Wednesday, August 19, 2015

Low output voltages from CRT monitor

Dear Sir,
I got CRT monitor. I have checked the voltages up to the big capacitors and everything is alright. When I check the voltages on the secondary side of the Transformer. I did not get good voltages.

I removed one leg of the diode and to check the voltage I heard a big sound in the neck board (one capacitor blown).

I don’t know why that happened. I am following your book which I have purchased online. Please help me. Do you want me send you some pictures of the board. This board is different from what you have shown in the book.
I will wait on your positive reply.

Best Regards,
M.Imran-India



Hi Imran,
It is not advisable to remove any diode on the secondary side and apply power, this this because one of the diode rectify the line which is monitored (I mean the feedback circuit) and therefore if you remove this diode you will disable the feedback circuit and hence the voltage will no longer be regulated and therefor will shot to the roof.

I think this is what happened in your case, I suggest you solder back the diode and apply the power again and start again to troubleshoot the monitor.

I highly recommend you use a 100 watts bulb across the main fuse holders when applying the power for the first time just in case the control i.c/switching transistor (primary side) has also shorted.

If the bulb brightness goes dim this means the switching component survived but if you find the bulb remain bright even after a minute means there is a shorted component on the primary side with the main suspect being switching component used in that monitor.

If you confirm the primary side is again running and you still have low output voltage I suggest you remove the H.O.T transistor and apply power briefly and if you find the secondary side voltages have increased could mean the actual H.O.T is shorted.

Regards Humphrey


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