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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 22nd May 2015, 10:09 pm   #1
govjohn
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Default Servicing tip

When I was about 13 or 14 I used to pester the people in the workshop at the back of a radio & TV dealers in Woodford. Rather surprisingly they put up with me and taught me how to repair sets. In particular I was shewn* a method for checking for checking 'that capacitor' which I haven't read about in this forum although other people must be familiar with it. It work best with sets using a chassis and you need a meter and an insulated handle thin screwdriver.
Connect the meter across the cathode resistor and note the voltage, whilst watching the voltage short the anode of the voltage amplifier valve to chassis with the screwdriver, the cathode voltage of the O/P valve should dip momentarily and then return to the original reading, in this case the capacitor is OK. Then use the screwdriver to short the grid of the O/P valve to chassis, if the reading falls the valve is drawing grid current and experience will show* weather to do any thing about it. I am probably trying to teach some people on here how to suck eggs, but for those who haven't come across it it takes less time to do than to read about it. On sets with printed boards use a short lead to earth the two points. Try it and see. John.

*What is the correct spelling here?
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Old 22nd May 2015, 10:31 pm   #2
paulsherwin
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Default Re: Servicing tip

Shew/shewn is correct but archaic. Show/shown is the modern spelling.

The trick you describe really dates from the time when standard workshop meters couldn't measure the grid voltage accurately. You could only measure it with a scope or VVM. Modern digital meters do allow direct measurement but it's still useful to know these old shortcuts.
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Old 22nd May 2015, 10:40 pm   #3
kalee20
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Default Re: Servicing tip

It's a good technique, though it needs to be used with care. Some radios don't have the output valve grid leak returned to chassis anyway, there may be a partial source of negative bias such as from a back bias resistor. Shorting the grid leak resistor itself is preferable.

Quote:
Originally Posted by govjohn View Post
...I was shewn* a method for checking for checking 'that capacitor'...

... experience will show* weather to do any thing about it...

*What is the correct spelling here?
Either is correct though shew and shewn are rather archaic these days.

But weather is wrong, it should be whether.

Last edited by kalee20; 22nd May 2015 at 10:43 pm. Reason: Crossed with Paul S!
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Old 22nd May 2015, 11:51 pm   #4
petervk2mlg
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Default Re: Servicing tip

Fowler's Modern English usage says that according to OED "shew" was prevalent in the 18th century and not uncommon in the first half of the 19th, so it's definitely obsolete now, apart from in some legal documents.
I doubt any of us would come across this spelling nowadays apart from in the pages of the Radiophile.
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Old 23rd May 2015, 12:07 am   #5
govjohn
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Default Re: Servicing tip

Oops, thanks about whether though I thought it looked wrong.
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Old 23rd May 2015, 9:30 am   #6
Mike Phelan
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Default Re: Servicing tip

I often use John's method, but instead of shorting the anode of the previous stage to chassis, I short the output valve grid to chassis and check if the cathode voltage changes as per Kalee's post.
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Old 25th May 2015, 7:16 pm   #7
Leon Crampin
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Default Re: Servicing tip

Shorting the output valve grid to chassis will mask the effect of any gas current in the valve. Usually, in an old radio which has seen better days, excessive output valve current will result from either a leaky coupling capacitor or grid current - or both. The leaky capacitor will often damage the valve and produce grid current.

The procedure suggested above which entails shorting the anode of the driver stage to chassis will keep the output valve operating under its real-life conditions, provided the coupling capacitor is not actually shorted. A leak of a few Meg Ohms will not affect the test result.

Having exonerated or replaced the coupling capacitor, the acid test is a DVM connected between the output valve grid and earth - assuming normal biasing arrangements. Any positive voltage greater than a few tens of mV, especially if it increases over time is a sure indication of gas current or grid emission - both bad news.

Larger valves such as the KT66 will tend to have a higher value of positive voltage on the grid, especially when operated incorrectly with excessive values of g1 - k resitor or unsuitable biasing, as in the Quad amplifiers.

The real danger is a positive g1 - earth voltage which inexorably increases as the valve gets hotter - and hotter...

Leon.
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Old 25th May 2015, 7:31 pm   #8
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Default Re: Servicing tip

Readers of The Radiophile will be accustomed to shew/shewn its very much the style of Chas Miller!
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Old 25th May 2015, 8:59 pm   #9
kalee20
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Default Re: Servicing tip

Oops - red-faced Kalee20!

I misread the initial post, taking the previous stage anode to chassis - I read as taking the output grid to chassis. Stupid, especially as I actually use the anode-chassis trick myself!

This works and negates the cautions in post #3.

Leon Crampin's points are highly valid - gas current, leakage in valveholder, are real (even if slightly less likely than 'that cap' problems) and ought to be checked for.
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