AVO CT160 calibration & faults

[John_BS]

Just a couple of issue to share following a refurbishment & safety mods to a CT160.

1. The switch contacts SC across R14, the resistor nornally in series with the meter, were permanently closed. This would not be apparent to an operator unless they were familiar with the behaviour of the instrument when measuring Ia etc. The switch is intimately bound up in the Gm control.
(Symptom: very agressive meter movements when setting up valve test).

2. The capacitors C1 and C2 should be replaced, as they become leaky and exhibit high loss. C1 is actually two 0.047uF 200v caps in series, so can be replaced with a single 22nF >400V polyprop.

(Symptom: inter-electrode leakage measured with no valve plugged in)

3. The 8uF electrolytic which is employed as part of the load for testing diodes should also be replaced: the value is not critical, but in this example the capacitor fitted was a 450V part which measured high C on a tester, but also high esr. The maximum voltage on the capacitor is less than 100V, so this can be replaced with a smaller 10uF part.

(Symptom: good diodes read lower than expected on test).

John

[Dekatron]

Good notes!

I always recommend to replace all capacitors with modern Polypropylene capacitors.

Regarding point 1: I have heard of, and also seen myself, that on some CT160 the switch wafer that shorts R14 has been mounted in the wrong position, either the whole switch rotated 180 degrees, flipped over, or the rotating part of the wafer had been rotated 180 degrees compared to the position it should be in, or a combination of these, which also resulted in R14 being shorted in the wrong position and of course opened in the wrong position when the gm-wheel was turned.

This has probably happened after someone has replaced the potentiometer on the gm-wheel and forgotten which way the wafer should be installed in and just put it back as they didn't realise that it was important to put it back in the correct position. I don't think that AVO would have delivered many of these with the wrong position from the factory but you never know. All I have seen myself and seen photos of have had the solder reworked so I haven't seen any which had this fault as new with untouched solder. On some occasions I have suspected that someone has put an incorrect switch wafer there, possibly from some other equipment, probably the original one was broken.

So, always take a lot of photos or make a drawing of how things are placed before you dismantle them.