Derwent 9" television

[FERNSEH]

Circuit diagrams of the timebaes in the McMichael 512.
The frame timbase is very similar to the circuit in the Derwent.
In the line timebase the line hold control is on the other side of the transformer secondary, otherwise it is similar to the Plessey and Derwent circuits.
Line sync enters through C57 and the frame sync through C51.

DFWB.

[line sync]

Hi folks , its about time I came back to this thread and finished it off.
If you remember the chassis was working but the line hold pot was getting hot.
There not being any service information available it looked like the only option was to rewire the line o/p stage to the very similar Plessey chassis as david suggested but I didn`t want to do this so I put the chassis aside intending on coming back to it with new ideas.
I think it was January when I showed the circuit diagram I`d drawn to some friends who are more clued up than me and asked if they could see a reason why the line hold pot was over heating.
Apart from " have you changed this valve and are these caps good " nothing was obvious until I said " I don't suppose it matters that there is no core in the width coil does it ".
Well that was the reason for my problems as my friends pointed out there would be more current flowing threw the circuit with out the core.
When I got this set there was no core in the width coil so at some point in the sets past this must have been robed or fell out .
The core is a long round rod that slides into the width coils like the arrangement used in the pye B18T / BV20 for the line amp adjustment.
I had no idea how long this core should be so a lot of time was spent experimenting with different lengths.
I didn`t have the right size of core so I had to sand 1" long cores down to a smaller diameter on a belt sander.
I couldn't believe what a difference fitting this core made , of course it opened out the width but also altered the line frequency to the point that I had to change the value of the 1K resistor in series with the line hold pot.
The line hold pot is now running a lot cooler.
I must learn more about coils and tuned circuits as I thought leaving the width coil with no core would only affect the width.
While playing a film threw the set I was aware that there was not much volume.
I established the sound o/p stage was ok so it must be an alignment problem in the earlier sound stages.
I spent ages coping the sound and vision stages down onto paper so that I would know what to adjust ( remember no service info available )
There are two rf coils in the sound channel and adjusting one of these was just like finding a preset volume control , what a difference that made.
The chassis was now finished and was fitted back into its cabinet along with its tuner , the tuner being very fiddly to fit .
Although the cabinet has quite a few deep scratches it responded well to a going over with linseed oil.
The cream paint around the crt window had to be repainted as did the crt rubber mask.
Last job to do was make a back cover as the set didn`t come with one.
I made the back from mill board and having got a picture of an original derwent back , I tried to copy this the best I could.
Making the box for the crt neck was a real challenge but it turned out well.
I was lucky with the ventilation slots as there is a business where I work who have a press and just happened to have a slot shaped tool for it.
Because they knew me they let me use this machine and while I was there I did a back for another tv as well.
I could have a nice little earner making back panels for other collectors if I had a machine at home like that.
Over all I`m very pleased with this little set.

[Jac]

Nice write-up. Thank you.
A very good result with some hints for me to keep in mind.

I hadn't heard about linseed oil for buffing up a cabinet.
How do you apply that? Do you immediately remove the excess after application?

Thanks.

Regards,
Jac

[Heatercathodeshort]

What a fantastic result! A really good effort and such a rare little receiver. Well worth all the struggle and pain. The Derwent design team [who were very high caliber] would have been amazed if they had been told in 1952 that an example would still be working in 2016! Regards, John.

[McMurdo]

Lovely job all round, the chassis does look like a new one!! Everything looks quite sensibly laid out from the bits I can see.

[German Dalek]

Hey, thats a real beauty.
Thanks for sharing with us.
I never should say "no" to this set.....

German Dalek

[line sync]

Thanks for the nice comments guys.
I really like this derwent television so I spent a lot of time doing a fussy restoration.
Jac the linseed oil is just applied and wiped off after a minute or two.
I was experimenting with attenuators on this set to day and would you believe it a fault has happened.
After the set has fully warmed up the picture starts to smear and looks like the pictures below , the second picture with the contrast at maximum.
The camera didn`t capture the picture fault accurately.
I switched off for 5 minutes and when I switched back on again all was fine and then the fault slowly starts to show again after a few minutes.
I`ve checked connections and wiggled valves but this has no effect so I`m now thinking there is an RF stage valve playing up.
Does anyone have any ideas before I start pulling valves ?
It was working fine until I put the back on.

[Sideband]

Could be a heater/cathode leak in one of the valves (hope it's not the tube....), the clue being the few minutes taken for the fault to develop after switching on.

[Heatercathodeshort]

Not the usual symptoms with a heatercathode short in a Mullard tube but you never know.
Remove the tube heater connections and supply the heater via a 6V battery or if you have one, a special low capacity isolation transformer. That will prove the point. An ordinary transformer will result in very poor definition due to the capacity of the windings to earth.
John.

[line sync]

Thanks for your help sideband and john.
It turned out to be the PCF80 in the tuner , but testing it on the valve tester showed no faults but putting the valve back in the set again and the fault returns.
After refitting the replacement valve I ran the set for another hour and all was ok.

Robin

[Sideband]

Good news then! Glad it was an easy fix. Funny how these fault wait until you've put the back on...!

[6.3volts]

Congratulations on your restoration. What a superb looking set.
Your work making a new back has inspired me to look at a few of my sets with missing backs with a view to making my own.