Stromberg Carlson X-21 M4B project.

[Tom9589]

Audio tapers are logarithmic. They will work, but the adjustment will not be uniform like a linear taper. The adjustment range of the audio taper will be "bunched" at one end.

[Electronic M]

I think it's Mark Oppat who has the parts to recreate most vintage radio potentiometers.

If you're feeling adventurous you could try finding a new dual audio taper pot mechanically identical to individual linear pots from the same maker and swap the resistance elements between the three.

[Yamamaya42]

There is yet another option,
https://i.imgur.com/VwqCuEK.jpg

You can see just how ugly that pot looked before I tried to clean it.
There is no reason both adjustments MUST be on the same shaft, the HOR LIN , option is not installed in this set, so, if I can't get the original pot to behave, and find a suitable dual concentric replacement that's linear, I can just split it and put it in 2 locations.

[Yamamaya42]

Another thing I have noticed while poking around under this thing, the yolk has been replaced before, splices in all 4 wires underside.

[Yamamaya42]

IF tubes replaced, new bypass caps, and new AGC tube,

https://i.imgur.com/Zipl6L3.jpg

now much more stable with RF input, but still has a tenancy to oscillate on the 95v line, gonna add one more electrolytic cap.

sound was still weak, after replacing the old caps in the sound area, did find a bad 86 ohm resistor on the plate of the audio output,( checked at 105 ohm ) that did help a bit, but still not very loud, it may just be the crappy speaker.

I WILL have to change the AGC pot, It's unstable.

[Yamamaya42]

To clarify, what I mean by oscillation is, some weird harmonics in the 95v line, that tend to show with RF input as unstable picture and sync, it can be fine for some time and then just go nuts, and I can get it to stop by briefly shutting off the RF modulator and turning it back on.

As mentioned before, the source for 95v is a bit odd in this set, v8 & v15 with one filter c2b.
https://i.imgur.com/YseSmi3.jpg

C2 b 40uf the only filter on the 95v line, is on the front of chassis, focus socket is at the back, (95v is being used,)
v8 1/2 the source of 95v is on the left side, v15 the other half is on the right.
I'm hoping that adding another filter cap at the focus socket may help things run more stable.

For the most part, when tuned in and NOT oscillating, it has a pretty good picture for it's age.

[Yamamaya42]

Well, video via RF input is now pretty stable and OK, but I have not been able to get sound to work, still very quiet.

It is NOT the tubes, and I have been through the whole area and replaced every resistor that was less than 10% what should be, but it still is very weak sounding, tried tweaking the sound IF, that was no help.

https://i.imgur.com/c1mGoQR.png

been all over it , voltages are OK, but poor sound.
i suspect the cap hidden in L24 may be causing a problem.

a new coil can be gotten but at this point I feel it's not worth the trouble, esp since there is a easy way to feed audio into this set via an RCA input and would only take re-wiring a switch.
https://www.youtube.com/watch?v=6puItmZnQsk

this is with RF input and direct audio, ( faulty audio IF bypassed ) and a decent speaker! sounds great!

keep in mind, I have not put on the centering magnets yet, so the final screen adjustments are not done.

[Yamamaya42]

I am assuming this has to be a mistake...

https://i.imgur.com/sdXG704.png

VTVM to point D, common to chassis, input 4.5mhz at point C, adjust input till 1.5 volts is read, keep there for the remainder... BUT UM...

https://i.imgur.com/c1mGoQR.png

POINT D HAS A - VOLTAGE ON IT!
and is tied to a negative side of an electrolytic cap.
so, should that be, adjust generator output until -1.5 volts is seen at point D?

THEN adjust for max defection?
confusing...

[old_tv_nut]

Looks like a mistake to me. I would say if you connect the VTVM reversed of what they say, that should give you a positive reading. If you look at the diode conduction, it seems like the voltage at point C [EDI T - oops, point D] must be negative compared to chassis.

Then tune A11, A12, A13 for maximum while reducing the input level to keep the reading near 1.5 volts.

[old_coot88]

I may be succumbing to another geriatric grand mal brain fart since this has been over 50 years ago. But the few times I've replaced that small electrolytic in a ratio detector, the cap was always 'upside down' with the positive end to ground.

Originally Posted by old_tv_nut

...If you look at the diode conduction, it seems like the voltage at point C must be negative compared to chassis.

Um.. don't you mean point D? Going by direction of conduction >, the diode is gonna put an excess of electrons (negative voltage) on pt. D

Compare to diode direction if pt. D were positive.

Again, I may be all wet, so all bets are off.

[old_tv_nut]

Sorry for the typo, yes, point D should be negative. I don't even see point C (the input) on that clip of the schematic.

[Yamamaya42]

Originally Posted by old_tv_nut

Sorry for the typo, yes, point D should be negative. I don't even see point C (the input) on that clip of the schematic.

point C is tied to the far side of that 5pf cap, (c58) and goes directly to the video detector diode, before any peaking coils.
https://i.imgur.com/goeH862.jpg

[Yamamaya42]

I am seeing barely perceptible contrast smear on bright objects on the screen, (to the left/right ) this is seen with both RF and direct video input, not sure if this is a sign of a real problem or not, or if this is just the standard for a set this old.

[Yamamaya42]

looking better

replaced a peaking coil
https://youtu.be/rDpXewiOVNM

RF audio now working after new L24 coil ratio det.
Speakers that were with the set are shot, not sure what I can replace them wit, I just may use a big ext one.

[old_tv_nut]