Phillips CM8833 Troubles

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Kazzie
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Phillips CM8833 Troubles

Post by Kazzie » Sun Sep 16, 2018 2:48 pm

My monitor just gave up on me.

It was connected to my A420/1, displaying a static screen, when the picture spontaneously disappeared, the power LED extinguished, and the normally very-very high-pitched whine plummeted significantly. By the time I got my phones spectrum analyzer on it, it was ringing around 2.5kHz, and is dropping steadily over time (whether remaining switched on or left off for a while). The electron gun doesn't appear to be firing anything at the screen. No funny smells have escaped, and the box is warm, but not unusually so.

I'm not going to have the time to open her up before next weekend, but does anyone have any pointers on where I need to look inside? (While I've discharged tubes and worked inside CRTs before, it's always been on the surrounding gubbins, as opposed to the HV stuff itself.
Last edited by Kazzie on Sun Sep 16, 2018 2:49 pm, edited 2 times in total.
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AndyF
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Re: Phillips CM8833 Troubles

Post by AndyF » Sun Sep 16, 2018 3:49 pm

Without specific knowledge of this monitor although I've seen them in years past (and they are a decent thing usually with Stereo out in some cases, at least the ones I saw commanded relatively high prices in good nick) , the biggest clue I saw in your post was the fact the power light / neon / led (whatever it is) went out.

To my initial thoughts of mind, this points towards a problem with the initial power supply circuitry hopefully, possibly even the LV (12V? maybe) output for the audio/power led etc/ , I'm mainly thinking here of a open circuit rectifier or cap perhaps. I guess all I'm saying is with the description I'd expect to find the 'concern' near the mains input area not further down or near the HV area**

** HV as in "more than mains aka tripler area
Last edited by AndyF on Sun Sep 16, 2018 3:50 pm, edited 1 time in total.
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Re: Phillips CM8833 Troubles

Post by Kazzie » Sun Sep 16, 2018 4:10 pm

AndyF wrote:
Sun Sep 16, 2018 3:49 pm
Without specific knowledge of this monitor although I've seen them in years past (and they are a decent thing usually with Stereo out in some cases, at least the ones I saw commanded relatively high prices in good nick) , the biggest clue I saw in your post was the fact the power light / neon / led (whatever it is) went out.

To my initial thoughts of mind, this points towards a problem with the initial power supply circuitry hopefully, possibly even the LV (12V? maybe) output for the audio/power led etc/ , I'm mainly thinking here of a open circuit rectifier or cap perhaps. I guess all I'm saying is with the description I'd expect to find the 'concern' near the mains input area not further down or near the HV area**

** HV as in "more than mains aka tripler area
That matches my suspicions, having had a quick general read on the interweb about CRT issues. (This one is a model with Stereo output, and a hand-me-down from my old high school.)

There's a service manual available online, which contains a circuit diagram, component list, and instructions for calibrations after repair (but the technician is expected to be able to repair it from the diagram).

I'll gladly accept any further guidance or pearls of wisdom from those who opened up CRTs back in the days when looking at an LCD display meant you were using a calculator.
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AndyF
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Re: Phillips CM8833 Troubles

Post by AndyF » Sun Sep 16, 2018 4:45 pm

Someone else will be able to advise on that point I expect. I looked at the manual and thankfully the power supply diagram is separate from the rest, the bottom part of Page-16 , if it was me that is where I would start, being aware that mains voltage or higher is exposed , usual safety precautions apply here, main one being if in doubt = don't! , I know you probably know that but I have to type it for my peace of mind sorry. :oops:

:)

One thing that might help others to help you , what do you have in terms of testing kit ? I'm assuming a multimeter, do you have a 'scope or a logic probe / anything else ?
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1024MAK
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Re: Phillips CM8833 Troubles

Post by 1024MAK » Sun Sep 16, 2018 5:05 pm

As with anything else, after a good visual inspection, the next step is to work out (by testing) what power supply rails are okay and which ones are missing or low. No need to worry about testing the EHT though.

If the tube heater is not glowing, that normally means it has lost it’s supply.

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Re: Phillips CM8833 Troubles

Post by Kazzie » Sun Sep 16, 2018 6:34 pm

AndyF wrote:
Sun Sep 16, 2018 4:45 pm
Someone else will be able to advise on that point I expect. I looked at the manual and thankfully the power supply diagram is separate from the rest, the bottom part of Page-16 , if it was me that is where I would start, being aware that mains voltage or higher is exposed , usual safety precautions apply here, main one being if in doubt = don't! , I know you probably know that but I have to type it for my peace of mind sorry. :oops:

:)

One thing that might help others to help you , what do you have in terms of testing kit ? I'm assuming a multimeter, do you have a 'scope or a logic probe / anything else ?
Multimeter, check. My el-cheapo oscilloscope died a few weeks ago, but I can cart the monitor in to work and use a 'scope there (as long as I don't tell the H&S people what I'm repairing).
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)
Acorn System 1 home-made replica

Kazzie
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Re: Phillips CM8833 Troubles

Post by Kazzie » Sun Sep 16, 2018 6:50 pm

1024MAK wrote:
Sun Sep 16, 2018 5:05 pm
As with anything else, after a good visual inspection, the next step is to work out (by testing) what power supply rails are okay and which ones are missing or low. No need to worry about testing the EHT though.

If the tube heater is not glowing, that normally means it has lost it’s supply.

Mark
I've just plugged the monitor back in for two minutes, and while it's still buzzing at me, I can't detect any heat convecting out of it.

It seems that this failure may not be as fatal as I initially feared. Thank you for your pointers and reassurance.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM (mid- restoration)
Acorn System 1 home-made replica

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