Commodore 1902A "Vertical Collapse" problem

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MM007
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Commodore 1902A "Vertical Collapse" problem

Post by MM007 » Thu Sep 20, 2018 8:26 pm

Hello. I pulled my 1902A out of storage in hopes of using its composite input with my old NES (my CRT sucks with 240p composite) and found a thin, bright, horizontal line in the middle of the screen. Looking it up, I learned that this is called 'vertical collapse'. There was also a crack in the plastic on the top of the back casing where it slopes, but I do not recall if it had that when it went into storage nor if it is related.

I understand the risks of the high voltage with the electron tube and that it needs safely discharged already, but I was wondering if anyone knew where I could start looking for ways to fix the 1902A. If that can be fixed, I'd also like to look into other mods like S-Video, RGB, and other options...but at minimum I want to have a CRT that works with composite again.

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LoneWolf33
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Re: Commodore 1902A "Vertical Collapse" problem

Post by LoneWolf33 » Sun Sep 23, 2018 2:53 am

Hello,

It seems the problem is probably related to one or more of the following:
- missing vertical sync pulse generation, or the pulses are not reaching the vertical driver stage
- vertical driver stage
- missing 26 Volt power supply of the vertical driver stage
- vertical deflection coil on the CRT neck is open/shorted or it's connection(s) is(are) interrupted

IC 402 is responsible for vertical sync pulse generation (plus a bunch of other stuff),
VSYNC pulse output is pin#9.

IC 302 is responsible for the vertical driver pulse formation. It receives the VSYNC pulses on pin#2 thru an RC integrator.
The vertical deflection coil is connected between pin#6 of this IC and the driver stage
containing two power transistors (TR367 and 368 respectively).
Collectors of the power transistors were connected together directly. With the monitor is powered OFF you
can check the resistance between the power transistor collectors and pin#6 of IC302. Resistance readings
between infinite and approx. 560 Ohms indicates open circuit towards the vertical deflection coil.
IC 302 (@ pin#5 and 9) and the driver stage both require 26V supply.

An oscilloscope may come in handy when checking the sync pulses.

Note: apart from the ~24 000V CRT anode voltage (which has the appropriate insulation)
there are several points in the working set, where potentially lethal voltages are present (above 100 Volts).

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Re: Commodore 1902A "Vertical Collapse" problem

Post by MM007 » Thu Oct 04, 2018 10:24 pm

Thank you very much for the information. Hopefully I can figure it out then. Are the chips the kind that have off-the-shelf replacements? I'm assuming not.

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Re: Commodore 1902A "Vertical Collapse" problem

Post by LoneWolf33 » Fri Oct 05, 2018 3:03 am

Hello,

I think they're still available, or at least the IC402 (TDA2595) which was also used in TVs as well.
You can make sure by checking the exact IC types on the packages and google them.
IC302 is mounted adjacent to a heatsink at the centerline of main PCB, near to the rear edge of the panel,
below the CRT neck.
IC402 is on the flyback transformer side, almost at the front edge of the main PCB.

IMHO not the ICs causing the problem, but keep us updated.

Good luck ! :)

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Re: Commodore 1902A "Vertical Collapse" problem

Post by rogger007 » Fri Oct 05, 2018 4:02 pm

Nice post

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