[R-390] R-390 (not A) transformer

[Drew P.]

Tom B wrote:

"I am working on an R-390 (not A).  Could someone tell me the DC resistance of the transformer primaries? The radio blows the AC fuse and I am worried that it might be the transformer. With the transformer disconnected, I read 1.7 Ohms between terminals 1 and 2, and 1.2 Ohms between 3, and 4."

A good test for this type of problem is to connect a large, say, 100 w, 120v incandescent light bulb (if you can find one nowadays) across the fuse holder, fuse removed. When thusly connected and then powered, a bright bulb indicates a short circuit and limits the current to a safe level. Once the bulb is connected, try removing loads from the secondaries of the transformer. First and easiest would be to yank the 26Z5W rectifiers. IIRC, the transformer in the non-A is non-unpluggable (as per the -A) and so to remove further loads you may need to unsolder leads. After each load removal, retest with the light bulb. When it goes dim, you've found the secondary load at fault. 

If bulb is still bright with all secondary loads removed, try disconnecting the primary supply leads and then connecting AC line to transformer primary winding (either one of the dual primaries would suffice) via a line cord and the bulb in series. If dim, transformer is not shorted, but you've a short in the AC wiring in the radio. If still bright, prepare a requiem for a dead transformer. 

I've heard of the feedthroughs for the transformer terminals on the -A breaking down and becoming conductive to the transformer's case; I don't know if the non-A's transformer is susceptible to this malady. In any event, if you've a shorted transformer, you've nothing to lose by exploratory surgery on the terminals. Hey, maybe you'd even perform and document a DIY rewinding job and post a tutorial for us on Al's R-390 site, as others have done for the audio output transformer and mechanical filters. 

Drew