Avo8 Mk3 ohms/100 circuit question - UK Vintage Radio Repair and Restoration Discussion Forum

SeanMcK · 29th Sep 2008, 9:33 pm

Hello
I just acquired a 'dead' one. I gave it a quick test on some voltages and they seem ok, however the ohms/100 is dead. A look inside shows that the 9ohm resistor has been cooked (R32 or V depending on which circuit diagram is used )

Ok I know I wont be able to get the actual resistor but I can start by getting close the the value. My concern is choosing the correct wattage.
Does anyone know the specified wattage of this resistor. The manual I have for the Mk3 shows the old wound wire resistors where as this meter has the more modern axial lead resistors. Also I have one circuit diagram which shows a pair of diode in parallel with this resistor and reverse biased with repect to one another.

Why are these diodes there?

Having seen the cooked resistor I cant help but wonder if they are some over volt bypass for the resistor.
Thanks

SPCh · 30th Sep 2008, 10:32 pm

Okay; it's not as profound as it seems.

I don't know the "official" wattage spec of the 9 ohm resistor, but in normal use (about 80mA loop current) it should not carry more than about 60 mW. The two reverse diode pairs (BY124s all) are a protection against overload to ensure that the fuse blows before the movement is damaged. They will kick in at a loop current of about 160mA, i.e. when the movement is driven at about twice full scale - so when measuring the "resistance" of a 1.4 volt source (I think we can assume where the designers 'were coming from'). They won't offer much protection against measuring the resistance of the mains !

If you replace the resistor with almost anything 9 ohms, and the quad-diode pack, the meter will work quite happily if you treat it with respect. There's a (highly unofficial) mod if you want to protect it against serious abuse. It's also a good idea to down-rate the fuse from 1 amp to 250 or even 150 mA !

29th Sep 2008, 9:33 pm	#1
SeanMcK Retired Dormant Member Join Date: Sep 2008 Location: Londonderry, Northern Ireland, UK. Posts: 58	Avo8 Mk3 ohms/100 circuit question Hello I just acquired a 'dead' one. I gave it a quick test on some voltages and they seem ok, however the ohms/100 is dead. A look inside shows that the 9ohm resistor has been cooked (R32 or V depending on which circuit diagram is used ) Ok I know I wont be able to get the actual resistor but I can start by getting close the the value. My concern is choosing the correct wattage. Does anyone know the specified wattage of this resistor. The manual I have for the Mk3 shows the old wound wire resistors where as this meter has the more modern axial lead resistors. Also I have one circuit diagram which shows a pair of diode in parallel with this resistor and reverse biased with repect to one another. Why are these diodes there? Having seen the cooked resistor I cant help but wonder if they are some over volt bypass for the resistor. Thanks Last edited by Darren-UK; 29th Sep 2008 at 10:03 pm. Reason: To accord with Forum Rules.

30th Sep 2008, 10:32 pm	#2
SPCh Rest in Peace Join Date: Nov 2004 Location: Antwerp, Belgium Posts: 345	Re: Avo8 Mk3 ohms/100 circuit question Okay; it's not as profound as it seems. I don't know the "official" wattage spec of the 9 ohm resistor, but in normal use (about 80mA loop current) it should not carry more than about 60 mW. The two reverse diode pairs (BY124s all) are a protection against overload to ensure that the fuse blows before the movement is damaged. They will kick in at a loop current of about 160mA, i.e. when the movement is driven at about twice full scale - so when measuring the "resistance" of a 1.4 volt source (I think we can assume where the designers 'were coming from'). They won't offer much protection against measuring the resistance of the mains ! If you replace the resistor with almost anything 9 ohms, and the quad-diode pack, the meter will work quite happily if you treat it with respect. There's a (highly unofficial) mod if you want to protect it against serious abuse. It's also a good idea to down-rate the fuse from 1 amp to 250 or even 150 mA !