Repair CRT monitor Commodore 1084S-P

[decoder_82]

I am trying to repair a Commodore 1084S-P. The service manual can be found here
https://gona.mactar.hu/Commodore/monitor/schematics/Philips_CM8833_Service_Manual_scanned_by_Gona_PDF_by_Guru.pdf
                               
Basically the symptoms are: power led is green but there is no image

First investigations done:

- HV is present (confirmed when discharging the monitor)
- Filament are not glowing at the neck of the tube
- no visible cold solder joints
- all capacitors are still original ones

Looking at the power supply board:

- when disconnected from the main board I observed 125v, 26v and 16v - which is expected voltages.
- when connected from the main board: 56v, 11v and 7v

Where should I look next? what could cause this drop of voltage when connected to load ?

[sausage]

Is there the usual high pitched whine of the flyback? Or is that ok? Good hope if that part is intact.

I did a 1081, it's not a 1084 but some tips might be gained from here: https://blog.waynejohnson.net/doku.php/amiga_1081_monitor_repair

Did you maginify the joints. Really none cracked or dry?

* Check voltages and expected values from anything that looks dodgy.
* ESR test on all big to mid-sized caps. You said all original. I'm guessing a good area to start.
* Check input signals all along their paths to the video decoder IC.

[decoder_82]

Thanks for the reply. There is no noise from the flyback. That's already a good thing.
I have connected a 60W light bulb on the power supply board and the 125v was still there. I assume that the issue does not come from there.
Next I will double check for cold joints and ESR of the caps.
What is strange is that almost all voltages from the power board are affected.
 

[decoder_82]

OK so I checked ESR value of the larger caps and nothing too bad there.
When taking measurements of voltage at different points I always get too low voltage.
Does it mean there is a component shorted somewhere ? Wouldn't that trigger the over voltage protection on the power board?

[sausage]

That's a good question, I'll leave that for others

But I notice this schematic has voltages marked on the ICs. Might be worth printing it out and marking the voltages that don't match up. Some would have to be good?

[inse]

As the PSU is able to power a light bulb, there is a chance that the PSU is OK and one of the power rails is in overload.
You can also interrupt the 125V rail and see if it powers up then.
Have you tried measuring the other two supply voltages for excessive current draw?
Does anything run hot?
If you have a power meter, measure mains power consumption.
Repaired a 1084 myself recently, but it does have a different power supply.
Sorry, I modified my posting now the ideas are kind of mixed up, but I hope you get an idea how to proceed.

[decoder_82]

Thanks for the reply !
My isolation transformer has ammeter and it does not look it draws a lot of current. It is analogue and barely see the needle moving (like with working monitor).
Looking at temperature of component it seems the HOT (TS467) and the connected heat sink get hot. I did de solder it and tested and it did not look shorted. Which is strange now that I see it gets hot.

[decoder_82]

So I tried to remove TS467 and turn on the monitor. All the voltages are good without this component. I will order a replacement and see if it resolves the issue. 

[decoder_82]

Actually I had a replacement for TS467 from a previous repair.
After replacing TS467 with a new transistor, the same issue happened, it gets hot and all voltages drop.
Does it indicate there is an issue with the 125v line ?

[inse]

A faulty HOT is only one of many possible defects.
Have you checked the base drive signal?
The HV transformer also creates an additional supply voltage, -20V which may be overloaded or shorted.
The resonance or snubber circuit might also be at fault.
Check C467, 468, 470 for cracks.
Usually the HOT does not die by itself, but this cannot be excluded.
Finally, the HV transformer may have a winding short…

[inse]

Is this the same monitor you had fixed in December?

[decoder_82]

Is this the same monitor you had fixed in December?

No it is a different one (not exactly the same model so I cannot swap parts).

So far I have tested many resistors, diodes and capacitors that were close to the 125v line. I did not find anything bad.
At the flyback not sure on wich pin there should be -20v but I did not find it anywhere.
Does it mean the flyback is faulty ? I don't have a flyback ring tester.

[inse]

OK, lets try to track down the culprit.
I have to admit I'm out of the TV business for a very long time...
First, lets summarize:
The power supply can deliver 125V to a decent load (25 to 40W bulb)
Once the flyback is connected, the voltage breaks down and the HOT does as its name.
If you are drawing excessive current, R469 should get hot as well.
D484 supplies the -20V but I have not figured out for what...
Have you checked D467, D468, C471?
The main problem with FBTs used to be isolation faults as far as I remember.
The base drive signal for the HOT looks as depicted in the manual?

[decoder_82]

Indeed the power resistor R469 gets hot as well.
Voltage at D484 is around -6v (instead of 20v)
D467, D468 and C471 have been checked and are good.
When I put the scope on the base of the HOT I get the expected shape as depicted in manual (waveform 18).
Is there a way to see if the FBT has a shorted winding ?

[inse]

Unfortunately, I dont have a recipe for checking winding shorts.
In the workshop, suspicious FBTs were simply swapped, no in-depth fault analysis.
No obvious damage on the FBT, like pimples or cracks?
The integrated HV diodes may have failed as well, you could do a resistance check on the HV outputs.
Make sure everything is discharged to avoid unpleasant surprizes.
I suspect, those spare parts are not so available any more?

[decoder_82]

There is no obvious sign of damage visible externally at least.
I will try to replace FBT and HOT and see how it goes.
The good news is that those FBT are still being sold. In this case a HR7506 

[inse]

HR Diemen - I didn‘t know they still exist :-)
Crossing fingers for successful repair

[decoder_82]

Yes I will post updates here once I will have the part.

[inse]

Did you also check the circuit around TS474/478?
Have you measured oscillograms 15..18, too?

[decoder_82]

Yes, I have controlled components around TS474/478. They look good.
Voltages at TS474/478 are below what is in the service manual (as almost like everywhere in the circuit)
I will check again oscillograms 15 and 18 to be sure.

[inse]

Just asking whether you would see any signal irregularities in case there was a winding short.
But I have no clue about it, I have to admit...
How much is the spare FBT?

[decoder_82]

Good question indeed. So far waveforms look as expected. Amplitude seems a bit off in some areas (smaller than expected)
Anyway once I will have the new flyback I will measure resistance on old and new one and confirm that there is a short. From what I read this could work (https://www.repairfaq.org/sam/tvfaq.htm#tvtofly).
Replacement is not too expensive (around 50euros)

[decoder_82]

Replacing the flyback transformer resolved the issue.
With the multimeter I was able to confirm that there was a short in the flyback.
Thanks for the help !

[inse]

Thumbs up to the successful repair!
I’m glad the FBT has been a good investment.