avo: CT-160 (CT160); Valve Tester calibration

ID: 176446
This article refers to the model: Test Set Electronic Valve C.T.160 (AVO Ltd.; London)

avo: CT-160 (CT160); Valve Tester calibration 
03.Nov.08 21:39
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Paul Cornelissen (NL)
Articles: 9
Count of Thanks: 16
Paul Cornelissen

Dear members,

For a few days I'm trying to calibrate my AVO CT160 valve tester. I downloaded all the mauals and articles I could find, read the calibration parts several times. Did the calibration step by step.  I checked all the resistors in the instrument (up till now I didn't check the capacitors and potmeters). Replaced the calibration resistors R3 and R4. And last but not least I made a calibration tube (ECL80) in the way it is described in the AVO manual. Checking the instrument with my new made calibration tube I found out that on the low end the Ia measurements from the AVO is a little to high (10%). In the middel it is oke and with higher Ia settings values from the AVO are lower (15%) than expected. Gm measurement is fine. Anyone who has experience with calibration of a AVO CT160? A little help is appreciated.

Regards,

Paul

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CT160 
04.Nov.08 08:28

Roy Johnson (GB)
Articles: 284
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Dear Paul,

I have not had experience of calibrating the CT160, but have dealt with several of the other AVO models.

The most frequent problem is the meter movement itself and I always start by checking the basic current calibration with a current calibration source.   The 10 to 15% is a typical error and seems to be due to the magnetic field change with age.   However, the apparent linearity in your case is strange but this may be because other components have been altered in an attempt to compensate the basic sensitivity and resistance.

Unless the current calibration and the resistance of the meter circuit is correct, calibration on these meters (at least the earlier ones) is impossible.   Some adjustment by the magnetic shunt on the movement can usually compensate and once this is done the calibration stays accurate for many years.

It is rather strange that AVO - of all companies - should have had this problem with meters up to and including the 163.  It may be that the problem had been resolved by the time of the 160 and that your fault lies elsewhere, but I suggest that you start at this level, if not already done,  before attempting further compensating adjustments. 

Good luck!

Regards,

Roy 

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avo: CT-160 (CT160); Valve Tester calibration 
04.Nov.08 21:16

Paul Cornelissen (NL)
Articles: 9
Count of Thanks: 8
Paul Cornelissen

Dear Roy,

First of all, thank you for your response. In the coming weekend I'm going to find out if the meter itself is still oke. That was something I didn't check up till now. I am going to use your suggestions for testing it.

Regards,

Paul

 

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