bloody Classic II, bloody analog board, again! Horizontal size broken.

[Cam]

So the analog board I finally got working a couple weeks ago? Is broken again.

I powered up the Classic II this morning, and the image on screen was oversized on the horizontal axis. I tried the horizontal size pot on the back and nothing happened. I expected the size to shrink/expand on the screen, side to side.
Has anyone out there had a similar problem?

I looked at the schematics that are out there and can't find the PP2 pot on them, but can find the RP8 resistor that's attached to PP2 on the analog board so I'm going to start from there.

 

[bpetersen]

Sounds like similar but opposite problem to what I just posted about. Did you ever find the cause?

 

[Cam]

Sort of?
Resistor RP48 was a charred mess, so I replaced it. The new one died a smokey death too, so there's a short somewhere on the AB (I think), but it's anything but obvious.
If I can locate the short, I'm reasonably sure clearing it and replacing the resistor again will fix everything. I got very frustrated and set it aside to look at "later" and really should get back to it . . .

 

[bpetersen]

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 other side.

 

[Cam]

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 other side.

Thanks! Yes, I'm on the rev B board and your advice sounds like a solid place for me to start. Might be time to move it from the 'someday' pile and onto the bench.

 

[bpetersen]

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.

 

[Cam]

Hopefully I'll get some time this weekend to look at the beast. Just started a new job and 'spare time' is a thing of the past!