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Old 13th Dec 2018, 12:47 pm   #1
David Simpson
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Default Vintage Siemens Hearing Aid

Just wondering - has anyone attempted the repair/rejuvenation of an ACOS 1 3/4"(43mm) dia crystal mic insert. As described in the April Edition 2007 Page 19 of the VMARS Signal Magazine ? (Steve Ward GW1 XVC)
I've had a shot of an old one which only produced 50 - 100mV P-P on my scope, with a hefty whistle. Now gives approx. 200mV.What a lot of delicate diss-assembly. Found that the approx. 1cm sq of crystal had become un-attached from the casing. Re-glued with superglue.
I'm hesitant to pdf copy the article here, don't want to upset VMARS.
Steve Ward's mic was a Type 43, but my old thing was just stamped ACOS & MIC above the logo. He achieved 600mV, but his mic looked a bit newer than mine.

Regards, David
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Old 16th Dec 2018, 6:06 pm   #2
ionburn
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Default Re: ACOS Crystal Microphone

I have a mic insert labelled ACOS MIC 43 which is crystal. I have never used it (or had to repair it), but I just tested it and got similar results to Steve. I must admit though that testing is a bit variable with whistles. I used my sig gen with a receiver producing the whistle
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Old 18th Dec 2018, 6:01 pm   #3
David Simpson
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Default Vintage Siemens Hearing Aid

Following on from my thread post "ACOS Crystal Microphone" on 13th Dec.(Which possibly Moderators might like to combine into this new thread ? Please), thought I'd give more info as to why I needed to repair the ACOS mic in the first place.
Back in 1959 someone gave me an old external battery powered lapel button mounted working bakelite hearing aid, as I had just started to become interested in things radio/electronic as a schoolboy. Many years ago the original two carbon mic inserts became u/s. I've just recently re-discovered this wee gem in an old box.
A VMARS chum gave me the ACOS, plus advice that a cheapo wee LM386 minute chip circuit board(less than a tenner from Hong Kong) would handle the i/p & o/p impedances, and leave room for making my own 12V button cell pack. All done, but not much volume out of the old earpiece or a modern plastic one.
The hearing Aid was made by Siemens & Halske AG of Berlin. Internet research reveals that their original "Double Microphone" one - the "Phonophor" came out in 1913, and a later version - the M22 came out in 1925. This one of mine I reckon is from the 1930's, but I'm open to more informed advice. The ACOS fits superbly at one end, and the vacant mic space left takes the LM386 & the homemade 12V battery. Just a novelty project, but I must admit that I'm out of my comfort zone with micro-electronics. I had intended building just a simple OC71 friendly miniature veroboard AF amp, but my chum reckons I should wake up to 21st century technology.
Any vintage audiologists out there ? Its approx. 3" x 6" x 5/8" thick - - i-phone size.

Regards, David

PS Attn "ionburn"
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Last edited by David Simpson; 18th Dec 2018 at 6:05 pm. Reason: Add'l info
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Old 19th Dec 2018, 10:49 am   #4
ionburn
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Default Re: Vintage Siemens Hearing Aid

I see the reason for the completely different microphone now. I have some circuit diagrams around for (I think Mullard) circuits for hearing aids from the Germanium transitor era (OC57 -> 60), but nothing of that age, and no experience of carbon microphones. With modern microcircuits almost anything could be fitted in a case that size.
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