Murphy V659 LOPT Substitution

[Murphyv310]

Hi.
With yet another oil filled LOPT giving up the ghost it was about time I came up with a solution.
I've opened up many of these in my time and had about 40% success.
This time I thought no, time to see if a jelly pot will work.
My wife Judy must think I've abandoned her but she is happy to see me getting back to more things. I can say after a good few hours this works very with a Thorn 900 jelly pot. I also used the stabilised width circuit from the Thorn. I'll post up the circuit in a day or two with some pictures as well.
I've done a YouTube video showing the work, still to be tidied and the EHT rewired.
https://youtu.be/jMKGGdd8ezo

[ppppenguin]

Those jellypot LOPTs seem to be one of the best designs ever. Thorn certainly got them right.

[Geordie McBoyne]

I have the remote control version of Trevor's Murphy and the line output transformer is giving trouble. The picture starts to fade after less than one hours use. Another Murphy, a V410 which is the 90 degree CRT development of the V310A. That set was exhibiting the same fault as the 19" V659 but now the line OP transformer has gone short circuit to chassis.
And just to compound the Murphy awfulness there's a customers V280 in the shop needing a replacement U25 EHT rectifier and the valve is inside the oil filled can.

Geordie McBoyne.

[Geordie McBoyne]

Those jellypot LOPTs seem to be one of the best designs ever. Thorn certainly got them right.

I can't ever remember replacing one. Thorn was certainly ahead of the game in 1964. Some very early production 900 series receivers did employ a conventional transformer. Similar to the one used in the 850 series.

Geordie McBoyne.

[Murphyv310]

Hi.
My stock of jelly pots is now down to one 1500 left.
This might be the last one I can do unless I'm able to source more.

[Geordie McBoyne]

The attachment shows the stabilised line timebase employed in the 1957/58 Peto-Scott model TV1720.  An OA71 diode is used to rectify a portion of the flyback pulse.  Maybe this circuit could be considered for the Murphy V659.

Geordie McBoyne.

[Murphyv310]

Neat circuit David.
May try it on the next set if I get some more jelly pots.

[Murphyv310]

Hi all boxed up and running on test for the last few hours in the shed which is at 28c, really warm.
Good solid results with a tiny hint of striations.
Will draw out the diagram later.

[Murphyv310]

Hi.
Here we have the circuits of the two different TV timebases and a rough idea of what to do.
I will explain as best as possible.

On the Murphy firstly remove the LOPT, U26 holder and EHT lead. If the original LOPT is not going to be salvaged just snip the leads as close to the LOPT as possible, Remove one PK screw that holds the LOPT bracket at the lower part of the line TB section and remove the bracket.
On the 900 LOPT remove the LOPT from the bracket and remove the lip on the bracket by simply bending it off, dress the rough edge with a file. Turn te bracket upside down and fit refit the LOPT to the bracket, you'll need two thin washers under the LOPT to the bracket to compensate for the recessed holes on the bracket, as the LOPT wont sit properly without them. The LOPT can be mounted in the angled section of the chassis with two PK screws, make sure it is mounted to clear the the removable cover. How you want to do the EHT diodes is up to you. I used a little pick up cartridge box and potted three diodes and the cap with hot melt done in layers.
Once fitted now do the following. The 30P19 anode goes to "E" on the jellypot, using the blue lead, the U193 cathode goes to "D" using the Yellow lead. The Gray lead from the scan coils go to "A" not L39 but the LOPT end of the choke. Red/White goes to "C" on the jellypot. Fit a 330pf 2kv cap between "A & B" of the jellypot. Remove the following leads from the set that you have cut Brown, Green & White from their respective components.
Remove R316 220k. Follow the 900 circuit for the stabilised EHT, I used 100pf pulse caps for C105 & C106. R129 is 1.5m, R133 is 820k which goes to "A" on the new LOPT. The VDR has a green paint spot on one end, it is not polarity conscious. You need 25-40v negative volts for the linearity control, where you removed R316, on my set the mains dropper was partially O/C and the rectifier supplies negative volts to the heaters so I just tapped off around 60v fed this via a 18K resistor to a 100uf cap with a 30v zener across it to give -30v for the control. Lastly add a 0.22uf 250v cap for "S" correction in series with the Grey lead from the scan coils.
Time to check the work and test. I advise using a variac set to 160v initially, gradually increasing it once scan is seen. If all goes well at 180v the width control will hold the width constant with rising input volts.

Hope this all helps

[Murphyv310]

Here is the potted EHT rectifier assembly.