VOGONS


First post, by Skip94

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Hi all
I'm hoping to get some pointers from someone who has experience in repairing CRT monitors, as information online seems to be very lacking.
I decided starting a new thread was probably best for this, as it is very much a tangent from the thread I posted the other day on the same monitor. Gateway 2000/Tatung CM-1495 CRT
Basically, I would like to attempt to repair this Gateway 2000/Tatung CM1495 monitor.
I picked this up at the weekend and after making a 9 to 15 pin adapter, tested it, but no luck.
Symptoms are; power LED comes on, the cathode heaters start to glow, but there is no activity at all on the screen. The power switch seems to have no effect, power LED stays green all the time.
Now, I am well aware that repairing a CRT can be dangerous business, but I am comfortable with safely discharging the tube before working on it and I am also comfortable in taking voltage readings safely on live boards. I have a basic knowledge of electronics, the function of various components etc and I am happy desoldering bits and taking resistance readings etc. Currently the only test equipment I have is a digital multimeter, but I keep meaning to invest in a basic oscilloscope, so this may be the time.
What I lack is real world circuit diagnosis experience, so any pointers to recommended items for testing is greatly appreciated.
As for the monitor!
On the inside it is significantly more complex than I was expecting, with 4 main PCB's. There are 2 fuses on the power supply PCB, but both of these are fine.
I'm fairly certain that I'm not getting HV, as it doesn't seem to create any static on the front of the screen, which is something I seem to recall from the last time I used a CRT. I however do not have the equipment to test the HV at the anode.
There seems to be power getting from the power supply to the board with the flyback transformer on it, there are 2x 3 pin connectors joining the two and I am measuring 23V and 115v DC on one and 4.9v and 115v DC on the other. I have not yet found a repair manual though, so I don't know if these voltages are correct though.
There is no obvious damage on any of the boards, no burnt out components, no leaky/bulging caps and no cracked solder joints.
The only other advice I have found online involves the horizontal output transistor, which in circuit tests open circuit/high resistance between the collector and emitter one direction and 330 ohms in the other. From what I have read, this is roughly what I should be expecting.
As for where to go from here, I'm not sure. I would really like to get this working, so any advice would be greatly appreciated.
Thanks
Andrew

Reply 1 of 2, by Tiido

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When there's no HV it means the biggest transistor that is nearby on a heatsink is most probably dead and needs replacing with a similar kind one. It seems it isn't the case in this monitor though. Line deflection and HV generation are intimately tied together on most monitors, if this part doesn't work, nothing does so the problem is probably still related.

When that transistor is dead the "Why" needs to be figured out too. There are a number of components in the basic circuit, including some big fat capacitors which may be the cause of the death of the transistor or maybe were killed by it. It is told that when one replaces one part in that area, one should replace all of them. One of those capacitors have 1kV+ voltage rating, others should be 400....200V. You cannot really measure much stuff live in there because the flyback pulses in the line deflection+HV generation are usually exceeding what a normal multimeter can do. I actually killed a multimeter this way when I was younger and inexperienced 🤣.

If no fault is found in there, the problem is in the sync part instead. There probably is one simple chip that does all of the sync processing. An oscilloscope will be most helpful in this part of the things. Normally that chip listens for a flyback pulse from one winding on the HV transformer output and outputs line drive signal through a simple transistor amp and a transformer that then drives the actual line switching transistor. It could be the power supply to these preceding stages is absent or some other fault is related, but without an oscilloscope it is a pain to figure anything out.

There's always the possibility that everything actually works, but the problem is that the HV transformer itself is dead and no anode voltage is produced anymore. This would be the worst scenario, since you probably are not gonna find a directly compatible transformer and making some other work takes effort and requires some knowledge about what the original is like...

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Reply 2 of 2, by Skip94

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Thanks Tiido
I think what I need to do is pack it away fir a little while, I have quite a bit on my plate currently ad I'm just starting some construction work. I need to do some research and then invest in a decent oscilloscope. Once I have that, I'll have a proper dig into it. At the moment I'm pretty much working blind.
Cheers
Andrew