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Old 28th May 2017, 7:57 pm   #1
neikeel
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Default AVO CT160 care advice

Hi

My first proper post here, so please be gentle with me!

I have two projects that I need to get up and running, the first is my AVO CT160 and the other a Hunts CRB3.

I generally work on guitar valve amps as hobby and as a human plumber for my day job.

The AVO is generally in good condition and works in all respects except for the Gm measurement which consistently seems to read low (I can advance the dial to the 1mA/V point but new EL34s should have Gm of 11mA/V seem to be testing at 4. Same goes for EL34s.
I have taken a look inside and can see that some of the resistors have been replaced and the backlight bulb for the meter is a screw type with wires soldered to the end.
I have the excellent guides and schematics but am struggling with identifying a number of the resistors as they have no bands or lettering and a couple of wirewounds are scorched to the wire coil.
Is there a photograph with labels of the resistors around?
I plan on putting some protection in for the meter but otherwise keeping it simple.
Any tips?
The Hunts is (naturally!) going to need a recap and when I get the AVO reworking I can test the magic eye!!! Again thanks to people here for sharing the manual and diagrams.
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Old 28th May 2017, 10:31 pm   #2
Dekatron
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Default Re: AVO CT160 care advice

You can find the VMARS manuals here: http://www.vmarsmanuals.co.uk/archive/files_index.htm, manuals 1279-1286 are about the AVO valve testers and 1282 contains some pictures on where some of the resistors are placed.

The lamp should be mounted like you describe it. Be careful when poking around inside the CT160 as the lamp holder is connected to the mains voltage.

If you can take a photo of the resistors you are asking about it will be easier to help.

There are some photos and a nice circuit diagram of the valve panel here: https://sites.google.com/site/wongkm...n-gui/home/avo

I can recommend you to remove the moving coil meter before you try any repairs and when you have finished the repairs you should put a 3250 Ohm resistor in its place and measure the voltage across that resistor, or the current through it, to see that everything looks fine before hooking up the meter so you can check that everything is fine.

While the meter is out you can check to see that it has the correct FSD current as well as the correct current at both the 1mA/V line and the SET-AC line.

/Martin
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Old 29th May 2017, 9:49 am   #3
David Simpson
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Default Re: AVO CT160 care advice

Hello Neikeel,
Have a good look at the vast amount of CT160 info which can be accessed via "Search".
Martin has wisely advised you to take great care of the meter - they are now horrendously expensive to replace & jolly fragile. Also please take note that CT160's, like their Avo VCM siblings, are mains AC testers. Full Peak to Peak 50Hz AC Voltage waveforms from the transformer are on the meter connections and the Anode circuitry. Please take extreme care.

Regards, David
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Old 3rd Jun 2017, 2:15 pm   #4
neikeel
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Default Re: AVO CT160 care advice

Thank you David and Martin.
I did some searching before asking and found the factory user and service manuals as well as the excellent guide that Martin put together.
It was layout views I struggled with, the schematics look pretty simple, the photographs on the Japanes site were most useful.
I also wanted to distil the 'must do' mods vs optional extras as I did not really want to go with diode replacement of the valves, just a properly calibrated working unit.
I am pretty familiar with high voltage equipment but extra reminders about safety are always valuable.
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Old 3rd Jun 2017, 9:17 pm   #5
David Simpson
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Default Re: AVO CT160 care advice

Hello again Neikeel,
The CT160 was designed for military use. Its basically a Mk4, shrunk to fit inside a military grade dustproof case. As a consequence of its compactness & gasketed clamshell casing it is rather awkward to work on. However, its a jolly good valve tester.
Some years ago, whilst recovering from a nasty shoulder op, I drew up a simplified diagram, one handed, during my re-hab. Technical document standard it is not. But it explains the basic functions.

Regards, David
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Old 4th Jun 2017, 9:00 pm   #6
neikeel
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Default Re: AVO CT160 care advice

Thank you David.

Interesting block diagram of the functions of each aspect of the tester and how they interact, the manual does describe the functions but in a less integrated way.

I will study and use.

Cheers
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