CTM640 blowing 2A fuse F501 looks like STK7308 problem

[peterjm]

Basically I have a serious short and after testing a number of capacitors, resistors, diodes I have come to the conclusion that the issue must be the STK7308 IC.

I have measured the resistance on pins 10,11,14-15 and I get a short no matter which way I connect the probes. Boo Hoo.

Please can someone confirm that this is a dead STK7308 IC and what other components must be replaced if the STK7308 has died? I am on a budget so any cost savings would be welcome.

Thank you in advance.

[Bryce]

It's highly likely that the HV transformer has shorted inside and taken the STK with it. Rarely worth trying to fix unless you have a spare CTM640 to take parts from.

Bryce.

[peterjm]

Thank you Bryce.

Do you mean part T501 Tx. 8142004? Is there an easy way to test it to see if it is still operating within specification?


I would really like to get this old screen working if possible. I completely understand that it might not be worth spending much on but if I can be fairly sure the biggest expense is the STK I will buy one.


Otherwise what parts would you recommend for connecting to a PC monitor?


Peter

[ComSoft6128]

Is this component in the CTM644?


If so I have a working monitor heading for the bin and could remove the PCB and send it off to you with you paying the postage only.

[Bryce]

Thank you Bryce.

Do you mean part T501 Tx. 8142004? Is there an easy way to test it to see if it is still operating within specification?


I would really like to get this old screen working if possible. I completely understand that it might not be worth spending much on but if I can be fairly sure the biggest expense is the STK I will buy one.


Otherwise what parts would you recommend for connecting to a PC monitor?


Peter

Yes, that's the part. This has most likely failed / shorted. There's not much else that could cause the STK to fail (although it's also possible that the STK failed on its own).

Bryce.

[peterjm]

Is this component in the CTM644?


If so I have a working monitor heading for the bin and could remove the PCB and send it off to you with you paying the postage only.

Thank you for your kind offer.

The CTM644 does have the same switching regulator (STK7308) but not the same transformer (8142006 instead of 8142004). So your board would be useful as it has one of the components and it might be only that one I need to replace.

As your monitor is working would you be happy to ship the whole monitor? I understand if that is more hassle than you would like.

Peter

[ComSoft6128]

Just the PCB I'm afraid.

[peterjm]

Just the PCB I'm afraid.

Please can we make the arrangements to ship your PCB? Thank you.

[ComSoft6128]

OK,


PM me your contact details and I'll get it off to you in about a week.

[peterjm]

I have finally found time to do some work on my CTM640 board.

A few weeks back I removed the switching regulator (STK7308) from the CTM644 board ComSoft6128 kindly sent me and soldered it into my board. Sadly I powered the monitor and nothing happened. But one good sign was that the 2A fuse (F501) didn't blow!

So out came the multimeter again and I started testing voltages. The part of the circuit before the transformer (T501 Tx. 8142004) was powered but nothing else and it looked like the switching regulator wasn't working. I finally found a zener diode (D507) that needed replacing and used one from ComSoft6128 board.

That did the trick. My old monitor is now working!!!

Thank you for all your help Bryce and ComSoft6128. Without a spare board I wouldn't have been able to fix it.

I think it does need some calibration as the picture isn't quite as good as I remember. If anyone has any good monitor calibration tips I would be grateful to hear them.

Thank you again.

Peter