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Help diagnose CM8833-II monitor

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nztdm

Experienced Member
Joined
Mar 17, 2018
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New Zealand
Hello

I am repairing a friend's Philips CM8833-II (PAL). I believe this is the same as the Commodore 1084S-P1; it is not the 1084S-P.
It has a somewhat intermittent fault where it won't turn on.
It worked in the past, and yesterday when they brought it to me.
Today, it won't work again. It's not heat related.

When it works it looks perfect and draws about 50W.
When it doesn't work, the power LED (powered by the 12V-C rail) will flash momentarily then the monitor will emit an odd high pitch sound and only draw 5W. When an power is removed, the sound changes over a few seconds, then the power LED will flash once more at the end.

Here's a video. Turn your volume right up.
https://photos.app.goo.gl/3AocYpdrZTEN4riG7

I can't see bad solder joints and flexing the board doesn't affect it.

If someone knows of this fault, that would save me a lot of time shotgun replacing parts.
Yoke measures fine and I can't see corrosion.
Ideally it's just bad caps.

Cheers
 
Probably the best initial move (rather than shotgun replacing) is to check the Electrolytic caps in the power supply section with the ESR meter and replace any obviously suspect/defective ones and re-test.
 
Hugo Holden said:
Probably the best initial move (rather than shotgun replacing) is to check the Electrolytic caps in the power supply section with the ESR meter and replace any obviously suspect/defective ones and re-test.
Click to expand...

Thanks once again.

I will do that soon. I think I have every replacement anyway. This monitor is much harder to service than the previous version though.

On a side note, I have a yoke on its way from a Samsung tube from that Canadian ebay seller. It has the same resistance and inductance and will fit the Philips tube in my 1084S-P. Here's hoping I can get it aligned well!
 
nztdm said:
On a side note, I have a yoke on its way from a Samsung tube from that Canadian ebay seller. It has the same resistance and inductance and will fit the Philips tube in my 1084S-P. Here's hoping I can get it aligned well!
Click to expand...

It is good we finally found that fault.

You should also save that video you took or post it on you tube under the title of Shorting Horizontal Deflection Yoke Coil in CRT Video Monitor (or similar) , it is not that often (or at all) that the effect of a horizontal deflection yoke coil shorting out, is captured on video at the moment it actually did it, making that a very special video.
 
Hugo Holden said:
It is good we finally found that fault.

You should also save that video you took or post it on you tube under the title of Shorting Horizontal Deflection Yoke Coil in CRT Video Monitor (or similar) , it is not that often (or at all) that the effect of a horizontal deflection yoke coil shorting out, is captured on video at the moment it actually did it, making that a very special video.
Click to expand...

Just uploaded that video: https://www.youtube.com/watch?v=GkQ2A5ad-eI

And also, I have just recapped the CM8833-II, and it now works again. I have it running and will see if it lasts.
I tested it just before I recapped, and it didn't work.
I tested the large caps in a component tester and didn't notice anything odd, but it is a cheap junky component tester. All ESR were milliohms for those large caps.
 
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nztdm said:
Just uploaded that video: https://www.youtube.com/watch?v=GkQ2A5ad-eI

And also, I have just recapped the CM8833-II, and it now works again. I have it running and will see if it lasts.
I tested it just before I recapped, and it didn't work.
I tested the large caps in a component tester and didn't notice anything odd, but it is a cheap junky component tester. All ESR were milliohms for those large caps.
Click to expand...

It is interesting how often all of the physically large caps are ok.

Probably, if there is a certain amount of electrolyte or water vapor loss via the seals over decades, if that was some fairly constant rate, the smaller caps would dry out first. In addition the larger capacitor is a better thermal radiator, because as a physical object they would have, like a larger heatsink, a lower degree C per watt temperature increase than a small sized one. Also with lower range ESR's will dissipate less heat for any pulse current.

Recently I repaired a Large LG flat screen TV that was 15 years old. 25 surface mount 47uF 16v caps on the signal processor board were very high ESR, but all the physically larger caps were ok. I opened some of the faulty caps up for inspection and they were as dry as parchment paper in there. But they had not visibly leaked (as they often do). Dipping the de-cased capacitor in deionized water immediately restored their normal capacity and ESR. So I was thinking, if you were stuck in a space capsule, or with no spares, these caps could have been temporarily fixed, by drilling a tiny hole in the top and injecting a drop of deionized water with a needle & syringe.
 
Hugo Holden said:
It is interesting how often all of the physically large caps are ok.

Probably, if there is a certain amount of electrolyte or water vapor loss via the seals over decades, if that was some fairly constant rate, the smaller caps would dry out first. In addition the larger capacitor is a better thermal radiator, because as a physical object they would have, like a larger heatsink, a lower degree C per watt temperature increase than a small sized one. Also with lower range ESR's will dissipate less heat for any pulse current.

Recently I repaired a Large LG flat screen TV that was 15 years old. 25 surface mount 47uF 16v caps on the signal processor board were very high ESR, but all the physically larger caps were ok. I opened some of the faulty caps up for inspection and they were as dry as parchment paper in there. But they had not visibly leaked (as they often do). Dipping the de-cased capacitor in deionized water immediately restored their normal capacity and ESR. So I was thinking, if you were stuck in a space capsule, or with no spares, these caps could have been temporarily fixed, by drilling a tiny hole in the top and injecting a drop of deionized water with a needle & syringe.
Click to expand...

And you can notice that most small caps such as 10uF 50V, can only be had in ~2000hrs 105C, whereas almost every large value (especially low voltage, high capacitance), can be had in 10000hrs 105C. The large high voltage caps such as >100uF 400V+ are often like the small ones, and top out at 5000hrs.
 
I'm the "lucky" owner of the cursed CM8833-II. Many thanks to both of you for the troubleshooting help so far.

It died again this morning while I was using it. The by-now-familiar symptoms: immediate loss of display and power LED (the screen flashed bright white momentarily, in the exact same way it does when turning off) accompanied by the whining noise. Since I was sitting in front of the screen at the time, I immediately powered it off. After letting the unit rest for a couple of minutes, it powered on and the display is working fine again.

Question: if the X-Ray protection/shutdown circuit activates (due to either excess HV in the flyback or due to a faulty shutdown circuit) could this be causing the "whining" noise?

I'm speculating that the PSU could be rapidly oscillating, as first the X-Ray shutdown input activates and kills the PSU output, followed by the X-Ray shutdown input going inactive as the flyback voltage decays, and then quickly followed by the PSU starting up again, only to be killed yet again by the X-Ray circuit. Is this plausible?
 
It is possible. But it is unlikely the EHT voltage would suddenly shoot up, it can happen.

The peak level of the flyback pulse (and EHT) is determined by how much magnetic field energy is stored in the lopt & yoke, at the end of scan, this is determined by how long the HOT is turned on for per cycle (does the width appear to increase just prior to the shutdown ?), and that stored magnetic field being transferred to the tuning capacitor's electric field. This is the capacitor/s on the HOT's collector circuit, the voltage peaking half way through H flyback. If that tuning capacitor's value drops, the EHT increases , so there could be a dry join or defective capacitor connection or defective capacitor, but its less common they go intermittent, but its not impossible.

I guess you could monitor the output from the Xray protection circuit and see if that is the thing that trips off the shutdown. Or say, when it was in that state if the Xray protection circuit was in fact producing an output keeping the supply in an oscillating shut down mode.

It would pay to examine the schematic and see for the particular psu, where any shutdown signals enter and where they come from. Some supplies have over-current detectors, some over voltage detectors and other supplies nothing.
 
The links to your reply from my email or the site's notifications says I don't have permission. I had to go to the thread manually. Some bugs with the new site I guess.

Here's the power supply schematic: https://imgur.com/6sQWMUm
And the X-ray protection circuit: https://imgur.com/3ThcZ21

Hmm. Both times I got it working, I had touched up the HOT joints.
The first time I touched up the joint was when no caps were replaced. I noticed the middle pin had hardly any solder, but I didn't touch it up perfectly as it wasn't fully taken apart so it was hard to see. Then the monitor worked for a bit.
The second time was when I did the recap 2 days ago. I touched it up properly since I could see better.
Maybe it's going short, but some heat is fixing it temporarily. Just one idea to add. I have replacements to try (as well as for the PSU switching transistor).
 
Hugo Holden said:
(does the width appear to increase just prior to the shutdown ?)
Click to expand...

No, the picture went from "absolutely fine" to gone in just an instant.

I keep thinking about the really weird symptom: when the display goes into its dead state with the whine, once power is removed the power LED will briefly flash on and the whining stops and the screen sounds like it's just about to turn on normally....
 
kdr said:
No, the picture went from "absolutely fine" to gone in just an instant.

I keep thinking about the really weird symptom: when the display goes into its dead state with the whine, once power is removed the power LED will briefly flash on and the whining stops and the screen sounds like it's just about to turn on normally....
Click to expand...

For further clarification, this is shown in the video on the first post (must have volume turned up).
 
Okay I have it back in my hands now.
No effect replacing HOT.

There is 0.72V across SCR Gate and Cathode. Is that enough to trigger it?
There is 0.78V across SCR Anode and Cathode. Looks like a diode drop - maybe the SCR is being triggered.

Refer to video in first post. The high pitch squeak and power LED blink at the end is the PSU coming up. The voltage shoots up and I can see over 70V on the DMM for a moment. This could point to the SCR also.

Look at transistor 7470 on the PSU schematic. It says there should be 0V at the collector, but there is 0.72V as measured before (the collector connects to SCR Gate).

Someone's quote online:
"Talking of SCR crowbar circuits. I recently had to repair my Philips CM8833-II monitor as it wasn't powering up fully and emitting a loud whining noise. People were telling me it would be the flyback, but I didn't want to take their word for it and order one without checking things first as they're quite expensive.

I found a BT151 500v SCR and it's associated 15V zener diode both shorted. Also, the TDA3505 chip was shorted between it's red input and ground. I replaced the 3 components and the monitor is working fine now. But the original zener diode was a 1% accuracy type, and I could not source a 1% accuracy one so used a 5% one. I'm wondering if the crowbar circuit could be overly sensitive now? Do you think I should keep trying to source a 1% accurate diode or just leave it be? "

Edit: Just tested the SCR and two 15V zener diodes and they're fine (unless SCR goes short with a large potential across it).
 
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nztdm said:
Okay I have it back in my hands now.
No effect replacing HOT.

There is 0.72V across SCR Gate and Cathode. Is that enough to trigger it?
There is 0.78V across SCR Anode and Cathode. Looks like a diode drop - maybe the SCR is being triggered.

.
Click to expand...

It would seem like the SCR has been triggered. You could disconnect the SCR Anode, and try the monitor and see if it runs normally, at least briefly and check the supply output voltage while it is running. It is unlikely this would damage anything for a quick test. If the monitor worked, you could then test the SCR and zener. (If the monitor's H scan width is correct, the HT must be correct, the EHT will also be correct, that is if the tuning capacitors on the collector of the HOT are normal)
 
Hugo Holden said:
It would seem like the SCR has been triggered. You could disconnect the SCR Anode, and try the monitor and see if it runs normally, at least briefly and check the supply output voltage while it is running. It is unlikely this would damage anything for a quick test. If the monitor worked, you could then test the SCR and zener. (If the monitor's H scan width is correct, the HT must be correct, the EHT will also be correct, that is if the tuning capacitors on the collector of the HOT are normal)
Click to expand...

I powered it for just under a second. There was a quiet pop and my multimeter watching the 128V rail needed rebooting. I believe the SCR has been tripping for a reason, hmmmm
I don't notice any blown parts.
 
Hmm. I replaced the flyback with no effect. I wonder what it is!

So, we know the PSU starts when HOT is replaced with a 60W 240V bulb, so the trouble is around flyback area or a part derived from such.

EDIT:
Just shotgun replaced all ICs except the audio ones, as I had spares and am out of ideas, and was also worried turning it on without SCR that time could have damaged them. No change.
The three logic ICs tested good in chip tester.
Also changed 7812 regulator, PSU switching transistor, and optocoupler.
So it must be a small transistor, a diode, or a ceramic/film cap (or unlikely, a resistor).
Retrace timing cap tests as 470pF, but can't test under its rated voltage. Will add replacement to cart just in case.

EDIT 2:
Fixed for now. Likely was the LOPT after all, but when I tested without SCR (with old LOPT installed), it killed my new HOT without me knowing. Replaced the HOT again and it fires up.
Will put back together properly and leave it running for a bit.
 
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