u5000 R760 Smoke

flip41126

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Well I just got U5000 monitor back together. Originaly started with the thin line across the screen. Did the cap and other 3 updates. Hooked it up, turned it on, nothing form the screen. The game did start. Could only adjust B+ @ TP202 to 119.2 volts min. This is running 25 kHz and should be at 117.2 volts. Also, checked TP203 and am only getting 1.72 volts. Turned off then back on. There was some wierd red flickering lines on the screen. Then blank. Then R760 got red hot and started to smoke. Opps.
 
R760 and the associated components (Q708, Z702) are a quick and dirty voltage regulator that provides the initial voltage needed to start up the horizontal circuit. The circuit supplies roughly 8-9 volts to get the horizontal circuit running, once running, 15v from a secondary of the flyback takes over, supplying the needed current for normal circuit operation.

This startup circuit cannot supply any significant amount of current for any extended periods of time, if the horizontal circuit doesn't start up for any reason, the startup circuit, which r760 is part of, will continue supplying current and start burning up.


At this point I would go over the entire horizontal circuit. It's hard to be specific, since anything that stops the horizontal circuit from running will give you these symptoms. After checking for cold or bridged solder joints, I would look for improperly installed components (backwards cap, etc..) then I would suggest checking all the transistors and diodes in the horizontal circuit for shorts, then open resistors and so forth.

It's a bastard of a chassis to work on, so I do wish you the best of luck with it.

D
 
It's a bastard of a chassis to work on, so I do wish you the best of luck with it.

That is all.....

j/k
Had one kick my tail for quite some time with the same burning up resistor, wish I had my old notes handy.(that was 5-6 years ago now)
Check, or better yet just go ahead and change Z705, Q708 and Q703.
I say go ahead and change because sometimes they will check fine, but still be bad.
Z705 and Q708 are right near that resistor, but Q703 is near the back edge of the chassis.

And re-cap it if it hasn't already been done.

EDIT: Just saw your other post......

Well I just got U5000 monitor back together. Original started with the thin line across the screen. Did the cap and the other 3 updates. Hooked it up, turned it on, nothing form the screen. The game did start. Could only adjust B+ @ TP202 to 119.2 volts min. This is running 25 kHz and should be at 117.2 volts. Also, checked TP203 and am only getting 1.72 volts. Turned off then back on. There was some wierd red flickering lines on the screen. Then blank. Then R760 got red hot and started to smoke. Opps.

Double check all of your previous work and THEN check the above.
 
Well, I count 29 different components to check on this circuit. What fun it was to find them all and check the solder joints. All looking good. Will not be able to play with it till Monday. Oh well. So, if I am to understand correctly, when the circuit does not start up, the monitor will not turn on? Or, maybe, do weird things for a very short time till smoke comes out?
 
Kevin, If you happen to fall apon your old notes, it would be greatly appreciated. Thanks for the input. All of my work, so far, has been checked and looks correct.
 
d710 d712 d713 Q708 Q703 Z702 r760 all new. Tick Tick Tick. Back to the drawing board.
D713 did show a open when I was checking them.
 
Hummmm, maybe. Would that cause the ticking noise??? I have another U5000. That I did repair and works great. I could try a flyback swap and see what happens.
 
Believe it or not, if it's ticking now and that resistor isn't smoking anymore, you ARE making progress.

Those symptoms indicate that the deflection circuits are at least trying to start up now, but something is drawing too much current causing the switch mode power supply to shut down.

Have you tested the HOT yet? The next place to look would be the HOT and to check for sorted components around the HOT. If the HOT is shorted, you may have a bad flyback, but on this model I have seen the HOT fail for no real reason at all and I have seen bad flybacks, can go either way. You can check to see if it's shorted, and you can also try and pulling the HOT, and power up the chassis with it out (only run it a short period of time like this), if the ticking stops with the HOT pulled the issue is around there somewhere.

D
 
Moving forward. Tried a known good flyback. Up went R760 and R765. Oh the smell of smoke before bed gets my wife going. JK. Anyways, found Z702 is shorted. Other diodes in this circuit tested good. Replaced Z702, Z709, Z711, R760, R765, Q708. Fired it up. Almost works right. The screen is rolling up and down. I can not adjust this out. Tried different good remote boards. Notice. I seem to recall seeing something about this out there.





Note : Manual shows R760 to be a 100 ohm 1/2 5%
Wells Gardner bullentin for the power supply updates this to a 470 ohm . Yeah, just
noticed it durning this last session. There are many other updates in this kit as well.
 
Replaced IC U601 with new TDA1771. Pic is looking good. However, I can not get the pic centered. Fills the top 3/4 of the screen. The vert position will not change. I can get it to go into a fine line or will 3/4 of the screen. No in between. Changed the vert position pots. Nothing.
 
Note : Manual shows R760 to be a 100 ohm 1/2 5%
Wells Gardner bullentin for the power supply updates this to a 470 ohm . Yeah, just
noticed it durning this last session. There are many other updates in this kit as well.

Reviving a thread from the dead....
BUT.... what value is R760 supposed to be???
 
Increase it if you want. If it died, check R765 and Q708 while you're there, as well as the diodes nearby...
 
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