10-09-2018, 04:17 PM
Hi Terry,
The set has been on its side whilst Ive been servicing it, I rotated the screen pictures (the wrong way) before posting them.
Ive noticed some circuit elements are a little different from the schematic, some component values and configurations.
There are some response tailoring inductors in the video output amplifier, R33 is not fitted.
R77 and R78 are higher values than listed.
The frame height modification does not appear to have been done on this set.
There are also some differences which look like the schematic is just plain wrong, as noted by Terry.
I am using a mains isolation transformer since the 'scope is earthed ;-)
I can change the cogging effect by altering the value of the cathode decoupling capacitor on the video amplifier. The least cogging appears when the cap is not fitted, next best is a 2uF, worst is the 1000pF as on the schematic. of course the gain and frequency response are all changing when I do this. The scope I'm using doesn't like HT voltages so I'm going to use another with a wider input range when I look further. Also Ive not ruled out an interference problem causing it. The set is a can of Hunts worms, there are still plenty to change - so far vie been concentrating on the worst culprits stopping the set from running, any o/c decoupling caps have yet to be addressed. Perhaps the problem will go away when I feed a signal through the aerial socket, reducing the level made little differeance.
Some news on the EHT regulation, Ive found another PL81 in my tin of valves and this one gives much better results, full brightness just gives a slightly bigger picture albeit a bit fuzzy. I've ordered a new EY51 to see if this is going to help. The LOPT gets very warm in operation, I don't know what is normal. Previous change to PY81 made little change.
Today Ive cleaned the case a bit and made a new bottom panel. Could someone please suggest if it should have ventilation holes in it?
Thanks for the schematic stitching Terry, I need a bigger printer to do it justice - I'll take it to work tomorrow.
Some more pictures. Corrected screen shots, inside LOPT can, exterior condition, etc. in my album here: T143 photos
Kind regards, David
The set has been on its side whilst Ive been servicing it, I rotated the screen pictures (the wrong way) before posting them.
Ive noticed some circuit elements are a little different from the schematic, some component values and configurations.
There are some response tailoring inductors in the video output amplifier, R33 is not fitted.
R77 and R78 are higher values than listed.
The frame height modification does not appear to have been done on this set.
There are also some differences which look like the schematic is just plain wrong, as noted by Terry.
I am using a mains isolation transformer since the 'scope is earthed ;-)
I can change the cogging effect by altering the value of the cathode decoupling capacitor on the video amplifier. The least cogging appears when the cap is not fitted, next best is a 2uF, worst is the 1000pF as on the schematic. of course the gain and frequency response are all changing when I do this. The scope I'm using doesn't like HT voltages so I'm going to use another with a wider input range when I look further. Also Ive not ruled out an interference problem causing it. The set is a can of Hunts worms, there are still plenty to change - so far vie been concentrating on the worst culprits stopping the set from running, any o/c decoupling caps have yet to be addressed. Perhaps the problem will go away when I feed a signal through the aerial socket, reducing the level made little differeance.
Some news on the EHT regulation, Ive found another PL81 in my tin of valves and this one gives much better results, full brightness just gives a slightly bigger picture albeit a bit fuzzy. I've ordered a new EY51 to see if this is going to help. The LOPT gets very warm in operation, I don't know what is normal. Previous change to PY81 made little change.
Today Ive cleaned the case a bit and made a new bottom panel. Could someone please suggest if it should have ventilation holes in it?
Thanks for the schematic stitching Terry, I need a bigger printer to do it justice - I'll take it to work tomorrow.
Some more pictures. Corrected screen shots, inside LOPT can, exterior condition, etc. in my album here: T143 photos
Kind regards, David
