Fixed! It was a bad QL2. I had already replaced it twice with parts I got on eBay. Replacing with a Mouser part completely solved the problem. Other than the floppy, everything works now.
If others have bad analog boards, I’d be happy to debug and fix them. I have so much hard won experience...
Here is the oscilloscope output at the deflection yoke for vertical and horizontal. Does anyone have correct traces I could compare to? Or feedback on whether this is clearly bad or not?
Since your last post, I managed to get an oscilloscope to debug my problem (similar but on the horizontal). I at first had trouble getting the 60Hz traces too, but I realized I just wasn't triggering on them. Adjusting my scope, I got the one on pin 4 fine. The signal on pin 3 is the same...
Nice! Let us know how it goes. The one other component to check in that area is the capacitor connected to QP6. I took a look at the size of the resistor you are blowing and it it looks like a 1/4 watt or smaller -- it would definitely would burn out if it was connected between 29V and ground.
It seems strange that you get a good 60Hz signal out of pin 8 but don't get a 60Hz signal in from the digital board at HS1.
I was reading this thread on VCF, though, that discusses debugging this area of the circuit and could be useful...
I'm assuming you are on the second board revision of the analog board, and RP48 is connected to QP6 which goes to ground. Talking to my EE friend, the most likely short that would burn RP48 given the schematics would be between the 29.5v on one side of RP48 and the ground connected to QP6 on the...
See attached photo. I tuned it with the seven pots as best I could. It has been cleaned, recapped both boards, etc (separate thread on that) to get this far.
Is anyone familiar with this problem? Without knowing what I am talking about, it feels like it isn’t scanning all the way across. Maybe...
I solved it! Replacing the QL2 (IRF740 power MOSFET) with a new one from Amazon (Anchor Electronics does not stock this, sad Mac...), and putting everything together, I get a screen! Now I need to:
1. Get a keyboard! I wasn't sure I would get far enough to need one. Anyone in the CA Bay Area...