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murphyv310 · 6th Nov 2020, 6:24 pm

Well I got an unexpected hour this afternoon..
Ken's work is nice and tidy and there is no issues with what he's done.
I did a couple of cold checks first. I didn't like the look of R15 an HT feed it read spot on but drifted badly when warm so I replaced it. Also one of the 1 Meg AGC resistors drifted with pressure on the meter leads, common enough with these PCB resistors so I replaced both. On power up the set worked reasonably well for a couple of minutes. Then the fault appeared, a bit like motorboating. The issue with light was really easy to see. So a poke with the scope revealed the issue. The oscillator waveform was perfect but on Pin 6 of the UCH81 Heptode anode the resultant waveform was full of hum. I suspected a PCB leak from pin 5 to pin 6. After a scrape with a flat screwdriver the fault cleared with a big improvement in sensitivity. The UCH81 valve base was also in a poor state. A new base has been fitted, the scratchy tuning cap sorted out and a good hour run as perfect.
I've cleaned the PCB as best as possible and am happy to sign off the repair.

https://youtu.be/4Jm7Jfk9D_s

6th Nov 2020, 6:24 pm	#100
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: PYE R33 Over Voltage. Well I got an unexpected hour this afternoon.. Ken's work is nice and tidy and there is no issues with what he's done. I did a couple of cold checks first. I didn't like the look of R15 an HT feed it read spot on but drifted badly when warm so I replaced it. Also one of the 1 Meg AGC resistors drifted with pressure on the meter leads, common enough with these PCB resistors so I replaced both. On power up the set worked reasonably well for a couple of minutes. Then the fault appeared, a bit like motorboating. The issue with light was really easy to see. So a poke with the scope revealed the issue. The oscillator waveform was perfect but on Pin 6 of the UCH81 Heptode anode the resultant waveform was full of hum. I suspected a PCB leak from pin 5 to pin 6. After a scrape with a flat screwdriver the fault cleared with a big improvement in sensitivity. The UCH81 valve base was also in a poor state. A new base has been fitted, the scratchy tuning cap sorted out and a good hour run as perfect. I've cleaned the PCB as best as possible and am happy to sign off the repair. https://youtu.be/4Jm7Jfk9D_s __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member Last edited by murphyv310; 6th Nov 2020 at 6:52 pm.