Re: VCM 163 No.1. Problem while calibrating
 

I have checked the faulty amplifier board again and can find no faulty component, have also tested continuity between components. I am thinking that something is failing under load which components are likely to do so?
I have decided to put the working VCM 163 back together and focus on sorting out this faulty amplifier board.
Than you for getting me this far.

Re: VCM 163 No.1. Problem while calibrating
 

If it works in the Cal. position as you say and then fails in the other positions it looks to me like there is a problem in the wiring to/from the mA/V switch and not in the oscillator/amplifier boards.

Re: VCM 163 No.1. Problem while calibrating
 

By the way, do you have have a tube in the tester which you are testing while you try the other positions on the mA/V switch?

Re: VCM 163 No.1. Problem while calibrating
 

I have now put the good amplifier board back into the working VCM163 as I did not want to damage it while working on the mA/V switch on the non working VCM163.
I now have two problems to solve.
1. A faulty amplifier board.
2. A problem in the mA/V switch area.
I did test a valve in the faulty VCM163 (when it had the good amplifier board installed) and the result was what it should have been if I took into consideration the the mA/V meter was reading 1 (not zero) before testing the valve. I will try and find where that reading of 1 is coming from but the faulty amplifier board must be mended first.
I hope all the above is not too confusing.
Haydn

Re: VCM 163 No.1. Problem while calibrating
 

I have checked the faulty VCM163 amplifier board again and replaced all the transistors with new 2n2926 transistors. Is it ok use 1n4148 diodes in place of the original 1n914's?

Re: VCM 163 No.1. Problem while calibrating
 

Yes, you can use 1N4148.

Re: VCM 163 No.1. Problem while calibrating
 

The transformer measured 41.3ohms and 0.4 ohms.
Would it be possible to obtain a transformer of this specification and if yes, where?

Re: VCM 163 No.1. Problem while calibrating
 

At what temperature did you measure those values?

Did you check the inductance too?

It's not that far from the values that AVO stated in their documentation, so my guess is that it is just fine:

----------------------------------------
Amplifier Transformer:

Use Universal Bridge TF 1313, Marconi, for the following measurements.

PRIMARY:

Resistance: 0,2 Ohm +/-20% at 20°C, Inductance at 10KHz 99.5uH, Q-factor: 2.4 +/-10%

SECONDARY:

Resistance: 45 Ohm +/-10% at 20°C, Inductance at 10 kc/s 99.5mH +/-2%, Q-factor: 14 +/-10%
----------------------------------------

I don't know of anyone who can supply another transformer, of any kind, for an AVO nor anyone who can supply the two NTC-resistors on the oscillator board.

Re: VCM 163 No.1. Problem while calibrating
 

I have again checked every component on the amplifier board. I have checked all the tracks for continuity. The only thing I have not done is check the inductance of the transformer. It is a mystery there must be a fault somewhere on the board but for some reason I cannot find it. Like I said earlier I will happily post the board to anyone who has the skills to sort it out. I'd be happy to purchase a replacement amplifier board if anyone has one available.

Re: VCM 163 No.1. Problem while calibrating
 

Can you post some photos of the amplifier board, top and bottom?

Please also post the serial numbers of the two VCM 163's you have, and if possible photos of the working amplifier board for comparison.

Re: VCM 163 No.1. Problem while calibrating
 

You could also try to replace the 2N2926 transistors with modern BC549C.

Re: VCM 163 No.1. Problem while calibrating
 

Hi!

Looking at the drawing of the Amplifier/Oscillator board I was able to refer to, you could perhaps use an RM10 pot–core kit as a basis for winding a new transformer, working out the turns needed for Primary and Secondary by back– calculating from the Inductance value specified using the AL given from the datasheet for the core kit you pick!

I don't think the exact turns ratio has to be followed "chapter and verse" – it doesn't actually appear to be quoted from the data you posted, but it's reasonable to go by the voltage ratio suggested, giving a turns–ratio of 2 :1

Chris Williams

PS!

I have got a TF1313 if you want me to try and make you a replacement oscillator transformer for you to try!

Re: VCM 163 No.1. Problem while calibrating
 

Martin
I have ordered some BC549's.
The amplifier board from the working VCM163 is identical to the one I have attached as I checked them side by side.
Would you change all the original capacitors even if they test OK?
Chris
The problem is the amplifier board as the oscillator board is producing the correct signal. I am not sure whether I am capable of making a replacement transformer for the amplifier (if it proves to be the problem).

Re: VCM 163 No.1. Problem while calibrating
 

2N2926 were very early silicon transistors with absolutely terrible gain spreads, hence they were grouped and marked with different coloured dots. They were also an early attempt at plastic encapsulation. I would definitely opt for something a bit more modern. The oscillator is a Wien Bridge design, C1 C2 R1//R2, R13 set the frequency, but most important for this application are R12, R19 and the thermistor which set the amplitude. If this shifts in level, then all your Gm measurements go out. You really do need the right thermistor.

T1 needs to have good isolation because it isolates the oscillator from the grid bias voltage. Its turns ratio needs to be close to right, or else the cal pot will run out of range.

On the amplifier board, T1 is resonated, so you have to get the inductance right. Generic small signal silicon diodes will be fine 1N914 or 1N4148. You can probably get away with most reasonable gain silicon transistors.

Why don't I see any high stability close tolerance parts in this area? Or an adjuster? Do they rely entirely on setting the grid drive oscillator to suit the amplifier/meter sensitivity?

T1 has to have very good isolation because its primary will be at anode potential.

I think the original T1 was built in a Mullard 'Vinkor' ferrite pot core. A modern re-interpretation would need a two section bobbin without 'window' gaps between the sections in order to get high voltage isolation.

The grid drive transformer looks to be a conventional iron laminations job.

This thing doesn't really give the feel of a precision measuring instrument, but don't tell that to the people auctioning them.

David

Re: VCM 163 No.1. Problem while calibrating
 

@David (Radio Wrangler)

Excellent description of the design! Those old transistors were a problem, AVO wrote a list near the circuit diagram which color code you should use for which transistor.

There are just a few close tolerance parts, the big styroflex capacitor (1.99uF), and two smaller ones on the oscillator board (1990pF).

The amplifier ferrite pot core transformer has a trimmer at its center.

@HBWOODY:

I've never seen heatsinks on the transistors - did you put them there yourself?

Do check the value of the big styroflex capacitor, it is known to break in these testers going open circuit or only getting half of its value due to one of the pins connecting to the aluminium sheet inside coming loose. Handle it with care so you don't break it yourself!

Did you replace the two diodes at the back of the board two, they are placed at the secondary of the transformer and if one of them is shorted or open circuit I know that you get strange behaviour from the amplifier.

Do look for dry joints on especially the connecting pins going to the wires, some like pin 4 & 5 look a little bit poor on your card, check the rest too.

Re: VCM 163 No.1. Problem while calibrating
 

Thank you for that input David.
The heat sinks were already attached. The big capacitor C1 measures exactly as it should (measured out of circuit). I have checked the diodes on the secondary of the transformer and they were ok, I will still replace them with some new 1N4148's . I checked continuity on all the connecting pins to all components. I get continuity across pins 1 and 2 and across R1 and C1 (is this as it should be?). I have re- soldered (with care) all solder joints and run solder along all the tracks on the circuit board. I will install the BC549C which should be with me in a day or so.

Re: VCM 163 No.1. Problem while calibrating
 

A little known trick is that polystyrene film capacitors have a non-zero temperature coefficient, but some ferrites (Siemens N28 comes to mind) happen to have an almost equal but opposite temperature coefficient, and so the temperature sensitivity of a tuned frequency is almost nulled. I wonder if that pot core is in a similar material? Were AVO up to that level of subtlety?

Polystyrene film capacitors undergo a permanent value shift if they get taken over 70 to 80C... typically several percent.

David

Re: VCM 163 No.1. Problem while calibrating
 

I'm not sure how well tunes the circuit is, AVO seems to have been more interested in getting a temperature stable design from the early hand drawn/written documents that I've looked at. They don't mention anything special about the tuning and some tests that a friend made showed that the amplifier was happily accepting a wide range of frequencies with a flat response - not my area of expertice, never liked logarithmic papers/curves. :)

But I know that the amplifier is easily disturbed by mains noise from some 4KHz up to 80KHz as I have had several people telling me that when they turn of their solar power inverters (8-24KHz range) the needle falls to zero from whatever extra signal it was detecting. I have worked with modern electricity meters that communicate over the mains and they add a lot of signal on the mains voltage in the upper range and also when they fail in their communication devices they broadcast on a very large spectrum. There are also tons of cheap LED lamps that broadcast on a lot of frequencies, some of those we found out about when the electricity meters were completely drenched with noise so they couldn't communicate properly and also from all of the cheap wall warts that add their own broadband spectrum of noise even down to these low KHz ranges.

Making a small extra power supply, or even just replacing the zener diode with a 12V regulator like the LM7812 plus a full wave rectifier and some smoothing/decoupling capacitors to the oscillator board power supply removes most of the effects from mains noise. I've tested it but I don't have much problem with noise where I live so I kept the original design.

Re: VCM 163 No.1. Problem while calibrating
 

Have replaced the transistors with BC549C and there is no change in the behaviour of the amplifier board.

Re: VCM 163 No.1. Problem while calibrating
 

Well, this was a hard nut to crack.

Right now I can only recommend you to fully trace the wiring in the circuit and also check every component again, if necessary de-solder them to measure them out of the circuit. Take special care to check all of the switches so they work properly.