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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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10th Feb 2020, 10:42 pm | #1 |
Nonode
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,039
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What were Transfilters?
Hi all,
I've just remembered something I'd forgotten all about. In ( I think )Practical Wireless, in 1967, was a project to build a "Transfilter Portable". What were they? An IFT with an inbuilt filter? Cheers Aub
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10th Feb 2020, 10:47 pm | #2 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 1,558
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Re: What were Transfilters?
I have a vague memory that they were ceramic filters that could be used in place of tuned IF transformers - could be wrong though.
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10th Feb 2020, 10:52 pm | #3 |
Nonode
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,039
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Re: What were Transfilters?
Ah ok. Thanks. That sounds likely.
Cheers Aub
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11th Feb 2020, 12:46 am | #4 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
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Re: What were Transfilters?
They were early piezo-acoustic ceramic filters made by Brush-Clevite. I think it was their brand-name. Makes them sound a bit grander than they were. Toko and Murata took over the market.
David
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11th Feb 2020, 1:32 am | #5 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,433
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Re: What were Transfilters?
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11th Feb 2020, 9:33 am | #6 |
Nonode
Join Date: Mar 2005
Location: Nuneaton, Warwickshire, UK.
Posts: 2,039
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Re: What were Transfilters?
Thanks Frank. I wondered if someone would do that. Love those plastic covered batteries.
Aub
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11th Feb 2020, 10:14 am | #7 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,433
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Re: What were Transfilters?
They improved selectivity by using a bypass filter in the emitter circuit of an IF amp instead of just a capacitor. I understand how that works, just never thought about it before. Or if I have seen it I have forgotten all about it which is very possible.
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11th Feb 2020, 3:32 pm | #8 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,998
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Re: What were Transfilters?
There were at least two styles of 'Transfilter' - one was a 2-terminal device which replaced the normal emitter-resistor-bypass-capacitor. The idea being that at the series-resonant frequency of the transfilter the emitter was essentially grounded at the intermediate frequency so gicing good gain, but at other frequencies when the Transfilter wasn't resonant the gain of the transistor was reduced by the degenerative-feedback from the emitter resistor.
The other type was a 3-terminal device used as a replacement for the 'traditional' IFT between the collector of an IF amp and the base of the next-amp-in-the-chain. Their big selling-point to the receiver-manufacturers was that - unlike wound-component IFTs, there was no alignment needed, so cutting production-costs. Several Heathkit receivers of the 1960s used them. Attached is a section of the schematic for the GC1-A "Mohican' that shows both the inter-stage-coupling Transfilters and the emitter-resistor-bypass variety. |
11th Feb 2020, 4:14 pm | #9 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
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Re: What were Transfilters?
Those three-terminal filters wiere used by Blaupunkt (and possibly others) in car radios both at AM and FM(VHF). They were made in 455, 460, 465, and 470kHz versions for AM, and nominally 10.7Mhz (which could be between, IIRC 10.65 & 10.75Mhz) for VHF. If replacing a ceramic filter (They did fail occasionally), obviously one of the correct frequency had to be used.. I may still have some somewhere)
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11th Feb 2020, 5:32 pm | #10 |
Heptode
Join Date: Apr 2003
Location: Ayr, Ayrshire, UK.
Posts: 631
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Re: What were Transfilters?
Yes, here's some pictures of one I repaired from a Mohican - it had become 'intermittent' and responded to cleaning of the disc and contacts with IPA then reassembly with the disc rotated by 90°.
Andy
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