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Commodore 1702 colors problem (no blue?)

drg

Member
Joined
Oct 23, 2020
Messages
40
Location
Wroclaw, Poland
Hi,

I bought Commodore 1702 monitor, however the colors are strange, for me it looks like there is no blue. I tried few cables, front and rear connectors. With console (Nintendo Gamecube) it's the same. All front potentiometers work, however no changes in terms of blue color. Any ideas?

Best regards,
Pawel

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The service manual is available online here: https://gona.mactar.hu/Commodore/mo..._Manual_Preliminary_314004-001_(1984_Oct).pdf.

I think you are correct - the blue has gone.

I assume you know that the GameCube and cables work on another monitor (?) thus demonstrating it is the monitor?

If so, then you will need to troubleshoot the blue cathode drive logic. If you are not competent to do this, I would suggest finding a TV repair shop to help you out.

All the 'smarts' is done inside a single IC (IC301 - HA11247) and the cathode drive transistor is Q351. The problem may be associated with a simple resistor, capacitor or inductor associated with the IC and transistor. Failing that, it is probably the CRT itself.

It might be worth checking R357 (12k 2W). This is a power resistor - so it could have burnt out. However, it could have burnt out because of another component fault.

Dave
 
Last edited:
Yes, Gamecube and my other C64s work with another monitors (1084 and small TV with AV plug).

I was thinking there are some easy fixes with some potentiometers or so.

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Possibility of course.

The blue drive is a fixed resistor - but the three cut-offs are variable.

You could try 'twiddling' the blue cut-off potentiometer - but note the position before you start of course and (if it doesn't have any effect) put it back again.

It looks like that some of the component identifiers on the PCB picture differ slightly from those in the technical manual I have located...

It would still be worth doing a visual inspection of the components on the PCB first though (to see if any are burnt or otherwise show signs of distress).

Dave
 
I didn't think it would to be honest.

Did you checkout the service manual I referenced? There are some quite good fault-finding tips in here (look for the text 'no blue raster') despite the fact that the PCB component identifiers don't fully align.

It would be worth checking the voltages on the collectors of the three transistors (using a multimeter) to see if there is any gross discrepancy between the red and green (that works) and the blue (that doesn't work). This voltage could be up to 350V DC.

Dave
 
Hi,

I was able to find and fix cold joint and blue is there again!

I marked it on red.

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On the picture perhaps it looks like too blueish, but I think it's ok and to adjust.

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Thanks for insights!
 
The simplest problems are always the best!

Well done!

Yep, have a tweak to get it just how you like it.

Enjoy!

Dave
 
It looks like the emitter of the blue drive transistor was a bad connection then. From my reading of the fault finding guide, that would give you no blue raster. So that does make sense when comparing the identified failure and the fault symptom (and, of course, the fact that it is working now!).

I would, however, clean some of the flux off some of the soldered joints I can see though to prevent further potential problems in the future.

Dave
 
Thank you Dave, good point. I will clear the board with IPA and for last time I will check cut off potentiometers, because I touched them a little bit.

All the best!
 
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