I made some progress on my Color Classic. I was able to pick away the glue that was preventing me from adjusting the trimpot. I was then able to adjust it to get B+ to 71.0V. The resistances between the pot pins were 372, 372, and 0 ohms (previously the measurements were 517, 517, and 0 ohms). Obviously, I’m not able to determine the trimpot’s value in circuit. It’s likely a 1K pot, as YMK suggested.
At the new pot setting the raster distortion was reduced, but not completely. I raised B+ to 71.9V and got pretty good raster after some additional video adjustments on the back. Satisfied with the results, I put the CC back together switched on the AC. I saw a whisp of smoke, and immediately switched off line power. After that, the CC wouldn't boot. A few checks revealed no +12 and +5 voltages from the analog board. Oddly, I didn't see any signs of a burned component on the board. Did I do something wrong putting the CC back together? I don't know. I understand there's no schematic for the CC's AB, so it's going to be a challenge fixing it (sigh).
After smoking my CC analog board, I have no +12- or +5-volt outputs to the logic board. Diagnosing the problem is proving to be a good learning experience. Please correct me if I’m wrong, but it looks like the CC’s analog board uses a 4A full-wave bridge rectifier (DP1) on 60Hz AC line voltage. I put the scope on the rectifier’s + and – pins and observed + and – half-wave forms -- 180 deg out of phase I assume. I concluded that the rectifier is good.
The minus pin of the rectifier is fed to the Source pin of MOSFET QP7 (IRFBC40), and the positive pin of the rectifier is fed indirectly to a switching controller IC (IP7) which controls the Gate pin on the MOSFET QP7. The MOSFET’s Drain pin is connected to a primary winding (pin 2) of the low voltage (larger) transformer. With the scope I saw a pulsing high frequency voltage superimposed onto the input to pin 1 of the big transformer. That didn’t make sense, so I pulled QP7 out of the board and tested it, found it to be defective (the resistance between the Source and Drain pins is 63 ohms regardless of the Gate voltage. I pulled identical QP1 on the high voltage side to compare and found it to be good. Unfortunately, IRFBC40 is obsolete and unavailable. Has anyone found a substitute that works?
I'm still trying to find the problem with my Color Classic analog board which I smoked a few days ago. I get no +5 or +12 volts, nor am I producing 22KV to the CRT. While looking for bad components I found what looks like a burned resistor. I remove it and it measured 0.11ohms. What's curious is that it's located on the AB labeled LP14 with a coil symbol as shown in the photo. LP14 is not a component on the list of AB components. Can anyone tell me what the value of this resistor is? The best I can make out the bands is BROWN BLACK BROWN SILVER (100 ohms 10%). Can anyone confirm?