Pentranic 1427 (CH-288) Flashing

[JoshCRT]

I had a Pentranic 1427 with bad tube & yoke, was told it was working, turned out the chassis was dead (clicking).

Fuse was good, HOT shorted. Replaced the HOT and all electrolytic caps and it fired it up on a 27" (flat display) test tube. I can hear HV, can hear deflection, can hear a minor sizzle/arcing, screen extremely dark and something was flashing/pulsing on screen. Poked around and found D703 on the neckboard to have an odd reading (0.2 - 0.3), didn't have any new so pulled one from another chassis/neckboard (reading 0.4 - 0.5).

Monitor fires up, displays an image (which looks really sharp!) but the minor sizzling or arcing is still present and the monitor is flashing. It looks like the flashing is related to the vertical deflection.

I took a couple video; one & two
NOTE: The image is not out of sync! It's hard to explain, but the image itself is stable. The brightness appears to be rolling as it flashes (if that makes sense).

My next step is to replace the vertical IC but I will have to order one (parts chassis's IC is already gone). Was curious if anyone has ever seen this before.

Thanks



Old post below, now irrelevant. CH-288 switchable schematic shows differences between difference sizes. 26" chassis will work on both 25" and 27" tubes.

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I acquired a monitor from a vendor that had been dropped. The tube and yoke are completely destroyed but the chassis appears to only have some minor damage. I was told it was working beforehand.

The chassis is a Pentranic 1427, dual res (manually select 15khz/25khz), based on the Rodotron CH-288, with a 26" sticker on one of the filter caps. I have a two nearly identical monitors, but the 25" equivalent -  Pentranic 1425. Both of my 1425 monitors have the same 26" sticker on the filter caps.

Does anyone know what the differences are between the two? Yoke differences? Can I run the 1427 on a 25" tube? Upon brief inspection of both 1425 and 1427 chassis', I cannot see any difference.


[JoshCRT]

Figured out chassis compatibility, found a test tube, monitor now flashes. Modified original post to reflect issue.

Any help is much appreciated!

[grantspain]

the fault is the brightness changes on bright screens to dark screens?

[JoshCRT]

The entire screen flashes like it's flip-flopping between normal/ok brightness to dark/low brightness. It rapidly does this multiple times a second.

It doesn't appear to be an HV or B+ issue, B+ is stable and the size of the image does not change. The image does not jump or lose sync. All controls appear to work as expected; sizing, V & H sync adjust, even brightness and contrast - although the latter two have no effect on the rapid flashing.

[grantspain]

could be something to do with the tube heater current, is there a resistor on the heater line?
if you lower the screen volts does the issue resolve itself?