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Old 8th Jul 2012, 4:21 pm   #1
Karsten
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Default AVO CT 160 modifications

In the attached .pdf I have described and fully documented the modifications I have made to my tester. The major one is the replacement of the Vg potentiometer by a potentiometer – digital panel meter combination which has several additional advantages. This may be of interest for those folks who struggle with a more or less defective and irreplaceable Grid voltage potentiometer.
The 90 Ohm backing off potentiometer has been replaced by a modern 100 Ohm standard conductive plastic type. Its alignment with the existing etched scale values has turned out to be easy and accurate.
Another modification is the use of the built in analogue meter for direct measurement of anode current.

- Karsten -
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File Type: pdf Modifications.pdf (1.11 MB, 564 views)

Last edited by Karsten; 8th Jul 2012 at 4:30 pm.
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Old 14th Jul 2012, 8:02 pm   #2
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Default Re: AVO CT 160 modifications

Karsten, this was good reading during my vacation!

I like every effort into making a working AVO valve tester from a non working one and as you say some parts are made of "unobtanium".

This shows that it is possible to make a fully functional AVO valve tester without the original parts and even to add something to it while you are at it. Protecting the meter while at the same time making it easier to read the Anode current is alwaus welcome.

Could you please add some pictures of the inside of the CT160 which shows how you mounted the meter and all other components as that will help others if they are going to do something similar.

I'm glad that you have shared your findings so others can benefit from it!

/Martin
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Old 16th Jul 2012, 5:37 pm   #3
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Default Re: AVO CT 160 modifications

The modifications report has been updated and many pictures added.

- Karsten -
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Old 16th Jul 2012, 7:09 pm   #4
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Default Re: AVO CT 160 modifications

Excellent!

Now it is much more easy to see how it all has been placed inside the case and what it will require of anyone who wants to make the same or similar modification/s to their CT160.
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Old 18th Jul 2012, 9:17 pm   #5
Karsten
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Default Re: AVO CT 160 modifications

For those who are in need of a working RV2 grid bias potentiometer :

From the described mods I still have this pot which I do not need any more. It has two scorches, one between 0 and 1 Volt, a second one at 5 Volt. The potentiometer track is ok. The overall resistance is 10.8 KOhm. I have repaired both defects by securing the resistance wire with Araldite ("Uhu Plus"). The scorch at 5 Volt cannot even be felt when turning the potentiometer, it is ok now. The scorch at 1 Volt is worse. The minimum bias voltage is 1.4 Volt but above this minimum the pot functions normally. I think this is not too much of a drawback as most valves are measured with higher bias voltages anyway.

I have measured the U=f(angle of rotation) characteristic and prepared an Excel sheet with the results. It can be used to make a new dial because the original one has become inaccurate, particularly in the 0 to 5 V range. I will include the original dial disk so you can use it as carrier for the new overlay.

PM me if you are interested.

- Karsten -
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Old 18th Jul 2012, 10:09 pm   #6
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Default Re: AVO CT 160 modifications

Here comes the potentiometer characteristic. The "ideal" black line resembles the existing dial scale inscription. The red line is the measured characteristic. Obviously, a new dial needs to be made.

- Karsten -
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File Type: pdf Vermessung ausgebautes Meter.pdf (10.8 KB, 282 views)
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Old 13th Aug 2012, 6:54 pm   #7
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Default Re: AVO CT 160 modifications

With some help from outside there is also a nice scale now.

- Karsten -
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Last edited by Karsten; 13th Aug 2012 at 6:58 pm. Reason: Added more pictures
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Old 15th Sep 2012, 5:53 pm   #8
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Default AVO CT 160 modifications

In my posting above I had used the V1/V2 supply voltage to power the DVM 210 panel meter. With this source the meter displays overflow ("1") when testing direct heated valves. The effect was first observed with a CK1007 rectifier valve. It is is apparently caused by the meter's >100 MOhm input impedance in conjunction with the stray capacitance between the V1/V2 heater winding and the heater winding for the tube under test. Both windings are close to each other on the same transformer T2.

The meter is fed from a small separate transformer now and gives correct readings. The transformer's internal resistance is sufficient to limit the zener current so no extra resistor is needed.
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File Type: pdf Schaltplan Mods 0912.pdf (349.6 KB, 236 views)

Last edited by Darren-UK; 16th Sep 2012 at 7:50 pm. Reason: Attachment replaced at Karsten's request.
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Old 15th Sep 2012, 7:00 pm   #9
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Default Re: AVO CT 160 modifications

Thank you for posting this correction as I was just thinking of modifying one of my own CT160's with a similar arrangement as yours.
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Old 21st Sep 2012, 6:03 pm   #10
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Default Re: AVO CT 160 modifications

This may, or may not, be of any interest but why not replace the Vg pot with a sine/cosine pot? Minor snag in that it only needs to rotate 90 degrees so some type of 3:1 reduction is needed.

Just been playing with one of these and it gives a reasonable match to the Vg curve, obviously smooth rather than straight with break points.

Or a log pot?

Sure this has been talked about but it is finding it, problems of a large and active forum!

Bob
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Old 21st Sep 2012, 8:20 pm   #11
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Default Re: AVO CT 160 modifications

A search for "ct160 grid volts" here on the forum shows a few different ways of how to replace the grid volts potentiometer in the CT160. Mike Phelan has a nice solution where you modify a 25K potentiometer which means that you can use the original dial.

Forum member "einovaan" made a design similar to the AVO Mk IV with a range switch and a linear potentiometer and I myself used a 10-turn potentiometer, both of these two solutions make use of a new dial and in my case I used a counting dial.

One old, and original, grid volts potentiometer that I had in a CT160 I owned previously even had the text "Log." stamped into the metal case, I have seen similar photos on the Internet with this text too.
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Old 21st Sep 2012, 8:49 pm   #12
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Default Re: AVO CT 160 modifications

Almost any potentiometer which has a characteristic with a higher resolution at the beginning (i.e. low angle of rotation) will do. What you will have to modify in most cases however, is the scale. You need to make an overlay for the original silk screen print. One method to accomplish this is to scan the original scale and shift the tic marks to their new position with Corel Draw. This is how an unknown member of the German mikrocontroller.net forum made the new scale (see above). Thanks again for his effort!

@Dekatron: my pot is also stamped "LOG" (see the photo above) even though it is not logarithmic but stepwise linear.

Last edited by Karsten; 21st Sep 2012 at 8:51 pm. Reason: minor text correction
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