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Old 21st Dec 2009, 12:17 am   #121
oldticktock
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Default Re: Newb Stumped on a Derwent

6v between 4 & 5

the other end of the resistor went to pin 4 of the 42
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Old 21st Dec 2009, 12:20 am   #122
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Default Re: Newb Stumped on a Derwent

Question - is there a resistor between Pin 4 and Chassis right now. Yes or no?

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Old 21st Dec 2009, 12:25 am   #123
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Default Re: Newb Stumped on a Derwent

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Question - is there a resistor between Pin 4 and Chassis right now. Yes or no?

Cheers,

Steve P.
yes
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Old 21st Dec 2009, 12:26 am   #124
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Default Re: Newb Stumped on a Derwent

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Originally Posted by Station X View Post
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. ] ]
Post 87 says 75 has its own bias resistor of 5000 ohms

Last edited by electrogram; 21st Dec 2009 at 12:28 am. Reason: missed a word
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Old 21st Dec 2009, 12:27 am   #125
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Default Re: Newb Stumped on a Derwent

Does it measure OK? 500 Kohms or so?

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Old 21st Dec 2009, 12:28 am   #126
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Default Re: Newb Stumped on a Derwent

510k to be exact or 510,000
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Old 21st Dec 2009, 12:30 am   #127
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Default Re: Newb Stumped on a Derwent

Fine. Have you got a scope I wonder?

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Old 21st Dec 2009, 12:32 am   #128
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by electrogram
Post 87 says 75 has its own bias resistor of 5000 ohms
It actually says he measured 5000 ohms, which isn't the same thing. Also the drawings show the cathodes of the 42 and the 75 connected together. It doesn't look right to me, but that's how it is.
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Old 21st Dec 2009, 12:35 am   #129
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Default Re: Newb Stumped on a Derwent

Quote:
6v between 4 & 5
That's low, it should be the same as the cathode voltage ie 23V. Is THAT capacitor disconnected? If not measure again with THAT capacitor disconnected. Get rid of the surplus 510K resistor too.
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Old 21st Dec 2009, 12:35 am   #130
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Default Re: Newb Stumped on a Derwent

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Fine. Have you got a scope I wonder?

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I have a solatron ct160 but believe me that was bought with the intention of using when i'm grown up. also not been powered up for 3 years so will need to do so slowly on the variac
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Old 21st Dec 2009, 12:39 am   #131
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Default Re: Newb Stumped on a Derwent

hmm
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Old 21st Dec 2009, 12:40 am   #132
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Default Re: Newb Stumped on a Derwent

Pity. We could have looked for the Audio.

It's unlikely that THAT capacitor is faulty, but it happened to me once, so it's possible.

Cheers,

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Old 21st Dec 2009, 12:45 am   #133
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Default Re: Newb Stumped on a Derwent

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Pity. We could have looked for the Audio.

It's unlikely that THAT capacitor is faulty, but it happened to me once, so it's possible.

Cheers,

Steve P.
your up for giving me pain arn't you ....... can you imagine numpty boy and an oscilloscope. I bought a book "Understanding oscilloscopes and display waveforms", these guys are mad!!!
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Old 21st Dec 2009, 12:47 am   #134
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Default Re: Newb Stumped on a Derwent

Im really wondering if we could have a piccy of the whole underside of the chassis - this is just an overgrown US midget set, so really shouldn't present this much trouble getting the audio stage to work reliably!

I also think we need to standardise on some terminology here.....
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Old 21st Dec 2009, 12:49 am   #135
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Default Re: Newb Stumped on a Derwent

He's right, we should. But, looking at the picture that OTT drew, I would like to disconnect the tone control and try it.

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Old 21st Dec 2009, 12:49 am   #136
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by Sean Williams View Post
Im really wondering if we could have a piccy of the whole underside of the chassis - this is just an overgrown US midget set, so really shouldn't present this much trouble getting the audio stage to work reliably!

I also think we need to standardise on some terminology here.....
Underside already posted not sure how far back probably page 4,5 or 6. On the main holding (domestic) you can see all the attachments i thin its "derchas"
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Old 21st Dec 2009, 12:51 am   #137
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Default Re: Newb Stumped on a Derwent

a photo was taken here it is again

Oops it won't let me load same one anyway its not correct now as the 50uf is back in place the right way round.

Last edited by oldticktock; 21st Dec 2009 at 12:57 am.
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Old 21st Dec 2009, 1:01 am   #138
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by Station X View Post
Quote:
Originally Posted by electrogram
Post 87 says 75 has its own bias resistor of 5000 ohms
It actually says he measured 5000 ohms, which isn't the same thing. Also the drawings show the cathodes of the 42 and the 75 connected together. It doesn't look right to me, but that's how it is.
Are you saying cathode current of 42 passes through both 470 and the 'measured' 5000, but in that case the 510k on pin 4 of 42 might better be returned to junction of the two resisrors the 540 and 5000, that is pin 5 cathode pin of 75, therefore the only error in post 53 is that 510k was connected to pin 5 not 4 of 42

Last edited by electrogram; 21st Dec 2009 at 1:12 am. Reason: missed a word
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Old 21st Dec 2009, 1:11 am   #139
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by Station X View Post
Quote:
6v between 4 & 5
That's low, it should be the same as the cathode voltage ie 23V. Is THAT capacitor disconnected? If not measure again with THAT capacitor disconnected. Get rid of the surplus 510K resistor too.
as requested and 23v is the result

Last edited by oldticktock; 21st Dec 2009 at 1:16 am.
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Old 21st Dec 2009, 1:22 am   #140
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Default Re: Newb Stumped on a Derwent

So with THAT capacitor connected you get 6V between pins 4 and 5 of the 42, and with that capacitor disconnected you get 23V between pins 4 and 5?

If that's the case then THAT capacitor is faulty.
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