Correct use of AVO 8 MK2 2500V measurements.

[bigfathairyvika]

Hi All,

Had the need to measure ac to 900 and 1800v today.
Connected my probes to the bottom left red common terminal and the top right 2500 ac terminal and measured my voltages.
Set dc range to ac and ac range to 250v ( but I don't think this actually matters )
Measuring what I'm sure is about 980v ac , I get 600v.
I did do a quick check with a digital meter which flashed the display to say stop doing that. But it did read 980v.

Have I connected up wrong?

Mark

[bigfathairyvika]

Update: just tested the mains voltage on the 250 and 1000v settings and it's wrong!
Damn...

Dc ranges read OK , well I only have a 12v battery here at the moment , haven't tried 2500v dc range.

[bigfathairyvika]

First thoughts... diode in bridge open circuit?

[Craig Sawyers]

The manual says to measure on the 2,500 V range, the voltage switch (either dc or ac) should be set to 1000V.

[bigfathairyvika]

From the schematic, the ac voltage setting only affects the reading from the bottom right connector.
As long as it is set to voltage all should be ok.

As the meter reads incorrectly on only ac voltages I definitely suspect a diode in the bridge and will now investigate.

[bigfathairyvika]

OK, diodes appear OK.

Did more tests using low voltage inputs.

238vac on 250v range = 150v measured
24vac on 100v range = 22.5v
12.2vac on 25v range = 12.5v

Only thing common above 100v range is 150k resistor and the range switch.

On my mk2 there are additional resistors in series with the resistors on the schematic.

So, checked resistance used to bridge from connector.
100v = 99k
250v = 250k
25v = 6.47k
All within spec!

[The Philpott]

Voltage readings that are low by that much suggest a switching problem (the stacks of leaf switches behind the rotary selectors.)

The circuit diagram shows the correct open/closed sequences.

Dave

[bigfathairyvika]

Originally Posted by The Philpott

Voltage readings that are low by that much suggest a switching problem (the stacks of leaf switches behind the rotary selectors.)

The circuit diagram shows the correct open/closed sequences.

Dave

Thanks for mentioning that Dave, I have found a thread on here also mentioning the cam switches for a simular problem.
Will check and clean tomorrow.

[The Philpott]

Although the rotary selector* shafts are quite tight in their brass bushes it is sometimes (Prior to dismantling) possible to identify this problem by feeding in the reference voltage and rocking them* at north and south positions, thus marginally changing the connectivity between the leaf contacts.

Dave

[bigfathairyvika]

OK, I have got it down to the "fg" pair of contacts.

I verifed this by altering the "zero x100" control which should only alter readings on x100 ohms as the "fg" contacts bypass the pot and a 3.3k resistor.

Altering the control altered the ac voltage reading!

I checked the cam adjustment screw which was slightly loose. So, I reset the cam as per the service sheet to TDC whilst x100 setting is set.
This has allowed adjustment of the "fg" contacts possible as previously they were near touching when they shouldn't have been.

The contacts look like they are touching but they may be high resistance.
There may be a problem with the "abc" contacts, but I will short out each set of contacts next to verify the bad connection is the switch and not the wiring.

Bed time now.... so more tomorrow.
Hopefully fix this before Thursday as I'm back at work then.

[bigfathairyvika]

Avo fixed.
"fg" contacts cleaned with paper wetted in brasso, then polished with dry paper.

[The Philpott]

Excellent. I forgot to mention the cam rotor screw(s)- there's a crinkle washer under the head, but they can on rare occasions come loose.

Curiously the cam rotors are not pegged or keyed in position, so are capable of slipping out of synch with the selectors in the event of the screw/s coming loose! Easy enough to reset the timing if you've got the circuit diagram..

Dave