PET 2001-32N Nonlinear Picture

[CommodoreZ]

Hey y'all, I set up my PET 2001-32N for the first time in awhile, and with newfound confidence, I decided to tackle the issues plaguing the unit.

The most glaring of which were the following:

1. The picture feed on the display would drop out, and come back with a smack (figured it was a solder joint)
2. The top line or two of text would be squished horizontally (sounds like a capacitor)
3. The top few lines of text wouldn't be at the very top of the screen, and would also be squished vertically

I put it on the bench, opened it up, and replaced electrolytic capacitors C3, C9, C12, C13, C15, C17, C22, & C23, which has solved problems #1 & #2. I would have replaced the remaining 3 electrolytics if i had them in my stockpiles. Problem #3 was not solved by the capacitor replacement...


So I cranked up the brightness knob past where you would normally want to set it, and the horizontal lines across the screen showed me more about problem #3. The whole vertical deflection is nonlinear.

I've hooked up my scope to a few spots to see what kind of wave forms I get, and they mostly look okay, with proper voltage ranges. But based on the crude drawings in the schematics, I'm seeing that the wave forms near the vertical yoke (pictured), and near the base of Q10 aren't quite the right shape.


Long story short, I'm not sure what the next step in my repair process should be. Any ideas what I should test & try next?

 

[CommodoreZ]

Update Time!


I threw together a composite video adapter, and hooked it into the PET to confirm that the signal going to the monitor was alright. The linearity issue was not present on the other display

I've ordered the remaining 3 electrolytic capacitors, including the odd 10μF nonpolarized electrolytic capacitor. I've also ordered replacement transistors for the vertical drive section.

I'll keep y'all posted on if those new parts fix the nonlinearity.

 

[gertk]

Is the 12 Volt supply OK ? Maybe Q8 is bad ?

Otherwise just check each of the components in the vertical circuit, non electrolytic capacitors, resistors, transistors and such.

 

[CommodoreZ]

The 12V supply is looking fine actually. Clean stead 12V, with very reasonable ripple. I've tested a few of the resistors and diodes, however I don't have a good way to test the transistors at present. I can check the other capacitors when I've got everything apart for the transistor upgrade.

I agree, Q8 sounds like trouble, so I'm swapping that out when the parts arrive.

 

[CommodoreZ]

Good news, everyone!

I replaced all five transistors in the vertical deflection circuit (Q6-Q10), along with the two remaining semi-common electrolytic capacitors on the board. Sure enough, I have crisp clean linear video displaying on the screen!

Look how beautiful that is. Honestly, I don't see any reason to replace the nonpolarized electrolytic at present, considering the video looks as good as it does.

 

[KC9UDX]

Very nice!

I wouldn't do any more work unless something goes wrong.

 

[gertk]

Looks great and if it ain't broke, don't fix it