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Old 20th Dec 2009, 10:34 pm   #101
Station X
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Default Re: Newb Stumped on a Derwent

OK. There is a 0.1uF capacitor from the anode of the previous stage (AF Amp) to the control grid of the 42.

It's called that capacitor because it always gives trouble and goes leaky. The reason for this is that it leads a hard life with the full HT across it.

Disconnect it at pin 4 of the 42, reconnect the 510K from chassis to pin 4, power up and check the 42 voltages again. If they're OK THAT capacitor is faulty.
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Old 20th Dec 2009, 10:35 pm   #102
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Default Re: Newb Stumped on a Derwent

Hi Old TT, PM me if you are still stuck and the snow has subsided. You can then arrange to bring it over some time and I'll go through it with you.

Best regards, Ed
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Old 20th Dec 2009, 10:37 pm   #103
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Default Re: Newb Stumped on a Derwent

Graham, the 75 cathode seems to be connected to the 42 cathode? I wonder if that's correct? Anyhow, the 50uF is goint to be important as any AC on the cathode would play havoc with detection and AGC.
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Old 20th Dec 2009, 10:37 pm   #104
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by Station X View Post
OK. There is a 0.1uF capacitor form the anode of the previous stage (AF Amp) to the control grid of the 42.

It's called that capacitor because it always gives trouble and goes leaky. The reason for this is that it leads a hard life with the full HT across it.

Disconect it at pin 4 of the 42, power up and check the 42 voltages again. If they're OK THAT capacitor is faulty.
surely it won't be faulty its a new modern yellow one rated at 750v I only put that in replacing an old waxy one a couple of days ago.
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Old 20th Dec 2009, 10:43 pm   #105
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Default Re: Newb Stumped on a Derwent

If it helps you find a service sheet from maybe some of the more knowlegeable of the 30's sets, the radio you have looks as though it could have been by Central Equipment Ltd.

The unit they released in 1935 was the Derwent "international five" , different looking model and a table top but the same aeroplane dial.

Just a suggestion to save your backside anymore damage

Mike
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Old 20th Dec 2009, 10:44 pm   #106
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Default Re: Newb Stumped on a Derwent

Unlikely. If you have a signal source to hand, put it in at Pin 4 (Control Grid) of the 42.

Cheers,

Steve P.
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Old 20th Dec 2009, 10:45 pm   #107
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Default Re: Newb Stumped on a Derwent

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Originally Posted by Ed_Dinning View Post
Hi Old TT, PM me if you are still stuck and the snow has subsided. You can then arrange to bring it over some time and I'll go through it with you.

Best regards, Ed
Cheers Ed, If I don't get anywhere I will take you up on that, as you say if the snow goes. We got it bad out in the sticks, Im about 3 miles from Tow Law and we always get hit

This investigation and assistance is fascinating a real education and baptism of fire if not also confusing, a real attack on the senses.
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Old 20th Dec 2009, 10:46 pm   #108
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Default Re: Newb Stumped on a Derwent

The two cathodes are probably connected as the two valves share a common cathode resistor and bypass capacitor. This is however an unusual arrangement.

I'm faced with the problem of trying to explain why connecting a 510K grid leak resistor pulls down all the voltages. This can only happen if there's a voltage on pin 4 and the grid leak is pulling it down to chassis. I can't see how this can happen unless that capacitor is faulty. Try disconnecting it as I suggested and recheck the voltages on the 42.

We still have the problem of the cathode voltage of the 42 being rather high and the 42 apparently drawing excess anode and or screen current. It's possible there may be other faults. Disconecting THAT capacitor will effectively isolate the output stage and may help to track down the fault
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Old 20th Dec 2009, 10:48 pm   #109
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Default Re: Newb Stumped on a Derwent

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Unlikely. If you have a signal source to hand, put it in at Pin 4 (Control Grid) of the 42.

Cheers,

Steve P.
Tried that, hope i was doing it right. Used the lead you talked me through making last night. Earth end on chassis and other end on pin.Nothing doing
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Old 20th Dec 2009, 10:57 pm   #110
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Default Re: Newb Stumped on a Derwent

Sorry I can't help but this an interesting and active thread so don't give up I want to know how its fixed, but if you wait for thaw in Tow Low we'll be here until July
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Old 20th Dec 2009, 10:58 pm   #111
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Default Re: Newb Stumped on a Derwent

Interesting disconnecting the 0.1 cap i found buried inside a heat shrink another 510k which would have been wired to chassis except we had cut that end. But as we had snipped the chassis end previously it will not help us but does show a component error was there.

I'm worried now i'm not getting DC readings at all AC on one side of the RECT 80S on DC on the other has the 80S now given up or what .................... I think I'm going to call this a day soon and start with a fresh head.

Last edited by oldticktock; 20th Dec 2009 at 11:05 pm.
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Old 20th Dec 2009, 11:05 pm   #112
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Default Re: Newb Stumped on a Derwent

could another 510k be the grid leak for the 75
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Old 20th Dec 2009, 11:12 pm   #113
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Default Re: Newb Stumped on a Derwent

maybe not, a prvious post said grid cap is connected to vol pot
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Old 20th Dec 2009, 11:24 pm   #114
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Default Re: Newb Stumped on a Derwent

Ok i'm shutting down for the night, somethings not right no DC at all now, not even on the rect, or i've stuffed my avo on dc.

head fried ass kicked need to go and mope
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Old 20th Dec 2009, 11:54 pm   #115
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Default Re: Newb Stumped on a Derwent

Ok I lied, im hooked I canna give it up i'm official a radio geek bed who needs it.

Ok the AVO seems to hate DC now what I did I don't know. Went to my test equip cupboard and pulled out my mains powered Cossor instrument reads AC DC-DC+ and OHMs

42 readings

pin 1 cannot get a reading
pin 2 250
pin 3 270
pin 4 0
pin 5 23
pin 6 cannot get a reading

anyone know why the avo 7 gave uo on dc? still reads ac and ohms
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Old 20th Dec 2009, 11:59 pm   #116
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Default Re: Newb Stumped on a Derwent

Still high is the Cathode voltage. The valve should be lit or there wouldn't be a cathode voltage.

With the set OFF, remove the 42 and measure to deck - Pin 4 and Pin 5. Post the readings.

Cheers,

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Old 21st Dec 2009, 12:01 am   #117
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Default Re: Newb Stumped on a Derwent

Quote:
42 readings

pin 1 cannot get a reading
pin 2 250
pin 3 270
pin 4 0
pin 5 23
pin 6 cannot get a reading
Looks good. What voltage do you measure between pins 4 and 5?

I assume the 50uF is back in its correct position ie negative end to chassis and positive end to pin 5 (cathode) of both valves?

Quote:
Interesting disconnecting the 0.1 cap i found buried inside a heat shrink another 510k which would have been wired to chassis except we had cut that end. But as we had snipped the chassis end previously it will not help us but does show a component error was there.
Where did the unearthed end of the other 510K go to?
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Old 21st Dec 2009, 12:09 am   #118
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Default Re: Newb Stumped on a Derwent

yup 50uf back as stated

not sure what you mean" between" pins 4 & 5

the additional 510k would have been to chassis right at the start of this thread but mid way we cut that end so it would have been negated
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Old 21st Dec 2009, 12:09 am   #119
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Default Re: Newb Stumped on a Derwent

Steve P.

The 75 is a triode and two diodes in the same envelope. They have a common cathode. Therefore the cathode current of all four valves (42 and 75) will flow through the cathode resistor. I'm wondering if it should be a lower value than 500ohms? It might be a body, tip spot type of 450 ohms.
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Old 21st Dec 2009, 12:12 am   #120
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Default Re: Newb Stumped on a Derwent

Quote:
not sure what you mean" between" pins 4 & 5
One lead of the meter to pin 4 and the other to pin 5.

Quote:
the additional 510k would have been to chassis right at the start of this thread but mid way we cut that end so it would have been negated
Yes, but where did the end which is not connected to chassis go to? I'm trying to establish what its function was/is
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