Zenith Chromacolor II loss of picture control after warmup

[RetroHacker]

I have a Chromacolor II, chassis 19JC48. The set works well, but after it's warmed up for a while, it will suddenly go to a very dim image, looking exactly like the picture control is turned all the way down. At this point, the picture control has no effect, but the brightness, color and tint still work. When this first happened, I hit it, and it popped back to working for a minute or two - I've not been able to repeat this, however. Turning it off and letting it sit for several minutes - it will work fine when powered back up. The problem seems random as to when it will happen - but it never happens when the set is cold and just turned on. Sometimes it'll happen after it's been on for an hour, sometimes two - this morning it only took about five minutes.

Obviously, this seems like a bad connection. I've been through this thing, I've traced the picture control back to the 9-88-03 luma module, and I can clip onto that and verify that yes, the resistance changes as the picture control is turned. The connections seem solid to the control and the stakes on the module connection, I have not been able to observe that going open. The resistor in series with the picture control seems fine and has not drifted. I've pulled the module and gone through it carefully, and resoldered a ton of what appeared to be bad connections - but the problem persists. I cannot make it go away when it crops up by wiggling or flexing modules, tapping or banging on anything. I went ahead and replaced all of the electrolytic capacitors on the 9-88-03 module - one of them was fairly bad, testing almost twice it's proper value - but again, this did not fix the problem. I'm beginning to wonder if there's something wrong with the ceramic resistor module (105-114-01), as that thing gets *hot*. I did try hitting the IC (221-96) with freeze spray when it fails, and could not change the problem. I'm kind of afraid to freeze that hot ceramic module though - that seems like it might break.

Unfortunately, I don't have the schematic for this set, this is one of those ones that Sam's doesn't have scanned in already. I've been working off schematics for a later Chromacolor II console that uses a lot of similar parts, but it has a 9-88-02 instead of the -03, and it doesn't have that weird ceramic resistor module either.

If anyone has any ideas (or the schematic for the 9-88-03 module), I'd appreciate it.

-Ian

[zeno]

Usually the ceramic resistor got cold joints. Happened to all of them.
Every one that came in got resoldered. You could also try cleaning
the Chromatic switch if it has one.
Often there is another resistor pack on the 9-89 too, check that.
I should have the Sams & will look into it later.

73 Zeno
LFOD !

[RetroHacker]

Thanks! I've already resoldered all of the joints to the circuit board, but I have not tried resoldering it at the resistor itself - probably have to remove it or some of the nearby components to get in there to solder that. It *looks* OK, but, as we all know, looks can be deceiving. I've tried wiggling and tapping it while it's on, but no change.

Also, I spoke too soon... now the fault seems permanent, fired it up a minute ago and it's dim. The solid fault will hopefully be easier to find. I did go through and check all the diodes on the board, they're good.

This set doesn't have a chromatic switch.

And yeah - that resistor on the output module gets super hot too, and good idea, I should resolder that also.

Do these fail internally (like the conducive part cracking on the substrate)? Or is it just solder joints?

Thanks for the help!

-Ian

[RetroHacker]

I completely desoldered the resistor module, and resoldered all of the pads on the ceramic module, both sides - and stuck it back in and resoldered it. Still not working. I'm wondering if perhaps one of the printed resistors are actually damaged or open.

I also pulled the 9-89 as well. Definitely a lot of lousy looking solder joints, but none actually broken. I resoldered that board as well. It did not fix this problem, but it probably would have caused it's own issues at some point down the line.

If you can find the schematic for the 9-88-03, that would be a huge help, so I can check the elements of that resistor module. I'm still suspicious of that part, and I sure hope what's in there is documented.

Thanks!

-Ian

[damen]

check and clean the service switch

[damen]

9-88

[JohnCT]

There were several versions of the 9-88. The later ones with the ceramic module were more trouble. If you ever try to solder the ceramic module, you need silver solder to do this.

As far as I can recall, most 88 modules are interchangeable.



John

[RetroHacker]

Going through and checking the ceramic resistor against the schematic, I found that the 1 meg section between pins 1 and 10 is open. I replaced this by tacking a separate 1 meg resistor across the board. Unfortunately - no change. The set hasn't been intermittent in a while, now it's just always dim with no change from the picture control.

I did check voltages at the video IC 221-96 against the schematic, and they all seem about right, and I can adjust the voltage at pin 16 with the picture control.

I'm beginning to wonder if something in that chip might have failed - possibly due to the ceramic resistor having that bad section? Unfortunately I don't have another one of those chips, so I'm probably going to have to order one to try swapping it out. All the other components on the board test good, and every other section of the ceramic resistor checks OK on the meter.

-Ian