AVO VCM MK3. Meter needle not moving when switched on.

[Station X]

Something amiss there then.

What do you measure at the various taps on the transformer? As Lawrence has stated, expect 1.1 times the voltage on the switch.

EDIT.

I don't think the 0V end of the winding is connected to chassis, so you'll have to measure wrt the 0V terminal.

[ms660]

Also the reference measuring point can sometimes lead up the garden path, what reference point are you measuring from? Transformer 0v or somewhere else?

Lawrence.

[Station X]

Edit and post crossed.

[HBWOODY]

Have found the 125volt supply to RV6 on the screen voltage selector so all is well there.

[Station X]

Where are we at then?

You have 125 X 1.1 VAC on the wiper of the screen grid voltage switch with respect to the 0V connection, but only 15.7 VAC at RV6 wrt the same point?

[Craig Sawyers]

HBWOODY, it might prevent some confusion if you mark up the voltages you're measuring on the circuit schematic. And post that as an attachment.

Craig

[HBWOODY]

Craig I am confusing myself.
The Mk3 was functioning but the meter failed. Used a spare meter which I serviced with the patient help of members. Shortly after the meter was installed a new fault appeared. I was going through the voltage checks, heaters good, anode volts good, screen volts good when moving on to checking the "grid volts" could not get the required volts. I also found that when switching on the needle did not rise to the SET position. At the moment I am trying to follow the circuit to check for a failure somewhere. I can see no burnt out components.
When I get a chance I am going to start again and work my way from the Leakage switch. I suspect one of my failings is overcomplicating things and loosing focus.

[HBWOODY]

I have been looking at the attached jpg of the Set circuit for the CT160 to help me understand the SET circuit of my Mk3. Is there a similar circuit for the MK3 available?

[HBWOODY]

Hope this PDF shows up

[ms660]

Originally Posted by HBWOODY

I have been looking at the attached jpg of the Set circuit for the CT160 to help me understand the SET circuit of my Mk3. Is there a similar circuit for the MK3 available?

It's basically a halve wave circuit for the grid voltage when the valve under test is conducting and a full wave circuit for the meter when in the set position.

Don't forget that for the meter needle to reach the set position on the scale the meter switch has to be set to a current range other than the 120mA or 180mA range, if not then the supply to the meter will in effect be half wave which will result in a much lower indication than otherwise.

If I get time I'll draw a simplified schematic and post it up on here.

Lawrence.

[ms660]

Excuse the scrawl.

Excuse the scan, had a major PC foul up a while back, lost my original scanner procedure, did this through some crumby Windows app.

Hopefully you'll be able to see what's what.

Lawrence.

[Dekatron]

The circuit for the Mk III is almost identical to the CT160, the only differences are in the component values chosen around the mA/V potentiometer and the addition of the step-values for the Grid volts potentiometer. R7-R14 form a voltage divider chain where the potentiometer RV5 with in parallel R15 is inserted at different steps so that it adds the step voltage to the variable voltage. In the mA/V circuit a few resistors, R3-R6, has been added to adjust the scale of the mA/V to make it fit.

I made a simple schematic by just cutting out the portions not needed for the SET AC position, note that the "Meter Switch" is set in the 100mA position to make sure that the common ground is set correctly, that the "Leakage" switch is set in the SET AC position so that there is a connection between H- (10) and the Meter connection (9) in the circuit in this position so that it forms a complete circuit for the current to flow.

You can of course redraw this even more but I usually use the circuit diagram as a base for these when checking an instrument as it is somewhat easier in my opinion as you can see where the wires and components are compared to the full circuit. The includd PDF-file contains the complete circuit diagram as well as the simplified with the cutouts and you can switch between them on the screen to compare them easily.

[HBWOODY]

Martin, Lawrence the simplified circuit breakdowns are a great help. I have only had a few spare moments to look at them, they will enabled me to follow the circuit in the actual MK3. When I have some spare time I will have to study how to measure and check components etc in the circuit. There is most likely a simple explanation to the failure and I am determined to find it. Reason I cannot focus on the Mk3 at the moment is the failure of my tractor (diesel pump seized), it takes priority so my Mk3 is feeling a little neglected.
Thanks again for your help.

[HBWOODY]

Lawrence as you are Cornwall based would you consider sorting the problem for me if I have to throw in the towel? Still working on my tractor at the moment.

[ms660]

Originally Posted by HBWOODY

Lawrence as you are Cornwall based would you consider sorting the problem for me if I have to throw in the towel? Still working on my tractor at the moment.

Sorry, I can't, I can only advise via the forum, if you have a missing or low voltage then hopefully it should be easy enough to nail down, for the needle not reaching the "set area" and lack of grid voltage you need to establish whether or not the correct AC voltages are being supplied to the rectifier and that includes the heater voltage (Eg: make sure the heater is lit)

I cant find any info as to what the AC supply is to the rectifier for the grid winding should be but it should be at least that of what's supplied from the HT transformer via the link to the other cathode of the rectifier (137 volts AC) To measure them at the rectifier cathodes I would first measure them with respect to the 0 volts connection on the HT transformer, and to avoid any confusion the meter switch should be switched to the 100mA setting.

Lawrence.

[HBWOODY]

Thanks for the prompt reply. Apology not needed I respect your position and I apologise for approaching you in the first place. I will investigate your suggestions when I have sorted the tractor (hopefully only a couple of days).
I have been studying the circuits sent by yourself and Decatron and I am able to follow it on the actual MK3
One area I have found which I am not confident is where R19 connects to the meter. It looks to me that R19 connects to the meter on the top (moving) section of the circuit selector switch and the meter connection to the contact plate on the underneath of that switch section. I do not get continuity when testing between the top contact plate and the bottom contact plate of that section. I find continuity between top and bottom contact plates of all the other sections of the circuit selector. Hope the above makes sense.

[ms660]

Yes, R19 doesn't connect directly to the meter in the tester, the simplified schematic I did was just to show the general layout of things without having to show all the switches being shown and from that you can see on the full schematic what connects to what.

Basically the circuit is that of a full wave rectifier, the full wave voltage is needed to deflect the meter needle to the Set area on the meter scale but in effect only half of that voltage is used when doing a valve test, that's because the valve only conducts for one half of the cycle.

Lawrence.

[HBWOODY]

Still waiting for a tractor part this morning so decided to tackle the MK3 using the simplified circuits and measuring advice received from members. It did not take long to discover that RV4 had failed. Will replace RV4 and go through the setting up and calibration procedure when I have some more spare time and report back on the outcome. I was close to giving up so thank you again for keeping me on the right track.
As you can imagine I am a happy bunny at the moment. If I have success with the tractor in the next few days I will be treating myself to a few indulgences before moving on to some other problem to sort out.
This lock down has certainly helped my focus.

[Dekatron]

Nice that you found one fault and that it is an easy fault to fix.

[HBWOODY]

Have replaced RV4, set up the working voltages and neg grid volts which are now as they should be. When adjusting RV6 to to get the needle to the correct position (SET ZONE) it gets very close, 85ua rather than the required 90ua on the meter scale, so that is one problem I have to solve.