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Old 11th Feb 2017, 5:55 pm   #1
mrrepel
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Default AVO CT160 burnt resistor / shorted

Hi all,

I was granted a CT160 as a gift but is was not working. As of now i have the following working / faults:

-After turning it on it goes to the calibration mark (not shown in movie, after 5 minutes it is on the mark)
-Heater measurement works

Cold insulation testing:
I've made a little YouTube movie about the testing.
https://youtu.be/BhT9GO-UBZg

As you can see, the insulation testing shorst out. To be sure it is a problem with the tester and not the tubefittings / selector i've desoldered the tube board.

On closer inspection i've found a burnt resistor.

The values are

The measured values are top to bottom:
R3 630K (11M+600K parallel)
R4 660K (660K)
total value 1,29M

R1 1,75K (burnt resistor + 18K? )

Am i correct with the resistor places ( R1,R3,R4 ) as stated in this scematic ? http://jacmusic.com/Tube-testers/AVO...g-editiert.JPG

I've replaced the R3 / R4 with some resistors i had lying around that combined to 1,32M. The meter calibration is a different than before, but no "big" changes.

I've replaced the burned and bodged in resistor with 2350R but still no changes. When i switch to A/R, C.HR, C/H the meter is shorted. Only on S/R the meter is'nt shorted.

For the ones paying attention to the meter, yes that is a MK4 scale in a CT160 meter. My MK4 had a broken meter and is working perfect now. I want to get the CT160 working as well so i can have both
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Old 11th Feb 2017, 6:56 pm   #2
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Default Re: AVO CT160 burnt resistor / shorted

Problem found

The clamp on the resistor was touching the chassis
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Old 11th Feb 2017, 7:45 pm   #3
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Default Re: AVO CT160 burnt resistor / shorted

Problem #6...

The potentiometer of the negative grid volts is broken. It seems that it is burned out. When i was testing the machine i got "choppy" feedback so i opened it up.

Finding a 10K lineair 5W potentiometer is no problem, but finding a logarithmic one... Any thoughts ?
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Old 12th Feb 2017, 5:10 am   #4
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Default Re: AVO CT160 burnt resistor / shorted

After som (lot) of reading i've found that the potentiometer is a special wound one and not replaceable.

I found this thread https://www.vintage-radio.net/forum/...ad.php?t=85697 but my knowledge is to small to make these modifications.

I did found this thread https://www.vintage-radio.net/forum/...ead.php?t=3481 but no further information. Does anybody have more information on the board / scematic used ?
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Old 13th Feb 2017, 10:23 am   #5
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Default Re: AVO CT160 burnt resistor / shorted

It is almost impossible to find an original potentiometer for the grid volts control.

The simplest solution is to use another 10k linear potentiometer with the same degree of travel, rotational angle, and turn the aluminum scale upside down and mark new positions for each grid volts setting on that side.

You won't get the same resolution for lower grid volts, which is why the original potentiometer has different resistance ranges, but with your own markings you can mark the most common values.

I've also used a 10-turn potentiometer with a counting-knob and made my own list for what voltage each position on the knob corresponded to, the 10-turn potentiometers are not perfectly linear so a setting of say 100 does not always correspond to 100 Ohms on a 1k potentiometer so therefore I used the list to correct for any differences.
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Old 13th Feb 2017, 10:37 am   #6
Craig Sawyers
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Default Re: AVO CT160 burnt resistor / shorted

I have no idea where I downloaded this from, but this is a procedure for faking the correct law using a linear wirewound pot.

Craig
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Old 13th Feb 2017, 10:45 am   #7
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Default Re: AVO CT160 burnt resistor / shorted

Just remembered the way the Mk2, 3 and 4 got around the custom pot problem was to have a Vg range switch and a linear 10k pot.

Although it would not be original, since a hole would have to be added for the switch, and a new scale made for the pot, that is one alternative to the above.

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Old 13th Feb 2017, 11:07 am   #8
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Default Re: AVO CT160 burnt resistor / shorted

If you look at this post: https://www.vintage-radio.net/forum/...86&postcount=6 (whole thread here:https://www.vintage-radio.net/forum/...ad.php?t=68488) you see one modification with a range switch.
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Old 13th Feb 2017, 11:32 am   #9
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Default Re: AVO CT160 burnt resistor / shorted

Quote:
Originally Posted by Craig Sawyers View Post
I have no idea where I downloaded this from, but this is a procedure for faking the correct law using a linear wirewound pot.

Craig
From here:-

https://www.vintage-radio.net/forum/...ead.php?t=2505
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Old 13th Feb 2017, 11:34 am   #10
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Default Re: AVO CT160 burnt resistor / shorted

Aha - the law faking mod that I sent the word document for was posted here https://www.vintage-radio.net/forum/...ad.php?p=47426 by Mike Phelan back in 2005.

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Old 15th Feb 2017, 5:49 pm   #11
mrrepel
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Default Re: AVO CT160 burnt resistor / shorted

Thanks for all the reply's. I've ordered a 25K potentiometer from Ebay and will try the mod with the windings and parallel resitors. Will be back soon
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Old 16th Feb 2017, 7:06 pm   #12
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Default Re: AVO CT160 burnt resistor / shorted

I was just about to say "Instead of using silver conductive paint, which is fragile, you can get silver loaded epoxy"

Then I looked at the price - cheapest I could find was close to £70 for a couple of ml!!!!

Silver loaded paint from Maplin is £11, which seems a bit more realistic.
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Old 22nd Feb 2017, 1:20 pm   #13
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Default Re: AVO CT160 burnt resistor / shorted

Today i recieved the potentiometer. Sold as a wirewound, opened is and it was a carbon one aaaarghh....

Can not seem to find a suitable 25K potentiometer. If anybody had any thoughts / something lying around, please
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Old 22nd Feb 2017, 2:26 pm   #14
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Default Re: AVO CT160 burnt resistor / shorted

Colvern CLR4000 series and also Clarostat RA30 series will probably work fine, you can find them on Ebay sometimes. I've not built this exact replacement so I can't say how well these will fit with the rotational angle and the aluminum scale though.
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Old 22nd Feb 2017, 4:56 pm   #15
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Default Re: AVO CT160 burnt resistor / shorted

Thanks for the quick reply. Just found some nice NOS potentiometers. I am curious if it would be possible to wind the resistor wire from the "broken" potentiometer in to a donor potentiometer. I'll try that first, if not, i'll make the splices.

For those who are searching for the same potentiometers see http://www.ebay.co.uk/itm/COLVERN-WI...3D162390958946
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Old 22nd Feb 2017, 5:36 pm   #16
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Default Re: AVO CT160 burnt resistor / shorted

I tried rewinding these potentiometers and even if I started from the spliced points and marked those places on the new supports I couldn't find a reliable way of rewinding even if I did it by hand - there is a lot of wire to keep on spools while rewinding the middle wire. One more thing is that the resistance has probably changed due to the partial overheating, at least it had done so on the potentiometers that I tried to repair. I also tried to just make supports on each side of the partially burnt support to try to keep the wires in place and then tried to fill the partially burnt support with epoxy glue but it did not turn out very well as it was very hard to get an even track for the wiper to move on, it might have been a good idea to put some kind of support beneath the top of the wire to keep it level but I didn't think of that before the glue had set.

Maybe these things that I tried can keep you from making the same mistakes. I'll keep my fingers crossed that you succeed with your repairs.
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