Today's Mystery Guest (RCA CTC-4)

[old_coot88]

If still wonky after adjusting T115 cores for best display, see what effect adjusting T114 core has.
Also check voltage on V125 plates (pins 1 and 6). Should be about 36V.
If not correct, check resistors
-
R256 (110K),
R258 (180K)
R262 (150K)
R283 (47K),
R282 (3.3K)
R286 (3.3K)


.

[Phil Nelson]

I spent some time adjusting T115 with two diddle sticks while watching the color bars, but didn't reach Nirvana. Lots of colors, but no clear path to the right ones. I think there is more going on here than mere misalignment. The lock doesn't seem very strong, either. The bars were prone to start pulsing, and tweaking the reactance coil did not always cure that in an obvious way.

I checked some voltages on V123, the 6AG7 demodulator driver:

pin 4: 0 (0 expected)
pin 5: 7.1 (6.2 expected)
pin 6: 175 (230 expected)
pin 8: 248 (280 expected)

The "expected" voltages are from the RCA schematic; the Sams manual gives lower voltages for pins 5 (5.5v), 6 (200v) and 8 (250v).

T114's secondary has continuity, measuring about 4 ohms from terminals A-B.

I checked the plate voltages of V125, the 12BH7 R-Y/G-Y demodulator:

pin 1: 33v
pin 6: 16v

I tried scoping the signal coming from T113, the bandpass transformer, and following it through the demodulator driver transformer (T114). Here is the waveform at the grid (pin 4) of the 6AG7 demodulator driver:



Here's the waveform at the plate (pin 8) of the demod driver:



Here's the waveform at terminal A of T114, the demodulator driver transformer:



And here's the waveform at terminal D of T114:



Out of curiosity, I put the frequency counter at these points. For what it's worth, it measured 3.80 MHz at the plate of the demod driver tube, 3.69 MHz at terminal A of T114, and 3.74 MHz at terminal D of T114.

Next stop: checking resistors.

Phil Nelson

[Phil Nelson]

Originally Posted by old_coot88

check resistors
R256 (110K),
R258 (180K)
R262 (150K)
R283 (47K),
R282 (3.3K)
R286 (3.3K)

I checked those (in place) without seeing anything alarming.

Maybe I will break down and install the replacement quadrature transformer. But first, I want to mess around with the breadboard circuit a little more, to see how close I can get to the ideal frequency. If there's any component tweaking to be done, I'd rather do it on the breadboard.

Phil Nelson

[old_coot88]

Originally Posted by Phil Nelson

I checked those (in place) without seeing anything alarming.

Did you clip one end loose to check them? ('Specially the high value ones. The 3.3K ones not so much.)

I checked the plate voltages of V125, the 12BH7 R-Y/G-Y demodulator:

pin 1: 33v
pin 6: 16v

The 16V is 'waay low, less than half the expected. What about the grid voltages (pins 2 and 7)? Both expect the same, about -20V.

[Phil Nelson]

I lifted those resistors to re-check, and didn't find anything more than 10% off spec, but I replaced them anyway. Now I have replaced most of the resistors on the video board hanging off the side of the chassis.

None of the voltages on the 12BH7 (including plate voltages) are very close to spec. These circuits have a bunch of 2-watt resistors connected to the +380V line. I don't keep many 2-watt resistors on hand, so I'm off to the local store to restock.

Phil Nelson

[old_coot88]

Holy !@#$%^&. I didn't see those. Are those the three paralleled 82Ks feeding the pin 1 plate? Your voltage reading there was near spec.

And I didn't see R259 (120K) feeding pin 6 plate, which is 'waay low.

[Phil Nelson]

And don't forget the two paralleled 82Ks on pin 7 of V124B (B-Y amp). Kind of ugly construction in that neighborhood. Maybe they got a good deal on 82K/2W resistors.

Phil Nelson

[old_coot88]

Yeah, and considering that 2W carbons are know to char and drift downward, it'll be interesting to see what those actually read.