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37-650 Current Symptoms - Printable Version

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37-650 Current Symptoms - Robbie Roberts - 02-19-2006

Here's the latest on the 37-650 situation... Apparently I did something right: I have been receiving on one of the SW bands- 5.8 to 11.5. Mainly religious stuff, and slightly off - I was receiving one station on 7.75 which said it's broadcast was at 7.65, so .1 difference. Not major, probably just a dial issue.

I am also getting signals of low strength on the other SW band(12-16) but AM/Broadcast is still nothing but junk. I will try more adjustments tonight on the set. I am trying an antenna- the base antenna from my 39-116! I just rewound it with its original wire, just to have something organized to work with...

Comments? Suggestions?
Thanks!
Scott

- Robbie Roberts - 02-20-2006

Well, I might not pick up 1210 AM in Philadelphia(15 miles by air) but I can pick up Radio Ukraine International...
Scott

- Robbie Roberts - 02-20-2006

I've been fiddling and twiddling, and I am almost up to "Maxwell's Silver" hammer time after a couple weeks of this! AAARRRGGGHHH!!!!

SW works as described above, the other SW works on part of the band, and AM/Broadcast is right out- nothing seems to help!
I Am Going To Bed.

Scott

- Chuck Schwark - 02-20-2006

Sounds like you still have leftover bandswitch contact issues or BC band osc coil issues if the OSC works on Shortwave and you receivce stations.

- Robbie Roberts - 02-20-2006

So, what should I pull and check out? Oscillator plate from the subchassis?

Scott

- Chuck Schwark - 02-20-2006

Oscillator tube seems fine and working on SW bands (from your msg above), so checking the "plate" will not show anything, unless the voltage there goes away on AM mode which points to a band bandswitch contact(s) problem.

Perform continutiy checks on the bandswitch contacts, the BC band oscillator coil windings, and wiring associated with the bandswitch contacts for the BC circuits.

SW is working and the set works in SW. It's obviously the AM portion that's got a problem; dirty/open/loose/shorted bandswitch contacts; mis-wired BC osc coil/padders, etc.

Also check for dirty/shorted padder/trimmers in the BC osc circuit.

- Robbie Roberts - 02-20-2006

Thanks- BC had been working pretty well, other than volume, before the breakdown- Is there a possible capacitor or resistor which may have overloaded and blown in the BC circuit only, as a result of strain due to passing signal through the 3,300M Ohm resistor that was in place of the 330K number 46? Something that affects only the BC band? I've checked those I can see, but am not sure which would be which: I am still learning how they all work together, but am still having trouble following how each circuit goes- I tried to follow a path on the schematic from antenna to speaker, and only confused myself worse!

Everything else has been checked and rechecked- I cleaned all the contacts in the wafer switches well, and went through all the coils, etc. during the 2nd teardown. The 16-16A-10-4 adjusters are about the only things which haven't been torn down, although they have been cleaned somewhat. But they had been working previously! I just can't seem to get them to align properly. I get squaks burrs, and squeals at intervals across the dial on BC.

The other SW band is coming in ok now, too- daylight hours- Even listened to one of the UTC time tone stations, it was between 14.8 & 14.9, although I don't remember where it should be, and I did not hear a station identifier. But it was not one of the Canadian ones: voice was in US English only. I also was picking up WYFR, Okochobee, FL around 12.5, and Radio???Espana up near 16, so I think the two SW's are close to where they should be.

Thanks again!
Scott

- Ron Ramirez - 02-20-2006

Go and do what Chuck has suggested, which is excellent advice.

There is a remote possibility that you may have a burn (carbon track) on one of the switch wafers. This happened to me on a 37-650 last year. As with your radio, this set initially tried to work, then quit. Eventually, and by sheer luck, I found (via continuity checks which Chuck has suggested that you do, and this is one reason why you need to do this) that a carbon track had formed from B+ to ground on one of the oscillator section switch wafers, and had burned a pit in the phenolic. Using a sharp knife, I scraped out the carbon until I had no continuity between the terminals where the track had formed. I then filled the cut area and the pit with epoxy. Reassembled, end of problem.

Diagnosing and repairing a carbon track is not a job for a novice; it isn't easy for someone who has been doing this for years, either, believe me. Good luck. You may need an Elmer on this job.

- Robbie Roberts - 02-20-2006

An Elmer? I will look into the carbon track concept, and pull the Oscillator section for a look-see...

Thanks!
Scott

- Ron Ramirez - 02-20-2006

Elmer = a mentor. Someone more experienced who can be there with you to help you out. No relation to Elmer Fudd. Icon_lol

I was going to suggest revisiting the AM band coils, but I think it's worth looking into the possibility of a carbon track, since I've been down the same path before and that was the problem on the set I was working on...your mileage may vary.

- Robbie Roberts - 02-21-2006

I'm looking over the wafer switches, but not seeing anything yet. I am probably going to pull the BC coils, and recheck them... With that thought in mind, I do recall replacing the leads on one section of the RF section BC coil- I think it was leads 3&4, on part 9, the BC RF Trans coil. They were the thin woven kind, sort of like the braid on shielded wire, but much lighter. I replaces it with a size about twice/3X's as thick- Could THIS be modifying the resistance to an unmanagable point?

Is it possible to swap in the one from the 38-3 chassis I have parted out?(I am going to print out the specs on that in a minute, but thought I'd ask...)

Scott

More info - Robbie Roberts - 02-21-2006

Regarding the BC RF coil #9- the #1&2 leads should ohm out at 130, and are 131, so I feel they are fine. HOWEVER, the ones I replaced the leads on, 3&4, should be 5.0 and they ohm out at 14.0

Sounds a little high- I think I will replace the new leads with some from another coil, closer to the original thickness, and see if I get a drop in resistance, get it closer to what it should be...

Could this resistance be my problem??? Now I will have to go check the other BC coils in the subchassis for similar defects, though I only remember the one set of leads being replaced.

Now, I do l know I seem to have no change whatsoever when I adjust the Var. Cond. on the RF board(number 10) so I am thinking it might be in this section...

Scott
PS- When I checked the 38-3 specs, they have completely different numbers.

Further Results - Robbie Roberts - 02-21-2006

I removed my replacement leads, and grafted on a donor set removed from the 38-3 coil: I now have a resistance reading of 6ohms- down from 14, so I feel it is much closer to spec. However, I have just noticed a tendency of this particular (digital) multimeter to read 1 ohm when the multimeter test leads are connected together, unless "scrubbed" against each other a second, at which point it goes to 000 as it should for a direct connection between test leads. So, with this +1ohm deviation factored in, that brings the actual resistance to about 5 ohms, which is called for in the schematic.

I don't recal having to replace any other coil leads, but I am going to run a quick check of the other BC coils while I have the subchassis out HOPEfully for the last time this endeavor. I think if I have to take this subchassis out one more time, I am going to inatall quick disconnects!...

Comments?
Thanks!
Scott