Hantarex MTC900E High Voltage - Can't turn down

barakawins

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I'm working on this hantarex 900E chassis. The high voltage should be 115vdc. I'm getting 145 - 146 vdc and it's giving off a wave to the picture. I thought it might have been the fyback. It's not. Changed the flyback to a new one and same result. Second, I recapped the entire chassis. Still same problem. I though Capacitor RV12 which regulates the voltage would be the problem. I pulled it out of circuit. It's a 1k pot. I'm getting .98k so it's close enough to tell me this works. Any other ideas on what is causing this DC voltage to remain High? What regulates the voltage aside from the pot?
 
What regulates the voltage aside from the pot?

That would be the "Voltage Regulator".

I'm not familiar with this monitor chassis, so you'll have to look at yours... but all B+ supplies are regulated by a voltage regulator (either a transistor or an IC) - the pot simply controls the circuit. There is only a tiny amount of voltage on that little pot - a semiconductor does all the heavy lifting.

-Ian
 
voltage regulator

Well, I took the voltage regulator out and tested it out of circuit. It tests good in diode test.
So far I've done the following:

Changed the flyback,
Tested the voltage regulator and HOT with a meter. Tests good.
Did a full cap kit
tested the B+ trim pot out of circuit and it is a 1k pot. I got it to measure up to .98k so this pot works.

I'm stumped anyone know any other thing that could cause voltage to go to 145 vdc and I can't regulate it? It's driving me nuts. There's a wave in the screen due to too much HV.
 
I glanced at the schematics earlier - there are like three other small transistors in that circuit - did you check those too?

-Ian
 
thought

I think I may have found the culprit. The power regulator is a TIPL762 and it's mounted to the chassis. Someone installed a TIPL751. I don't think this is a good cross. Now, I found that the TIPL762 is impossible to find. Well, I happen to have one but it's not in the metal casing but a regular transistor. I'm wondering if I could rig this to work with this chassis. Before I go this route, I put in a call to zanen electronics for a good cross. I believe this is causing the wave problem. Anyway, I did check out a few transistors in the circuit which seems good. I think I have to replace this TIPL751 with the correct TIPL762 first. I'll report back after install.
 
Nope

I haven't found the culprit yet!!! Turns out TIPL751 came with Hantarex monitors as voltage regulators. Urghh. Anyone having Hantarex 900E experience please chime in. I'm getting 145vdc which is too high causing the screen to be wavy and I can't seem to figure out how to cure it. I already changed all caps including C11 main filter cap. I already took TIPL751 out and tested the voltage regulator in diode test with a meter and it checks out good. I pulled out the Trim Pot at RV12 which is a 1k pot. It tested good and came up with .98k in ohms test. Now I'm lost. Any other ideas on what to check?

Thanks.
 
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