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38-610 Code 126 signal issues - Printable Version

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38-610 Code 126 signal issues - BrendaAnnD - 05-01-2014

I've been working on this set for a few days now. I don't have the service packet from Chuck yet, but the radio is essentially the same as the schematic from the Code 121-122 schematic.

I finally have the radio stable enough that all three bands are good, except above 19 MHz, where the LO seems to die. I'm going to order a new 6A8 to try to take care of that.

Also, I now have enough gain to get some noise floor on all bands.

Now here's the fun part(s):

With no antenna attached, I get signals on all bands, but at lower volume levels.

With my random longwire attached, the stations get very much louder. The AGC is developing good negative voltages to the grids of the 6A8 and 6K7. Unfortunately, that voltage does not seem to actually DO anything to regulate the input signal.

Add to this that when the longwire is connected, I have input overload across the BCB to the extent that there are spurs all across the dial.

The antenna coils all seem to be fine. The primaries are all continuous and the secondaries are continuous and also tune properly.

The 6A8 cathode resistor measures 106 ohms, whereas it should be 100 ohms. Well within tolerance.

Any ideas? This one really has me scratching my head. Other than the total lack of AGC action (despite a good range of voltages being developed: -1.5 to -13.5V), the set seems to perform quite well.

RE: 38-610 Code 126 signal issues - Chuck Schwark - 05-01-2014

Did you check the continuity of the IF cans P & S windings?
Could account for low volume and possibly AGC voltage not getting to a tube grid??

Just typing out loud . . .Chuck

P.S. Packet should mail out over weekend.

RE: 38-610 Code 126 signal issues - BrendaAnnD - 05-01-2014

Chuck,

Not really any volume problems, and there is good AGC voltage getting to the grids, it just doesn't seem to be DOING anything, at least not as it should be. The various coils are some of the first things I checked, given Philco's proclivity for open coils.

I did change out the 1M resistors in the AGC circuit. They had drifted quite high. This did help with overload, but did not completely solve the problem.

Also, here's the oddest thing.. it seems to be worse on BCB. The difference between a short antenna and a long one seems to be much less on the SW bands, which though they don't have the degree of normal background noise I'm used to, seem to be much more "even".

I suppose it's possible I have a weak/gassy tube in the IF and/or detector, but I am getting, as I said, a very good range of AGC voltages.

Oh.. I doubt that the packet will mail out over the weekend, since I haven't made it to the post office with the payment as yet (post office is different than our mail room, and I don't get there as often).

RE: 38-610 Code 126 signal issues - Chuck Schwark - 05-02-2014

Packet is still to be created, as it turns out.
Work and life and work conspire against the best laid plans….
Just made another note to see first note I made...Icon_rolleyes

Careful tube checks may be in order. Also, try removing tubes and doing a continuity test from inside each pin's socket terminal to the lead(s) soldered to it. Maybe a broken socket contact between the wafers??
WAG on my part.

If it happens to be the first tube to get AGC, then nothing changes from there on back to Det.

If it's not a bad tube(s), it's gotta be sumthin'
[insert head scratch emoticon here]

Chuck

RE: 38-610 Code 126 signal issues - gregb - 05-02-2014

Brenda, I am having the exact same issue with a couple of my sets as well. I am at a loss for what to try next as all coils are good, all resistors are good and I have installed all new tubes with little to no improvement.

Gregb

RE: 38-610 Code 126 signal issues - PAradiogeek - 05-02-2014

I agree with Chuck. There has to be some sort of a bad connection. Are the tube pins oxidized, or the socket connections loose?

Jon