Sync Separator Diode Replacement
 

TITLE CORRECTION: HORIZ AFC DIODE REPLACEMENT AND ASSOCIATED PROBLEMS

I’m working on a ‘57 Travler portable b&w which appears to have major syncing issues. The diode shown below was open and needed to be replaced.

https://ibb.co/sKw2zqP
The stock diode was a Selenium, was open and I replaced it with a Silcicon Schottky 1N5408.

I assume this is the sync separator diode? The waveform somewhat looks okish, read at the “-“ end of the diode, but jumps around and looks unstable.

No waveforms are read at both pin 2s of the Noise Canceller (V6) and Sync Sep (V5) and I’m concerned that this may be the wrong diode to use, voltage drop compensation aside.

Waveforms look ok before the Very Osc and after the Vert Outputs.

Is the Schottky ok to use or should I source a different type, even a Selenium? I know my Schottky is installed correctly with the correct polarity at least.

Image of Diode, negative leg waveform.https://ibb.co/W28sG08

https://youtu.be/fM1fGqM99WEMovie of same

Which way did you install the diode? Seleniums are often marked with a plus sign(+) on the cathode end. With Si diodes, the banded end is the cathode.

I have the cathode indicating band corresponding to the cathode “+” indicator on the Selenium in the schematic.

In other words, the cathode side of the Si diode is in the same position of where the “+” side would’ve been on the Selenium. I guess I’m aware of “discrepancy” between how Seleniums used to be notated on schematics. Anyway, if I orientate the Silicon the opposite direction, hardly any signal is displayed on screen, which was the situation prior to replacing the bad Selenium diode.

Looks like the diode is used as a phase comparator (normally a dual diode is used). Never seen a single diode used there before. The circuit depends on a feedback pulse which would come in via C50 and R60.

If there's no waveforms present at the sync sep and noise cancellor stages, the the signal loss must be somewhere 'upstream' of there. Probably be helpful if you could post a fuller schematic.

Maybe the schematic is incorrect if it’s just showing a single diode. Sams are not known to be perfect...

Here’s a couple shots of the full schematic for this model:

https://ibb.co/SXnLWdX

https://ibb.co/4FXh7Rv

It’s weird, I have a schematic for a year later model, and that one labels it as a Horiz AFC, but one of the legs (of the dual diode) has no connection. Schematic otherwise looks identical.

https://ibb.co/m40Ftj9

Now we know it’s a Horiz AFC. Wish I could change the thread title.Thanks for the help by the way!

Does the vert lock in ? If so odds are its not in the sync sep stage.
Diode probably could use any little signal diode like the one
used as the video detector. It should just pass the positive H sync
pulses.
BTW any markings or part #s in the Sams ?

73 Zeno

It’s weird but after some component replacements yesterday, on stuff that had drifted way out of tolerance, I can’t adjust the vertical anymore. After these replacements the picture seems to have degraded, even though the replacements are close to spec. I’m using a BK NTSC pattern generator and it’s almost impossible to get just the horizontal lines dialed in, let alone the staircase pattern as that pattern is repeated vertically. This set is a nightmare.

Nice strong CRT and deflection though.:D

https://ibb.co/PwdVKSrPic of Lines pattern on screen (sorry for angle)

Pic of staircase or bars

https://ibb.co/XCgTxLm

Maybe the pics spell out the problem better for the veterans

What kinda display do you get using a regular signal source (e.g., VCR, cable box etc)?

Pretty much the same. Barely discernible, washed out image that repeats horizontally.

Note this set is some kind of transitional model or oddball. Aside from an occasional different value, the set, although it matches the schematic that I posted, looks more like a later model, with the tuning knob on top, not on the side.

The dead diode I replaced was a dual diode (series arrangement) with one leg unconnected, just as in the year later schematic. The year later schematic has an almost completely different tube compliment. I can post that one as well.

I’ve pretty much replaced every wax and electrolytic capacitor and only left in originals that tested within tolerance. I haven’t really messed with the ceramics.

"Repeats horizontally" suggests the horz sweep is 'waay off frequency. I wouldn't use a pattern generator, but rather a regular signal source to get it on frequency first. Then see what the actual video looks like. You can tell a lot more that way.

You indicate absence of any waveforms at the sync sep. Sync is picked off the video chain right after the video output. Check peaking coils L7 and L9, and coil L8 for continuity for starters. And check plate voltage on the video output (pin 9).

Thanks. I have indeed used a regular signal source, RF on the antenna terminals, IF, and composite in the IF chain and after the detector. Same result.

Per your suggestion, I did test resistances on coils L7-9 and everything tested fine.


Plate voltage (varies with Contrast control) tests normal.

If L8 is shorted and video is getting through, should still work.

I would agree that the first thing is to check the horizontal oscillator frequency.

P.S. Wow - talk about corner cutting - only 2 video IF's and both sound and sync taken off the video output AFTER the contrast control. Yet, they spent money on a noise canceller stage. Must have had an extra tube section they didn't know what to do with.

Agree with old_tv_nut on all points. After you get the horiz on frequency, let's see what the actual video looks like.

I’ve tried adjusting the Horiz freq numerous times, following the Sams guide, by centering the Horiz lock and then moving the Horiz freq. Just can never get sync, and always overlapping pictures or “xmas tree” effect.

Wondering if I should put the output of the Hor freq coil on the freq counter.

The only thing I can think of is either the coil is bad or there still remains a faulty component or lead that needs to be traced down.

Either that or the Horiz osc tube needs replacing. This tube, tested fine for emissions and Gm on both a Hickok 533 and Mercury tester. I have one 5u8 on order anyway, just to make sure it’s not a defect the testers didn’t detect.

Yes, use a frequency counter, or just use your scope first to see if the frequency is even close.

So...progress...in fact, a big step!

Using my trusty counter I did find the Hor freq coil was set very low. The coil after it, which acts as a kind of range setter was way off as well. Having dialed these two coils into range, 15.500-750 ish and the H lock into a mid position, i was able to get rid of this pesky Horiz sync problem and my multiple pictures have gone away.

Big learning moment! Give the dang frequency counter some love!

Now I can see the line, crosshatch and staircase patterns.

Vertical lock remains a problem, especially as the control no longer affects anything. Vert linearity and height work though.

Please enjoy these progress pics:


https://ibb.co/fY1pqX4
https://ibb.co/5W7QDPd
https://ibb.co/rp1WJfg

Adding a short video

https://youtu.be/9mdcOzbqWaw

Discovered that pins 3,4 were shorted somehow to each other on the Vert osc / output tube. Fixed that and much of the noise and some instability went away.

Still have the issue of a split vertical picture. Still syncing issues to be addressed.

https://ibb.co/rpWHvYR