#166
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Ignore the reactance tube for now, or at least keep the input grounded for the moment per the adjustment procedure so you can focus on getting the oscillator running. It's obvious you have no output from it, that's why you have zero grid volts on V124A and also sagging plate volts- that tube is probably turned on 100%. Make sure the .01uf caps to ground coming off pin 2 are not leaky (C2110/C211), then see if you can lift one of the primary winding wires of T115 (pin B or E) and measure resistance. It's possible your CW transformer has shorted or something, but tuned circuits can be deceptively hard to troubleshoot despite their apparent simplicity.
One last note: although the inductors in the CTC-4 chassis are not the dreaded white ones that always go bad like the CTC-2(B) chassis had, it's always possible you have a bad inductor still. Make sure L42 checks in at the proper resistance, and also ensure C209 isn't leaky.
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Evolution... |
#167
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Thanks, we hear and obey
Phil Nelson |
#168
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There's also a couple of .001 caps (C219 and C220) across the secondaries of the CW xfmr. One of those could be wonky.
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#169
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I believe C219 and C220 are inside the T115 can, but as chrisWA6TV mentioned, little micas inside xfmr cans have been known to go bad, especially the "naked" kind that are uninsulated mica sheets. I replaced some in the audio section of an Admiral set.
My back is just about played out for the day. Tomorrow I will grunt the chassis back onto the workbench and go on a cap-and-resistor hunting expedition. miniman82 mentioned C209, which I already replaced, but that reminds me -- depending on what schematic I look at, that cap's value is given as 4, 4.7, or 6.8 pf. Here's the C209 that I removed: I hate the codes on these little dogbone-shaped caps (and I have never found a bad one, but I replaced it anyway, while I was soldering on that terminal). What I had on hand was a 5 pf mica, so that's what I installed. I'm hoping that this code translates as 4.7 pf and that a difference of .3 pf doesn't matter here. If I'm wrong on one or both counts, someone tell me now, and I'll just put the old one back in. Phil Nelson P.S. The resistance across L42 is 10.6 ohms, which doesn't seem outlandish. The schematics don't give a resistance for it, only the notation 12 muh. Last edited by Phil Nelson; 04-08-2015 at 06:58 PM. |
#170
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Check the CW trans, T115. It's a simple circuit, but the answer isn't always obvious.
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Evolution... |
Audiokarma |
#171
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Quote:
Since L42 goes to pin 3 (cathode), pin 3 should be showing 10.6 ohms to ground. Were you measuring the coil with one end disconnected? If so, could there be a short inside the socket from pin 3 to ground? Last edited by old_coot88; 04-08-2015 at 08:40 PM. |
#172
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Sorry, I've been flipping back & forth between the Sams & RCA schematics and jumbled some numbers.
The 10.8 ohms reading was across L127, the reactance coil. That is with everything connected on that coil. Resistance from pin 3 to ground is 2.1 ohms. That's across L42 with everything connected. The RCA manual has no resistance chart, but for what it's worth, Sams says resistance at pin 3 should be .4 ohms, where I now measure 2.1 ohms. I did some checking of T115, the CW transformer. Resistance across the primary (from B to E) is 7.0 ohms. Resistance from F to A (F to ground) is .1 ohms. Resistance from D to C (D to ground) is .1 ohms. This is with everything connected to T115. Someone has worked in this area before. C202, the 120-pf cap on pin 6, looked like a replacement, attached hook-and-loop style to a stub left on the terminal. R246, 22K, is another obvious hook-and-loop replacement. The crystal may also have been replaced; one of its leads was much too long, and with strange bends -- not neatly dressed as you'd expect from the factory. Phil Nelson Last edited by Phil Nelson; 04-08-2015 at 11:16 PM. |
#173
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The schematic indicates there should be three ferrite slugs inside the CW xfmr. Are all 3 present and intact? No doubt they've been diddled with if somebody's been in there before.
Also, you stated R246 is 22K, where the schematic shows it as 2.2K. Is the replacement red-red-orange (22K) or red-red-red (2.2K)? In any case, that would not stop the oscillator from running. Last edited by old_coot88; 04-08-2015 at 11:39 PM. |
#174
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You're right, it's 2.2K (red-red-red). When I lose the ability to read and type, that's a sign to break for the night
T115 has one adjuster slug accessible from the bottom and a second accessible from the top. The RCA manual says that the bottom adjuster is used in color AFC alignment and the top one for B-Y phasing. Both slugs are present and can be moved (the settings may be completely wrong, of course). The Sams schematic seems to show that the two secondaries are adjustable, labeling them A64 (top) and A60. No label is given for the primary. I don't know why the primary is shown with an "adjustable" symbol if no adjuster is accessible. Phil Nelson |
#175
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Could it be a slug is missing from the primary? Or is there a big enuff hole to allow its exit?
(Edit) On second thought, it does look like an untuned primary, since there's no cap in parallel with it. Last edited by old_coot88; 04-09-2015 at 11:41 AM. |
Audiokarma |
#176
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Only the secondaries are tuned, not sure why the schematic shows it is. Never caught that before, but it doesn't matter. Keep looking for bad parts, I'm sure you'll see something obvious sooner or later.
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Evolution... |
#177
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My guess is that the problem is in the cathode and grid area of the 3.58 MHz oscillator. If possible measure the component values, especially the cathode inductor L42 , its capacitor C213, the 4 pf feedback cap C209, the crystal and its resistor. Or maybe some conductive goo on the tube socket pins? My 21CT55 oscillator cathode tuned coil had been mis-adjusted and caused the oscillator to stop often.
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#178
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I will keep plugging along. On the positive side, I tried dtvmcdonald's trick to test the crystals, and they all look good, including the one that was installed in the set. Each of them shows a distinct peak on the scope at around 3.58mc.
I replaced a couple more components around that tube, without making any improvement. This updated schematic shows the 5 caps and resistors replaced there. None of those components was grievously out of spec -- well, R249 measured 54K instead of 47K -- but at least I can cross those off the list of possible suspects. Of the remaining components connected immediately to that tube, we also can eliminate the crystal, which tested OK, and resistor R248, which measures a hair over 47K. Edit: I just saw the reply from Zenith6S321. One thing I haven't tried yet is disconnecting L42 to measure it. I can try that later on, along with giving the socket another good hard cleaning. Phil Nelson |
#179
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Quote:
In that photo, the scope was connected to terminal F on T115. By turning the bottom adjuster of T115, I could increase the amplitude up to a point, beyond which it fell off again. When I moved the probe to terminal D, the waveform looked about the same, although I didn't try the top adjuster. None of which helps me figure out why the oscillator doesn't oscillate, but I'm taking it as evidence that T115 is operational, anyhow. I had to move the adjuster quite a long way from its original position to find the peak, but perhaps that isn't surprising, given that many of the components attached to T115 have been replaced. Phil Nelson |
#180
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By sheer elimination, it begins to look like maybe the resonance point of the L42/C213 tank circuit is too far off from 3.58 to allow oscillation.
Do you have a signal genny that's reasonably well calibrated, and can be manually swept thru 3.58mc? If so, you might try sweeping the tank and watch for the resonant point on the scope. (But first disconnect the tank from the tube cathode and C209. Connect the genny and scope to the top of the tank.) |
Audiokarma |
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