Thanks a lot for your comments, Hugo. Indeed, yesterday I was not sure whether it would be all right to power on with the neck pcb board disconnected, so I didn't proceed, just to be safe, even though my guess was that the transformer was likely to work properly. Today I found from googling that for small CRTs it would not be too dangerous. And at the right moment, just when I returned home, you confirmed it clearly. Thanks! I will post again after I confirm that the transformer works well.
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Need help - repairing a 12-inch monochrome CRT monitor (Daewoo DM-120MWA)
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Thanks a lot for your comments, Hugo. Indeed, yesterday I was not sure whether it would be all right to power on with the neck pcb board disconnected, so I didn't proceed, just to be safe, even though my guess was that the transformer was likely to work properly. Today I found from googling that for small CRTs it would not be too dangerous. And at the right moment, just when I returned home, you confirmed it clearly. Thanks! I will post again after I confirm that the transformer works well.
I was about to have dinner first and then do the test, but changed the plan ^^;;;
Readings with the new transformer are as follows. (Everything is connected but the CRT board.)
HOT collector voltage:
HOT emitter current:
I am a bit curious about the "two peaks" in the collector voltage waveform. If I understood it correctly, the ideal waveform is half of the sine curve, which has one peak. The H drive signal at the base and the current reading look unstable at the end of the "half sine curve". Again, if I understood it correctly, this is when the damper diode turns on ... so I wonder it is acceptable or it indicates some problem of a particular part. Or if it is because of the lack of my measuring skill.
The current drain at R618 is 0.55A. I think I can now safely replace it with a new 0.47 ohm.
Readings with the new transformer are as follows. (Everything is connected but the CRT board.)
HOT collector voltage:
HOT emitter current:
I am a bit curious about the "two peaks" in the collector voltage waveform. If I understood it correctly, the ideal waveform is half of the sine curve, which has one peak. The H drive signal at the base and the current reading look unstable at the end of the "half sine curve". Again, if I understood it correctly, this is when the damper diode turns on ... so I wonder it is acceptable or it indicates some problem of a particular part. Or if it is because of the lack of my measuring skill.
The current drain at R618 is 0.55A. I think I can now safely replace it with a new 0.47 ohm.
I've just found a crack in the CRT, unfortunately. It is at the very end of the neck, where the pins are. (So the crack is hidden when the neck PCB socket is connected to the tube.) Perhaps I should have checked this earlier ...
I really, really wonder whether one can get a CRT in 2023. The label of my CRT is not readable at all similarly to the original flyback, but I found the model number from https://oldcrap.org/2020/05/17/daewoo-dm-120m-green-monitor/ :
I really, really wonder whether one can get a CRT in 2023. The label of my CRT is not readable at all similarly to the original flyback, but I found the model number from https://oldcrap.org/2020/05/17/daewoo-dm-120m-green-monitor/ :
Last edited:
andy
Experienced Member
Monochrome CRTs are pretty interchangeable. I wouldn't worry too much about finding the exact same type number.
Hugo Holden
Veteran Member
Good news, the transformer is working normally, Notice how the collector current doesn't start to rise until about 1/2 way into the scan, unlike the case with the transformer with the shorted turns, where it occurred as soon as the transistor received Base drive. This is a convenient method to check for shorted turns in a transformer without having to do a ringing test.
The dip in the collector waveform is perfectly normal and an excellent example of it, it relates to what is known as 3rd Harmonic tuning, I explained in that PET VDU restoration article on about page 31:
With air in the tube, the final anode connection will have been arcing across the gun to the lower voltage electrodes there. Possibly, this is what damaged the original transformer if it was left turned on like that for some time, the transformer would have overheated and developed the damaged insulation and shorted turns. The video output transistor might have got a hammering from this arcing, but that is easily replaced if that is the case.
All you need now is a replacement CRT. I will have a look around.
The dip in the collector waveform is perfectly normal and an excellent example of it, it relates to what is known as 3rd Harmonic tuning, I explained in that PET VDU restoration article on about page 31:
With air in the tube, the final anode connection will have been arcing across the gun to the lower voltage electrodes there. Possibly, this is what damaged the original transformer if it was left turned on like that for some time, the transformer would have overheated and developed the damaged insulation and shorted turns. The video output transistor might have got a hammering from this arcing, but that is easily replaced if that is the case.
All you need now is a replacement CRT. I will have a look around.
Hugo Holden
Veteran Member
One place to source donor monochrome 9 " and 12" CRT's is from the video monitors used in the security industry. Many of these used Orion CRT's. But the guns are nearly always the same or very similar and can be adapted to work.
A typical 12" example would be something like this:
The only thing is you would have to settle for a White Phosphor (P4), rather than the Green(P31).
A typical 12" example would be something like this:
Samsung Ssc-12m B/w Observation Monitor 38w for sale online | eBay
Find many great new & used options and get the best deals for Samsung Ssc-12m B/w Observation Monitor 38w at the best online prices at eBay! Free shipping for many products!
www.ebay.com
The only thing is you would have to settle for a White Phosphor (P4), rather than the Green(P31).
I didn't know that. Thanks a lot!
Hugo Holden
Veteran Member
There is a thread somewhere, where I drew up the gun diagrams of these sorts of CRT's and how to identify the pin connections looking through the glass, so it can be checked before the substitute tube is tried.
Generally; there are two heater connections H,H (most tubes in this range have 12V heaters), the cathode k, the g1 grid, first anode A1, then the focus electrode F which is a cylinder , however on either side of this cylinder is the final anode A2, connected to the internal aquadag and EHT connection on the tube bulb. Often on the schematics the final anode's relationship to the focus electrode is not drawn in.
The main thing is to try to find a tube with the same mounting ears/configuration, faceplate perimeter geometry & curvature, neck and deflection angle, to make it a good fit. Most 5,9, and 12" types of a similar vintage are compatible. Later they went to more rectangular, flatter faces with higher deflection angles, these are more problematic to fit.
Hugo Holden
Veteran Member
Well, are you in luck.
I just got an email back from newtubes on ebay, and they have just one of your CRT's in stock, in green too.
Here is a typical newtubes auction for another type of crt:
If you go to that auction and contact the seller, they will be able to sell you your tube on a buy it now.
I just got an email back from newtubes on ebay, and they have just one of your CRT's in stock, in green too.
Here is a typical newtubes auction for another type of crt:
New CRT for Wavetek Stabilock 4031 4032 Svc Monitor | eBay
Find many great new & used options and get the best deals for New CRT for Wavetek Stabilock 4031 4032 Svc Monitor at the best online prices at eBay! Free shipping for many products!
www.ebay.com
If you go to that auction and contact the seller, they will be able to sell you your tube on a buy it now.
Thanks for your efforts! I didn't expect this, honestly. I will contact the seller. I am somewhat worried about the price (other CRTs they sell seem more expensive than buying this VDU which is working). But we will see if it's within my budget...