Electrohome G07 CB0 is dark

[ncflagg]

I have a PC VGA'd to an Electrohome G07 CB0 and it's pretty dark. There's some missing video at the top and bottom (or, left and right if mounted horizontally). It's about half as bright as the pics indicate.

History: I installed a cap kit and flyback about 7 years ago and it looked great at the time. It then went largely unused and was stored in a closed garage. While it was stored, the PC that was installed died and I recently rebuilt one. The video config is the same as before; I cut a VGA cord and soldered it into a blank Jamma connector. Soft 15khz is configured. Picture is stable, but way too dark to play. I tried upping the Screen adjustment on the flyback, but it never gets much brighter and just washes everything out. I tried upping the Screen pot also without the VGA connected, and it was still dark.

I'll appreciate ideas on what to look at next.

Pics:

[Ken Layton]

On the monitor main board, test or replace resistors R516 (150k @ half watt) and R517 (180k @ half watt). If either or both go bad, you'll have a dark picture. They either go "open circuit" or radically increase in value (out of tolerance).

[ncflagg]

You're a treasure, Ken. I'm positive you've helped me before (maybe even with this very monitor).
I'll check out the resistors.

[Ken Layton]

R517 sits underneath R522. So you have to temporarily unsolder R522 just to get R517 in/out.

[ncflagg]

Finally got around to, well, screwing this up big time.

I removed and tested said resistors. They tested at the values Ken mentioned, so I looked over the board and other than being just, filthy, I couldn't spot anything by eye. I put the resistors back and powered on the monitor. I was looking at the board when I powered it up and saw a little puff of smoke from the area I was working. After I put the smoke back in..

Smoke seems to have come from one leg of FR401. Then, I noticed I swapped R517 and R522's positions.  I pulled all three and tested again. Only R517 was bad (tested at 5 ohms), so I built a new one out of two 330K, one 10K and one 4.7K 1/4W resistor. Screen is now brighter, but a tad smaller than before.

How many GoFundMe's should I start?

[Ken Layton]

Vertical collapse is usually caused by that 68 ohm 2 watt resistor going open circuit.

Here is where to buy it:

https://www.arcadepartsandrepair.com/store/components/resistors/68-ohm-2-watt-wirewound-fusible-resistor-3-pieces/

https://www.twistywristarcade.com/monitor-components/747-68-2-watt-fusible-resistor.html

[ncflagg]

I pulled FR401 and it still tests at 68ohm. I went ahead and built a 70ohm 2W resistor (just for testing, I know it needs flameproof resistor permanently) and the vertical collapse continues.

[ncflagg]

Inspecting more, R526 (1K 1/4W) is what burned. Trying to replace that one..


Edit: No change. Still have vertical collapse.

[Ken Layton]

Maybe the two vertical output transistors have gone bad?

If that little tantalum capacitor next to the IC (underneath the metal shield) goes bad, you'll get vertical collapse. It's typically NOT included in some capkits.

[ncflagg]

-sigh-  Vertical collapse was another symptom of my apparent dyslexia. I had my custom 180k resistor in the R522 position. Once I put my custom resistor 180K 3/4W in R517 and put the old R522 back in place, the vertical collapse is now fixed.

I'm now back to the dark picture issue; went through all the monitor controls again. I'm also noticing that the picture pulses occasionally. About every 5 seconds or so (not super regular), the picture shrinks by a few percent and then immediately goes back to the usual size.

I'm going to explore another possibility, that there is no problem. My VGA cable is connected directly to the input of the monitor which definitely used to work. The video card which used to work was killed when that PC died. I'm starting to wonder if that video card just happened to put out more voltage, and all the replacements I've tried do not (1v vs 4v, perhaps). I'm ordering a VGA amp and will test that.

[ncflagg]

Success! Nice and bright now.
Welp, now I know what the +B1 pot is for. Voltage was below 120v. Followed the manual and got it set to 119.x V. I think that was the issue the entire time. I notice that unlike tuning all the other pots, the effects from this one have a lag of several seconds. I'll guess that at some point I tuned all the pots (including that one) maybe right before I stored the thing so I never noticed.

Also, didn't need the video amplifier.

Ken, thanks again for your assistance. Much appreciated.