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Old 19th Dec 2009, 11:11 pm   #21
Steve_P
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Default Re: Newb Stumped on a Derwent

Sorry I've been away. Watching something on TV. (Abuse by PM please - Strictly Come Dancing.)

Cathode Resistor and Capacitor : swap them both. If they are both there. I don't know the set.

The valve details are available here: http://www.r-type.org/exhib/aaa0788.htm

There should be a negative voltage at Pin 4. If it's positive then there is a voltage getting on the grid which won't do the valve a lot of good.

If it's positive, lift the wires and check the valve - it should be Zero or negative. If it's positive, you may well need a new valve. But check the voltage on the wire too and make sure it's not got a positive voltage on it.

If it has, trace this back and you'll come to a capacitor known as 'that capacitor'. Do change it...

Cheers,

Steve P.
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Old 19th Dec 2009, 11:27 pm   #22
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Default Re: Newb Stumped on a Derwent

Cheers all and especially Steve for your help this evening. I sort of feel like i've learned something although its not too well organised in the grey matter yet. Hmmmmm TV choice........... say no more.

I'm fried and think i'm going to pick this up in the morning, I wont be doing much else as we have had about 6 inches of snow on top of the last lot we had up here, looking like a white one

will try and make sense of the good advice.

All the best
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Old 19th Dec 2009, 11:30 pm   #23
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Default Re: Newb Stumped on a Derwent

Derwent was a Co-op brand, as for manufacturer..... later stuff I think was plessey/rediffusion?
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Old 19th Dec 2009, 11:48 pm   #24
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Default Re: Newb Stumped on a Derwent

Don't worry OldTickTock. I'd be the same if someone gave me a clock to look at! We're nearly there. (Oh and Dilys insisted I watch it. It's a small price to pay for changing the kitchen into a workshop!)

Read this lot :

http://www.vintage-radio.com/repair-...ion/index.html

A lot of it you won't need on this set, but it simplifies all the stuff in the 1940s books.

A Co-op brand - well the later ones are Plessey, but the earlier stuff, well who knows. Just get the valves from here: http://www.r-type.org/ and that'll do unless someone has the data. I'll look in my paper stocks tomorrow.

Cheers,

Steve P.
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Old 20th Dec 2009, 10:56 am   #25
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Default Re: Newb Stumped on a Derwent

I think we need to vereyfi that the output valve is the correct type,there is something odd 340 volts on g1 ? can you check on ohms range pin 5 to chassis olso what is feeding pin 4? .also trace pin 2 where does it go ? all with the unpluged ofcourse.
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Old 20th Dec 2009, 11:24 am   #26
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Default Re: Newb Stumped on a Derwent

Brrrr its cold......

Vinrads

pin 2 on the 42 seems to be linked to the 75 valve which is fed from the mains transformer.

pin 4 seems to be wired onto a long wire rail (rail is about 1.5mm diameter) lots of components and wires terminate onto this. I noticed that one end finally ends up on one of the 4 pins which is the 4 pin loom connector that hooks the chassis up to the speaker.The other end seems to go to a dual tag which goes to chassis, the other tag goes to a can (coil).

pin 5 tried getting an OHMS reading even on the meg range backing of Q as advised on the AVO. Seems to be infinite resistance or i'm doing something wrong, when the probe is touched on the pin there is a tiny and i mean tiny needle kick but nothing to be read.
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Old 20th Dec 2009, 11:49 am   #27
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by Steve_P View Post
Cathode Resistor and Capacitor : swap them both. If they are both there. I don't know the set.

The valve details are available here: http://www.r-type.org/exhib/aaa0788.htm

There should be a negative voltage at Pin 4. If it's positive then there is a voltage getting on the grid which won't do the valve a lot of good.

If it's positive, lift the wires and check the valve - it should be Zero or negative. If it's positive, you may well need a new valve. But check the voltage on the wire too and make sure it's not got a positive voltage on it.

If it has, trace this back and you'll come to a capacitor known as 'that capacitor'. Do change it...
I had already replaced the cathode cap (0.01uf) with a new one and replaced the in series resistor a 220000 ohm resistor which terminates onto a wire rail that goes to one pin of 4 pins which is the 4 pin loom connector that hooks the chassis up to the speaker
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Old 20th Dec 2009, 12:32 pm   #28
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Default Re: Newb Stumped on a Derwent

the plot get's thicker! hope the snow is'nt ,can you tell if any of the wireing has been altered? lets start again with the 42 base is it a six pin with two thick pins together,these are the heater connections if the valve lights up ok you know these connections are ok, that leaves 4 pins the only two that you should have HT on are pin 2 anode which comes from the output transformer and pin3 grid 2, make sure you are counting the pins correctly start with one of the heater pins the next pin clockwise wants to be a thiner pin this is pin two, a pic would be helpfull, regds Mick.
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Old 20th Dec 2009, 12:49 pm   #29
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Default Re: Newb Stumped on a Derwent

valve 42

pin 6&1 heaters pin1=15-30v with meter set to read ac - pin 6 no reading
pin2 anode=320v with meter set to read dc
pin3 grid 2=340v with meter set to read dc
pin4 grid 1-0 no readings when meter set to either ac or dc
pin5 cathode=30v meter set to dc
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Last edited by Station X; 20th Dec 2009 at 1:32 pm. Reason: Voltage readings pasted in from following post.
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Old 20th Dec 2009, 1:23 pm   #30
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Default Re: Newb Stumped on a Derwent

Oops took to long to edit that post so here are the reading so ignore the above soz station x

Ok with head cleared and pins understood the newb ventures once again into the fray

valve 42

yes six pin looking at valve top down its lit

pin 6&1 heaters pin1=15-30v with meter set to read ac - pin 6 no reading
pin2 anode=320v with meter set to read dc
pin3 grid 2=340v with meter set to read dc
pin4 grid 1-0 no readings when meter set to either ac or dc
pin5 cathode=30v meter set to dc
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Old 20th Dec 2009, 1:26 pm   #31
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Default Re: Newb Stumped on a Derwent

Try putting a signal in on Pin 4 (Grid 1). Does it stay or does it go?

Cheers,

Steve P.
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Old 20th Dec 2009, 1:27 pm   #32
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Default Re: Newb Stumped on a Derwent

Any chance of moving the yellow capacitor out of the way and taking another picture? It obscures a lot of wiring detail.
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Old 20th Dec 2009, 1:36 pm   #33
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Default Re: Newb Stumped on a Derwent

better view I hope
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Old 20th Dec 2009, 1:42 pm   #34
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Default Re: Newb Stumped on a Derwent

view from another angle
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Old 20th Dec 2009, 1:48 pm   #35
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Default Re: Newb Stumped on a Derwent

You can see the output transformer with the tone correction capacitor wired across the primary. One side of the primary goes to the HT rail (thick wire) and the other side should go to the anode of the output valve. Try to draw the circuit based on that.

EDIT.

There's a wire from the HT rail to pin 3. That'll be the screen grid connection.
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Old 20th Dec 2009, 1:57 pm   #36
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Default Re: Newb Stumped on a Derwent

can anyone explain and is it not significant what happened last night
.... I accidently set the avo 7 to read ac, with the ac dial set to 10 range touch pin 5 (cathode) with the probe and wow station heard loud albeit a bit a tad distorted this can be trimmed better with the tone. If I change the range up to 100 the signal goes.
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Old 20th Dec 2009, 2:00 pm   #37
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Default Re: Newb Stumped on a Derwent

is the 220k ohm red red yellow, resistor connected to pin4 (grid1) and the other end to chassis make sure that thick wire goes to chassis ,i take it the 220k is ok it looks new,what is the reading from pin 5 (cathode) to chassis in ohms with set off.
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Old 20th Dec 2009, 2:02 pm   #38
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Default Re: Newb Stumped on a Derwent

Based on your readings 30V DC seems rather high for a cathode voltage. I can only assume that connecting your meter pulled down the cathode voltage to a lower value which made the set work.

I don't know the biasing arrangements for this radio, but normally I would expect to see a resistor between cathode and earth and another between grid 1 and earth.
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Old 20th Dec 2009, 2:05 pm   #39
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Post Re: Newb Stumped on a Derwent

Sounds to me as if you have a simple coupling fault, "that cap" if it has not been changed, the Avo is giving an alternative path being AC coupled, the higher range just means the coupling is too small and attenuates the signal.

Or it could be the valve grid is slowly being driven more and more negative by a bias fault until the valve is cut off. A high HT on the anode would result.

Geof
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Old 20th Dec 2009, 2:10 pm   #40
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Default Re: Newb Stumped on a Derwent

We'd better establish whether the thick wire is earth or HT. With the set on what voltage do you get between chassis and the thick wire?
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