dissassembling a meter.... arrrg!

[bradb]

Hello everyone,

I'm working on a Federal AM-864/u and the meter is bad. I'm trying to have a go at fixing it. It measures open, resistance wise.

Here's a picture of it.



I CANNOT get the meter/magnet part out of the housing. Has anyone dissassembled one of these things...

also, anyone got a spare to save me some time? F.S. 1.01mA

Thanks!!


trying to get the hang of posting images/links here..

 

[bradb]

ug, just figured it out after 2 days.. the electromech part sits on a plastic part that sits in the metal housing... the plastic part and metal housing look like 1 piece, thats what got me. now i've got it apart and only have to deal with a grouchy stuck nut, its always something..

bb

 

[bradb]

Here is the inside of my meter. I measured a good one and its roughly 100Ohms.

Are the insides of these things standard? I need another one that's 1.01mA full scale.

here are some pics:

http://www.imagehosting.us/index.php?action=show&ident=471789

http://www.imagehosting.us/index.php?action=show&ident=471797

http://www.imagehosting.us/index.php?action=show&ident=471800


help?

 

[PRR]

> Are the insides of these things standard?

Of course not. That would make life easy.

It isn't even a VU meter. It uses a variation of the pre-1939 standard. My meters are calibrated for 6mW but into 500 ohms, not 600 ohms like yours. Standards are such fun.

Get a bright light and try to find the break in the winding.

When you give up, get a cheap plastic 1mA meter, pull the guts out of the case, prop it up in the old case, and put the scale in front. The resistance is not critical, and I'm sure there is no difference between 1mA and 1.01mA. But you do want a meter with about the same size and a similar deflection angle. If the new meter has a wider deflection angle, zero-trim the left end, put a stop at the right end, and use ~300 ohms shunting the meter to reduce its sensitivity to equal the working meter.

 

[BYacey]

Meter surgery is a very delicate task. Wipe all your tools with the sticky side of some masking tape to remove any metal filings or swarf that might be on your tools. One little filing in the movement and you may as well throw it out.
I have tried to resurrect a number of D'arsonval movements in the past and had limited success. The bearing tension, coil spring tension are all critical adjustments. Hardly worth the effort unless it's something really special. I like PRR's suggestion, put a new motor under the hood.

 

[bradb]

[quote author="PRR"]> Are the insides of these things standard?

Of course not. That would make life easy.

It isn't even a VU meter. It uses a variation of the pre-1939 standard. My meters are calibrated for 6mW but into 500 ohms, not 600 ohms like yours. Standards are such fun.

Get a bright light and try to find the break in the winding.

When you give up, get a cheap plastic 1mA meter, pull the guts out of the case, prop it up in the old case, and put the scale in front. The resistance is not critical, and I'm sure there is no difference between 1mA and 1.01mA. But you do want a meter with about the same size and a similar deflection angle. If the new meter has a wider deflection angle, zero-trim the left end, put a stop at the right end, and use ~300 ohms shunting the meter to reduce its sensitivity to equal the working meter.[/quote]

I found a broken winding, tried moving it to the solder post and it broke off never to be found again...

I'll step up my search for a cheap 1mA meter... I've heard alot about ballistics, sensitivity etc. with meters.. can you enlighten me on what this stuff means and how it is affected? How/why did you choose 300 Ohms?

thank you!

 

[SSLtech]

[quote author="PRR"]My meters are calibrated for 6mW but into 500 ohms, not 600 ohms like yours. Standards are such fun.[/quote]

"We love standards... -We have LOTS of them!"

:green:

Keef

 

[NewYorkDave]

Sorry to be the humorless pedant, but the 0dB = 6mW/500 ohms can be considered an obsolete standard. It was superseded in 1939 by the VU meter standard we still use today. It did persist a while afterwards in the film industry, though.

 

[BYacey]

With an adjustable meter drive amp, it can be scaled to whatever you want it to be. Not many people connect an un-buffered meter direct to the audio line anymore.