Need some advice repairing a CRT Black & White TTL

gehrig · Post by **gehrig** » Thu Apr 17, 2008 8:34 am

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).

Edit: I noticed last night that a lot of voltages at my test points were dropping slowly. And my 30+ is up to 33+. But, that can be adjusted with the V Bias Pot.

zaphod7501 · Post by **zaphod7501** » Thu Apr 17, 2008 6:51 pm

You really need an Oscilloscope for most troubleshooting on an oscillator circuit, DC voltages don't mean much.

On the other hand, it's a safe bet the oscillator isn't running at all (no voltage on emitter of Q10).

The simplest of failures would be the electrolytic capacitors or a broken V.hold control (or the leads to the panel where it is located)

R56 could have increased in value
C36 could be open or leaky (or c33)

There is a crude way of injecting a drive signal that might not be fatal: putting your finger on a probe, touching the base of Q10. (or through a .047 capacitor)

gehrig · Post by **gehrig** » Fri Apr 18, 2008 7:52 am

I noticed that there is a pot between R56 and 73+ called vertical size (100k). Adjusting that helped a lot of the voltage readings on the left side of the circuit. My main concern right now, is the 30+ going to the vertical output. That is waay low. Not sure why that is.

zaphod7501 · Post by **zaphod7501** » Fri Apr 18, 2008 7:11 pm

Q13 could be shorted.
One troubleshooting method on this type of circuit is to open the loop.
(probably by opening/lifting the base of Q12)
This should allow the oscillator to start up.

If Q12 and Q13 are OK then opening the base should cause both to turn off, allowing the collector of Q13 to rise to 73v.
The only way there is 9v on the emitter Q12 is if Q12 is shorted or Q13 is leaky or R63 is open. But it's hard to tell from DC voltages.

R63 and/or C39 could also be open.