Sync Separator Diode Replacement

[Jon1967us]

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.

[Jon1967us]

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

[Jon1967us]

https://youtu.be/fM1fGqM99WEMovie of same

[old_coot88]

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.

[Jon1967us]

Originally Posted by old_coot88

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.

[old_coot88]

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.

[Jon1967us]

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

[Jon1967us]

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!

[zeno]

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

[Jon1967us]

Originally Posted by zeno

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.

[old_coot88]

Originally Posted by Jon1967us

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.

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

[Jon1967us]

Originally Posted by old_coot88

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.

[Jon1967us]

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

[old_tv_nut]

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.

[old_coot88]

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