Megger ID battery model - UK Vintage Radio Repair and Restoration Discussion Forum

Refugee · 15th Apr 2017, 12:41 am

I have two of these in various states of repair.
The first one does not have a model number unless it is under the service label. It has three resistance ranges and a bad meter movement.
The potential donor for the movement shows model number BM7 and has a well rotted battery compartment and only two ranges.

I would like to find out what model the three range one is and if circuits can be got for them.

Stockden · 15th Apr 2017, 9:54 am

The 3 three range one looks like a BM6. There's more about them in this thread.

Hugh

WME_bill · 15th Apr 2017, 11:36 am

Avo Meggers. The three range model look like BM6. The two range BM7. As the details I see I posted back in March 2011, under heading BM6 Megger.
I suggest a bit of reverse engineering is required for the BM6 circuit. The vertical circuit board is clearly the oscillator, generating the high voltage, and likely to be the same as the BM7.
The supply for the other lower ranges will come from the battery, tapped off by a resistor chain - the ones you are puzzled by.
I have the circuit for the BM8, but it is not directly helpful. It offers 5 different test voltages, 50v & 100v obtained by a resistor chain tapping across the 250v chain. Then the higher 250v, 500v & 1000v by switching the rectifiers to give voltage doubling. The BM6 mid-range uses 8.5v, which I would expect to come from the 9v battery more or less direct.
The transistors are general purpose germanium. ACY21 family. Quite non critical.
I have a note that later models used the silicon BC214. If you are really stuck, I can find some Ge TO5 Texas GA396 as used a lot by Solartron or GET885, all of which would do. Or even find some ACY17-21 family.
Take the offered meter. It is 250ua, 175ohm on the BM7. On the BM8, the circuit says it is 10ua. I am a bit suspicious that this is not a mis-print for 100ua. 10ua are very very rare indeed, and Avo seemed generally to keep to 30ua or 50ua, using the centre pole movement they purchased with Taylor Instruments.
wme_bill.

Refugee · 15th Apr 2017, 12:46 pm

The inverter is working in the BM6 as I can hear it. I have got some ACY series transistors and might well have the right ones. The main problem with the BM6 is the movement.
The BM7 has got so much corrosion in the battery holder that my only option will be to buy several cheap replacement battery holders from the shops or the web and cut them up.
Most of the battery springs remained stuck to the batteries when I flicked them out into the bin.
I will start a thread for each model as I do them.

15th Apr 2017, 12:41 am	#1
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Megger ID battery model I have two of these in various states of repair. The first one does not have a model number unless it is under the service label. It has three resistance ranges and a bad meter movement. The potential donor for the movement shows model number BM7 and has a well rotted battery compartment and only two ranges. I would like to find out what model the three range one is and if circuits can be got for them. Attached Thumbnails

15th Apr 2017, 9:54 am	#2
Stockden Heptode Join Date: Mar 2008 Location: Forest of Dean, Gloucestershire, UK. Posts: 648	Re: Megger ID battery model The 3 three range one looks like a BM6. There's more about them in this thread. Hugh

15th Apr 2017, 11:36 am	#3
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Megger ID battery model Avo Meggers. The three range model look like BM6. The two range BM7. As the details I see I posted back in March 2011, under heading BM6 Megger. I suggest a bit of reverse engineering is required for the BM6 circuit. The vertical circuit board is clearly the oscillator, generating the high voltage, and likely to be the same as the BM7. The supply for the other lower ranges will come from the battery, tapped off by a resistor chain - the ones you are puzzled by. I have the circuit for the BM8, but it is not directly helpful. It offers 5 different test voltages, 50v & 100v obtained by a resistor chain tapping across the 250v chain. Then the higher 250v, 500v & 1000v by switching the rectifiers to give voltage doubling. The BM6 mid-range uses 8.5v, which I would expect to come from the 9v battery more or less direct. The transistors are general purpose germanium. ACY21 family. Quite non critical. I have a note that later models used the silicon BC214. If you are really stuck, I can find some Ge TO5 Texas GA396 as used a lot by Solartron or GET885, all of which would do. Or even find some ACY17-21 family. Take the offered meter. It is 250ua, 175ohm on the BM7. On the BM8, the circuit says it is 10ua. I am a bit suspicious that this is not a mis-print for 100ua. 10ua are very very rare indeed, and Avo seemed generally to keep to 30ua or 50ua, using the centre pole movement they purchased with Taylor Instruments. wme_bill.

15th Apr 2017, 12:46 pm	#4
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Megger ID battery model The inverter is working in the BM6 as I can hear it. I have got some ACY series transistors and might well have the right ones. The main problem with the BM6 is the movement. The BM7 has got so much corrosion in the battery holder that my only option will be to buy several cheap replacement battery holders from the shops or the web and cut them up. Most of the battery springs remained stuck to the batteries when I flicked them out into the bin. I will start a thread for each model as I do them.