DaveWM
08-27-2011, 03:37 PM
got this one a while ago. Metal cabinet, the orig problem was a shorted bypass disk cap that was taking out the B+. New cap and new power resistor got it back up and running. Rather than the normal shot gun recap I decided to keep this one as orig as possible, the idea being I wanted to get better as trouble shooting. It only has a couple HV ceramic tubed paper caps, two filter cans and a handfull of smaller black electro caps. After watching for a while the 1st defect that happened was in the AGC, got super critical, sure enough one of those 4mf single electros was open.
Todays problem has been presenting for while, there was a bit of a smear in the pic to the right, would get better after being on for a while, which was a clue. I checked the schematic and there was a 50uf in the cathode circuit of the video amp. Normally I assume when this goes open you just lose gain on the Y signal and you lose contrast and get a weak pic. In this case the contrast worked fine so I had not suspected that one. I used a technique described by VK member CTC17 that uses an audio signal passed thru the suspect cap, sure enough it sounded weak an muted. I tacked on a new cap and the smear was cleared. I found the cathode bypass cap in the audio tube was weak as well (the set did seem a little weak in audio) so replace that one as well.
Next up was it had developed a case of the bends so I checked the filter caps. The audio test was not conclusive as they passed sound but my ear is just not that well calibrated. I tacked on a couple 80ufs one the two main filter caps (the one before and after the choke). this cleared up the bends so I will have to rebuild those cans for a proper fix.
the only remaining problem is the tint control. It seems to lack the ability to drive the screen to the green, it goes to purple no problem. as I turn towards green, the color shift some, but it also starts to lose color gain.
fortunity its has range enough for perfect skin tone, but its not righ. I did try adj the phase coil that the tint control connect to, I can shift the purple more or less but it does not add to the green. There is a .01 cap that couples the tint control to the phasing coil, I replaced it think it may have been leaky, but not effect. Its not the pot on the hue or the connection of the pot to the chassis as those would be a green shift.
going to try a new 6JU8 but it tested fine. going to try doing some VTVM checks around the burst amp and chroma sync phase detector.
Todays problem has been presenting for while, there was a bit of a smear in the pic to the right, would get better after being on for a while, which was a clue. I checked the schematic and there was a 50uf in the cathode circuit of the video amp. Normally I assume when this goes open you just lose gain on the Y signal and you lose contrast and get a weak pic. In this case the contrast worked fine so I had not suspected that one. I used a technique described by VK member CTC17 that uses an audio signal passed thru the suspect cap, sure enough it sounded weak an muted. I tacked on a new cap and the smear was cleared. I found the cathode bypass cap in the audio tube was weak as well (the set did seem a little weak in audio) so replace that one as well.
Next up was it had developed a case of the bends so I checked the filter caps. The audio test was not conclusive as they passed sound but my ear is just not that well calibrated. I tacked on a couple 80ufs one the two main filter caps (the one before and after the choke). this cleared up the bends so I will have to rebuild those cans for a proper fix.
the only remaining problem is the tint control. It seems to lack the ability to drive the screen to the green, it goes to purple no problem. as I turn towards green, the color shift some, but it also starts to lose color gain.
fortunity its has range enough for perfect skin tone, but its not righ. I did try adj the phase coil that the tint control connect to, I can shift the purple more or less but it does not add to the green. There is a .01 cap that couples the tint control to the phasing coil, I replaced it think it may have been leaky, but not effect. Its not the pot on the hue or the connection of the pot to the chassis as those would be a green shift.
going to try a new 6JU8 but it tested fine. going to try doing some VTVM checks around the burst amp and chroma sync phase detector.