After working without issue for decades, my Commodore 1702 monitor went dark, I decided to re-cap the thing, ever the tried-and-true fix all. And sure enough, re-capping the boards restored the image, with caveats: 1) there's no color, and 2) retrace lines are visible. Now I'm stuck t'shooting likely sources of the fault(s).

None of the basic stuff is the cause (e.g., the color pot on the front panel, front/back video inputs, cables, video selector switch). Obviously the problem is limited to video signal processing, so that limits the number of caps a/o area(s) of the PCB to scrutinize significantly.

I can think of just two ways to mess up a re-cap job: 1) installing the incorrect spec cap, and 2) installing a cap in reverse polarity. In one instance, installed a cap that is not the same uF as the cap it replaced--4.7/250 new vs. 10/250 old--'cause that's what the kit provided for this location on the pcb.

Anywho.. back to the symptoms at hand...
Anyone know what might cause a color monitor to display no color and show retrace lines?

 

hi 1,
Which capacitor Idents did you replace as shown on the circuit?
E

 

The kit provides 39 caps; I replaced all caps in the kit per https://wiki.console5.com/wiki/Commodore_1702

 

Download their service manual (bottom of their Wiki) and check the components that have hints of "no color" in the comment boxes. Some of the caps (C301, C306), if open (cold solder joint?), will result in "no color".

 

Retrace lines are usually caused by excessive Screen voltage (A1) , no colour will be in the colour killer/ demodulator section.

 

Some of the caps (C301, C306), if open (cold solder joint?), will result in "no color".

C301 and C306 are both ceramic caps; I replaced neither.
I did, however, replace electrolytic caps C303 and C305.

The handwritten notes in the service manual advise C303 and C305, if open, will result in "no color".
I removed both and measured capacitance. C303 (100uF/25v) reads ~97uF--normal; but C305 (0.47uF/50v) reads ~487.8uF!

 

Post photos of the screen so we can see the symptoms.

 

C301 and C306 are both ceramic caps; I replaced neither.
I did, however, replace electrolytic caps C303 and C305.

The handwritten notes in the service manual advise C303 and C305, if open, will result in "no color".
I removed both and measured capacitance. C303 (100uF/25v) reads ~97uF--normal; but C305 (0.47uF/50v) reads ~487.8uF.

C305 is too high in value, and it's directly connected to the colour killer.

 

C301 and C306 are both ceramic caps; I replaced neither.
I did, however, replace electrolytic caps C303 and C305.

The handwritten notes in the service manual advise C303 and C305, if open, will result in "no color".
I removed both and measured capacitance. C303 (100uF/25v) reads ~97uF--normal; but C305 (0.47uF/50v) reads ~487.8uF!

View attachment 291826

View attachment 291830

That reads as 487.8nF, NOT uF, so is within tolerance for 0.47uF!

 

C301 and C306 are both ceramic caps; I replaced neither.
I did, however, replace electrolytic caps C303 and C305.

The handwritten notes in the service manual advise C303 and C305, if open, will result in "no color".
I removed both and measured capacitance. C303 (100uF/25v) reads ~97uF--normal; but C305 (0.47uF/50v) reads ~487.8uF!

View attachment 291826

View attachment 291830

Nope, that 487.9 is in Nano Farads, not uF. That works out to 0.489uF, close to 0.47uf rating
Always check units!

 

nanofarads...

i've never seen capacitance shown with the nano prefix. i simply took it for granted that my DMM was displaying microfarad. But sure enough, upon closer inspection there's a little "n" on the display. Thanks for pointing this out.

C303 and C305 are within spec. The hunt continues.