bought a 220v Roland Jazz Chorus 55 that was adapted for Canadian use with a voltage up-converter. It looks like the power transformer is dead. Pilot light does not come on and the secondary coil reads zero volts. Primary coil reads 235v. Fuses and switch are ok. (I wonder whether the 15 volts over-voltage on the primary blew out the secondary?)

Can I put in a new tranny that will convert the amp to 120v/60hz? The current 220v transformer seems to output 24v/1.5A at two points. As far as I can tell, those two 24v leads power the entire amp. Can I just get a 120v transformer with two 24v/1.5A outputs and plunk it in? The service manual says that 220 and 120 PC boards are the same. It *seems* like they both run on 24v.

I've built three tube amps, so I'm not a complete noob, but this is my first SS amp and my first Roland, and I've never worked with Euro voltages. I'm good at following directions, though.

Thanks!