Stag PPZ universal programmer repair

Please note that for safety reasons repairs to electronic equipment should only be undertaken by a qualified electronics technician

Warning, this device uses high voltage that can cause injury or death, do not touch it when power is on. Even after power has been removed sufficient time must elapse until the high voltage has dissipated before working on it.

The first time that I powered up this Stag PPZ I could see that it had a problem with the CRT display. The screen display was being clipped so badly that it felt like peering through a keyhole. The text was only visible if the brightness knob on the back was turned up as far as it would go, but unfortunately the horizontal flyback lines were then also visble and brighter than the text. In addition, the screen brightness slowly improved for a couple of minutes after being powered up. The PPZ was turned off while I thought about the many problems that I had discovered.

Before going any further, which would involve touching the CRT and/or the CRT drive PCB, I left the PPZ turned off for several hours.

In the past I had fixed a few CRT televisions with various faults and recognised that the clipping was caused by the deflection yoke on the CRT being positioned incorrectly. I opened the case, it is held together by three screws on the underside at the front which allows it to open up like a suitcase, and found that the yoke had slipped down the neck of the tube, probably while in transit. The yoke was able to slide easily as the clamping screw had not been tightened. I pushed it back up so that it almost touched the tube and tightened the screw.

The brightness/flyback problems can be caused by internal faults in the CRT so I carefully pulled off the multipin connector from the neck of the CRT and with a multimeter checked that the resistances between a pin and all other pins was high, except for between the two filament pins of course. They were all fine so I concluded that these problems were caused by the drive PCB.

I turned the PPZ on and the display covered the entire screen and the brightness was slightly better although it still had the flyback problem.

On the CRT drive PCB is a potentiometer VR1 labelled 'Brill', so using a small insulated screwdriver I adjusted this slightly first one way and then the other but it had virtually no effect so I left it roughly where it was before. The PPZ was turned off and left until I had the time to return to it.

Some days later I disconnected the connectors on the PCB, removed the four nuts holding it in place and lifted it clear, although some wires stopped it being removed completely. The capacitor C12 seemed to have leaked in the past so I replaced it along with other larger value capacitors. Transistor TR1, a BF459, was interesting since it did not seem to be original as it had been adapted to the PCB with its legs bent around to fit. After checking how TR1 was connected I realised that it was responsible for blanking the beam scan during flyback and so was suspected of causing the display problems. I tried to obtain a replacement BF459 but it was unobtainable so I replaced the PCB and put the PPZ aside.

Update: 14/03/18

After using the PPZ for testing with StagView for over three years I happened to check for BF459s on ebay and found a seller with genuine unused examples. I removed the driver PCB again and desoldered TR1. I tested the replacement BF459 using the diode test setting on my multimeter and it measured a forward voltage of approx 0.6V for base to collector and base to emitter. When the removed BF459 was tested it gave a reading of 0.6V for base to collector but only 0.1V for base to emitter indicating some kind of internal damage. A new BF459 was fitted and at the same time I replaced the 0.1uF 1000V RIFA capacitor C4 which was starting to crack.After turning the PPZ on the display was much brighter, too bright with no flyback line except if the brightness knob was turned up. I suspected that the 'Brill' potentiometer VR1 had been adjusted in the past and now it needed to be adjusted back to where it should be by turning it in a clockwise direction. The display is not perfect, there is slight distortion that I believe is caused by the yoke being able to move sideways because there are no jams keeping it in a fixed position, but it is useable now.

VR1 is the potentiometer next to the wires and TR1 is on the right in the picture below

In the picture below the digital camera shows the display as having varying brightness probably caused by the slow refresh rate of the PPZ.

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