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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 23rd Sep 2021, 12:08 pm   #1
suebutcher
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Default Hearing aid Bosch Omniton MT80 SP.

I bought this old Bosch hearing aid recently. Made about 1975, I think. It worked once the battery muck was removed, and it amplifies distant voices quite well. But I'm not sure if the AGC is working correctly, because the wearer's own voice tends to overload the output circuit. If you look at the trimmer panel on the side, there's a microphone gain control (GC), a tone control (TC), and what I assume is the AGC (PC). But adjusting the PC trimmer affects the gain at all volume levels, which doesn't help close voice distortion. Inside it's all common or garden discrete components, so it seems serviceable. Advice, anyone? Where might I find a circuit diagram?
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Last edited by Cobaltblue; 25th Sep 2021 at 10:09 am. Reason: title change requested by OP
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Old 25th Sep 2021, 10:19 am   #2
Cobaltblue
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Default Re: Hearing aid Bosch Omniton MT80 SP.

I did look into hearing aids a few decades ago having picked some up cheap at the Boot sale, for simple devices they can be amazingly sophisticated.

Many have a power saving circuit that switches off the output stage unless sound reaches a threshold as well as an AGC.

I spy tants and I have a very suspicious nature when stuff has tants in it.

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Old 26th Sep 2021, 10:52 am   #3
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Default Re: Hearing aid Bosch Omniton MT80 SP.

Yep, the only non-tantalum electrolytic in there is a replacement. It wouldn't be hard to replace the rest. Is age on its own a problem with tantalums? Because I've plenty of unused tants in my spares.

I've improved on the performance somewhat. Gain down to -20dB, then AGC to -8dB, which gives less distortion on close voice without reducing the volume of a distant voice.
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Old 26th Sep 2021, 1:06 pm   #4
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Default Re: Hearing aid Bosch Omniton MT80 SP.

I doubt age is the problem as I had a fairly recent one from Maplin fail (lasted 10 years), but rather inadequate voltage rating, i.e. putting 6V rated parts on a 5V rail, or 16V parts on a 12V or 15V rail will increase the failure rate.
I should point out I have lots of 30+ year old test gear with tants and so far only the one item has had a failure (OK it happened again as I didn't uprate the part I fitted).

I've seen far more problems with small electrolytics, the seals can fail after 20 to 30 years, the electrolyte escapes and corrodes traces on multilayer boards. So far I've seen Sprague, Panasonic, Rubycon & Nichicon fail in this way (including NOS).

David

Last edited by factory; 26th Sep 2021 at 1:17 pm.
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Old 26th Sep 2021, 1:19 pm   #5
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Default Re: Hearing aid Bosch Omniton MT80 SP.

I'm far from being an expert on old hearing aids, but the design looked dated for 1975, especially given that Bosch were a premium manufacturer. I did a bit of googling and found several references to 1970, though even that seems late for a 'shirt front' hearing aid.

https://hearingaidmuseum.com/gallery...ndex-bosch.htm
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Old 27th Sep 2021, 2:07 am   #6
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Default Re: Hearing aid Bosch Omniton MT80 SP.

The date's a guess. The serial number starts with "750..." and the colour of the case is beige and dark brown, a typical late-70s appliance scheme. Manufacturers kept making body hearing aids like this for people with severe hearing loss. This one is quite powerful, far too loud for Carol at full volume.
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