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Old 22nd Dec 2009, 5:17 pm   #261
PJL
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Default Re: Newb Stumped on a Derwent

It looks like you are nearly there. Speakers of this age often have no dust cover over the voice coil and need a bit of a vacuum to get them free again. Rarely the spider needs adjusting. Take the speaker off and lets have a picture of the front of it.

The valve might have had a little bit of air in it after many years of disuse. The air is absorbed by the gettering when the valve is left runnning for a bit. You do need to keep an eye on it and should leave the set on for an hour with the meter connected and check the voltage stays low. The 42 gm is about 2.5mA/V so don't let pin 4 exceed 4V.

I still wonder if the AGC is working with the new bias arrangement. Tune into a strong station and try with a short aerial (a foot) and then a long aerial. The long aerial should sound best but if it gets distorted the AGC isn't working properly.
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Old 22nd Dec 2009, 5:22 pm   #262
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Default Re: Newb Stumped on a Derwent

Given that OTT has a VVM he could measure the AGC voltage directly on strong and weak stations.
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Old 22nd Dec 2009, 5:22 pm   #263
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Default Re: Newb Stumped on a Derwent

Quote:
Originally Posted by PJL View Post
It looks like you are nearly there. Speakers of this age often have no dust cover over the voice coil and need a bit of a vacuum to get them free again. Rarely the spider needs adjusting. Take the speaker off and lets have a picture of the front of it.

The valve might have had a little bit of air in it after many years of disuse. The air is absorbed by the gettering when the valve is left runnning for a bit. You do need to keep an eye on it and should leave the set on for an hour with the meter connected and check the voltage stays low. The 42 gm is about 2.5mA/V so don't let pin 4 exceed 4V.

I still wonder if the AGC is working with the new bias arrangement. Tune into a strong station and try with a short aerial (a foot) and then a long aerial. The long aerial should sound best but if it gets distorted the AGC isn't working properly.
Hi PJL spooky, Just got my tools out and was about to remove the speaker from the cab to have a look. will post a pic
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Old 22nd Dec 2009, 5:23 pm   #264
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Default Re: Newb Stumped on a Derwent

Graham, I agree but that would mean working that bit of the circuit out too...
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Old 22nd Dec 2009, 5:35 pm   #265
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Default Re: Newb Stumped on a Derwent

Pictures of speaker out. Looks ok lightly dusty but no rips or cone coming away anywhere
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Old 22nd Dec 2009, 6:23 pm   #266
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Default Re: Newb Stumped on a Derwent

By the way as mentioned earlier should I remove the earth wire I introduced when I put a new mains cable on the Cossor Valve Volt Meter?
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Old 22nd Dec 2009, 6:54 pm   #267
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Default Re: Newb Stumped on a Derwent

Try moving the cone gently in/out with your hands, you may be able to hear the scraping.

What I do is use the hose attachement from the vacuum but you need to put a hand on the cone whilst you bring the hose close as you can do some damage. If that fails I use some thin plastic sheet cut like a taper and work it around the gap moving between the holes in the spider.

Be careful as it's easy to damage the speaker.
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Old 22nd Dec 2009, 7:08 pm   #268
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Default Re: Newb Stumped on a Derwent

Just went rummaging in boxes and found the AVO 2 Panel valve tester and manual. Will try and sus how to work and test the 42.
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Old 25th Dec 2009, 11:35 am   #269
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Default Re: Newb Stumped on a Derwent

Just want to wish you all a Merry Christmas, and thanks for the help.

The Derwent is downstairs playing Old Time radio shows via my little A.M. Transmitter, albeit a little rough around the edges on sound quality,its a 100% improvement on how it was. Special thanks to Station X for his hard work at patience with me.

I guess i will slowly figure out its little quality issues in the new year.

I cannot thank you all enough
All the best
OldTickTock

Last edited by oldticktock; 25th Dec 2009 at 11:48 am.
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Old 25th Dec 2009, 1:45 pm   #270
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Default Re: Newb Stumped on a Derwent

It's nice to see a "result" from a thread which has reached 269 posts. Generally when they get into triple figures it's a lost cause.

Let us know if you're able to sort out the sound quality problems.
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Old 22nd Jan 2010, 3:10 pm   #271
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Default Re: Newb Stumped on a Derwent

Hi Guys,

An update on the Derwent....... The nightmare is over!

As you may know after station X solved the main issue, I was left with the task of resolving the outstanding issues. After replacing virtually everything I was left with two suspect valves. I tested the 42 & 75 on my AVO 2 panel, they were shown as replace. I took delivery today of NOS 42&75 valves tested them in the AVO and both showed as good.

Installed them both in the Derwent and fired it up, low and behold it sounds fantastic all the issues are now resolved and this radio sings again, in great part this would not have been the case without the support of the forum and most particular Station X and his reverse engineering skills and his modification to rectify the phantom menance who got here before me.


Running on soak test now as it's never been on longer than about 30 mins before, but I don't think I'm counting my chickens when I say I think this is the end


Regards
OTT

Last edited by oldticktock; 22nd Jan 2010 at 3:20 pm.
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