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Old 21st Feb 2020, 6:09 pm   #1
Superscope
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Default AVO meter EM272

Does anybody on the Forum have any experience with the AVO EM272 Electronic Multimeter?

It reads very low.
50% at least on Higher AC & DC Ranges, and at least
25% on lower Ranges.

Can't get the Ohms range to Zero, only get it to around 25% FSD.

I assume it will be a problem with the Current Amplifier somewhere.

So before I get started, I thought I would ask around for any pointers on where to start or look out for.

There are Six Transistors in this Meter.
BC 214 x 2 Obsolete but equivalents are available
BC 184 x 2 Still Available

But, it has FRB 935 x 2 Cannot find any information on this Transistor.
Data-sheet or Equivalents.

Does anybody know anything about this Transistor?

I may not need it this Time, but no harm in preparing.


Ian
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Old 21st Feb 2020, 9:53 pm   #2
pmmunro
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Default Re: AVO meter EM272

Superscope,

Do you have the EM272 Service Manual? It's not much help with information on the FRB935, simply listing TR1 & TR2 as "Matched pair of transistors" AVO part No. 6120-440.

Please let me know if you need a copy of the manual.

PMM
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Old 21st Feb 2020, 10:03 pm   #3
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Default Re: AVO meter EM272

BC214 are still available, 60p from Cricklewood. I've got quite a few if you get desperate. Good luck on finding the true identity of the FRB things.

Andrew
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Old 21st Feb 2020, 10:11 pm   #4
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Default Re: AVO meter EM272

FET's for very high impedance inputs? Les.
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Old 21st Feb 2020, 10:24 pm   #5
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Default Re: AVO meter EM272

Thinking about it, was FRB a Ferranti part number? I bought a load of ex-mil transistors several years ago and some of those I'm sure were marked something like that.

Andrew
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Old 21st Feb 2020, 10:30 pm   #6
Superscope
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Default Re: AVO meter EM272

PMM,

I have a copy of the Service Manual, but as you say, it is not much help.
It doesn't even correctly described the disassembly of the Meter.
Fortunately, that part of the puzzle is fairly easy to work out.

My meter may possibly (wishful thinking) only require adjustment of the
Current Amplifier, but in the manual the adjustment procedure mentions measuring
the Voltage and adjusting for Zero between the points X on the PCB drawing.
I cannot find any points marked X on the actual PCB or on any of the Drawings in the
Manual, so at the moment, I can't even try adjusting the Amplifier.


Andrew,

Yes, the BC's or their equivalents seem fairly easy to get, at least at the moment.
The FRB's are a bit of a mystery.
Hopefully I don't need them, but it would be nice to know as much as possible
regarding this Meter.






Ian
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Old 21st Feb 2020, 11:02 pm   #7
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Default Re: AVO meter EM272

I don't recall off hand, but some FRBxxx transistors were used on the output of a TQ scope, a DM63 from memory Something like a BF259 would be electrically similar.. NOT FRB935 though.
As you have two, which MAY be OK, one of those cheap (£10 ish) Chinese all singing testers would quickly tell you the basics (npn,pnp, gain, pin out).
Size for size, the FRBxxx in the TQ looks much lower power than it is (hence BF259, 459 etc.) being OK to replace a small black device, though they did use a small black"wing" heatsink to help.
Les.
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Old 22nd Feb 2020, 12:26 am   #8
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Default Re: AVO meter EM272

BC214 and BC184 are jellybean types, nothing special. There are many alternatives.

David
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Old 22nd Feb 2020, 10:32 pm   #9
Superscope
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Default Re: AVO meter EM272

The Circuit Symbol on the Schematic suggests that the FRB-935 is a regular
Bipolar Transistor, not an FET.

As suggested, I will try to test it with one of those $10 Chinese Component testers.
Actually, I will try to test all the Transistors.
Not sure how well that will work in Circuit, but worth a look.

As I mentioned, the FRB-935 may not be at fault, but the adjustment procedure shown
in Section B refers to checking and setting the Voltage to Zero between points X on the
PCB Diagram, but unless I'm missing something, there is no X written anywhere on the Drawings or the actual PCB!
So the set up procedure is a bit meaningless at the moment.



Ian
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Old 23rd Feb 2020, 12:57 pm   #10
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Default Re: AVO meter EM272

Avo EM272
I agree with RadioWrangler. These transistors are standard low noise, high gain audio types, absolutely jelly bean. Take your pick.
The two FRB935 which are worrying you, are clearly a manufacturers special selection, so will not appear on any catalogue or list.
But looking at how they are used, I suggest that they have been matched for similar gain, within 20%, and then clipped or stuck together to keep them at the same temperature.
Test a selection at low current (about 10ua I think in that circuit).
When selecting transistors or FET, I have found that from six, taken from the same manufacturing batch (or bought at the same time), I can get two pairs sufficiently well matched.
You may find the article upon a Simple Electronic Multimeter by Linsley Hood in Wireless World, June 1972 which discusses a very similar circuit helpful.
I too have been puzzled by the mysterious "point X". I think it is between the bases of the balanced input pair. A very high impedance point, so use a valve voltmeter or CT471 or EA113 or similar as the book says.
Interesting the use of transistors as "super diodes", presumably to match the temperature coefficients across the circuit. The only commercial use which I know of them.
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