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Old 2nd Jun 2015, 7:22 pm   #1
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Default Derwent 9" television

I`ve been restoring this late 40`s early 50`s set and it shows a descent test card but the trouble is the line hold pot gets hot.
After the chassis has been running for half an hour , the area of the chassis where the pot is secured to gets warm and I think if it were not attached to the chassis frame it would get to hot to touch.
Now I didn't think pots were suppose to get hot like this.
When I first started on this chassis there were 3 wire wound resistors open circuit that had corroded and the wire unravelled .
I calculated there resistance by measuring the good part of the resistor that had not unravelled and fitted the values shown in the circuit diagram.
These resistors are the 6.8K screen feed to the EL38 , the 1K series resistor to the line hold pot and the 560K off the width coil.
I used a carbon type for the 560K.
I had calculated the resistor in series with the line hold pot to be 3 to 4K but I could only lock the picture by using a 1K so this is why i`m thinking something isn't right.
I don't have a circuit diagram for this set and the fact that Derwent made and maintained there own sets, I doubt that a diagram exists. To enable you guys to offer help I drew the diagram below.
The HT line is 256v and the 6.8K puts 75v on the screen of the EL38 line out put valve.
The Plessey mk1 chassis is similar and this is on trader sheet 984/T10.
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Old 2nd Jun 2015, 8:29 pm   #2
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Default Re: Derwent 9" television

Can you check the diagram, there is a s/c across the components in the grid of the EL38, well that's how it seems to me.
Frank
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Old 2nd Jun 2015, 8:31 pm   #3
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Default Re: Derwent 9" television

Hi Robin,
I reckon your best approach to solving the problem you have with the Derwent TV will be to rewire the line timebase circuit to the specification of the Plessey Mark 1. The circuit of the Regentone T15L in fact.
Is the chassis of Central Equipment manufacture or is it the Plessey Mark 1? The latter is easily identified by it's RF chassis. In the Mark 1 a 0.25mfd capacitor is wired across the 1Kohm line hold control. The secondary of the line output transformer feeds the scan coils via a 100mfd capacitor the other side of the winding goes to chassis. The secondary also connects to the line hold control and the suppressor grid of the EL38 (V11) via the 100Kohm line linearity control and a 27Kohm series resistor. The screen grid volts should be 70. Your HT voltage is correct.

I hope these notes help.

DFWB.
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Old 2nd Jun 2015, 9:05 pm   #4
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Default Re: Derwent 9" television

A follow up to my last post. The cold side brightness control should be returned to chassis via the resistor R65. The value can vary from model to model, depending which make and type of CRT is fitted, it's 4.7Kohm in the Regentone T15L.
The resistor connected to the bottom end of the brightness control in your set should be disconnected from the grid of the EL38 and returned to chassis. The line hold control will then be correctly wired across the 025mfd grid coupling capacitor.

DFWB.
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Old 3rd Jun 2015, 6:57 pm   #5
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Default Re: Derwent 9" television

Thank you frank for your reply.
As far as I know the circuit is correct but I may have made a mistake , I`ll have another look.
Thanks for your help david, I`ll change the brightness circuit to what you advise tomorrow and report back.
My chassis was made by central equipment.

Robin
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Old 3rd Jun 2015, 7:06 pm   #6
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Default Re: Derwent 9" television

Robin,
David knows far more than I do about theses sets, it is was just an observation but has David says that the line hold control is across the capacitor so it cannot be too far wrong. Make the changes he suggests, look forward to seeing the picture on the CRT.
Frank
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Old 3rd Jun 2015, 8:08 pm   #7
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Default Re: Derwent 9" television

Hi Robin,
I think this is the set Derwent kept in production into the mid fifties as an entry level model. Some sets were equipped with a thirteen channel turret tuner. It could of even been offered as a second set to well heeled rental customers.
The last picture of I saw of one these sets it had a Mullard MW22-16 CRT.
I'm looking forward to seeing pictures on your set.

DFWB.
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Old 4th Jun 2015, 7:51 am   #8
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Default Re: Derwent 9" television

I would imagine that this is a rare set. I have a Derwent radio, but have never seen one of their TV's. Any chance of some photo's?

Mark
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Old 4th Jun 2015, 2:39 pm   #9
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Default Re: Derwent 9" television

Circuit diagram of the line timebase of the Regentone T15L. As you can see it resembles the circuit drawn out by Robin in a previous post.

Quite simply what Central Equipment have done, they've bought in the timebase parts and the circuit design from Plessey and incorporated them in their own make chassis. A lot of TV manufactures used Plessey components.

DFWB.
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Old 4th Jun 2015, 8:22 pm   #10
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Default Re: Derwent 9" television

OK today I disconnected the brightness circuit from the EL38 grid circuit by just putting the bottom end of the brightness pot to ground via a 5K resistor.
The brightness control (1 meg) worked fine but the line hold pot still gets just as hot.
I think the only way to stop the line hold pot over heating is to rewire the line output stage as david suggests but the fact that the chassis is up and running means I`m tempted to leave it as it is.
The line hold pot is a wire wound type and can probably cope with these conditions , it certantly does not smell of burning.
I`m going to study that regentone circuit a bit more david.
This set originally used a CRM92 crt but that had been replaced a long time ago by an MW22-16 which was very low emission so was replaced by a better one.
The focus assembly just about focuses the picture at it max setting and you may notice on the test card picture below that the picture is not quite in focus.
Robin
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Old 4th Jun 2015, 9:03 pm   #11
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Default Re: Derwent 9" television

Looks like it has just come out of the factory.
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Old 4th Jun 2015, 9:29 pm   #12
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Default Re: Derwent 9" television

It didn't look that good when I first got it , lots of rubbing down and spray painting.

Robin
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Old 5th Jun 2015, 6:37 am   #13
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Default Re: Derwent 9" television

I can well imagine, it does look really good.
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Old 5th Jun 2015, 7:08 am   #14
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Default Re: Derwent 9" television

You have certainly done an excellent job on the chassis, I admire your patience!
Nice to see a set fitted with a mains transformer for a change.

Mark
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Old 5th Jun 2015, 10:37 am   #15
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Default Re: Derwent 9" television

I checked the temperature of the line hold control in the "upside down" Philco.
This set has the Plessey chassis. The control runs cool irrespective of the position of the slider. The fact the line hold control runs hot suggests a very high current is passing through the variable resistor and it would also suggest that current also flows through grid of the valve.
Very strange.
It might be worthwhile checking the 0.25mfd capacitor which is across the line hold control. A slight leak wouldn't matter much through.

DFWB.
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Old 5th Jun 2015, 9:24 pm   #16
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Default Re: Derwent 9" television

Thankyou for checking the temp of the line hold pot on the Plessey chassis david , I was thinking of doing this also.
Is this problem likely to load the lopt as it does get warm ?
All caps have been replaced.
If I were to alter the line output stage to match the Plessey circuit how would I connect the 2 wires from the width coil that go to line shift control, I don't have these shift controls on the derwent.
I think a lot of current is flowing because the 6.8K screen feed resistor to the EL38 gets really hot.

Robin
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Old 5th Jun 2015, 9:42 pm   #17
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Default Re: Derwent 9" television

Hi Robin,
The line and frame shift controls were only fitted on the Regentone T15L Mark 2 models. I do know this feature did result in some rewiring of the line output transformer. The circuit diagram is of the original Plessey Mark 1 chassis and is fitted in the Philco and also the Defiant version of the same model, so in many respects it resembles the Derwent circuit.
I don't think the line output transformer is the reason why the hold control runs hot.

DFWB.
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Old 6th Jun 2015, 2:49 pm   #18
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Default Re: Derwent 9" television

Hi Robin,
It's worth noting that in the Regentone T15 the scan coils are connected via a 100mfd capacitor to the EL38 grid winding of the line OP transformer. The other scan coil connection goes to the width control.
Also in the T15 the scan coils have a centre tap. That 560 Kohm resistor is in my opinion is too high a value to be of any use in that circuit. C30 and R52 (0.002mfd and 5Kohms) could be introduced into your circuit.
So it might be worthwhile checking the line scan coils in the Derwent TV to determine if they have a centre tap connection.

DFWB.
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Old 9th Jun 2015, 7:34 pm   #19
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Default Re: Derwent 9" television

Today I added a 0.002uf cap and a 5K resistor to the derwent connected as c30 & r52 on the circuit david posted (Plessey).
When this was tried the brightness level dropped slightly and the 5K resistor got hot very quickly despite being rated at 1W.
I forgot to look at the scan coils for a centre tap.

Robin
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Old 9th Jun 2015, 10:59 pm   #20
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Default Re: Derwent 9" television

Hi Robin,
The circuit of the McMichael model 512 resembles in many respects the Plessey Mark 1 and also your Derwent 9" TV.
The frame timebase circuit of the 512 is very similar to your set. The double triode is a 12AU7 which is also used in some late production Plessey sets. The line timebase is like the Derwent and Plessey Mk1 a self oscillating circuit and employs an EL38.
The circuit diagram of the McMichael 512 can be found on pages 460 and 461 in volume II of the Molloy and Poole Television servicing book.

I'll post up the circuit diagrams tomorrow.

DFWB.
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