Vision Pro KJ-1915 issue

ELutz

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I've got an older Happ Vision Pro model KJ-1915. I'm pretty sure this is a re-branded Kortec KT-1915. It came in dead...no tick...no nothing. I did a cap kit and found a bad diode in the 12 volt output section off the transformer. That brought life back. All voltages are now good, but I don't have an image.....All I have is raster. Looking through Randy Fromm flowcharts at other switching power supply monitors....this seems to point to a blanking issue. I'm not for sure what handles the blanking on this paticular model. Here's schematics for those curious...

http://arcarc.xmission.com/PDF_Monitors/Happ Controls KJ-XX15 13in and 19in Color Raster.pdf

I've swapped U201, Q405, and Q204 (I'm not for sure if this one is even in the blanking circuit)....still just raster.

I'm at a loss. Have I missed something?
Oh yeah, I am sending a good video signal to the monitor :)

Edward
 
Well, you could try U501. Is your +12 correct at the neckboard? What voltage are you getting on the legs of the three drive transistors?

I had a different monitor type have colors missing due to a bad resistor that dropped one of the voltages down by one volt....
 
Well, you could try U501. Is your +12 correct at the neckboard? What voltage are you getting on the legs of the three drive transistors?

I had a different monitor type have colors missing due to a bad resistor that dropped one of the voltages down by one volt....

OK, I just swapped the 4030...no change. 12VDC is present at the neckboard.
Transistor leg voltages...
Base = .030 (Give or take .005V)
emitter = 1.245VDC
1.285VDC
1.080VDC
collector = 203VDC

So, I notice the 185VDC line is running just over 200VDC. That juice comes straight off the flyback. It seems unregulated...so probably within tolerance. One thing I did notice....my raster is kinda a grayish-blue. Adjusting the six R/G/B pots on the neckboard do nothing. I assume they should change the color of the raster?

Edward
 
Well, the .030 on the base seems too low to me. Typically, on a CGA game, the video signal comes to the monitor at about 2.5vdc or so. By the time it hits the base of the drive transistor, I usually see other monitor types have about +6.0vdc on the base. I wonder if you have no colors because your color signals aren't there?

As you've already replaced U201, I think I'd start by tracing the red signal through from the input connector. Check the voltage on the red input pin, then on the base of Q201, then on pin 4 of U201, then on pin 26 of U201, then on the red connector to the neckboard.

You may also check C213 on the chassis and C704 on the neckboard. If either is shorted, it would pull the +12 to ground rather than allow it to do what it needs to do...
 
BTW - when I was working on a G07 with a similar issue, having too low of a voltage on the base of the drive transistors was giving out too high of a voltage on the collector - just like what you are experiencing. I also found that - when I turned the monitor off, when the screen got about half-size, I could see everything for a split second.

For me, the problem turned out to be a resistor in a parallel circuit that was off-value. In your case, those would be the 2.4k-ohm resistors at R218, R221, and R223, or perhaps something else there between U201 and the neckboard connector...
 
Mod, Big thanks for the help & suggestions!

OK, reporting back...parallel resistors all checked out good. I pretty much ohm-ed out all the resistors in the circuit associated with the U201 chip...all good. C213 and C704...good.

I started following the voltage as you suggested....everything is good up to U201...2.86V going into pin 4.....the output pin 26 was .028V. I lifted pin 26 out of circuit and took a reading...0...nothing.

I don't have any other monitors in the shop to compare readings. The U201 is a LM1205. The only other monitors I've found that use that chip is the newer Happ MTG series of the Vision Pro.....and the WG19K7302 (which is the same chassis)...which are both re-branded Tovis monitors.

Edward
 
Well. at least now you know why you have no picture. Now you just have to figure out why it's happening. I don't have time now to check the datasheet for that chip, but you may want to verify all the voltage inputs to it that are required for it to function properly if it's a new chip. If it's a used chip from a swap, then you might need to track down a new chip...
 
Is there a zener diode regulating the voltage to the 1205 chip? If so, I've seen those go bad several times on other monitors.
 
Thanks guys, I'll do some more digging.

Re: Ken; No zener.....15 volt output from the transformer...rectified to DC....into a 7812 for 12VDC. 12VDC line is good.

Edward
 
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