Hantarex MTC-900

Peale

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I have this Hantarex MTC0-900 that has just been kicking my butt. I worked on it over a year ago and put it away because it frustrated me so much. I had to dig it out of storage the other day, so I thought I'd get back to it.

Backstory: it used to display nothing except a full-on bright raster with no picture. After recapping and going over solder joints, now I can adjust the coarse brightness with the screen control, but can not do anything with the contrast or brightness controls.

I contacted Jomac about this, and he gave me a couple test points to see if I had voltage there.

@ SP5 and 8, tested together, I should get ~12VDC. I get nearly zip. If I meter from the heatsink on IC4 to SP8 I get 12VDC, like I should.

I have a thread @ BYOAC with my prior experience, but this thing has me beating my head against a wall. Any other pointers?

http://forum.arcadecontrols.com/index.php?topic=80194.0
 
Okay, assuming I ma using the proper schematic here:

Start by putting your black lead on the chassis ground somewhere and leaving it there, then on the leg of IC4 that connects to D17 and C77. That should have your +12. If not, then you have a problem.

If you have +12 there, check again at P22. If you have it there, follow it up to the Interface Board (IE). From there, it should be found on 1, then on to the neckboard at SP8.

if you lose it anywhere between one test point and the next, or it drops between on test point and the next, that may tell us the area we need to look at.

You could print out the schematic and use a highlighter to mark all the parts along that 12v line, and check them individually.

I had a G07 recently that had a problem with missing colors that was traced to one of the voltages to the drive transistors being off. Eventually I was able to trace through the circuit to find a parallel resistor that was off value. Replacing it fixed everything...
 
I can confirm there's some kind of weird grounding issue. I have another chassis, a 900E, that I can compare to, and I should be able to hit ground pretty much anywhere on this thing, using the 900E as a comparison. I'll be damned if I can find it though! I might run a bypass or something.

As for the schematic, that's what I do every time I work on any electronic item, assuming it's available.

I'll keep poking at it. Now that I KNOW it's a grounding issue, I can limit myself to that. It just doesn't make any sense!
 
I decided to see what would happen if I connected pin 12 of the flyback to the frame. It's ground, so...

Pretty sure I blew the horizontal output transistor. That's TR17, I think.

And it's possible that TR9 is bad too, or maybe I'm metering wrong.

Now to scavenge those off a parts chassis.
 
Now I'm not convinced it was bad. The one I took off a known working chassis reads the same as the removed from the borked chassis.

According to the schematic, I should be getting 111.5V on one leg and .23V on the center leg. IIRC I got nearly 180V (IIRC because I performed these tests about six hours ago).

And I'm not getting heater either
 
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