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Old 12th Jul 2021, 9:18 pm   #1
brassicGamer
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Default Commodore 1084-D Monitor Repair

Hi folks.

After working well for many years, my 1084-D (1989) has finally given up. It's been moved very frequently in the last year, as I was limited on space and I'm sure that's a contributing factor. Turned it on last week and it just didn't do anything. I have a lot of electronics experience, but this is my first CRT repair. I am fully versed in the risks and how to avoid them, however. I'm wondering if anyone can offer any tips on where to look next, as I have hit a dead-end. I don't have a scope to test with, so I'm limited to a multimeter, but I do have the NTSC schematic (mine is PAL, but the differences are very minor).

Essentially, there is no horizontal drive. So far I have verified the following:

- The power supply itself is working fine, and is delivering good voltages to the board.
- DC power is present on the main board - 110V is available from the power supply to the flyback at pins 1, 7 and 9. This means there is also 110V at the neck board terminals.
- I have measured 80V B+ at the horizontal drive transformer primary, but no voltage is present on the secondary. I removed it and it tests good.
- The HOT tests good.
- There is no noise from the flyback transformer.
- The horizontal combination IC is only getting 6V. This has activated the protection circuit, which is connected to pin 3 of the flyback.
- I'm measuring 1.4V on the ABL circuit.
- I've gone through and removed/tested a number of resistors and capacitors that could be causing problems but everything checks out. I have also used a magnifying glass to identify any connections that are dry/cracked and reflowed/replaced the solder where necessary. Mostly this board is in excellent condition, with hardly any dust and good, shiny solder.
- The 15V and 24V circuits are totally dead. These voltages should be coming from the group of pins 2, 5, 6 and 10 on the flyback AFAIK, but there's nothing.

My next step would be to remove the flyback and test resistance on the coils. I'm suspecting a dead flyback, although the working HOT suggests otherwise based on the classic set of symptoms.

If anyone could suggest where I could focus my efforts, or if they recognise the problem based on the set of symptoms I'd really appreciate it, as it's worth keeping this one going. Thanks in advance.
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Old 12th Jul 2021, 11:37 pm   #2
SiriusHardware
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Default Re: Commodore 1084-D Monitor Repair

No expert on these but based on your observation that the unit has been moved around quite a bit I would consider carefully checking for near invisible ring-shaped cracks in the solder joints around the pins of any items which are heavy and attached to the PCB only by solder. The pins of the line-output transformer would always be a first port of call in cases like this, but any other large heavy component which is only attached to the PCB by the solder on its pins should also be checked.

Cracks of the type I am describing can be very difficult to see unless you gently / slowly wobble the suspect component from side to side while looking at the solder joints on the underside.

I have a Philips CM8524 which I think may be very similar under the bonnet, used in my case with my Atari STs - I would consider it a disaster if mine stopped working.
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Old 13th Jul 2021, 11:14 pm   #3
brassicGamer
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Default Re: Commodore 1084-D Monitor Repair

Quote:
Originally Posted by SiriusHardware View Post
Cracks of the type I am describing can be very difficult to see unless you gently / slowly wobble the suspect component from side to side while looking at the solder joints on the underside.
Okay, thanks - I'll take a really good look. To be fair I have tackled most of the larger components, but I could have missed something. I've also noticed a few errors in the schematic I'm looking at. I made a comparison with a later one, and names have been duplicated, meaning there were certain key components in the flowcharts that I couldn't locate. I'm probably going to end up recreating the entire thing from scratch in KiCad so I can correct it / make it searchable. The originals were hand-drawn.
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Old 14th Jul 2021, 10:16 am   #4
Lloyd 1985
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Default Re: Commodore 1084-D Monitor Repair

Out of interest, what is the IC responsible for horizontal drive? And is it also responsible for vertical?

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Old 14th Jul 2021, 7:47 pm   #5
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Default Re: Commodore 1084-D Monitor Repair

Quote:
Originally Posted by SiriusHardware View Post
I have a Philips CM8524 which I think may be very similar under the bonnet, used in my case with my Atari STs - I would consider it a disaster if mine stopped working.
I think the 1084-D is a Daewoo design, not a Philips like other 1084 models, so probably not similar.
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Old 14th Jul 2021, 8:15 pm   #6
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Default Re: Commodore 1084-D Monitor Repair

Indeed Maarten,
IIRC it uses an STK 73XXX chopper IC unlike the Philips version/s.
I don't know if it was this version or not but one of this series needed the Amiga or whatever connected and running to bring the line time base up, pretty much more like more modern PC monitors do today, where as the earlier Philips based ones would fire up standalone.
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Old 14th Jul 2021, 11:47 pm   #7
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Default Re: Commodore 1084-D Monitor Repair

Quote:
I think the 1084-D is a Daewoo design, not a Philips like other 1084 models
Sorry, I was aware that my Philips monitor and Commodore's 1084s (that I have seen) look externally identical, but I did not realise the significance of the -D suffix.
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Old 15th Jul 2021, 4:10 am   #8
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Default Re: Commodore 1084-D Monitor Repair

As already mentioned, the most common problem on these monitors is either dry joints on the flyback transformer or cracked tracks around it.

Next on the list is a faulty flyback, they seemed to give up the ghost for no obvious reason and no pre failure indications of it going faulty.

Whether you will be able to get a replacement flyback transformer now, I don't know, it has been many many years since I've repaired one.
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Old 23rd Jul 2021, 3:46 pm   #9
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Default Re: Commodore 1084-D Monitor Repair

Quote:
Originally Posted by Lloyd 1985 View Post
Out of interest, what is the IC responsible for horizontal drive? And is it also responsible for vertical?

Regards
Lloyd
It's TDA2595. This is now the prime suspect, purely based on the fact that I've followed the service manual flow charts and eliminated every component except the IC. I have removed the flyback and tested the coils - they are behaving as expected. Given that the HOT is good, it's probably not the flyback anyway. I'm considering buying a spare anyway as it's bound to die eventually (if I can get it working).

The IC appears to be commonly available on eBay - even if it's not the problem, there's no harm in having a spare.
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Old 28th Jul 2021, 4:09 pm   #10
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Default Re: Commodore 1084-D Monitor Repair

Back to square one - the replacement IC has not resolved the issue. The symptoms are exactly the same so, unless the replacement IC is broken in exactly the same way as the original, I need to look elsewhere.

I was starting to think that I need a scope because the service manual has useful sample waveforms included. I then noticed that, in tiny writing I hadn't noticed before, the waveforms also indicate the expected voltage and I wish I'd noticed this sooner - the flowcharts only discuss B+ voltage, which is fine, but the other voltages coming from the power supply are wrong, so I think it must be the power IC. I know this is supposed to get hot (it's got a fat heatsink) but it's getting really hot. Obviously I'll check all the associated passives before I order anything. Infuriation!
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Old 29th Jul 2021, 9:48 am   #11
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Default Re: Commodore 1084-D Monitor Repair

I have a flyback that might fit this monitor if you need it.
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Old 2nd Aug 2021, 1:34 pm   #12
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Default Re: Commodore 1084-D Monitor Repair

That's good to know, thanks. How much would you want for it? Makes sense for me to acquire one when the opportunity presents itself.
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