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  #1  
Old 11-14-2010, 06:18 PM
bob91343 bob91343 is offline
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Hallicrafters S-20R

I am in the process of refurbishing this model. I have everything working except there seems to be a short in the grid circuit of the mixer stage. I have rewound some of the rf coils that were burned out but now am stumped.

Any suggestions appreciated.
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Old 11-14-2010, 08:40 PM
7"estatdef 7"estatdef is offline
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The input grid? or others?
I may be able to scare up a parts set.
Terry
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Old 11-14-2010, 10:16 PM
bob91343 bob91343 is offline
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I measure zero resistance from the middle tuning capacitor section to ground. There is a 33 Ohm resistor in the 6K8 grid and if I put an antenna on that grid cap the radio works. It appears that the 6SK7 rf amplifier is amplifying but its output isn't reaching the 6K8 grid.

If you have parts, I need a few things, like a Bandspread knob and the diagram of the external S meter. The speaker cone is torn (so far, patched with cement). I have repaired two of the rf coils that were burned out but I admit to not bringing them back to factory; one required 38AWG and I substituted 36. The other, a little bigger, still used 36AWG.
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Old 11-15-2010, 05:35 PM
7"estatdef 7"estatdef is offline
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Well for the fun of it you could cheat a bit by use a rf choke in place of the rf coil on the plate side of things. Use a small cap like 30mmfd to couple the rf to the grid coil on the 6k8. See if that will pass rf. Check for shorting plates on your tuning cap. Both main and BS caps
Terry
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  #5  
Old 11-15-2010, 08:04 PM
bob91343 bob91343 is offline
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The capacitor plates are not shorted. I feel the problem is that the resonant circuit in the grid isn't resonating. Whether that's due to a bandswitch wafer or what, I have not been able to determine. I do get gain in the first stage. I just can't couple it to the mixer.

I took off the grid cap, put an 8.2k resistor from grid to ground, and clipped an antenna to the grid. Radio works well, so all from there on is good. It's driving me bonkers. I removed the two burned out rf coils and rewound the primaries (not a good circuit because if you inadvertently ground the plate of the 6SK7 while measuring it, you burn out the coil.). Still no action and the trimmers have no effect. (This rewind operation is not for the faint of heart. I had to drill out the eyelets holding the trimmer assembly and replace with screws later on. Putting things back together required a midget with telepathic communication.)

Do you have a parts unit?
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Old 11-15-2010, 08:37 PM
7"estatdef 7"estatdef is offline
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Actually a friend of mine does. I would suspect that your rewinding coils may have made them out of resonance. Did you try to check the resonance with a dip meter? Got screen voltage on the 6SK7?
Terry
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Old 11-15-2010, 11:46 PM
bob91343 bob91343 is offline
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Yes the 6SJK7 does amplify. The windings I redid don't have resonating capacitors; those are on the secondary.

I don't have a dip meter but will see about measuring resonant frequency.

This unit was almost a basket case. It had rust all over, a warped-beyond-usability bandspread window, no dial strings, burned coils and torn speaker cone and missing knob.

I've got most of the problems resolved but this one is baffling me.

Ask your friend what he has and how much he wants. This radio isn't mine so I have to get the owner to pay for any parts; so far the cost has been zero.
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Old 11-16-2010, 05:43 AM
7"estatdef 7"estatdef is offline
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I think perhaps if there was enough current passed though the RF coil to open the winding then it may have damaged the band switch contacts. Maybe you can hard wire it so the contacts are not in play. I checked w/my buddy he has the S-19. Unfortunately that set doesn't use a RF stage.
Terry
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Old 11-16-2010, 01:37 PM
bob91343 bob91343 is offline
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I have inspected and scrubbed the bandswitch and think it's okay. Further, if the switch were damaged I would expect it not to connect the coils but it's doing that.

I guess I need to check for resonance and find out why it's not happening. The rf coils are such fine wire that I wouldn't expect it to damage the switch; the one for band 1 is 38 AWG, almost fine enough to burn out from current from an Ohmmeter. And I don't think the problem is in the primary of the RF coils; I feel the secondary is where my difficulty lies. I may try to pull the rf tube and send signal into where its plate connects to see if I can get energy transfer. Oh where is Bill Halligan when we need him? Did you know he started Radio Shack?
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Old 11-27-2010, 07:59 AM
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hvlee hvlee is offline
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Don't know how relevant this is...

I don't know how relevant this is since you're measuring a short, but I've only been able to get my S-20R to work with the 6K8 that came with it. I've tried others, both glass and metal, but only this one works. One of these days I'll be in trouble if this one fails and I can't find another which works.

Also, I had a problem with the bandswitch after cleaning with DeoxIT. Apparently I had created a bridge somewhere on the switch either conductive residue or a piece of something came loose. After seeing a bunch of warnings about DeoxIT, I've started spraying band switches with carburetor cleaner and then compressed air to remove the DeoxIT residue. This has cleared up a number of problems for me.

Harry, KT4AE
Maryville, Tennessee
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Old 11-27-2010, 02:26 PM
bob91343 bob91343 is offline
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Harry, thanks for the information. I also suspect a bridge on the bandswitch. But I have taken a break from it due to the holiday and now am about ready to tackle the problem again.

A suggestion on your 6K8 problem would be to check the diagram of the S-40 and substitute a 6SA7. It requires only minor changes, the biggest being a hole in the chassis to pass the grid lead down to the socket. Of course this wouldn't work if you want to preserve the original configuration. Why other tubes won't work is an interesting puzzle; does the oscillator run?

I don't ever use Deoxit. My solvent of choice is 1,1,1 trichlorethylene, which I have left from the days before hazardous substance regulations. I have had nothing but good luck with this stuff.

My biggest concerns are the missing knob and the torn speaker cone. I have restrung the dials and all looks pretty good otherwise. I made a new window for the bandspread dial, the old one being warped and dark yellow and cracked. So all that's left is getting the 6K8 grid circuit resonating again. I will try bypassing the bandswitch to see if that's the problem; if so, I have no idea where to get a new wafer.
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Old 11-28-2010, 08:28 AM
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hvlee hvlee is offline
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You're welcome and thanks

Thanks for the suggestion for a substitute. I've saved the post and will use it if it becomes necessary. I prefer to leave things original but I'd rather it worked.

Are you familiar with the Hallicrafters reflector? There's a lot more action than here and you might be able to find someone with parts or a parts unit. I've found a number of knobs there.

Go to www.qth.net , select "Email Mailing Lists" on the left. I think the process will be clear from there.

Harry
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Old 11-28-2010, 12:23 PM
bob91343 bob91343 is offline
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Thanks Harry; I didn't know about that. I put in my bid for subscription.

I am an old Hallicrafters fan, having owned an SX-25 for many years. I tried other receivers and kept coming back to that one. My transmitter for a long time was an HT-18. I have owned an Ht-37 or two also.
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  #14  
Old 11-28-2010, 04:46 PM
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Reece Reece is offline
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Just a suggestion: try CRC Electronic Cleaner from your auto parts store, rather than carb cleaner. Cleans, flashes dry fast, has no lube, won't hurt most plastics, is more specific for electronics.
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  #15  
Old 11-29-2010, 08:13 AM
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hvlee hvlee is offline
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Thanks

Reece:

Thanks for the CRC hint. Carb cleaner is a pretty good paint remover and plastic crinkler.

Bob:

I have several Hallicrafters also, though we don't have any of the same stuff except the S-20R. I think the SX-25 is a very interesting receiver and one of these days I'll get one. My wife would say, "Where are you going to put it?" if she didn't have problems of her own. I'm all-Hallicrafters except for a Johnson Ranger.

I've been a Hallicrafters fan ever since I was in single digits and was allowed to stay up late on Saturday nights and listen to "Judy Canova" on my dad's S-40A.

Harry
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