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Old 17th Nov 2020, 12:29 pm   #1
tjohnson
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Default Faulty CRT Computer Monitor

Hi All,

Hoping to get some advice.

I have an old Amstrad CM14 CRT computer monitor from about 1990.

I've had it in storage for about 2 years. When I pulled it out of storage the picture was distorting as shown in the picture below. Then I fiddled with the brightness and the picture competely disappeared and I was left with a high pitch whine coming from the monitor

I was then sent a replacement PCB without flyback which I proceeded to swap the flyback from the original PCB. Upon powering up the monitor there was a feint popping sound and there was a small flash from somewhere. The replacement PCB was of unknown condition and I didn't do any checks on it before installing but was told removed from working monitor many years ago.

I have limited experience of Tv CRT style monitors and hoping for some fault diagnosing advice.

I've found that ICP501 (ICP-N20) seems to be blown on both PCBs (open on multimeter), this seems to be some sort of current limiting fuse that utimately goes into the FB via the horizontal drive transformer and HOT

I've also found there seems to be a short on both boards between the base-emitter on the HOT Q405 (2SD1877). which I don't think is right, so I guess if this has blown on both boards it would pull large current through and blow this current limiting device. This doesn't seem to have any physcial damage and is a large transistor, 2sd1877.

Anyone have tips on testing the flyback and general fault diagnosis for this monitor? My main concern is that the FB is faulty and this is causing these components to blow.
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Old 17th Nov 2020, 1:24 pm   #2
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Default Re: Faulty CRT Computer Monitor

The photo looks like dried up capacitors in the power supply.
The short is the secondary winding of the driver transformer and is normal.
The decoupling capacitor can dry out on the power rail powering the primary of the driver causing the transistor to overheat.
The power supply often trips if the heater capacitor dries out as it is monitored for regulation.
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Old 17th Nov 2020, 2:37 pm   #3
Maarten
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Default Re: Faulty CRT Computer Monitor

The R2M is a crowbar diode so it will make the fuse blow if the voltage goes out of regulation too far.
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Old 17th Nov 2020, 3:25 pm   #4
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Default Re: Faulty CRT Computer Monitor

Thanks for the responses so far. Sorry for my ignorance but with does "R2M" mean?
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Old 17th Nov 2020, 4:06 pm   #5
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Default Re: Faulty CRT Computer Monitor

Quote:
Originally Posted by tjohnson View Post
Thanks for the responses so far. Sorry for my ignorance but with does "R2M" mean?
The zener D510 in the first schematic you posted
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Old 17th Nov 2020, 4:55 pm   #6
tjohnson
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Default Re: Faulty CRT Computer Monitor

Got it, didn't see that. I'll check the capacitors in the power supply section.
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Old 17th Nov 2020, 4:58 pm   #7
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Default Re: Faulty CRT Computer Monitor

I think the problem will be green spot corrosion on the line scan coils where the wedges are. Very common problem in small screen TV's in the 90's and I saw it on some cheaper monitors as well. The Horizontal Output Transistor usually fails if the monitor is run in this state.
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Old 17th Nov 2020, 6:35 pm   #8
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Default Re: Faulty CRT Computer Monitor

As can be seen in the first picture convergence is badly affected and I agree with Clydeuk that the rubber wedges in the scan coils are the cause. They are prone to absorbing moisture and become conductive, the resulting arcing often badly damaging the coils

John.
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Old 17th Nov 2020, 7:39 pm   #9
tjohnson
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Default Re: Faulty CRT Computer Monitor

Well I've had a look tonight and the spare PCB has a crack in it so that will never work!

I took some pictures of the tube neck, looks good to me. I suppose I could start popping out some caps and testing. I've used an ESR meter on some of the elctrolytics and they all seem to be ok.
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Old 17th Nov 2020, 10:00 pm   #10
Clydeuk
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Default Re: Faulty CRT Computer Monitor

Just to clarify, outwardly you will not be able to see the damage to the line scan coils as these are on the inside of the yoke assembly close to the CRT flare. You can see the 3 wedges though around the edge of the yoke. In the past I have found it possible to remove the wedges without disturbing the yoke where you will see where the arcing and decomposition has occurred. This is really only a quick check because the coils will be burnt where the wedge was.

To properly inspect the coils requires removing the yoke and then purity and convergence adjustment on refitting.

It does look like damaged coils judging by the poor convergence and trapezium shape of the raster.

If the coils have been damaged and you fit a new Horizontal Output Transistor you will get a repeat performance.
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Old 17th Nov 2020, 11:00 pm   #11
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Default Re: Faulty CRT Computer Monitor

Another vote for the scan coils. Seen that before!
My tip: Don't waste any more time and money on other components until you have taken the advice given and checked the coils by the rubber.
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Old 17th Nov 2020, 11:15 pm   #12
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Default Re: Faulty CRT Computer Monitor

Hi.

Regarding the scan coil problem, and like Clydeuk's experience, I used to encounter a similar situation in various 14" colour portable TVs such as Matsui and Amstrad (Orion type chassis). The issue related to the adhesive securing the rubber wedges to the scan coil. It was a type of impact adhesive similar looking to Evo-Stik, which becomes hygroscopic (absorbs moisture).

It tends to go from a honey yellow colour to dark brown and I think it becomes slightly acidic. It resulted in corrosion and shorted turns in the scan coils.
This same type of adhesive was also used to secure components to the main PCB and could cause a number of baffling faults if it becomes conductive. I had a nasty tuning drift fault in a VCR on one occasion caused by that glue. It took some time to pin down. Thereafter, I always eyed any components secured with that adhesive as a possible trouble spot.

Regards,
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Old 18th Nov 2020, 1:28 am   #13
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Default Re: Faulty CRT Computer Monitor

That glue is awful stuff.
The camera angle is wrong for me to see if there is any on those scan coils.
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Old 18th Nov 2020, 1:43 am   #14
tjohnson
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Default Re: Faulty CRT Computer Monitor

Oh yeah this is an Amstrad Orion unit and I can see the glue, it's a short of brown colour. Unfortunately it was stored in a garage so possibly the ambient temp and general dampness has caused this. Unfortunately sounds like this might be scrap!

Any tips taking the coils off? There are a couple of clamps near the magnets which I've losened but nothing is coming off easily and I don't want to force anything.
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Old 18th Nov 2020, 3:15 am   #15
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Default Re: Faulty CRT Computer Monitor

The way I used to do it was loosen the clamps fully, try and note the position of the magnet assembly then remove it with a twist. There is quite often some cloth-like tape around the neck where the clamp is to prevent the yoke from slipping once setup and tightened. I removed the rubber wedges first because the assembly can be stuck to these as well as the neck. Then a twist of the yoke usually frees it up. If it seems to be stuck tight you could try some heat from a hairdryer around the clamp area.

The scan coils were never available as a service item and were never intended to be removed, instead the CRT and coils were supposed to be replaced as an assembly.

I will admit it can seem daunting. I never took the neck off a tube trying to remove a yoke, but it's obviously possible, and when I used to do this for a living I had a lot of spare CRT's (usually from sets that were scrapped for this exact problem).

I would remove the rubber wedges before going further. You should be able to see where it's been arcing on the offending wedge once removed.
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Old 18th Nov 2020, 6:19 am   #16
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Default Re: Faulty CRT Computer Monitor

I have heard of a repair being done where the coils were removed and the green deposit carefully picked off and any bits of wire soldered back together with lots of resistance checks if there was more than one break and then plenty of epoxy.
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Old 18th Nov 2020, 9:35 pm   #17
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Default Re: Faulty CRT Computer Monitor

Quote:
Originally Posted by Refugee View Post
I have heard of a repair being done where the coils were removed and the green deposit carefully picked off and any bits of wire soldered back together with lots of resistance checks if there was more than one break and then plenty of epoxy.
Hi.

I was going to mention that earlier. I did successfully repair one scan coil that had only minimal damage/corrosion so was lucky on that occasion. ISTR carefully separating the shorted windings, soldering in a link wire and applying a small amount of Araldite to insulate and protect the join. It then needed carefully setting up to obtain the best convergence, particularly near the corners of the display.

When removing the scan coil, I first discharged the CRT's final anode to the aquadag braided earth strap and NOT the chassis. I used to do this by using a well insulated lead with a crocodile clip on one end and a low profile meter probe on the other. It is first clipped to the aquadag strap and then the insulated probe is carefully pushed under the EHT cap making sure your hand is well back on the insulated part of the probe. A crack is usually heard as it discharges. Repeat this a few times and leave in place. Then remove the CRT base board. Note the position of the multipole unit. This has a clamp held by a single screw. Fully loosen that and gently twist the multipole unit to free it.
Note the position of all rubber wedges and carefully remove the cloth tape securing the rubber wedges to the cone. The scan coil also has single screw securing its clamp. Again fully loosen this and carefully twist to free the coil.

When refitting I found the best way to bond the wedges to the scan coil was to use the right type of silicone rubber sealant, the sort that is neutral curing. Many silicone sealants contain acetic acid and these should be avoided at all costs. The wedges are normally held on the cone of the CRT by a cloth tape. The convergence will need to be set up before finalising the positions of the wedges.

Regards,
Symon.

Last edited by Philips210; 18th Nov 2020 at 9:49 pm.
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Old 18th Nov 2020, 10:07 pm   #18
tjohnson
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Default Re: Faulty CRT Computer Monitor

Spot on guys, I took this apart tonight, there was water on the wedges, they are visibly damp!

There is some burning of the coils, not sure what do do next tbh. I have never worked on a CRT, do these all terminate on the board circled in red?
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Old 18th Nov 2020, 10:36 pm   #19
Philips210
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Default Re: Faulty CRT Computer Monitor

Hi.

Glad you got the coil off OK. That burning doesn't look good I'm afraid. Cleaning the black deposit away with a nylon brush may reveal how far it's affected. It could be possible to cut the affected turns and soldering in links and insulate with epoxy resin but it is not a job to be taken lightly.

The terninals marked in your pics are the ends of the windings. The normal convention on many Japanese/Asian CRT scan coils is for the cableform to use Red and Blue wires for the line scan coils and Yellow and Green wires for the field scan coils. Note the Red wire is thicker as it is the high voltage side of the winding (similar potential to the line output transistor's collector). Sometime you'll find a RC series network connected across the line scan coil tags for damping purposes. If the burning is extensive then I don't think it is viable to attempt to rewind the line coils. They're a specially shaped winding to obtain the correct shape raster.

Regards,
Symon

Last edited by Philips210; 18th Nov 2020 at 10:49 pm.
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Old 18th Nov 2020, 11:20 pm   #20
tjohnson
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Default Re: Faulty CRT Computer Monitor

Thanks symon I think it's safe to say it's a lost cause, there's is atleast one broken wire and some have lost their covering. It's a shame really, I didn't know that monitors were prone to this issue otherwise I wouldn't have stored in my garage.
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