1970 Zenith 20BC50

[freakaftr8]

Well.. I might have pinned the tail on the donkey tonight.. I found what I believe to be a bad Pincushion correction amp transistor. NPN style, reads dead on my transistor checker. This is a high power transistor. I somehow overlooked testing this transistor early on.. Going to order it online.
Does this seem like if bad.. could cause this symptom? Other than this, I should ring test the yoke..

Looks as if in the sams this circuit carries into the pulse gate diode and

[kx250rider]

Without looking at the schematic, I'd say that you might be onto something... Is it shorted? Check it out of the circuit with a multimeter on diode scale (or 2K if no diode scale)... Measure from collector (most likely the case, if it's a T03 metal transistor), to base and to emitter. It should measure 600 ohms or so, one direction, but not the other, and not less than a few hundred ohms in either direction. (In some deflection transistors, there's a diode embedded inside, which will make the emitter to base read low in both directions, which is OK in that case, but not from collector to b or e, in any case).

Charles

[freakaftr8]

Actually it measures open all around. There is no change in raster with the transiustor in our out of circuit.

[freakaftr8]

Hey guys.
Well I got the new transistor in today for the pincushion correction amp. Shoot. No change. Guess I have to look upstrem and test voltages at the pins.

[andy]

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[freakaftr8]

Yeah. Charles kx250rider is looking for a yoke for this set. I got another ctc 17 to mess with. I'll get back on this set when I find a yoke.

[freakaftr8]

Hey all! Long time! I've had this Zenith in storage and wanting to get back into it. Please read earlier posts which led up to this disaster. Originally I swapped a dry electrolytic and put the new one in backwards.
Hope all of you have been well!
Haven't located a yoke yet btw

[Findm-Keepm]

Originally Posted by freakaftr8

Hey all! Long time! I've had this Zenith in storage and wanting to get back into it. Please read earlier posts which led up to this disaster. Originally I swapped a dry electrolytic and put the new one in backwards.
Hope all of you have been well!
Haven't located a yoke yet btw

Got a means to ring the yoke? Eico, Sencore, Paco, and others made flyback/yoke ringers, and there is even a kit still available. Pretty conclusive test for yokes - rings bad, it is bad (except for some toroidial GE and Philco/Sylvania yokes).

I'll grab the Sams and take a look again - seems like Charles was following your repairs quite closely too.

Welcome back Sean!

[freakaftr8]

Thanks Brian!

[zeno]

Sure looks like the yoke but almost too easy.
Keystoned pictures usually almost filled the screen
& this one is quite small. That makes me think
vert problem.
There are 4 lytics in the V out start with them.
Dont remember which but one causes odd problems
IIRC. Yokes were trouble but I saw few because I was a
bench tech so worked on very few consoles, just
jig work. Yokes got changed in the house.
If it were me I would take any old Zenith yoke
& mouse it in for a test. Most other 90 deg yoke.
probably would work enuf for a test also.
Also consider a ringer. Very accurate & useful.
Better yet an analist that does much more.

73 Zeno

[Username1]

I haven't seen anything about voltage readings on all the tube pins of the vert
output tube, and is the first grid on spec...? any DC getting in there will drive it
too hard, but still may allow some deflection..... And surely cause over current
draw.... Is there a current reading for the cathode...? I don't have a schematic
of this tv, so I'm only guessing at some of the comments here....

Also, on the yoke, I don't think it's the problem, but I see comments about
measuring vert winding, and horiz winding, but did you check for a short
horiz to vert windings....? Same with vert out transformer, Not in windings
but short one winding to ground...? or serious leakage to ground.....

I can't imagine where an open pincushion transistor would cause current draw
loading like that.... You have something on too hard.....


.

[freakaftr8]

Actually a few years back I did test the yoke and found no shorts. I will go on to testing the v out tube next. And v out xformer. Thanks guys

[freakaftr8]

Well hello all! Been a while now! Believe it or not, my daughter was only 6 months old when I started this thread! Shes 10 now and have had 2 boys in the process, ages 4 and 6. I decided to take on this beast again.. it's been sitting in the same place forever.

Anyone miss me? Lol.

Anyway still same issue. Keystone effect and low B+ off the 390v supply. something is loaded down. Resistor getting hot off the square of the filter capacitor, which is original. I should probably recap that section. if you remember reading back, I installed a 100uf cap backwards. it caused this after my mishap. the whole set seems to be loaded down. No sound of vert xformer, probably from low voltage. heres the kicker, if I put the tv into service mode via the switch, the raster line is cut short on the right side of the screen so horiz is cut too. Obviously from the 277v where the 390v should be. I lost the SAMS a member sent me years back..
With the yoke disconnected, still have the load down..

Any fresh ideas?

Thanks all

[Electronic M]

I might be able to scan you the schematic (though I think I loaned it to a friend with the parts chassis I have).

Definitely recap the lytics and vertical, and get a schematic. My 20CC50 table set is an excellent performed and virtually the same chassis so yours should be a very good set once you get the problems sorted.

[freakaftr8]

Dang chassis. I gotta pull it to read the ratings on the filters. Now I recapped half of them back in 09. Not sure why I left the square and one half moon on the original cans. That square, off of that goes to a paper yellow 475v 10mfd right off the cap I installed backwards I update soon.