Philips CM8833 Monitor, Sound Card Repair

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hatchcliff
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Philips CM8833 Monitor, Sound Card Repair

Post by hatchcliff »

I doubt the details of this repair will be of general interest, because I think the sound card is one of the least likely things to fail on the excellent but ageing CM8833 monitor, but it strikes me as interesting story of near disaster and unexplained mystery (like most of my hardware projects). Here’s what happened:

While I was restoring my A3000s some years ago, the mains switch on the CM8833 failed. The mechanical latch broke, so that it would not stay on. I fitted an exact replacement, reassembled the case, switched on, noticed a smell of burning - and switched off again in a hurry.

I reopened the case looking for signs of burning, but couldn’t see any. I also checked for stray blobs of solder, abandoned tools, snagged wires and incorrectly inserted connectors – but still couldn’t see anything wrong.

When I tentatively switched back on there was no burning, and the monitor worked – except with no sound.

So it seemed I had neatly substituted one fault for another. I had a lovely new mains switch, but a broken sound system - a classic Hamlet cigar moment, or in my case Tetley tea.

The sound card looked quite inaccessible, so I left things as they were, but I recently bought some new test gear and plucked up enough courage to take another look.

Actually the card turned out to be surprisingly easy to remove with a solder sucker. The picture shows it wired up with flying leads for testing.
Sound Card Wired for Testing.
Sound Card Wired for Testing.
On closer inspection I noticed that R845, nestling between the two heat sinks, was slightly blackened. It should be 4.7 ohms, but I measured it at 190 ohms, so it was clearly damaged. Any excessive current passing through this resistor has to dissipate through the two amplifier chips, suggesting that they were probably damaged too.

I replaced all three components and everything worked fine once more. :D
Sound Card Reinstalled.
Sound Card Reinstalled.
I’m happy with the outcome, but I’m left wondering what I did wrong in the first place. I don’t believe this was a coincidental failure. The only thing I can think of is that I inserted in the speaker plug incorrectly, and later on I inadvertently covered my tracks by removing it and reseating it correctly. You can see this plug in the bottom left of the first picture. It’s a little unusual in that it is possible to insert it the wrong way round, but as far as I can make out this would just swap the left and right channels and wouldn’t do any damage. Another possibility is that I managed to displace it by one pin, perhaps because I was viewing it end on when I plugged it in. This would connect one speaker in reverse polarity and leave the other channel open circuit. Could that have caused the problem?

Best Regards

Cliff
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lurkio
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by lurkio »

Great repair job, Cliff! I can't offer any helpful info on the sound card, being essentially hopeless at electronics, but I was wondering if you could supply any more details or photos of the power-switch repair, as I have two monitors very similar to yours -- were they all made with the same parts or even at the same factory? -- and both my monitors have a broken power switch! Both switches are currently pressed in by metal angle-brackets wedged under the monitor base. Not ideal!

EDIT: Oh, wait. I made a mistake. I don't think my monitors are quite the same as yours. Mine are both similar to the one pictured here:

Image
Last edited by lurkio on Sun Feb 08, 2015 12:52 pm, edited 1 time in total.
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1024MAK
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by 1024MAK »

Semiconductor based amplifiers don't normally have a problem with an open circuit speaker circuit.

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Re: Philips CM8833 Monitor, Sound Card Repair

Post by hatchcliff »

lurkio wrote:I was wondering if you could supply any more details or photos of the power-switch repair
I didn't take any photographs, but the replacement switch I bought from CJE Micros is listed here:

On/Off Switch for Many Philips and Acorn Monitors.

I can vouch for the fact that it fits the CM8833 Mk1. The listing says it is also good for the Mk2 and the Commodore Amiga 1084S.
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by DutchAcorn »

So have you managed to repair the sound card? If not I may be able to help with some parts?
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hatchcliff
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by hatchcliff »

1024MAK wrote:Semiconductor based amplifiers don't normally have a problem with an open circuit speaker circuit.
Thank's for this. It seems to rule out my best guess - so maybe this is going to remain a mystery.

I will be extra careful next time I open the case, but I hope that will be a long time in the future!
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by DutchAcorn »

DutchAcorn wrote:So have you managed to repair the sound card? If not I may be able to help with some parts?
Sorry - on second read I see that you already replaced the parts..
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by hatchcliff »

DutchAcorn wrote:on second read I see that you already replaced the parts
Yes, I did manage to get the spares, but thank you for your offer of help - it's very much appreciated. :D
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by CanonMan »

cliffh wrote: I didn't take any photographs, but the replacement switch I bought from CJE Micros is listed here:

On/Off Switch for Many Philips and Acorn Monitors
£24 each? The robbing b*st*rds!

The last time I bought a few, I paid about £2.50 each for them on eBay. They crop up on there every now and then, you just have to keep looking!
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by RobC »

CanonMan wrote:The last time I bought a few, I paid about £2.50 each for them on eBay. They crop up on there every now and then, you just have to keep looking!
Yes - think I paid something similar when I bought my first one. I then paid about £6 each (including P&P) when I bought 3 from http://www.donberg.co.uk. (Part number was SW28.)

Fitting them is really easy as long as you know what you're doing around CRTs. (If you don't then it's probably best to ask someone who does to do the repair for you.)

There's a tray that comes out (think you may have to undo a screw first) and the switch is mounted on the tray. It's then simply a matter of desoldering the old switch from the power cables, soldering the cables to the new switch and making sure you mount the new switch the same way as the old one.
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hatchcliff
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by hatchcliff »

RobC wrote:Fitting them is really easy as long as you know what you're doing around CRTs. (If you don't then it's probably best to ask someone who does to do the repair for you.)
Good advice, and an essential point to make.

I intended to tell a light hearted story, but on a serious note, there's a safety issue here. Perhaps my experience will serve as a warning to others, and maybe that’s a good thing. I certainly wouldn’t wish to encourage anyone to open up a CRT monitor if they are not sure they can manage the risks. (The warning, of course, does not apply to the numerous members of this forum who do have the requisite skills for this type of work.)

I think I will never know exactly what went wrong with my project, but having ruled out the things discussed above I can only conclude that something snagged when I closed the case and made contact where it shouldn’t.

I have a fairly strong sense of self preservation and a healthy respect for high voltages. However, I can’t claim to be a hardware expert. I’m sure some would say this a good reason to stay away from CRTs – and I wouldn’t argue with that.
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by george.h »

cliffh wrote:I have a fairly strong sense of self preservation and a healthy respect for high voltages. However, I can’t claim to be a hardware expert. I’m sure some would say this a good reason to stay away from CRTs – and I wouldn’t argue with that.
My father (old school TV engineer from the days when TVs had valves and the "TV man" came and repaired your broken one right there in your living room) has a very accurate image of a CRT base PCB from some 26" colour TV he was repairing - on his bald patch on top of his head! He didn't pull the chassis far enough out while working on it, leaned too far in over it and BANG!.....

Luckily the only damage was his bald patch...... :shock:
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Re: Philips CM8833 Monitor, Sound Card Repair

Post by 1024MAK »

Moving a bit off topic here (and I'm not sure that I can beat George's story)...

In my past, I have either discovered, or been sharply reminded that the following are painful things:
  • Unintentionally juggling a hot soldering iron by the hot end after knocking it out of the stand...
  • Dropping a hot soldering iron and being daft enough to have my trainer in the way, it burnt through the fabric on top of the trainer and found my foot :shock:
  • Working on a 240V AC fan heater (wall mounted type) circuit in a building (yes, I switched off the 16A MCB) and then finding out the hard way that the thermostat that controlled the contractor relay was wired to the still live lighting circuit (that hurt).
  • I was working on a very cold day, on an electrically operated drive system that operates outside in all weathers. While grabbing part of the structure, I forgot that it uses high power heating strips (so ice and snow does not cause problems) and the end of my finger touched the very hot heating strip. Ouch that hurt (I still have a slight mark on the end of my finger).
:arrow: So think first, and double check, before acting / touching :wink:

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Re: Philips CM8833 Monitor, Sound Card Repair

Post by george.h »

1024MAK wrote:So think first, and double check, before acting / touching
If in doubt, get some other mu.... person to try first! :lol:
1024MAK wrote:and I'm not sure that I can beat George's story
Well I still have a very odd looking finger print on one finger tip, from the business end of some iron or other in the dim and distant past! Sides of the fingers appear to heal MUCH better!

I would also advise against touching anything built by "a guy with a PhD" which has solid state relays mounted on stripboard (dangling on bits of wire) controlling linear mains PSUs with the underside of the strip board "shielded" from the metal 19" rack case by a PostIt note! Except with a sledge hammer!

As for my dad's "excitemet" I did say that luckily the only damage was to his bald patch. Thankfully the TV was unharmed, just as well as it was a "guvvy" job - aka a paid for repair outside of work :lol:
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