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Old 21st Dec 2009, 12:21 pm   #161
oldticktock
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Default Re: Newb Stumped on a Derwent

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Originally Posted by Station X View Post
PJL.

Quite possible I'd think. That O/P TX held in with one screw doesn't look quite right.

We have to work with what we have though.
Oh Yes forgot about that, I thought O/P TX with one screw was odd but what with everything did resgister. Why do people do this to these old gems its sad
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Old 21st Dec 2009, 12:22 pm   #162
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Default Re: Newb Stumped on a Derwent

Ok off must go going to get in trouble otherwise. Back soon
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Old 21st Dec 2009, 1:07 pm   #163
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Default Re: Newb Stumped on a Derwent

If we are redesigning then I suggest you disconnect the 75 cathode pin 5 and connect to earth although I would like to have seen a sketch of all the components connected to the 75 back to the IFT first.
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Old 21st Dec 2009, 2:15 pm   #164
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Default Re: Newb Stumped on a Derwent

I don't think the cathodes were ever connected, it is just the way the sketch in post 83 was drawn with the angle in the resistor lead accidently touching the wire of the 50 uf, perhaps OTT could comment on this
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Old 21st Dec 2009, 2:16 pm   #165
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Default Re: Newb Stumped on a Derwent

PJL.

I've just looked at some Philco circuits and the Model 60 seems to fit the bill. That uses grounded grids and a common negative bias supply.

As you say if further mods are needed we'll need a sketch/circuit of the components round the 75.
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Old 21st Dec 2009, 2:22 pm   #166
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Default Re: Newb Stumped on a Derwent

OTT. Could you take a high rsolution picture of the underside of the chassis and email it to me please. I'll PM you my email address. Thanks.

I'll try to reverse engineer more of the circuit.
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Old 21st Dec 2009, 3:32 pm   #167
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Default Re: Newb Stumped on a Derwent

I'm Back, glad that's glad that's over. Right picky underway
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Old 21st Dec 2009, 3:49 pm   #168
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Default Re: Newb Stumped on a Derwent

Station X pm large image sent.

For others following the plot, smaller images one of whole chassis. The other a magnified shot of 42/75
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Last edited by oldticktock; 21st Dec 2009 at 3:56 pm.
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Old 21st Dec 2009, 5:56 pm   #169
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Default Re: Newb Stumped on a Derwent

the probe on pin 5 of 42 still makes it a lot better.

leaving the set on I can hear lots of oscillation in the back which now has cleared. volume is poor if I turn it up it distorts. on medium to low you can make out the people chatting, although they sound like the have sandpaper in their voice box, if you know what i mean.
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Old 21st Dec 2009, 6:22 pm   #170
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Default Re: Newb Stumped on a Derwent

What are the voltages chassis to pins 2,3,4 and 5 of the 42?

What are the resistances chassis to pins 4 and 5 of the 42 (set off)?

What are the voltages chassis to pins 2,5 and the top cap of the 75 to chassis?

I'm attempting to reverse engineer more of the circuit, but it will take some time.
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Old 21st Dec 2009, 8:02 pm   #171
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Default Re: Newb Stumped on a Derwent

ok as follows, can hear radio 4 i think its got clive anderson on, as i said very quite

valve 42

pin 2........250v
pin 3........260v
pin 4........0v.................ohms see below
pin 5........10v......330ohms

had to read had avo 7 on 1MEG and backed off Q seems to read 10,000 but such a small defelection its hard to read.

Valve 75

pin 2......250v
pin 5......4-5v fluctuates
T/C........0v

It is radio 4 ,The Archers just came on.
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Old 21st Dec 2009, 8:06 pm   #172
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Default Re: Newb Stumped on a Derwent

Are those Voltage measurements you gave for Valve '42'? If so, there is something very wrong in that stage !! With respect to cathode, you should get readings of :- G1 = (approx) -12V, G2 = <250V, A = < 315V. Alternatively, with respect to chassis (assuming auto-bias), G1 = 0, G2 = <262V, A = <327V, K = (approx) 12V.
The Heater Voltage across pins 1 & 6 should read 6.3VAC.
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Old 21st Dec 2009, 8:26 pm   #173
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Default Re: Newb Stumped on a Derwent

When I first switched on and started measuring the 42 i noticed voltage was moving about. Just left it on now for half an hour and now readings as follows

42

pin2 250v
pin3 270v
pin5 15v

75

pin2....270v
pin5....15v

Last edited by oldticktock; 21st Dec 2009 at 8:52 pm.
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Old 21st Dec 2009, 9:01 pm   #174
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Default Re: Newb Stumped on a Derwent

Almost certainly the 75 bias needs sorting out. The set has been modified and has probably never worked since. Your figures show the 75 is not conducting at all (the anode pin 2 is at HT voltage so there is no current flowing in the 220K resistor) which is what I would have expected as it appears to be sharing the bias with the 42.

I would hang on till Graham has had a chance to work out the circuit. I am sure it can be got to work but it is probably going to need a few extra components. What would be useful is if you could have a go at drawing out the circuit, particularly around the 75. A wiring drawing similar to your others would do. You certainly chose the wrong set to learn on - or perhaps not!

The voltage between the grid and cathode controls the current flow between the anode and cathode. If the grid is too negative then the current flow stops altogether (cut-off) and the signal on the grid will not be amplified. The grid of the 75 should be set (biased) so that the right amount of anode current is flowing to put about 100v on the anode. This current can be easily calculated from I=V/R or I=(270-100)/220,000 = 0.77mA. You can then lookup the required grid voltage in the characteristics chart here http://oldradio.qrz.ru/tubes/foreign/03/75.gif which is quite difficult to read but I think Va=100V and Ia=0.75mA is about -1V.

If you measre the voltage on the top cap of the 75 then your grid voltage is the Vtc-Vpin5 which I would predict is -15V.

Last edited by PJL; 21st Dec 2009 at 9:25 pm.
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Old 21st Dec 2009, 9:15 pm   #175
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Default Re: Newb Stumped on a Derwent

another set of other surrounding readings

HT rail is now showing 270v
14v on + side of the 50uf
4 pin chassis to speaker loom

1 pin to HT rail 270v
1 pin 300v
1 pin link internal to external via rear wander plug 42v (fluctuates)
1 pin to O/P TX 42v (fluctuates)

O/P TX 4 tags two out two inner

outer nearest the 42 read 42-45v
outer nearest the tone pot read 42-45v
inner nearest the 42 read 270v
inner nearest the tone pot read 300v
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Old 21st Dec 2009, 9:18 pm   #176
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Default Re: Newb Stumped on a Derwent

The readings in post 173 for the 42 don't look too bad to me.

Unlike in post 171 the readings of both cathodes (pin 5) are the same. They should be, as they're joined together!

I'm a bit concerned that you can't measure the resistance from pin 4 to chassis. What do you get if you measure the 510K resistor which was removed from the circuit?

Are the valveholder contacts clean and making good contact with the valve pins? If they're not it could account for the flucuating voltage readings.

I'm trying to reverse engineer the circuit from the pictures you emailed me, but it isn't easy. As PJL says if you could sketch the wiring around the 75 it would be useful.
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Old 21st Dec 2009, 9:42 pm   #177
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Default Re: Newb Stumped on a Derwent

Here is another sketch.

Guess this set is going to be a mare then, shame it's a lovely looking set.
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Old 21st Dec 2009, 9:55 pm   #178
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Default Re: Newb Stumped on a Derwent

Quote:
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The readings in post 173 for the 42 don't look too bad to me.

Unlike in post 171 the readings of both cathodes (pin 5) are the same. They should be, as they're joined together!

I'm a bit concerned that you can't measure the resistance from pin 4 to chassis. What do you get if you measure the 510K resistor which was removed from the circuit?


Are the valveholder contacts clean and making good contact with the valve pins? If they're not it could account for the flucuating voltage readings.

I'm trying to reverse engineer the circuit from the pictures you emailed me, but it isn't easy. As PJL says if you could sketch the wiring around the 75 it would be useful.
It's not that I can't measure, I cannot read, my eyes.... the removed 510k reads but I can only just read it, looking with my bench magnifier its 510k on 1Meg range. looks the same as the pin 4 reading

valve pins and holder clean

much appreciated
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Old 21st Dec 2009, 9:58 pm   #179
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Default Re: Newb Stumped on a Derwent

That's useful. (The sketch in post 177 that is) Is there a direct connection between pin 2 (anode) of the 42 and the top inner terminal of the output transformer or does it go via the tone control?

I assume that the "speaker loom" now carries just two wires to the speaker and the other pins are redundant?
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Old 21st Dec 2009, 10:12 pm   #180
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by Station X View Post
That's useful. (The sketch in post 177 that is) Is there a direct connection between pin 2 (anode) of the 42 and the top inner terminal of the output transformer or does it go via the tone control?

I assume that the "speaker loom" now carries just two wires to the speaker and the other pins are redundant?
Yes pin 2 of 42 is wired to inner O/P TX tag the 250v one that has one end of the 0.0047.....0.047...tone pot
The chassis to speaker loom still carries 4 wires and looks original
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