Motorola 21BT130BE-1 vertical output

[cluelessgame]

I've had this set for a while and it ran great - 100% original/non-restored - but then one day the vertical died. After tedious checking of every component of the vertical section, it appears that the only component that is conclusively off is the vertical output transformer (much to my horror ). It produces a resistance of 23.4 ohms across pins 2 and 3, as opposed to the 600 ohms the Sams sheet calls for. The resistance across pins 1 and 3 are in spec, however. Am I mis-measuring this? I always thought these transformers only went open when they failed.


The Sams parts sheet identifies it as a MOTOROLA 25D65840A13 (-AF), cross-referencing it with
MERIT P-2853
STANCOR VO-108
THORDARSON 26S22
TRIAD A-133X

does anyone know if there's any hope at all of finding a replacement? or am I stuck with an 80 lb paperweight?

one thing that does seem a little peculiar is that I'm not getting any sort of signal on the input grid of the vertical output tube (V10), nor am I getting 25V or signal on the plate of the vertical multivibrator (V6) but all of the components and the tubes themselves check okay.

[tom.j.fla]

Did you test the transformer out of circuit? If tested in circuit a shunt path could give a false reading. One thing to test is R-65 V.D.R. that can kill vertical sweep, quick test lift one lead and see if the vertical returns. All the best, Tom

[zeno]

As Tom said lift 2 of 3 of the wires from the VOT to test the ohms.
They do go but not often. The Sams CAN be wrong also.
After a bad tube most likely its something in the feedback loop
like C41 or M5. Tubes are common, 6BL8 was a big Moto number
& 6GK6 also used in a lot of Zeniths.
You can drive the G-1 of the 6GK6 with the filament voltage & see
if it opens up a little. Or better yet use an analizer if you have one.....

73 Zeno

[cluelessgame]

those resistance readings were taken with the VOT taken completely out of circuit. I did attempt to test the VDR (R65) but did not get any sort of reading...I assumed that it was normal considering my Fluke and VTVM only puts out a few volts for resistance readings...but I've never encountered one before, and the Sams sheet doesn't give any indication of how a working one should test. I guess if that IS the source of failure, how would I go about getting the correct replacement? Two-thirds of it is white, with the other end having a black band and the end cap is red >> --{______white______|black|red}--
if anyone knows the VDR color code off hand.

I also ruled out the tubes being the issue as they test good, and I swapped them with others in the set with no change.

[zeno]

VDR's were a mystery part for us. Only remember changing
them in Zeniths both the vert & HV regulator. In 40 yrs
never learned how to test them. they were cheap so if you
had the symptom you just change them. Dont know of any
color codes, they were all just colored until later years when
they were a nice blue color with a Zenith part number.
BTW those OEM & NOS unopened go bad on the shelf its been
found.
BTW #2 M5 is probably like a Zenith integrator. The resistor
showing a cap symbol. It is 2 resistors with 3 caps. High failure
rate in Zeniths.
BTW #3 If you have total loss of vert I would not suspect the VDR.
The readings you got on the xfrmer point to it. Try hanging in
any old one from a B&W & see if it opens up.

73 Zeno

[cluelessgame]

I pulled and checked both C41 and C36 and both were within tolerance/no leakage. I also checked the resistance across pins 1 and 3 of the M5 integrator. It showed a reading of just under 200K, which I assume means its still in tolerance (the sum of the two resistors it contains is 150K).

As for the VDR, I hooked it up to my Sprague TO-6 capacitor analyzer set to leakage check. I slowly turned up the voltage and right around 275 - 300V it began to draw current. I don't exactly know if that's what it's supposed to do, but it would seem appropriate IF it's intended to clip the voltage if goes above the 250v supply...

I've searched around a bit for another transformer and although I haven't been able to track down the exact part number from any of the brands the Sams lists, I have found some that are off by one or two model numbers. I did come across an old post from 2011 on the antique radios forum (http://www.antiqueradios.com/forums/...c.php?t=159006) where someone was selling a lot of transformers and one of them was a Triad A-133X. according to the post, it has an 8:1 ratio. If I can't find an exact replacement, which is more important, the ratio, or the resistance?

the thing that still really bugs me (and keeps my hopes up that it may not be the transformer after all) is that this dead vertical sweep issue is semi-sporadic.
It first died on me about a year ago, but if I wait a month or so and then power it on, the vertical will work for about 30 min to an hour. In that brief time, you'll see the vertical height *lightly* flutter a little every once in a while, but otherwise it appears to work fine. But then suddenly the whole raster collapses to a bright horizontal line. It may be important to note that every time I observe the vertical fail, I notice that it collapses *somewhat* slowly... a rate comparable to the way some sets behave when you power them off and picture shrinks to a dot rather slowly. I feel like if the transformer was actually shorting, the picture would snap to a line instantly.

any ideas?

[DavGoodlin]

Did you try a flyback tester on the transformer. that DC resistance is an issue for sure.
There is one on a Sencore VA-48 that can pass fail on shorted turns. Multiple turns can short and flutter like that. I
observed this on a mid-50s Philco using a similar tube a 6CM6, after I recapped it. I had a junk chassis and the transformer fixed it.
You could use several versions (numbers) of this transformer as long as the sets tube is pentode like a 6GK6.

[dieseljeep]

Originally Posted by DavGoodlin

Did you try a flyback tester on the transformer. that DC resistance is an issue for sure.
There is one on a Sencore VA-48 that can pass fail on shorted turns. Multiple turns can short and flutter like that. I
observed this on a mid-50s Philco using a similar tube a 6CM6, after I recapped it. I had a junk chassis and the transformer fixed it.
You could use several versions (numbers) of this transformer as long as the sets tube is pentode like a 6GK6.

Was the yoke checked? Those yokes seemed to be trouble-prone.
That Motorola set itself was as good as the Zenith offerings at the time, except for, maybe the CRT.
I replaced a lot of Zenith yokes of the same era.

[cluelessgame]

Welp, I ended up tracking down another Motorola VOT that matched very closely in numbers (thanks, http://www.oldradioparts.com/) and vertical deflection returned at last!

the set still needs some tweaking:
-the audio buzzes with high-contrast text/guide screens,
-you can hear hissing with "sss" and "fff" sounds when people talk,
-contrast is not consistent from shot to shot,
-after running for a an hour or so the picture begins to go dark on the left end of the screen.

but otherwise it runs pretty well now.

one other thing I'm noticing that may need to be addressed sooner than later is that the picture will sporadically begin to pulse or wobble in size, almost as if the high voltage is fluctuating, yet the horizontal pitch and sync doesn't change. any thoughts?

[zeno]

Mostly sounds like AGC problems. Also this is a
common complaint with OSD graphics on old stuff.
Try the AGC or on some Motos the optimizer.
Also try reducing the RF input by unhooking one
end of the 75-300 ohm transformer. Sams often gives
an AGC range ( strong sig/ no sig ) for the tuner
& or IF grids, check that.

73 Zeno

[cluelessgame]

I tried adjusting the AGC and the buzz changed, but did not get any quieter. I attempted adjusting the coils in the sound IF, all but two are frozen (any ideas on getting them unstuck?), but the ones I could turn did help some. I plan on doing a full alignment but I am yet to possess an alignment generator. I'm looking at maybe purchasing a B&K 1077b off eBay, but haven't decided if that'll do all that I need it to do (I do have a Sencore VA-62 that'll do modern 40MHz stuff, so I don't necessarily need another test pattern generator, tho the VA-62 doesn't have a circle pattern).

another question...more of a fear that I have, is that since I haven't changed out anything else in the set, could something else going bad have caused the original VOT to short the way it did? all the capacitors appear to be early maroon plastic type, as opposed to the old paper/wax or bumblebees.
how hot should the VOT normally operate?
after about an hour of running it my infrared thermometer reads it at about 100-120deg, the power Xfmr reads about 140deg...the electrolytics, original, don't get very warm, and the sets from '66 so I would think they're not very leaky.

I'm also still noticing that the image size wobbles in and out (slightly, but noticeable) in a rhythmic fashion sporadically. basically, the image will be stable, and then every now and then it starts going "wub wub wub wub wub wub" (that's the technical term :P )...then stable again. but no signs of arcing or change in flyback squeal pitch

[Electronic M]

Plastic cased caps often have the same crappy paper guts as the older paper caps, and are comparably failure prone.