Need some advice repairing a CRT Black & White TTL Monit

gehrig · Joined: 17 Apr 2008 Posts: 8

I meant.. just remove the emitter leg and check B and C..

gehrig · Joined: 17 Apr 2008 Posts: 8

Hey.. just an udpate.. here are my voltage readings off of the transformer.

Guest · Posted: Fri Apr 18, 2008 5:30 pm Post subject:

Sorry I meant with Q13 removed place the probes on the print of the pcb where the legs would go if it was still installed.
Regarding your update voltages it seems to be a drop of 40v going somewhere. If Q13 was removed and you remeasured these points it would prove Q13 was doing this.

gehrig · Joined: 17 Apr 2008 Posts: 8

I see. Thanks for the info. I will give it a shot this weekend.

Guest · Posted: Sat Apr 19, 2008 12:25 am Post subject:

I pulled the collector on Q13 and powered on. The voltage reading jumped from 30 to about 70. So, thinking Q13 must be bad then? I have one on order..so thinking maybe I shouldn't try anything until I replace. What do you think?

vtech · Joined: 08 May 2006 Posts: 1264 Location: USA

Sure points to a bad Q13. ...still, no reason why you can't test it? (yes there are odd instances where the testing may not show it but definitely not in this case)
A leaky Q13 can easily be spoted with an ohmeter.

http://www.hobbyprojects.com/test_and_measurement/testing_transistors.html
http://www.kilowattclassroom.com/Archive/AN0007.pdf

Also, more than likely, you don't have to get an exact replacement number for Q13.

gehrig · Joined: 17 Apr 2008 Posts: 8

It was Q13.. I replaced and everything is up and running.

Thanks for the replies!