22nd Dec 2009, 3:11 pm | #241 |
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Re: Newb Stumped on a Derwent
Ok I will do #240 test first then post results, then I shall do #229 test
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22nd Dec 2009, 3:14 pm | #242 |
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Re: Newb Stumped on a Derwent
42 removed, 0 volts on either end of the 510k
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22nd Dec 2009, 3:16 pm | #243 |
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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 |
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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.
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22nd Dec 2009, 3:34 pm | #245 |
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Re: Newb Stumped on a Derwent
I think the first part of post 244 is rubbish on my part
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22nd Dec 2009, 3:41 pm | #246 |
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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.
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22nd Dec 2009, 3:42 pm | #247 |
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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 |
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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.
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22nd Dec 2009, 4:03 pm | #249 |
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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
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22nd Dec 2009, 4:08 pm | #250 |
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Re: Newb Stumped on a Derwent
VVM setting to read pin 4
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22nd Dec 2009, 4:13 pm | #251 | |
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Re: Newb Stumped on a Derwent
Quote:
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22nd Dec 2009, 4:24 pm | #252 | |
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Re: Newb Stumped on a Derwent
Quote:
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.
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22nd Dec 2009, 4:41 pm | #253 | ||
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Re: Newb Stumped on a Derwent
Quote:
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 |
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22nd Dec 2009, 4:48 pm | #254 |
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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 |
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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.
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22nd Dec 2009, 4:59 pm | #256 |
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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
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22nd Dec 2009, 5:02 pm | #257 |
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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.
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22nd Dec 2009, 5:07 pm | #258 |
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Re: Newb Stumped on a Derwent
yes external sounds better
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22nd Dec 2009, 5:11 pm | #259 |
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Re: Newb Stumped on a Derwent
There are numerous threads on fixing speaker problems.
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22nd Dec 2009, 5:17 pm | #260 |
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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 |