H-89 screen blank

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Darrell Pelan

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Aug 2, 2022, 4:02:00 PM (6 days ago) Aug 2
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Hi,

My screen went blank. It was a graceful failure over time which I thought was heat related. Finally, the cursor disappeared and there is no light at the CRT yoke. I dug through the manuals and found the following trouble shooting steps:
1. Brightness control (R1) turned down.
2. Anode voltage incorrect.
3. Grid voltages incorrect (G1, G2, G4).
4. No cathode drive.
5. Transistors Q901, Q902.
6. No video signal coming from terminal logic board.
7. IC U406.
8. Video circuits on logic board.
9. Diode D901.
10. No sync pulses coming from logic board.
11. Diode D209.

It's not number 1 :-). I've adjusted the Brightness control several times. Guess I'll start looking at the other steps. I thought it was heat related, so I placed a 5" fan on the side of the case a month or so ago. I turned it off while running errands just now and it was still dead after an hour.

Wait, You guys are awesome!! Simply typing this help message caused it to come back to life.  I only have a cursor since I'm using Les's H19 emulator to run the H-89 currently. At some point I'll put the system back together and a working screen will be important.

Any ideas on what is causing this random failure?

Thanks,

  Darrell
H89 Cursor Issue 220802.jpg

Glenn Roberts

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Aug 2, 2022, 5:50:28 PM (6 days ago) Aug 2
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Check the filament on the CRT (in a dark room). My guess is it’s cycling on and off. If so it could be as simple as cleaning the pins on the tube, but I suspect there’s a deeper cause of this problem. I’ve seen it on multiple of my systems. I don’t think it’s the filament itself but rather the power. I can’t recall if the filament voltage is a tap off the fly back transformer? But if so then perhaps the high voltage section is flaky? My first suspicions are always around the power supply…

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H89 Cursor Issue 220802.jpg

Dan Emrick

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Aug 2, 2022, 6:06:09 PM (6 days ago) Aug 2
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Since it seems to have come back (somewhat) on its own, any place were mechanical contact is made, such as CRT pin contacts as Glenn suggests, is suspect.  I've had trouble in that past with the wiper on the brightness control.  Sometimes it can be cleaned but eventually a new control is needed.  You could also check the flyback plug connectors or even the pins on the video board for excessive heat.

Looking forward to learning what you find out.

Dan

Joseph Travis

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Aug 2, 2022, 8:59:31 PM (6 days ago) Aug 2
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Hi Darrell,

I've seen this in many H19 / H89s (recently on my H19).  The most common causes are: 
(1) the power connector to the video board 
(2) the flyback connector 
(3) the large power resistor near the power connector
In all cases, check the solder connections on the bottom of the video board.  They are often poor or become pitted over time and fail slowly (like you described).  Good luck!

Regards,
Joe Travis n6ypc


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