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Need some advice repairing a CRT Black & White TTL Monit
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gehrig



Joined: 17 Apr 2008
Posts: 8

PostPosted: Thu Apr 17, 2008 1:33 am    Post subject: Need some advice repairing a CRT Black & White TTL Monit Reply with quote

I recieved an arcade with a broken monitor for free. This is really my first attempt at repairing.

Monitor: XM501-10 (CRT: Black & White). TTL

Scenario:

When powering up the monitor, all that is visible is a bright horizontal line that is in the middle of the screen. So, this seems to be a vertical deflection problem.

I did replace some caps from a kit that I recieved. No change.
Fuses are OK.

I replaced 3 transistors on the the vertical output circuit. Q10, Q11, and Q12.

My +30, +73, +150 are ok.

According to repair manual.. and using my voltage readings, it says "possible shorted driver or open oscillator transistor." Well, I replaced those still no go.

Anyone have any suggestions? Am I on the right track?

Here are my voltage tests (in red).


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Guest






PostPosted: Thu Apr 17, 2008 10:14 am    Post subject: Reply with quote

Q12 collector at supply potential as the emitter voltage is too high (not conducting). Check Q13. Interesting the emitter of Q12 and the base of Q13 have a difference of 0.5v though.
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PostPosted: Thu Apr 17, 2008 10:23 am    Post subject: Reply with quote

0.32v difference (can't count).
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gehrig



Joined: 17 Apr 2008
Posts: 8

PostPosted: Thu Apr 17, 2008 1:50 pm    Post subject: Reply with quote

I noticed last night, that the voltages are dropping very slowly at a lot of my test points. So, that maybe why they appear different.
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gehrig



Joined: 17 Apr 2008
Posts: 8

PostPosted: Thu Apr 17, 2008 1:53 pm    Post subject: Reply with quote

Oh.. also, I have not replaced Q13. My DIY store doesn't have it. I was thinking about ordering one.
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macam
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PostPosted: Thu Apr 17, 2008 6:25 pm    Post subject: Reply with quote

check R58, probably gone high
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Thu Apr 17, 2008 6:42 pm    Post subject: Reply with quote

Applying basic Ohm's law;
Look at 73V source line below T3. Based on your reading at the collector of Q13, you are dropping over 40V! aross the primary (57 ohm coil)? Provided there is no connection issue/cracked board etc, either there is serious leakage through Q13 or may simply be an open primary on the transformer which is rather uncommon---The [email protected]'s collector is waay too low.

Re-measure across T3 primary; is there infact ~40V difference on either side? & what if you
lift Q13's collector--- does the reading come back to ~73V on the trace?

No doubt there are several ways to t-shoot this fault;

Why not test Q13 w/an ohmmeter to know if it's bad or not
Do you have a scope to check for the typical Ramp wave form?
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PostPosted: Thu Apr 17, 2008 7:56 pm    Post subject: Reply with quote

Thanks for the responses.

I removed Q13 and checked with my DMM that has ohm readings before. It passed the tests. However, I read that this doesn't mean it couldn't be bad. Since the work load using an ohm meter (DMM) is so much less. I haven't even checked my transformer, due to the fact I don't know how. But, I am reading up on that now.

I recently purchased a scope, but still learning how to use it. The wave forms I was getting on the Q13 looked much like an AC wave form. All other transistors looked nothing like they should according to the repair manual.
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PostPosted: Thu Apr 17, 2008 8:26 pm    Post subject: Reply with quote

What is the voltage at both sides of R56 and voltage drop accross R56 and R58? Remove Q13 and state voltages at base and collector.
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gehrig



Joined: 17 Apr 2008
Posts: 8

PostPosted: Thu Apr 17, 2008 10:20 pm    Post subject: Reply with quote

I'll check look into the resistor voltage readings tonight.

I am a newb at this....I dont understand how I would check the voltages of the E and B of Q13 if it is removed. Do you mean just remove the collector leg?
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