Newb Stumped on a Derwent - Page 13 - UK Vintage Radio Repair and Restoration Discussion Forum

oldticktock · 22nd Dec 2009, 3:11 pm

Ok I will do #240 test first then post results, then I shall do #229 test

oldticktock · 22nd Dec 2009, 3:14 pm

42 removed, 0 volts on either end of the 510k

PJL · 22nd Dec 2009, 3:16 pm

It doesn't look good for the 42. Might be worth replacing the audio coupling capacitor and also checking that pin4 measures 510K to earth when switched off.

I just wonder if that is the problem being described:
Does the distortion get worse with higher volumes?
If you tune to a weaker station (or use a shorter aerial) , is it less or more distorted?
Does it distort if you put audio into the gram input?

Station X · 22nd Dec 2009, 3:21 pm

I've never used a VVM for measuring anything other than RF voltages. It wouldn't take much grid current (9.8uA) by my reckoning to develop 5V across the grid leak. A LOW impedance meter would probably kill this, but maybe not a VVM?

EDIT. 510K was measured yesterday and proved correct at the second attempt. I do wish we could get consistent results.

Station X · 22nd Dec 2009, 3:34 pm

I think the first part of post 244 is rubbish on my part

Station X · 22nd Dec 2009, 3:41 pm

According to post #244 with the 42 removed there's no voltage on pin 4 (Control Grid) of the 42. That says that the coupling capacitor is OK.

Could it be an anode to control grid or screen grid to control grid short in the 42? I think it has been described as rattlely.

oldticktock · 22nd Dec 2009, 3:42 pm

sorry guys for being a pain..... feel free to give up I've taken up more than enough of your time.

Just checked resistance with set off to pin 4 and it reads 510k this was read on an AVO 9. The AVO 9 has only one fault it went pop on AC a few months ago I had the switch set to low. Still reads DC and OHMs

Station X · 22nd Dec 2009, 3:51 pm

Well it wouldn't do any harm to use the AVO 9 to check the voltage chassis to pin 4 of the 42 with the 42 in and out. Even if the input reistance of the meter is low the worst it can do is pull the readings down, but we'll still get an idea whether the valve is positive.

With the valve out you could also try measuring the resistance between its various pins, although in my experience shorts only show up on high voltage.

oldticktock · 22nd Dec 2009, 4:03 pm

Just switched on, set the VVM to read upto 15v , common to chassis dc probe to 42 pin4 reads .9 of or 1 volt

oldticktock · 22nd Dec 2009, 4:08 pm

VVM setting to read pin 4

oldticktock · 22nd Dec 2009, 4:13 pm

Quote:

Originally Posted by Station X

According to post #244 with the 42 removed there's no voltage on pin 4 (Control Grid) of the 42. That says that the coupling capacitor is OK.

Could it be an anode to control grid or screen grid to control grid short in the 42? I think it has been described as rattlely.

The only valve which made a "tinkle" sound when rotated (cleaning) was the 6A7

Station X · 22nd Dec 2009, 4:24 pm

Quote:

Just switched on, set the VVM to read upto 15v , common to chassis dc probe to 42 pin4 reads .9 of or 1 volt

I assume that's with the valve plugged in? What does the AVO9 say?

At this stage were the set mine I'd test the 42 for internal shorts.

I'm going back to thinking about the 75 readings.

oldticktock · 22nd Dec 2009, 4:41 pm

Quote:

Originally Posted by Station X

Quote:

Just switched on, set the VVM to read upto 15v , common to chassis dc probe to 42 pin4 reads .9 of or 1 volt

I assume that's with the valve plugged in? What does the AVO9 say?

At this stage were the set mine I'd test the 42 for internal shorts.

I'm going back to thinking about the 75 readings.

Yep valve in place
avo 9 looks the same, although I find it harder to comprehend than the VVM here's a couple of shots the dial settings and the reading obtained.

Excuse me I feel the need to go outside again, stick my head in the snow and invite the locals to take a seasonal kick

oldticktock · 22nd Dec 2009, 4:48 pm

the raspy distortion goes if i stick the blunt end of a pencil on the speaker cone and lightly push. Sounds quite good doing that, just a bit of background whistle etc.

Does the length of aerial lead make a difference, I have a lead running form a great reel of bell wire theres a lot on the reel

You know with the pencil jobby, it's very acceptable. Not that i'm suggesting a bodge with a pencil, but is the speaker naff, I does not look it quite the opposite actually. Is it being overloaded

Station X · 22nd Dec 2009, 4:52 pm

Can you try another speaker in the external speaker socket then? You can't just unplug the existing speaker as the energiser winding acts as a smoothing choke.

The coil of wire will act as an inductance in series with the aerial, which hardly matters. It certainly won't give rise to audio distortion.

oldticktock · 22nd Dec 2009, 4:59 pm

I have an external speaker and on the back of the set there is a wander plug which you can locate in internal or external. I will do that

Station X · 22nd Dec 2009, 5:02 pm

Both the VVM and the AVO9 are showing small positive voltages on the 42 control grid. As there is no voltage with the valve unplugged this indicates a faulty valve which may or may not be causing the distortion. All you can do is check for internal shorts between pins.

As regards the 75, as I said earlier the cathode and therefore the grid voltage is a bit high. You can try lowering the value of the 1.8K cathode resistor say to 1.5K and then 1.2K to see if you can get closer to 2 VDC.

I think we've reached the limit of remote fault finding on this set. At least the fade out problem has been diagnosed and fixed.

A visit to Ed Dinning might pay dividends.

oldticktock · 22nd Dec 2009, 5:07 pm

yes external sounds better

Station X · 22nd Dec 2009, 5:11 pm

There are numerous threads on fixing speaker problems.

oldticktock · 22nd Dec 2009, 5:17 pm

I cannot thank you enough its been interesting and educational to say the least. A big thanks to all who have contributed to the cause of a newb. I have quite a few other sets and do hope that they are not as problematic. I think this thread has run its course.

Merry Christmas to you all
Regards
OldTickTock

22nd Dec 2009, 3:11 pm	#241
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent Ok I will do #240 test first then post results, then I shall do #229 test

22nd Dec 2009, 3:14 pm	#242
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent 42 removed, 0 volts on either end of the 510k

22nd Dec 2009, 3:16 pm	#243
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Newb Stumped on a Derwent It doesn't look good for the 42. Might be worth replacing the audio coupling capacitor and also checking that pin4 measures 510K to earth when switched off. I just wonder if that is the problem being described: Does the distortion get worse with higher volumes? If you tune to a weaker station (or use a shorter aerial) , is it less or more distorted? Does it distort if you put audio into the gram input?

22nd Dec 2009, 3:21 pm	#244
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Newb Stumped on a Derwent I've never used a VVM for measuring anything other than RF voltages. It wouldn't take much grid current (9.8uA) by my reckoning to develop 5V across the grid leak. A LOW impedance meter would probably kill this, but maybe not a VVM? EDIT. 510K was measured yesterday and proved correct at the second attempt. I do wish we could get consistent results. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2009, 3:34 pm	#245
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Newb Stumped on a Derwent I think the first part of post 244 is rubbish on my part __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2009, 3:41 pm	#246
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Newb Stumped on a Derwent According to post #244 with the 42 removed there's no voltage on pin 4 (Control Grid) of the 42. That says that the coupling capacitor is OK. Could it be an anode to control grid or screen grid to control grid short in the 42? I think it has been described as rattlely. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2009, 3:42 pm	#247
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent sorry guys for being a pain..... feel free to give up I've taken up more than enough of your time. Just checked resistance with set off to pin 4 and it reads 510k this was read on an AVO 9. The AVO 9 has only one fault it went pop on AC a few months ago I had the switch set to low. Still reads DC and OHMs

22nd Dec 2009, 3:51 pm	#248
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Newb Stumped on a Derwent Well it wouldn't do any harm to use the AVO 9 to check the voltage chassis to pin 4 of the 42 with the 42 in and out. Even if the input reistance of the meter is low the worst it can do is pull the readings down, but we'll still get an idea whether the valve is positive. With the valve out you could also try measuring the resistance between its various pins, although in my experience shorts only show up on high voltage. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2009, 4:03 pm	#249
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent Just switched on, set the VVM to read upto 15v , common to chassis dc probe to 42 pin4 reads .9 of or 1 volt Attached Thumbnails

22nd Dec 2009, 4:08 pm	#250
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent VVM setting to read pin 4 Attached Thumbnails

22nd Dec 2009, 4:48 pm	#254
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent the raspy distortion goes if i stick the blunt end of a pencil on the speaker cone and lightly push. Sounds quite good doing that, just a bit of background whistle etc. Does the length of aerial lead make a difference, I have a lead running form a great reel of bell wire theres a lot on the reel You know with the pencil jobby, it's very acceptable. Not that i'm suggesting a bodge with a pencil, but is the speaker naff, I does not look it quite the opposite actually. Is it being overloaded

22nd Dec 2009, 4:52 pm	#255
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Newb Stumped on a Derwent Can you try another speaker in the external speaker socket then? You can't just unplug the existing speaker as the energiser winding acts as a smoothing choke. The coil of wire will act as an inductance in series with the aerial, which hardly matters. It certainly won't give rise to audio distortion. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2009, 4:59 pm	#256
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent I have an external speaker and on the back of the set there is a wander plug which you can locate in internal or external. I will do that

22nd Dec 2009, 5:02 pm	#257
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Newb Stumped on a Derwent Both the VVM and the AVO9 are showing small positive voltages on the 42 control grid. As there is no voltage with the valve unplugged this indicates a faulty valve which may or may not be causing the distortion. All you can do is check for internal shorts between pins. As regards the 75, as I said earlier the cathode and therefore the grid voltage is a bit high. You can try lowering the value of the 1.8K cathode resistor say to 1.5K and then 1.2K to see if you can get closer to 2 VDC. I think we've reached the limit of remote fault finding on this set. At least the fade out problem has been diagnosed and fixed. A visit to Ed Dinning might pay dividends. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2009, 5:07 pm	#258
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent yes external sounds better

22nd Dec 2009, 5:11 pm	#259
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Newb Stumped on a Derwent There are numerous threads on fixing speaker problems. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

22nd Dec 2009, 5:17 pm	#260
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Newb Stumped on a Derwent I cannot thank you enough its been interesting and educational to say the least. A big thanks to all who have contributed to the cause of a newb. I have quite a few other sets and do hope that they are not as problematic. I think this thread has run its course. Merry Christmas to you all Regards OldTickTock