Stumped on G.E. H-77

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Stumped on G.E. H-77

Post by terrydec on Wed Nov 30, 2016 2:18 pm

I have restored the cabinet on a beautiful G.E. Console, the H-77. I also did a lot of work on the circuit. I had to remove the oscillator coil to replace some caps. However I tagged every lead and I am confident it is wired correctly. The audio section works great. I just put in a new 6SA7.

The tube compliment is: 6SA7, 6SK7, 6H6, 6SF5, 6J5, 6Y6.

When I put my signal generator on 455 across the antenna I get a clear audio signal out of the speaker. However when hooked to the antenna, nothing.

The front end is very complicated. There are multiple winding's on the antenna coil and oscillator coil, a 3 way band switch and individual push buttons. I hate push buttons. And of course the chassis is very crowded.

I assume from the symptoms and test results the problem is with the oscillator. However I am stumped as to where to go from here. The schematic I am working from is very rough. Should I pop that coil again and double check all of the connections? Any other ideas? Arrgghh!
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Re: Stumped on G.E. H-77

Post by terrydec on Wed Nov 30, 2016 2:54 pm

Oops I meant the Antenna coil, T1. The oscillator coil would be very hard, if not impossible, to remove or even test.
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Re: Stumped on G.E. H-77

Post by terrydec on Wed Nov 30, 2016 5:01 pm

Update. I have been playing around with the signal generator all afternoon and a funny thing happened. I disconnected it and got a very faint and distorted signal by touching the antenna. That means that everything essentially works. I hooked it back up to the loop and nothing. I keep feeling like I'm dancing around the solution to the problem. Maybe bad switches? I hope not.
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Re: Stumped on G.E. H-77

Post by terrydec on Wed Nov 30, 2016 5:33 pm

Humm. It just worked. How strange. I had the generator hooked up and turned off and absently moved the tuning cap and a station came in perfectly. Then the rest. Then the lead broke and I can get the signal back but it is very weak and distorted. Obviously it is a gain or impedance problem. Oh well, since I made it work it's just a matter of trial and error. One odd point. The external antenna terminal doesn't work at all.
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Re: Stumped on G.E. H-77

Post by terrydec on Wed Nov 30, 2016 6:11 pm

Fixed it. Broken wire on the extreme back of the antenna coil.

Never mind.
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Re: Stumped on G.E. H-77

Post by Greg Davis on Wed Nov 30, 2016 6:49 pm

That'd do it.
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Re: Stumped on G.E. H-77

Post by terrydec on Sun Dec 18, 2016 10:30 pm

Yep, back to sound and no signal. Arrgghh!!

Somebody said that it was an old set. "Yeah", I said, "But I fixed it. It should work"

I'm playing my cassettes through it, mostly my old radio shows. It sounds great.
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Re: Stumped on G.E. H-77

Post by terrydec on Thu Dec 29, 2016 4:23 pm

Don't you hate this...
The radio works great, then suddenly it stops.  Everything tests good. Time for the bench.  It takes the whole works, electromagnetic speaker, special antenna coil and the tank of a chassis. You poke around, take some measurements and drink more coffee.  Then the radio starts working. Wow! You let it play for hours. It works great. So you put the whole console back together again.  It plays for two days and then it stops, which it just did.  I hate that above all other ailments.  Time for the cold spray.

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Re: Stumped on G.E. H-77

Post by Bill Cahill on Thu Dec 29, 2016 11:13 pm

Do you have an ohm meter? These a cheap resistors. And, they mounted the resistors right on the tube socket.. I hate that.
solidly , with set un- plugged, of course, to each resistor under, and, above the chassis. Freeze spray might work, but, you said it's heat sensitive. check resistors starting with power supply, in normal specs they shouldn't be more than twenty percent.
If it s, replace it. Depends where it is. It may not be causing your problem, but, try that, after changing tubes, of course, The radio may have several problems.

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Re: Stumped on G.E. H-77

Post by terrydec on Fri Dec 30, 2016 1:24 am

Thanks- The tubes test excellent and I even replaced the 6SA7 with a new tube.
It was going 'pop' and changing volume. I'm going to wait until I can get some cold spray. It is a xxxxxx to get it out of the cabinet. The stupid "Beam- o - scope takes up half the bench and it won't operate without it. Fortunately the sides come off of the chassis which allows a lot of latitude. I hate intermittent problems more than anything else.
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Re: Stumped on G.E. H-77

Post by John Bartley on Fri Dec 30, 2016 9:41 am

Terry,

This suggestion is a bit "off the wall", but it did happen to me, so ....

Some years ago I had installed a set of points into an old snowblower. I test ran it indoors and it ran perfectly. Let it sit outside for an hour and it wouldn't start ... no spark. It took a long time to find that I had set one of the wiring lugs just close enough to the engine block that once it cooled off enough it would shrink, causing the lug to touch the block and ground the spark. Take it inside and it would expand enough to not touch the block and it would run.

So .... ?

While you are trouble shooting conventionally, also keep an eye out for wires, solder blobs, etc that might be physically close enough to another part of the circuit to intermittently short out?

just a thought ....

cheers

John

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Re: Stumped on G.E. H-77

Post by terrydec on Fri Dec 30, 2016 11:54 am

And a good one. I've done so much work I'm surprised it works at all. I REALLY hate intermittent problems, especially with a lot of push buttons and band switches.
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Re: Stumped on G.E. H-77

Post by terrydec on Tue Jan 03, 2017 2:51 pm

I want to use my scope to look at the output of the oscillator. I've never used this scope for RF even though it is a lab quality B&K Precision 1470. Where do I put the probe and do I need a DC blocking capacitor? If the oscillator is working then the problem is in the switching somewhere. There are three separate oscillator coils. I've also bought some cold spray. The darned thing probably has a loose leaf spring on some switch wafer.
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Re: Stumped on G.E. H-77

Post by terrydec on Sun Jan 08, 2017 8:51 pm

Aaarrgghh...no signal tracer. I feel like my arm is tied behind my back.
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Re: Stumped on G.E. H-77

Post by terrydec on Thu Jan 12, 2017 11:52 am

Question:
Does anybody have a test instrument with an RF probe? I need to know how to build one.
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Re: Stumped on G.E. H-77

Post by John Bartley on Thu Jan 12, 2017 1:38 pm

Hi Terry,

I have one set aside somewhere, but not handy without a bit of digging. The reason for the reply is to say that I had one apart to repair the wires and all I found was a diode a cap and a resistor inside an aluminum (Heathkit) body.

Try looking at these : https://www.google.ca/search?q=%22RF+probe%22&num=100&newwindow=1&safe=off&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjbrNDGkr3RAhUL3IMKHZ-pB2UQ_AUICCgB&biw=1467&bih=760

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Re: Stumped on G.E. H-77

Post by terrydec on Thu Jan 12, 2017 2:03 pm

Great. That's what I was looking for. I've bookmarked that page. There was one on eBay but it wasn't about to pay $40 for it.
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Re: Stumped on G.E. H-77

Post by terrydec on Thu Jan 12, 2017 6:06 pm

Making a probe is harder than it looks at first. It has to be shielded from stray RF. The example that I looked at was pretty good. It was a .01 cap, a 47k resistor and a diode, but the diode would only work up to 50 volts, obviously not high enough for tube work. However I have traced RF with my Micranta, and its RF jack had a similar circuit, a cap and diode, with the diode cathode on the amp side. The author in one example used a piece of 2" copper pipe. Pretty smart. Oh well, in for a penny, in for a pound.
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Re: Stumped on G.E. H-77

Post by terrydec on Wed Mar 08, 2017 2:45 pm

Arrgghh!

It is all set up on the bench again.  I've done a lot of work on this set and I have to remember that it did work, so there should be no problem there.  I'm afraid that it is the .05 cap underneath the osc coils.  That's another problem.  There are three separate coils, and that cap is totally inaccessible.

Sometimes I wish there was someone in the area who I could call and get another set of eyes on the thing.

Oh well, I guess it will sit there until I am inspired to climb into it again. I need a tiny insulated ladder. Shocked
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Re: Stumped on G.E. H-77

Post by terrydec on Wed Mar 08, 2017 5:48 pm

I know 'what' the oscillator does but the 'how' is a gray area. So, how do I tell if it is working? Everything in the circuit, including the tube, but not including the coil and bandswitch, has been replaced. There are a couple of mica caps but they look new. I know I didn't replace them. No bad solder joints. I'm not looking forward to pulling that switch. That's about all that's left.

While we're on the subject- How do you read the big square micas and barrel resistors again? I know they were just posted somewhere else.

What about that trick where you can put another AM radio right next to it and tell if it is working? Does it work?
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Re: Stumped on G.E. H-77

Post by willy3486 on Thu Mar 09, 2017 8:52 am

terrydec wrote:
 I'm going to wait until I can get some cold spray.    


If you don't have the cold spray you can substitute a can of air duster you use for computers. All you have to do is turn the can upside down and spray on the part. I have done that a few times on parts when I didn't have any of the freezit around.

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Re: Stumped on G.E. H-77

Post by John Bartley on Thu Mar 09, 2017 10:15 am

terrydec wrote:I know 'what' the oscillator does but the 'how' is a gray area.  So, how do I tell if it is working?

One way to see if an oscillator is working is :
- determine the IF frequency
- tune the set under repair to a low end frequency (say 600kc)
- the oscillator usually works at a frequency that is higher than the tuned station by the amount of the IF
- add the IF to that tuned number (ie : 600kc + 455kc = 1055kc)
- tune a portable radio that is in known working condition to just below that calculated number, set it very close by the tube set, then tune up slowly until you hear the "null", which should happen at that calculated number (1055kc for example)
- if you don't hear that "null", "swish", "whistle", then it's possible that the oscillator is not working.

I hope this isn't too confusing .... somebody else may be able to explain it better.

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Re: Stumped on G.E. H-77

Post by terrydec on Thu Mar 09, 2017 1:27 pm

No- it sounds good.
Thanks.
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Re: Stumped on G.E. H-77

Post by terrydec on Mon May 01, 2017 8:49 pm

Well-
This set has worked it's way back to the bench.  I thought this might be entertaining to some of you.  The set was working and something failed.  Now I have odd B+ values and no audio section.  I'm trying to think like my detective character Larry Digger-
What would cause odd voltages?  Caps could but probably not, since most of those caps were replaced.  
"If you take away what is impossible, what remains is the answer."
Resistor!
Now, how do you read these darned things?  Is it dot, body end cap? This is brown, yellow, silver.

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Re: Stumped on G.E. H-77

Post by terrydec on Tue May 02, 2017 2:52 pm

What is left is right. I replaced the 5Y3. My audio is back and the B+ looks good. Still no signal though.
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Re: Stumped on G.E. H-77

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