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Old 12th Mar 2020, 7:53 pm   #1
slabbi70
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Default Need to adjust CBM 8296 monitor

Hi,

I have some problems with the internal monitor of a CBM 8296.

The previous owner tried to repair the power supply and connected the 12v DC monitor connector directly to the 33v AC transformer. Poor 8296...

Well, after replacing some components (e.g. all capacitors and the UPC1031H2), the monitor displays a picture again but it is shifted a little bit to the left and the scan lines are visible.

Here I need some help. Any idea what component can be affected?

Thanks, slabbi.
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Old 13th Mar 2020, 3:49 pm   #2
SiriusHardware
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Default Re: Need to adjust CBM 8296 monitor

I don't know this computer at all but other images of the same machine working show green text on a black background. You have black text on a green background and the left edge missing.

I don't know if this system uses separate syncs or just combined video and sync, but if it is combined video and sync, one might almost think the video / sync signal had become inverted somehow.
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Old 13th Mar 2020, 6:41 pm   #3
slabbi70
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Default Re: Need to adjust CBM 8296 monitor

That's it! You are right!
Video is inverted.

I tried to connect the monitor to pin 8 of UD4 and the picture is fine.

So the error is somewhere is on the logic board:
- UD6 (74LS86, pins 1-10) seems to be ok (it is used as inverter).
- also UD5 seems to be correct
- the inverter UD7 works fine
Since the text is readable the character part of UC5, UC4 (parallel to serial) and UD4 (pin2-6) seems to be ok

Do you have any idea?
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Old 13th Mar 2020, 8:24 pm   #4
slabbi70
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Default Re: Need to adjust CBM 8296 monitor

Here are some pictures:

UD4-2-6 means: IC UD4, Pin 2 and Pin 6
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Old 13th Mar 2020, 8:25 pm   #5
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Default Re: Need to adjust CBM 8296 monitor

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Old 13th Mar 2020, 8:26 pm   #6
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Default Re: Need to adjust CBM 8296 monitor

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Old 13th Mar 2020, 8:27 pm   #7
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Default Re: Need to adjust CBM 8296 monitor

Any ideas? The components seems to be fine.
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Old 14th Mar 2020, 5:52 am   #8
TonyDuell
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Default Re: Need to adjust CBM 8296 monitor

I think you can invert the video (and/or the sync signals) by changing the soldered links on the inputs of that '86 XOR gate IC. Said links tie the inputs to the XOR gates either low (to the 0V rail) giving no inversion of the signal or high (to the pull-up resistor) which will then invert that particular signal.

I believe that was done as several different monitor PCBs were used (or might have been used), some need inverted video, some not.

However, if this machine (main PCB and monitor) has worked in the past then you have a problem elsewhere. If you are putting it together from parts (main PCB from one machine, monitor from another) then you may need to set the links.
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Old 14th Mar 2020, 9:42 am   #9
SiriusHardware
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Default Re: Need to adjust CBM 8296 monitor

Good spot.

As well as the video, which is obviously inverted, it may be necessary to invert (un-invert?) the separate hdrive and vdrive signals as well, to obtain correct positioning of the image.
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Old 14th Mar 2020, 9:47 am   #10
slabbi70
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Default Re: Need to adjust CBM 8296 monitor

Actually I did that.

Nevertheless the computer worked 20 years ago and changing the soldered links fixes the symptoms but not really the bug.

The oscilloscope says that all chips are working (the inverters are fine, the 4x XOR is fine, the FF seems to be fine). CBM changed the signal when switching from a 9" to 12" monitors. Maybe the logic board is fine and the problem is still the monitor?

The schematic says that black level is 5v and white level 0v (so the video signal is inverted to what the board delivers). But maybe the schematics are wrong and there is a fault with the transistors.

It would help to have the signals from a working machine I do not like to change chips that do not need repair.

CBM700_CRT_Schematics.pdf
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Old 14th Mar 2020, 9:50 am   #11
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Default Re: Need to adjust CBM 8296 monitor

@SiriusHardware

drive and vdrive are fine (UD6-3 and UD6-6). When changing the soldering link the picture is perfect.

If I were not a perfectionist now, I would leave it at that. But I am
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Old 14th Mar 2020, 10:06 am   #12
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Default Re: Need to adjust CBM 8296 monitor

The video input circuit is so simple that there is hardly anything which can be responsible for the video inversion, apart from possibly Q201/Q202/Q203 - it looks to me as though Q201 is not supposed to invert the video signal (it is PNP, so wired as it is, the signal on the emitter will follow the signal incoming on the base).

It may also be worth checking Q601 as well.

I admire your determination to find the real problem, that would also be me.
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Old 14th Mar 2020, 10:35 am   #13
slabbi70
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Default Re: Need to adjust CBM 8296 monitor

Yes, it is really simple. I have not seen such a problem before.

The monitor schematic says that the signal should be inverted and when I provide an inverted signal the picture is fine => monitor is ok

The computer was originally soldered to provide an inverted video signal, but delivers a non-inverted now. So I expected to problem on the logic board, but all components seem to work fine.

Weird.

I will check Q201-Q203 and also Q601 now.
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Old 14th Mar 2020, 10:36 am   #14
SiriusHardware
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Default Re: Need to adjust CBM 8296 monitor

Thinking about this more...

Item 1: You say that this computer and monitor worked together 20 years ago, but you probably only have that information second hand because you are not the original owner - or at least you are not the original owner of the monitor - post #1 "The Previous owner tried to repair the power supply"... Unless you have known the original owner for 20 years or more, you may never have seen this monitor and this computer working together. You may only have someone else's word for it.

Item 2: The monitor circuit diagram very clearly shows the sense of the video and sync signals it requires and there is no reason to suppose that this detail is wrong, but the computer (as found) is currently outputting signals, or at least the video, inverted, in the opposite sense to that which the monitor requires.

My unbiased conclusion would be that this monitor and this computer were not the original matched pair.

What was the 'fault' on the monitor which the original owner was trying to repair, I wonder? Was it just an inverted, displaced image?
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Old 14th Mar 2020, 12:48 pm   #15
slabbi70
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Default Re: Need to adjust CBM 8296 monitor

Yes, you are right. I have not seen both working together.

What I saw when I opened the computer the first time: The 12v DC rail (monitor) was connected directly(!) to the 33v AC from the transformer. After fixing the obvious broken components the monitor shows an picture again (inverted but better then the black one before).

And inspecting the logic board showed that this has not been modified in any way. From the part numbers the PCB has been used in a 8296 and the logic board is definitely a classic 8296. That's why I assumed that it has worked before.

But you might be right. Maybe it is a combination (there are several ones) that requires to chnage the soldered link. I do not know if the previous owner (maybe) changed the monitor PCB or the logic board in order to "repair" the computer (make one out of two).
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Old 14th Mar 2020, 6:09 pm   #16
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Default Re: Need to adjust CBM 8296 monitor

Yes I was wondering if the previous owner had attempted that ridiculous procedure of 'board swapping' and you now have a logic boar from one machine and a monitor board from another. Needless to say it's a lot easier to repair a machine starting from a combination that was known to work once.

Those polarity-inverting jumpers (and indeed the XOR gates used to select the polarityof the signals) were put there for a reason. Probably because some monitor board versions need the signals to be inverted, other monitor boards don't. At this point I think I'd just set the jumpers so that it works and enjoy the machine.
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Old 14th Mar 2020, 6:22 pm   #17
SiriusHardware
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Default Re: Need to adjust CBM 8296 monitor

And by the way slabbi70, well done on repairing the monitor.
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Old 14th Mar 2020, 7:22 pm   #18
slabbi70
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Default Re: Need to adjust CBM 8296 monitor

Thx!
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