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Old 12th Jun 2021, 6:46 pm   #2
Lloyd 1985
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Join Date: Jun 2008
Location: Coningsby, Lincolnshire, UK.
Posts: 2,814
Default Re: 1980's Sony PVM-1270Q monitor

Thought I should update this

After the first lot of experiments I decided to try and up the +B line a bit, I added some old SLA batteries and got up to 70V, still not quite 110V! After winding up the screen control I was able to get a line on the screen, confirming there was no frame scan, I checked the oscillator with the scope and sure enough there was nowt happening. I gave the oscillator output a poke with the meter set to continuity test and was able to make the line jump up and down on screen, so the frame output stage is good, as well as the line output stage.

Over on the VRAT forum it was suggested that if the frame stage was not working maybe the set would go into some sort of protection mode to stop a line getting burnt into the screen, so I went on a hunt to see if it was something to do with the frame stage, but as far as I can tell, the frame output stage only gets power once the line stage is running, as the voltage to the frame output IC comes from the LOPT, as does one of the VCC’s for the LA7802 IC, which has both line and frame oscillator’s in it.

IC 505 is responsible for ‘HV Hold down’, which connects to the LA7802’s ‘X-Ray protect’ pins, so I had a prod around there and found that the only voltage present was the 20V on pin 8, some of the others quickly flicked up by a volt or two when power cycling the set.

I did wonder if it was because the +B was high at over 120V, it should be 110V, so I decided to try loading the 110V line with a 40W lightbulb, that did bring the +B down, and I was then able to set the voltage to bang on 110V, but the monitor remains dead! Some of the voltages did come up on the pins of IC-505, but not by much.

I’m still thinking the LA7802 is toast, so I’ve ordered another, I can’t think of where else to go with this! The monitor had originally been used on a ship, so it’s been exposed to salty sea air, and there are a few components with rusty legs, a few transistors and some of the tiny resistors, but so far all have measured perfectly ok.

Regards
Lloyd

I also gave it a clean, and took a new photo of it in better light! Don’t suppose anyone has the control panel flap sitting in a box of junk?!
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