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Old 20th Dec 2009, 4:03 pm   #61
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Default Re: Newb Stumped on a Derwent

electrogram's post looks good to me.

The 220K resistor and 0.1uF capacitor are a red herring. It's the anode resistor of the 75 and associated decoupler.
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Old 20th Dec 2009, 4:06 pm   #62
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Default Re: Newb Stumped on a Derwent

Quote:
The cathode bias resisitor is a 1% type and seems to be green-brown-black-brown which is 5K
Isn't that a bit high for a cathode bias resistor? Something around 500R seems more likely.
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Old 20th Dec 2009, 4:11 pm   #63
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Default Re: Newb Stumped on a Derwent

Yes. It is. Chop it out and fit a 500 Ohm one.

I've been looking at other circuits for this era and 500 ohms is about right. Put a 1 Megohm between Pin 4 and Earth.

Then try it.

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Old 20th Dec 2009, 4:15 pm   #64
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Default Re: Newb Stumped on a Derwent

Steve.

It appears that a 500 or perhaps 540R resistor is already wired cathode to earth, but IN PARALLEL with the 510K. So perhaps the 510K is the grid leak, but goes to the wrong pin?
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Old 20th Dec 2009, 4:33 pm   #65
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Default Re: Newb Stumped on a Derwent

Possibly. It would cause a problem. But a new resistor will only work better, as we don't know what value it is. Should be yes, what is is, no. If it's open or high, then it won't do any harm.

When the set works, bring the pliers into play.

Cheers,

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Old 20th Dec 2009, 4:57 pm   #66
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Default Re: Newb Stumped on a Derwent

Just had a sanity break and i'm up for the next round ..... Oooo my head hurts will just read through and gather my thoughts and make sure I get all your advice clear, bear with me. So appreciative of your help.
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Old 20th Dec 2009, 5:10 pm   #67
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Default Re: Newb Stumped on a Derwent

Sorry guys so much advice came in it's left me a bit lost.

I think its the following

1. cut end of wire that joins 42 pin 4 to 75 chassis tag (as per Steve P) and confirmed by station x

2. cut other end at 42 pin4 and insert a high value resistor ( whats high 10k 20k 100k )

3 swap 50uf around as its wrong

4 510k resistor should be terminated on pin 4 of 42.

Last edited by oldticktock; 20th Dec 2009 at 5:16 pm. Reason: spelling
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Old 20th Dec 2009, 5:12 pm   #68
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Post Re: Newb Stumped on a Derwent

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Originally Posted by Station X View Post
Steve.

So perhaps the 510K is the grid leak, but goes to the wrong pin?
This might explain why the Avo makes it work, as a temporary substitute, this avenue should be explored more as it is the clue to why the valve is not working.

And I keep knocking my post of top spot
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Old 20th Dec 2009, 5:16 pm   #69
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Default Re: Newb Stumped on a Derwent

Quote:
1. cut end of wire that joins 42 pin 4 to 75 chassis tag (as per Steve P) and confirmed by station x

2. cut other end at 42 pin4 and insert a high value resister ( whats high 10k 20k 100k )

3 swap 50uf around as its wrong

4 510k resistor should be terminated on pin 4 of 42.
Almost right, but you can either shift the 510K from pin 5 to pin 4, or remove it altogether and connect a 1M resistor from pin 4 to earth. Either way you must break the link from pin 4 to earth.
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Old 20th Dec 2009, 5:17 pm   #70
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Default Re: Newb Stumped on a Derwent

Just steps 1 and 4. See what that does.

Cheers,

Steve P.
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Old 20th Dec 2009, 5:21 pm   #71
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Default Re: Newb Stumped on a Derwent

And step 3.
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Old 20th Dec 2009, 5:31 pm   #72
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Default Re: Newb Stumped on a Derwent

can I confirm that the pins on the 75 conform the same standard as the 42 in as much the heaters are the fat pins 1 & 6 so count clockwise round from pin one. Just want to make sure i get the 510 on the correct pin

Last edited by oldticktock; 20th Dec 2009 at 5:37 pm.
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Old 20th Dec 2009, 5:48 pm   #73
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Default Re: Newb Stumped on a Derwent

There is confusion here. The resisitor needs to be moved from pin 5 to pin 4 of the 42.

NOT THE 75.
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Old 20th Dec 2009, 5:54 pm   #74
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Default Re: Newb Stumped on a Derwent

Ok thats about to be done then I will fire it up ...... well lets home not really
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Old 20th Dec 2009, 5:59 pm   #75
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Default Re: Newb Stumped on a Derwent

Here's how the circuit should be.
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Old 20th Dec 2009, 6:05 pm   #76
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Default Re: Newb Stumped on a Derwent

I'm off to work on one of my own radios now. If the fix doesn't work check the DC voltages between chassis and pins 2 through to 5. Also the voltage between pins 4 and 5.

Post the results here.
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Old 20th Dec 2009, 6:12 pm   #77
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by Station X View Post
I'm off to work on one of my own radios now. If the fix doesn't work check the DC voltages between chassis and pins 2 through to 5. Also the voltage between pins 4 and 5.

Post the results here.
Thanks for all your help station x
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Old 20th Dec 2009, 6:26 pm   #78
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Default Re: Newb Stumped on a Derwent

Made changes as suggested, exactly the same scenario as before and when I did the avo trick it allows the station to be heard although this time its much more distorted the volume is on full

Last edited by oldticktock; 20th Dec 2009 at 6:45 pm.
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Old 20th Dec 2009, 6:28 pm   #79
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Default Re: Newb Stumped on a Derwent

Oh and voltage checks were the same
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Old 20th Dec 2009, 6:41 pm   #80
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Default Re: Newb Stumped on a Derwent

Actually pin 2 is now 315 instead of 325 and pin 3 is 335 instead of 345
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