Another Project?
 

One of my coworkers told me on Friday that he was going over to pick up a free old tv this weekend. He said it was in large cabinet with double doors, and that it was a radio phono combo, but the phono (and the drawer it came in) had been removed. Ideally, he wants to put his flat panel on top of this set, and have me get the TV/Radio working again. He would put his home theater gear in the gap where the phono had been. He also said the radio had a round dial, instead of the slide rule type typical of these sets.

Well, he emailed me some pictures this evening. Looks like fun. It is a GE model 802. Here are the pictures he sent me:

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Nice set, somewhat scarce too.
One just sold on eBay last week: http://cgi.ebay.com/1947-GE-model-80...d=p3286.c0.m14

I have the tabletop version without the phono, model 803, not my favorite set to work on.

Bah, I just got to work on this set, and it looks like the transformer is toast. I had pulled the rectifier and powered it up to see if the filament string was still good, and just as quickly powered it down when I didn't see any filament glow. A dim bulb test revealed a shorted transformer. Ohming it out, it looks like the filament string secondary is the problem.

Let this be a lesson in plugging in a questionable set! As found, the 5Y3 was burned up, and the power resistors coupled to the electrolytics were blackened, one had even blown apart! My guess is that someone in the past powered it up, and left it powered up, as smoked rolled out of the chassis, waiting for a picture to come up.

It's a shame, too. This had a very nice replacement crt, with a date code of 1952. It tests practically new.

Sounds like the packard bell in my avatar:

I guess it originaly had a fryed flyback,, When i bought it, someone had allready pluged it in and smoked the transformer and the 5U4. To make matters worse, the 17RP4 was weak. A nos fly and transformer and a crt later i got it running great.. :D

Steve

Well, I've talked to the set's owner, and between the option of getting his money back, or pressing on, he chose the latter. He really wants to see it working!

I have a few spare good transformers, but nothing that quite matches GE's design. The 802 uses two rectifiers, a 5U4 to put out something like 250 volts at 160ma, and a 5Y3 "boost" that puts out 425 volts at 85ma. If this were any other type of tv, I would probably have a match.

I'll dig into the original transformer first, and try to determine what is really going on with this thing. If I've only lost one of the secondaries, maybe I could patch in something else.

What are the chances on getting it rewound?

We've contacted a couple of rewinding services, and minimum cost for that would be around $250. That's a bit out of our budget, to say the least.

There's plenty of room to install a sub-chassis that would hold replacement power transformers below deck in that cabinet. Just be careful with the high voltage, hopefully you can get a transformer that supplies the high voltage that can fit on the TV chassis, and have the sub-chassis feed the tube heaters. This assumes nothing unobtanium is also bad...

Filament secondaries will measure near 0 ohms when good, so be sure that you aren't being fooled by that. Remove all tubes and try the dim bulb test.

If it is just a filament winding I would say that wouldn't be so bad. Should be space for a beefy filament transformer somewhere in there, and I don't guess they cost too much.

If it really is a shorted filament secondary, then a separate filament transformer won't help much since the short will still overload the transformer.

I agree with wa2ise though that ohming the secondary isn't a reliable test.

John

Well, this thing failed the dimbulb test days ago, I just forgot to mention that. I'd say it is pretty dead, since even with all of the tubes pulled, the simple act of powering it on with "no load" is enough to overload the transformer, as it rapidly heats up and makes a fairly audible hum to boot.

I hope to get into this a little more tonight-disconnect the tf leads and really see what is happening here, before going through transformer replacement hell.

I removed the transformer tonight and pried the bells off to see if I got lucky and it was just shorting out a bit inside the case. What a mess! The transformer is charred, with bits of ash raining out of the case. Whoever last powered this up must have left it on for quite a while before giving up, and I'll bet their house smelled great when it was over. Now that it is isolated, I can ohm it out much better, and it appears the failure is on the center tapped 5y3 secondary, but there is a lot more damage here than that.

On removing it, I found another item-a mystery winding that goes to the radio section. Looks like the transformers for the 801 (which I have the service info for) and the 802 are different. Guess I'll have to finally track down the SAMS for that. GE really loved packing in a lot of stuff in this transformer, I see that they used some shims to give the end bells the clearance they need.

My strategy now would be to make a power chassis, which would slip in where the record player used to be. I will probably have to buy two transformers, something like a 350-0-350 and a 450-0-450 and run them up to the main chassis through an umbilical cable. I can't be sure until I get the SAMS.

Sent you a P.M. concerning the schematic. :yes:

Be sure to check for what might have killed that transformer, like a shorted filter cap or such. It's possible the transformer killed itself, but you want to be sure there isn't a problem that would ruin a replacement transformer.

I've checked over the set's power supply, and I keep going back to the multisection filter caps. They all test bad, using my Heathkit cap checker. Certainly the e-cap section that was tied into the video IF bias supply was bad enough to cause a 2 watt power resistor to burn up into two pieces. The primary filter cap in the 5y3's filament string had also belched its guts out as well. So for now, I would just say that the transformer was killed recently by someone who powered it up with bad caps. After all, the set's owner was given this set for free, so it is possible the previous owner toasted it, knew it was badly damaged and gave up on it.

This leaves me with looking for a replacement power supply. Here is how the 802's tf broke down:

The plates of the 5u4 and 5y3 are shared on a single center-tapped to ground secondary.

The low voltage windings go to the 5u4's plates and put out 270 volts.
The higher voltage windings go to the 5y3's plates, supplying 440 volts.

There are two 5v windings for the rectifier filaments.

There is a center-tapped winding dedicated to the two filaments related to the phono preamp tubes.

There are two more pairs of filament secondarys, these are not center tapped:
The first filament secondary supplies 8 tube filaments and the dial lamps. This is all for the radio.

The second supplies 13 tube filaments for the television tubes.

So, I suppose to find the current handling of this transformer, I need to sit down with my tube manual and add up all of the filament currents in each string? Do the same for the plate/screen currents for the rectifier plate windings?