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Old 14th Oct 2018, 8:10 pm   #1
multigamma
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Join Date: Oct 2018
Location: Genova, Italy
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Default AVO CT160.......dancing meter during TEST

Hello, first of all I would like to give my compliments for this forum.....and apologize for my terrible English.
I'm an Italian vintage radio addicted and I own a valve tester AVO CT160, bought a decade ago.
I love this tester, but recently it has a problem for which I'm not able to find the reason.
In short: during the TEST process, the meter needle tends to dance at random, especially when the mA/V potentiometer is activated. This fact makes impossible to read correctly the valve efficiency on the green scale (the needle dance continuously).
I made the attached video to better explain the situation, that is the same with several types of valves (i.e EL84, ECL82, ELL80 etc.).
During the last days I've read a lot of pages about the CT160 on this forum, very useful to better understand the principles and the problems of this tester, but I didn't find anything about my problem.
I checked some parts potentially involved: RV1 (mA/V potentiometer), R14, R5, R2, R10 (anode resistor), the meter, some soldered joints, but in "cold" condition they seem OK.
Do you have any advice for me??
Many thanks for your help.
Roberto from Genova, Italy.
https://youtu.be/bbijzOw5uDU
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Old 15th Oct 2018, 6:37 am   #2
Dekatron
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Location: Linkoping, Sweden
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Default Re: AVO CT160.......dancing meter during TEST

Hi, welcome to the forum!

You should start by reading this article, it explains how a noisy contact on the gm wheel was troubling the writer: http://www.jacmusic.com/Tube-testers...0-1/index.html , he also says that he had a noisy wiper contact on the Grid Volts (Vg) potentiometer.

Before removing the switch mentioned above you should note its position so that you can get it back in the same position afterwards as it sits on the shaft of the gm-wheel potentiometer (I know of at least one situation where the switch had been turned upside down (!) and not working as intended).

You should also check the two Anode current controls for noisy wiper contacts and switch contacts.

Start by checking these things and let us know what results you find/get when cleaning them.

High contact resistance due to dirty/corroded contacts is quite common in these testers and since some currents are very low (microampere and below) high contact resistance results in high voltage drops (several millivolts up to several volts which shows on the meter).
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Martin, Sweden
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Old 26th Oct 2018, 12:07 pm   #3
multigamma
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Default Re: AVO CT160.......dancing meter during TEST

Hello Martin,
Many thanks for your advice.....and sorry for my late reply! Unfortunately I read your post only today.
Next days I'll perform the suggested checks and let you know about.
Regards
Roberto
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Old 27th Oct 2018, 12:31 pm   #4
John_BS
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Default Re: AVO CT160.......dancing meter during TEST

You have a stable reading when looking at anode current: when you move to gm reading the sensitivity of the meter increases dramatically.

The drift you observe is common and related to the thermal properties of the valve.

The "noise" may well be the result of running the instrument directly from your mains supply, which may be fluctuating slightly. I don't think there's much wrong with the CT160 itself, but cleaning the contacts won't do any harm. The gm potentiometer is the obvious candidate for cleaning (wiper to track).



John
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