Sobel T143 evaluation

[pwdrive]

In the schematic did you figure out how the boost voltage is obtained and the LOP screen voltage?

Lawrence.

[Refugee]

The cogging sure is in line with the castelations so it could also be getting thru the power supply too.

[djrm]

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

[Murphyv310]

Hi David.
Removing the cap from the video amps cathode will reduce its HF response, as pointed out earlier the very sharp changes will give HF transients that the circuit in the set is not designed for, one disadvantage of feeding video in at baseband with no filtering. I'm sure feeding the set correctly from an aurora or Franks Hedghog will be a cure. The main issue with sync coupling has been addressed as the capacitor leaks and makes the grid positive.

The base will have holes for ventilation.

[Terry]

In the schematic did you figure out how the boost voltage is obtained and the LOP screen voltage?

I replied to this some time ago but it seems to have vanished!

This circuit it bizarre!

I note that there is a vertical line of dots on every horizontal line immediately to the right of the LOPT as if two different schematics might have been spliced together but that still would explain the discrepancy between the plug diagram and the socket as they're both on the same section!

And it doesn't help with the errors I spotted earlier.

One thing is for certain: any set that was built as depicted in this schematic could not work!

Does anyone have an alternative version, I wonder?

[Terry]

The base will have holes for ventilation.

The holes in the bottom cover would normally be like those in the back and not easy to reproduce.As there should be no way that anyone could get to the underside of the chassis in normal operation, it would probably be better to leave it as you found it, without the cover.

Just a thought: if you have a known good electrolytic to hand, try putting it across the HT rail and see if it affects the cogging.

When I was 16, I had a Pye VT4 that suddenly lost its line sync in the middle of a programme. I changed just about everything, to no avail.

A colleague later looked at the set and traced the fault to the main smoothing! I've never forgotten that ...

[Murphyv310]

Hi.
I'd agree with terry re the bottom cover, without it the set will run cooler and with you already saying the LOPT is running hot some extra cooling won't go amiss.
The circuit is rather odd in places and I have seen numerous errors, the boost and screen wiring is odd at first glance but there are reasons its done this way. Time for Lawrence to give us a tutorial unless he's not cracked it..... so over to you Lawrence unless you throw in the towel!!

[djrm]

HI Terry,
Good ide to try adding smoothing to the HT, I've bee meaning to try this as there is a bit of a wave beating with the frame rate.
The line and frame hold controlls do indeed ineract a bit.

I think there is an interlock preventing the line output from operating if the octal coil plug is removed. 

I see there is a siilar set up for auction at present, this one has the turret tuner modification applied. It also has a replacement pegboard back. I could make a bottom cover out of that if I had some. Failing that perhaps I coould get my son to put some slots in it with his CNC machine.

David.

[Terry]

I think there is an interlock preventing the line output from operating if the octal coil plug is removed. 

Yes David, a permanent one! As Lawrence asked earlier:

In the schematic did you figure out how the boost voltage is obtained and the LOP screen voltage?

The line output valve screen grid is connected to pin 8 of the plug and the schematic shows a link in the plug between pins 8 & 4 but there is nothing shown connected to pin 4 of the socket!

[pwdrive]

Yes, my original question was a hint to the fact that their was a schematic error, eg: no screen volts, no boost (boost is boosted HT, no HT = no boost)

The other error I pointed out was the 1380 (ohm?) one of the cathode bias resistors in the ECL80 sound circuit, should probably have been listed as 180 ohm.

Lawrence.