Hantarex Polo 25" bad contrast / brightness (washed out)

[Deepblue]

I have a Hantarex Polo 25" standard res, which has a very strange contrast and brightness problem????

I have replaced IC1 (LM1203) and checked the video pre-amplifier section.
Chassis has been re-capped.

I can't seem to figure out why the image is so dull (see attachment)?

sorry about the image quality. It should be a crosshatch with color bars in the middle.

I have tried adjusting color cut-off as described in Hantarex service manual, but this doesn't help

[montech]

What are the Voltages on your CRT pins....Careful High Voltage present. I'm curios what your G2 and G1 read. Also check the Video Level at RGB  pins on CRT with a scope if you can. If you turn the screen down does it improve slightly? If so may be a bad CRT, but doubtful. I'm thinking this is a video level problem.

[Deepblue]

What are the Voltages on your CRT pins....Careful High Voltage present. I'm curios what your G2 and G1 read. Also check the Video Level at RGB  pins on CRT with a scope if you can. If you turn the screen down does it improve slightly? If so may be a bad CRT, but doubtful. I'm thinking this is a video level problem.

I have measured the voltages you asked for:
G2 measures about 1000VDC (measured with a scope) if measured with a multimeter (Fluke 183), it drops to about 500VDC???
G1 measures 19VDC with a multimerter. (See scope image)
Red Cathode is strange, I think you are right about video levels, but why? I have tested all components and replaced the LM1203 with a new one???

[montech]

I thought so. I think its a video level  (gain)l problem. So the things to do are look at the video level at the cathode while adjusting the contrast. it should change. If it does not then the contrast control circuitry is at fault. It is at far to small a level now. Does this have analogue or digital controls? Do you have a print I could look at? I forget the dc recovery circuitry in this one.
The impedance of the meter loads the G2 voltage to 500v -its normal if you can lower it with the screen control on the fly to about 250V. The G1 should be about 100V depending on the tube, so too low. I need to see a print.

[Deepblue]

I thought so. I think its a video level  (gain)l problem. So the things to do are look at the video level at the cathode while adjusting the contrast. it should change. If it does not then the contrast control circuitry is at fault. It is at far to small a level now. Does this have analogue or digital controls? Do you have a print I could look at? I forget the dc recovery circuitry in this one.
The impedance of the meter loads the G2 voltage to 500v -its normal if you can lower it with the screen control on the fly to about 250V. The G1 should be about 100V depending on the tube, so too low. I need to see a print.

This is great info, thanks
By print, do you mean the PCB or the schematic?? I'm not sure?

[montech]

If you can get me a schematic I could guide you through the circuit better. I have some Hanatrex schematics but not sure which one you have. Do you have a file of the schematic that you can upload or do you have a Model number on the chassis? Maybe someone else could chime in here too:)

[Deepblue]

I have the schematic here: http://fileshare.kims8bit.dk/POLO_MP_25_28.pdf
it's an 15kHz MP 25" version

[montech]

Thanks
If you follow from pin 6 on the fly you will see R9 and T1 going to pin 1 on the Video Processor(LM1203) This brings in a Horz pulse for the video circuitry pedestal level. Is it there?

[Deepblue]

Thanks
If you follow from pin 6 on the fly you will see R9 and T1 going to pin 1 on the Video Processor(LM1203) This brings in a Horz pulse for the video circuitry pedestal level. Is it there?

I guess it's Pin 14 you are referring to? the schematic is not very good right there
The pulse gets there, see scope image

[Deepblue]

I scoped the video signal on IC1 Pin 4, traced it back to the junction R142-R143

[Deepblue]

 and T103-Emitter

[montech]

I looks me like your losing the stairstep pulse between Junction 142 and 143 and pin 4. You have no pedestal level at the input to the IC, this is why I had you check the Clamp gate at pin 14.
If you look at the waveform at R32/33/34 given in the waveforms you see the voltage should be 80 p-p and you have a Horz pulse in that waveform which you are missing and your peak to peak is about 20V. What is the voltage at TP11? If its zero the zener is bad(ZD102) Are the drives turned all the way up? They should be and the screens should be about 1/3. Is IC102 working? What signal are you feeding it.?Do you have a 10bar staircase, like on the Sencore? Is R12 bringing proper bias to T2, T4 and T6?  What do you read at the base of these three transistors? Check these and get back to me....This one sure is challenging! I have attached a datasheet on the LM1203. It may help a bit.

[Deepblue]

I looks me like your losing the stairstep pulse between Junction 142 and 143 and pin 4. You have no pedestal level at the input to the IC, this is why I had you check the Clamp gate at pin 14.
If you look at the waveform at R32/33/34 given in the waveforms you see the voltage should be 80 p-p and you have a Horz pulse in that waveform which you are missing and your peak to peak is about 20V. What is the voltage at TP11? If its zero the zener is bad(ZD102) Are the drives turned all the way up? They should be and the screens should be about 1/3. Is IC102 working? What signal are you feeding it.?Do you have a 10bar staircase, like on the Sencore? Is R12 bringing proper bias to T2, T4 and T6?  What do you read at the base of these three transistors? Check these and get back to me....This one sure is challenging! I have attached a datasheet on the LM1203. It may help a bit.

I already tested the voltage at TP11 (it's 5V, so OK!)
I'm sorry to say, I don't have a nice pattern generator like the Sencore would really like to own one though

I was so fortunate to get some other Hantarex Polo chassis the other day, which didn't work either. I started to debug one of them but I was missing the remote board, so I used the one from this chassis.
To my surprise, this chassis had the same issue as the one I' having trouble with here
Then I thought..... this is strange, the exact same issue on two chassis, something's up!!!
I thought I would try another remote board, so I fetched one from another chassis, and SURPRISE it works....... 
I had already cheked all the pots on the defective remote board, so it had to be the board itself??? I then looked very closely at the PCB and found....... two hairline cracks......... !!!!
This had been the problem all the time. fixed the two tracks and now it works

Thank you for all your help  I couldn't have done it without

[Styx]

Can I ask how you prefer to fix the tracks when it's cracked? Are there better or worse methods? I love board repair, but I'm still new at it, and thought I'd ask.

-Styx

[cowboygames]

What's worked well for me is either running a jumper wire from a component on either side of the brake or a bit of wire right across the break and a solder bridge to cover it. Solid core telephone wire works great