Help with 1941 Farnsworth console?
 

I've had a Farnsworth CC-70 on the bench for a little while, dead set. Tubes light, never any sound. Tonight I just discovered the audio-out transformer has an open primary. Schematic snippet attached:

http://www.audiokarma.org/forums/att...8&d=1215231733

The field coil is ok and ohms out to 1600 just as the schematic says. Unfortunately the schematic says nothing about the DC resistance of the output transformer. The old one is infinity primary, and maybe 1/2 an ohm secondary. Neither one sounds plausible.

Any idea what the readings should be?

If I replace the transformer, should I worry about blowing it too? Is this common? What precautions should be taken.

And trivia-related, in the schematic I see the secondary driving 2 coils, labeled "VC" and "BC". I can guess VC is Voice Coil. I can't guess the other. Anyone?

Thanks!

"BC" is a hum bucking coil. I've seen the primary resistance of these transformers read from several hundred to around a thousand ohms. The secondary will generally read less than 1 ohm. I believe www.tubesandmore.com sells a universal output transformer designed for these old radios. Usually, these transformers will simply open up on their own. Just to be safe, I'd replace any caps in the audio circuit and I'd make sure the output tubes were not shorted. If in doubt, replace the tubes.

Right on with the posted response. Replace all the tubular capacitors and electrolytics if they haven't been done recently.

In order to get a decent bass response back in those days manufacturers would sometimes use some rather light-gauge magnet wire for OPT primaries, meaning frequent failure of same.

It's a single-ended output stage by the looks of things, and probably not too critical. While some output transformers may provide better performance in that application than others, just about any OPT cadged from a garden-variety receiver should do the trick in getting that old radio to play.

Thanks both for the info. :thmbsp:

The full schematic is on nostalgiaair.org. It's a single-ended 6K6 output stage. All tubes are nos or test as so. Electrolytics are new. Still has all the paper caps at the moment.

I found a 550-ohm primary, 0.5-ohm secondary transformer, approximately the same physical size, in the parts bin. It was attached to the PM speaker in the background:
http://www.audiokarma.org/forums/att...2&d=1215365701

The new xformer is shown attached, the old one is loose (shown for comparison). The transformer mounted with the windings vertical rather than horizontal, otherwise it looks pretty authentic/original. Primary was 550 Ohms, secondary, maybe 1/2 an ohm or so (I'm using an analog VOM, just noticeably deflects the needle off of 0). I don't recall anymore where the PM speaker came from, but it's the same size as the one I'm fixing, so hopefully the transformer is roughly the same power capacity.

The radio it will go into:
http://www.audiokarma.org/forums/att...3&d=1215365701

I want it to be a daily-listener more than a showpiece. I bet it will work as-is now, but just in case I blew the first transformer with a shorted paper cap, I think I'll play it safe and orange-drop the paper caps too. I don't see a third xformer around the junk bin... :nono:

There may be a cap from the plate of the 6K6 to ground - these will always take out the transformer if they short.

The 6K6 needs a primary impedance of about 7K - the transformer is probably 5K-7K, since the other likely value (2.0-2.5K) would have a lower DC resistance - more like 200 Ohms. I'm sure it will work fine.

Change those paper capacitors. They are a bomb waiting to explode. Even if they work now, they will fail sooner or later (probably sooner). I've seen cases where a paper cap would short and take the power transformer with it. Most of these old radios were not fused and any short on the B+ line can cause serious damage. Better to spend a few bucks now than big bucks in the future.

...Or even worse, take out an Unobtanium IF can or something...

I had a cap in basically the same position short on my Philco. It went from the plate on the ouput tube to ground. In my case, I have a push-pull amp but its the same idea. I was in the process of re-capping the set and plugged it in to check my work up to that point. Luckily I caught it before it took anything out, but the transformer did get warm.

If you really want to see what the transformer is, you'd need to calculate it's impedance. Basically take a known AC voltage and apply it to the secondary, and measure it on the primary. You use that to calculate the imepdance. http://www.radioremembered.org/outimp.htm

RCA book says 7600 ohms for the 6K6 at 250 volts. Its just AM radio though, so it doesn't need to be exact.

I'm looking at the schematic from nostalgiaair.org for this radio. If the 6K6 plate cap shorts, you end up with full B+ across the transformer primary. I wouldn't expect it to last one second under that condition. Should have the Orange Drops in hand by the weekend, will replace immediately.

I have the radio/phono combo version of this one. Your going to love it when its done as its a great performer! Mine pulls in stations that some of my other higher end radio's wont. It has a nice full bassy sound to it.
-Tony

Ok, the chassis is completely recapped now:

http://www.audiokarma.org/forums/att...7&d=1216663751

There was one totally-buried wax cap (circled in red) that I had no chance of replacing, in place. I moved it to an electrically-equivalent location (the rightmost, vertical orange-drop shown above):

http://www.audiokarma.org/forums/att...8&d=1216663751

The on/off switch was gunked/oxidized to the point that it no longer made contact. I had to split it open and clean it. While at it, I deoxxed the volume control carbon and wiper, so it should be scratchy-free:

http://www.audiokarma.org/forums/att...9&d=1216663751

Will put it all together next weekend probably. Hope it works!

Looks like you lucked out and got a radio with cloth wires. My Philco had rubber wires inside. What a mess to re-wire all that. I'm happy with the results but if I'd had my choice about it, it would *not* have been the radio I'd have picked for a first restore. In for a penny, in for a pound though. I got the radio free so if I blew it up I wasn't out anything.

Transformer, recap, and power-switch cleaning got'er done.

http://www.youtube.com/watch?v=xnQtFgdZsm4

It's been sort of a "catch-22" radio -- I didn't want to put too much effort into the cabinet until I knew it would play, and I didn't want to work too hard getting it playing until I knew the cabinet would restore. The cabinet had some horrific scratches on the top surfaces, I was able to sand and varnish very well. I still need to do the sides and down low, and I'll probably put a fresh coat of black on the inside too. The green columns are photo-finish, not real woodgrain. There's one scratch that's significant -- might be a real pain to get "right". And I have them too shiney right now -- the clear varnish I put on top looks right, but I think the columns ought to be satin or matte.

Then I suppose I'll want to detail the chassis -- repaint the transformer bell, polish the steel chassis, etc.

Gadget, yes this one had all cloth wiring -- except the 2 dial-lamp wires which were rubber. The wires crumbled into confetti first time I touched them. I would hate to see an entire radio that way.

Thanks all for the transformer advice and the orange-drop recap urging. It's turning out really well.