Recent content by jonschultz

There are several sort of silly things that could be wrong at this point, each having a low probability of being the root cause individually, but it's probably worth checking them. I would start by making sure that the gate of QP7 (pin 1) isn't shorted to ground (you can use pin 4 of IP7 again)...

No big deal, I found the problem really quickly and I have plenty of random replacement parts. It was a good lesson anyway, to be really careful in that area, especially when measuring relative to the rectified AC. It was fried while booted, so the logic board supply side was running.

Just a word of warning though: I just killed my QP7 MOSFET (the one with the small heatsink, just north of IP7) while probing around at IP7. This immediately also took out the fuse on the AC side. I was lucky to have a more modern acceptable replacement, a STP10NK70ZFP. This is probably not the...

For IP7 pin 6 on my system, with hard-power on but soft-power off, I measure -55V relative to chassis ground and 0V relative to the bridge rectifier's (DP1) negative terminal. With soft-power on and the system booted, I measure about -32V relative to chassis ground and +10V relative to the...

I think that whole section is floating relative to ground because it’s effectively on the AC side. The AC side and the secondary sides are fully isolated by the switch mode transformers (hence the use of optocouplers to send supply signals from the secondary back to the primary). It’s the...

Definitely seems like something is wrong with the supply section for the logic board power, right near IP7, though it's not necessarily the bad part. Everything sort of "north" of IP7 toward the front of the machine is related to logic board power, and it won't normally be running unless...

That's a good sign because it means the AC section and the CRT supply section are working. The logic board supply side could still have a problem. Which TDA4605 did you have discharge on? There are two, one for each of the switching supply transformers. In any case, I think the CRT side (which...

Are you now getting no voltage on RV26 and RF19?

When a CC seems to be this dead, I recommend checking some of the low-voltage supplies. Many of them will normally be running even without booting (shouldn't even require an installed logic board), you simply have to have the hard power switch on. You can check the 8.3V and 24V supplies first...

Over the years, there have been lots of discussions surrounding the infamous DL21 and DL22 18V Zener diodes on the Color Classic analog board and the damage they tend to induce over time from their high power dissipation. There's scorched PCBs, reduced life of nearby CF8, generally undesirable...

Well CP35 appears to be in the power supply section. I'm less familiar with this territory of the analog board, but I think CP35 might be a decoupling cap for the switching power supply IC nearby. If something went wrong there, then the almost nothing is going to get power. Time to start probing...

Yeah, the bulging is bad and a fairly typical problem on Color Classic analog boards, precisely because of the heat generated by those two nearby diodes. This cap is also part of the vertical section and could definitely be a source of your display problems. The diodes in question (DL21, DL22)...

Yeah I agree, the DF2 voltage is suspiciously high. Do you have the correct value for RF11, which biases DF2? I think it should be 370 Ohms.

Excellent suggestions regarding the voltages to check. I was also going to mention that checking a flyback-generated voltage would be a good verification, as well. Most are pretty large, but one is reasonably low for safely measuring with a DMM. That's the one on pin 11 of the LM2418T (the...

I measure about 22.7V across CF10 on my system. Like a lot of components on the CC analog board, it's not very easy to probe from the top side, especially with the machine running. I soldered some probe wires on the underside of the board and fed them out the back to my multi-meter.