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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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13th Mar 2018, 9:21 pm | #1 |
Pentode
Join Date: Feb 2018
Location: Cheshire, UK.
Posts: 121
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Armstrong 625 AM Aerial
I'm fixing up an Armstrong 626 amplifier/tuner. At some point in its past it has lost the AM aerial that has fixed onto the rear of the unit. From pictures I've seen it looks like a ferrite rod type enclosed in a plastic tube. I doubt if I'll find an exact replacement, but i thought I might fabricate a new one with an old rod and coil with some heatshrink. There are plenty of new and second hand ones about but they all seems to have two coils (MW/LW) with multiple taps off the coil. The 626 I have has a piece of coax hanging out where the missing aerial should be - so my question is what type of coil should i use on a new ferrite and which connections should I join to the coax?
BTW the spell checker complains about ferrite - it prefers fritter instead! |
13th Mar 2018, 9:59 pm | #2 |
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,944
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Re: Armstrong 625 AM Aerial
The Armstrong 600 series AM tuner was an upconversion design with a wideband RF stage - see: http://www.audiomisc.co.uk/Armstrong/600/600page4.html.
The feriite rod AM aerial was thus untuned. I guess that you might need to experiment with various turn numbers around a suitable ferrite rod in order to get somewhere near the original. Cheers, |
14th Mar 2018, 9:38 am | #3 |
Pentode
Join Date: Feb 2018
Location: Cheshire, UK.
Posts: 121
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Re: Armstrong 625 AM Aerial
Would anyone care to guess on how many turns of wire to start off with?
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14th Mar 2018, 10:28 am | #4 |
Nonode
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 2,038
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Re: Armstrong 625 AM Aerial
Looking at the cct, the input impedance will be fairly low. I'd start off with about 20 turns and increase in 10-turn increments, weighing up the performance on the same weak signal at every change.
I've never had much success with non-resonant ferrite rods, they pick up precious little signal.
__________________
Andy G1HBE. |
16th Mar 2018, 11:44 am | #5 |
Pentode
Join Date: Feb 2018
Location: Cheshire, UK.
Posts: 121
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Re: Armstrong 625 AM Aerial
I've ordered a 2nd hand ferrite, but while I'm waiting for it to be delivered I thought I'd see if I can get the AM tuner to work using some long pieces f wire plugged into the external aerial socket. All I'm getting in terms of audio is crackles and background noise. I'm a bit concerned that the AM circuitry isn't working or is incorrectly set up. There are an awful lot of tuning adjustments on the pcb and without specialist equipment and knowledge, I reckon I'm stumped with this. Any suggestions?
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16th Mar 2018, 2:34 pm | #6 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,966
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Re: Armstrong 625 AM Aerial
This is a 1972 model so should be after the AF11x era, but are there any Lockfit transistors inside? (Search the forum if you don't know what a Lockfit is).
Clean all the switch contacts thoroughly. |
16th Mar 2018, 6:28 pm | #7 |
Guest
Posts: n/a
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Re: Armstrong 625 AM Aerial
And don't twiddle any of the adjustments, they will most likely be near enough, I agree with Paul, clean the switches with (oddly enough) switch cleaner, not WD40. I am not normally a fan of specialist products where a standard domestic one will do, in this case the right stuff is important.
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16th Mar 2018, 9:07 pm | #8 |
Pentode
Join Date: Feb 2018
Location: Cheshire, UK.
Posts: 121
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Re: Armstrong 625 AM Aerial
No lockfits - just ITT BF241 in plastic D shaped enclosure with standard wire leads.
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