AVO Model 8 Mk 5 - UK Vintage Radio Repair and Restoration Discussion Forum

zcx2008 · 17th Jul 2008, 1:08 pm

I bought this meter as seen at a boot sale (£8), so I wouldn't expect it to be perfect.

I've now cleaned it up and checked it against the collection of junk that passes for test equipment here.

There are some faults and I need some advice to help fix it.

Here's the test results;

- all the DC Volts ranges seem to work OK, although the 300, 600 and 1000 ranges read increasingly high;

- all DC mA/A ranges seem to work OK;

- all AC Volts ranges seem to work OK except the 10 range which reads about 10% high;

- all AC Amps ranges seem to work OK.

Now the interesting stuff;

- fitted new 1.5v battery; connected 15v supply to the 15v battery contacts;

- the Ohms x1 range works but reads very high; a 10k resistor reads as 8.2, a 100k resistor reads 68 and a 1M resistor reads about 700; this range can be zeroed with the ohms x 1 pot;

- neither the Ohms x100 or Ohms x10k ranges work at all; the meter stays on the left hand end stop; neither range can be zeroed, not even a little bit.

I've removed the case and someone has been there before me. The thickest track on the flexible cable on the left hand side has vapourised and been replaced by a wire jumper.

The only burnt component I can see is the thick film resistor on the right hand side of the main board. I checked this and it's open circuit.

This is probably what's stopping the Ohms ranges from working properly, but without a circuit diagram I'm stuck.

Can anyone post a circuit please, or email me (Removed by Moderator) the circuit, or otherwise advise the value of the thick film resistor so I can make one up?

SPCh · 17th Jul 2008, 7:39 pm

I can post you a photocopy of the circuit diagram if you let me have your address by PM.

The ohms problems are clearly "hard" ones.

If the open circuit resistor you mention on the "right-hand side of the main board".
is the printed circuit component standing vertically at the extreme right of the outer switch board (the one with two ohms adjustment pots on it) that is certainly the cause of the ohmsx1 problem It should be "about 18.4 ohms, adjusted on calibration".

The ohmsx100 and ohmsx10k problems sound like TWO open-citcuits - one on each of their respective sides of the switch. (After the switch they use the same path to the meter as the ohmsx1).

If the lower DC voltage and all the DC current ranges are okay, the problem with the high voltage DC ranges can only be drifted resistor values (if they were reading low it could be due to dry joints or poor switch contacts).

With all but one of the AC ranges okay, the problem must be either a drifted-low R6 ("about 7k ohm, selected at calibration") or perhaps a leaky 220pF capacitor (which can simply be disconnected for low frequency measurements).

I hope this helps; good luck.
Be careful of the printed wiring it fails easily.

17th Jul 2008, 1:08 pm	#1
zcx2008 Triode Join Date: Jul 2008 Location: Bristol Posts: 16	AVO Model 8 Mk 5 I bought this meter as seen at a boot sale (£8), so I wouldn't expect it to be perfect. I've now cleaned it up and checked it against the collection of junk that passes for test equipment here. There are some faults and I need some advice to help fix it. Here's the test results; - all the DC Volts ranges seem to work OK, although the 300, 600 and 1000 ranges read increasingly high; - all DC mA/A ranges seem to work OK; - all AC Volts ranges seem to work OK except the 10 range which reads about 10% high; - all AC Amps ranges seem to work OK. Now the interesting stuff; - fitted new 1.5v battery; connected 15v supply to the 15v battery contacts; - the Ohms x1 range works but reads very high; a 10k resistor reads as 8.2, a 100k resistor reads 68 and a 1M resistor reads about 700; this range can be zeroed with the ohms x 1 pot; - neither the Ohms x100 or Ohms x10k ranges work at all; the meter stays on the left hand end stop; neither range can be zeroed, not even a little bit. I've removed the case and someone has been there before me. The thickest track on the flexible cable on the left hand side has vapourised and been replaced by a wire jumper. The only burnt component I can see is the thick film resistor on the right hand side of the main board. I checked this and it's open circuit. This is probably what's stopping the Ohms ranges from working properly, but without a circuit diagram I'm stuck. Can anyone post a circuit please, or email me (Removed by Moderator) the circuit, or otherwise advise the value of the thick film resistor so I can make one up? Last edited by Darren-UK; 17th Jul 2008 at 1:33 pm. Reason: Forum rule B12.

17th Jul 2008, 7:39 pm	#2
SPCh Rest in Peace Join Date: Nov 2004 Location: Antwerp, Belgium Posts: 345	Re: AVO Model 8 Mk 5 I can post you a photocopy of the circuit diagram if you let me have your address by PM. The ohms problems are clearly "hard" ones. If the open circuit resistor you mention on the "right-hand side of the main board". is the printed circuit component standing vertically at the extreme right of the outer switch board (the one with two ohms adjustment pots on it) that is certainly the cause of the ohmsx1 problem It should be "about 18.4 ohms, adjusted on calibration". The ohmsx100 and ohmsx10k problems sound like TWO open-citcuits - one on each of their respective sides of the switch. (After the switch they use the same path to the meter as the ohmsx1). If the lower DC voltage and all the DC current ranges are okay, the problem with the high voltage DC ranges can only be drifted resistor values (if they were reading low it could be due to dry joints or poor switch contacts). With all but one of the AC ranges okay, the problem must be either a drifted-low R6 ("about 7k ohm, selected at calibration") or perhaps a leaky 220pF capacitor (which can simply be disconnected for low frequency measurements). I hope this helps; good luck. Be careful of the printed wiring it fails easily.