Stromberg Carlson X-21 M4B project.

[Tom9589]

I always took out 3-4 of the individual strips and that was enough to gain access to the stationary contacts. No need to remove the whole drum.

[Yamamaya42]

Yes, I see that now, I just found a site that shows that they can be removed for cleaning.
https://www.rfcafe.com/references/po...march-1967.htm

This is my first time ever trying to clean this type of tuner, have seen mostly rotary switch type before.
Though the contacts ARE dirty and black in this tuner, I'm still not sure what is going on with the front end, by video injection, I have proven that all after video detector diode is OK and stable but tuner input is intermittent, which may or may not be due to contacts or dirty tube pins, (new clean tubes on the way) , or, it could also be out of tol resistors in the tuner/IF/AFC area.
Not really sure at this point, will have to see how it behaves with known good clean tubes in and cleaned sockets.
How it is behaving is a bit unusual something I have not seen before anyway, when the front end works, it works rather well, stable and solid for quite some time, but when it acts up, the screen goes to a blank raster, but only if it's being fed a signal on a given channel (rf mod into 3) , if I change away from chan 3, or shut off the rf modulator, the screen goes to snow, and back to raster when turned back on.
AGC problem? Noise cancel error? Flaky video detector? Or just a weird symptom contact problems perhaps?

[Yamamaya42]

Originally Posted by bandersen

Looks like they are pulling it from the cathode of the 12BY7 video output tube. They are using that tube as a voltage divider in other words.

I'm look as Riders vol 16. Not sure how it looks in Sams.

this may be what I have been looking for!
https://imgur.com/1NfY1AN

not on the 12BY7 video output tube, but on the 12AU7 sync phase inv,
it has the 1w 3.9k resistors on the plate and cathode, and is most likely the source of the 95v in this version of the set, which I have been looking and looking for, so I could replace those resistors to stabilize the 95v line.

[Tom9589]

Look at the revised circuit in the SAMS which you show in your last post. In the revised circuit the 12AU7 isn't even connected to the 95V line. R96 goes to ground, not the 95V bus in the revised circuit. The voltage dropping function is typically handled by the audio output tube.

[Yamamaya42]

Originally Posted by Tom9589

Look at the revised circuit in the SAMS which you show in your last post. In the revised circuit the 12AU7 isn't even connected to the 95V line. R96 goes to ground, not the 95V bus in the revised circuit. The voltage dropping function is typically handled by the audio output tube.

that's what i dislike about this SAMS. you do not know what is really in this set until you track it down.
i did a quick look, and the cathode is indeed tied to the 95v line and not ground in this set.
https://imgur.com/mVtjfmc

that being the white wire...
r96 checked as 4.7k in circuit, i got to replace both!

[Yamamaya42]

Originally Posted by Tom9589

Look at the revised circuit in the SAMS which you show in your last post. In the revised circuit the 12AU7 isn't even connected to the 95V line. R96 goes to ground, not the 95V bus in the revised circuit. The voltage dropping function is typically handled by the audio output tube.

the voltage divider you mention on the audio output is on another version of the set, tied to a 110v line, and can be seen in sams 0258-13n.pdf , the one for this set is, 0314_11n.pdf


I'm really not sure why they chose to create the 95v in such an odd way in this version

[Yamamaya42]

I'm defiantly going have to go go over this set for bad resistors, I tried again to bring in a signal in the tuner and it was still unstable, worked sometimes, and then blanking out to raster again, but only when there is RF input, 95v line is also unstable at this point, dropping to 82v when there is a pic locked in, and when it seems IF section collapses, it goes up to 109v, then when I remove the RF input, and the snow comes back to the screen, it goes back up to 95v.

And no sound at all from RF input.

But oddly enough, with direct AV input, the 95v is stable, and the picture is fine for a long time, I have had it on for 40 min or more.

https://www.youtube.com/watch?v=L7ypgEFao8s

the speaker was also moisture damaged so it also hardly works.

I'm sure I will find a lot of bad resistors

[Yamamaya42]

I think i may have found the problem.... and it's not good...

https://i.imgur.com/lIhTLGp.jpg ( ignore my finger in the pic again :O )

after looking around for bad resistors , and finding just a few.
I poked around some caps...
there are many of these 3 leggers in the IF section.

they SHOULD be 4000pf, but the 3 i checked were at 5.0 5.1 5.2 and leaky at low voltage. they are rated at 400, I don't have the means to test them at 400
seems to me that this will throw the IF all out of wack.

now I am debating, replace them and most likely face IF alignment? or stick with AV input?

[Yamamaya42]

Apparently, this TV is getting weirder and weirder, for it seems that the 95v line is not sourced from just v15 12AU7 or v8 12BY7 , but both... to me this just seems very problematic, and just asking for trouble to happen sooner or later, and have no idea why the hell they would do it like this, because it just seems like they are begging for things to fluctuate and oscillate, which is basically what I am seeing when I try to use RF input.

with v15 removed, 95v line drops to 82v , and with v8 removed, 95v line drops to 60v, with both removed, it's 13v.

Symptoms
direct a/v input,
95v line stable, picture is stable, no jitter or sync problems horz or vert.

RF input, tuned,
95v line unstable, fluctuating, 95v with video on screen or snow, over 100+ when IF fails to blank raster with rf input, horz & vert freq fluctuating, and horz & vert waviness seen in video on screen ( not seen at all with direct input )

Suspect bad caps in IF section may be causing problem, should be 4000pf but most testing over 5k (tried more than 1 tester, same result)
however, the original dual caps can't be found, nor can new 4000pf, but 3900pf seem to be all over the place, so I will guess they will be the ones to try, esp since 810-FG24C0G2W392JNT6 are much higher tol and more stable than what was in there in the first place.

[bandersen]

It's actually not that uncommon a power supply design. Every design needs multiple voltages. Typically the main B+ for the tuner, horizontal and audio output stages, Boosted B+ for the vertical output and a lower B+ for the IF.

You have a few choices for the lower B+, power resistor voltage divider, separate secondary power transformer winding, rectifier and filter or do what this design did. Use tubes as voltage dividers. I really don't think the fluctuation is any big deal. It's going to vary as the tubes draw more or less current in response to the incoming signal.

Tube circuits are designed with a wide tolerance. I wouldn't be concerning if readings differ +/- 20% from what the service info shows.

Those caps are ceramic and very reliable. I'd be surprised if they are bad. 3,900 is more than close enough. They are just RF bypass caps.

[Electronic M]

If you want to hurt your brain some more try to wrap it around the Motorola VT-71s voltage divider. They basically had 2 voltage rails and the lower was basically many different stages (the majority of the set) in series/parallel all collectively acting as a B+ voltage divider. It did its job well though.

[Yamamaya42]

I'm still a bit stumped by the odd behavior of rf input vs direct video feed, and the way when there IS video via rf in, its very unstable and wavy on the screen and 95v is all over the place.

Granted, all the IF tubes are original and I have new ( tested good ) ones on the way, also the AGC KEYING, and tuner tubes, I also intend to replace cathode and plate resistors on both source tubes for the 95v line, not done it yet. Right now, there is only 1 filter electrolytic capacitor on the line , as original design, 40uf, I may add another 5uf in there.

[Tom9589]

What would be really helpful is to use a 1077 B&K TV Analyst (or equivalent) and inject an IF signal directly after the tuner. Note the stability of the picture and the 95V bus reading. That would tell you whether the problem is in the tuner or in the IF section of the set. A problem in the AGC can also cause a TV to shut down as you have described.

BAnderson is right. These ceramic caps rarely fail. They are typically used for bypass and therefore their value is not critical. No IF alignment should be required after you replace them.

[Yamamaya42]

I intend to re-do the AGC KEYING area as well, especially C54, the .001 1kv that carries horz pulse to the AGC circuit from the fbt, even though it looks like it was replaced before, I do not trust it, this set spent countless years in a damp dirty barn, it had rust, cobwebs and roach castings in it, so far, 95% of every cap paper/ electrolytic, even some film ones replaced before, have been bad, but luckily it seems that only 10 to 15% of resistors are bad, mostly 1watt and above.

To give an example, at first, some of the pots were so rusty, that they would not turn before being sprayed down with deoxit before I could get them to turn

I have the 3900pf npo 5% caps ordered, should be here tomorrow, prob won't hurt much just to replace the bypass caps, & the “new” tubes mentioned are already on the way.
Will just have to see how it behaves.

[Yamamaya42]

Replaced a few more marginal parts last night, caps / resistors, found that the dual pot R9 for AGC/NOISE CANCEL is in sad shape, even after taking it apart and cleaning, replacing it is proving very hard, because, where as dual concentric 500K audio taper pots seem to be very easy to find all over the place, linear taper ones are about as easy to find as a goldfish in a desert at high noon.
Dare I use audio taper?

Most tubes ordered will be arriving today, so I will be able to try will known good (tested ) RF/IF tubes soon.