Armstrong 625 AM Aerial - UK Vintage Radio Repair and Restoration Discussion Forum

cooperman · 13th Mar 2018, 9:21 pm

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!

Synchrodyne · 13th Mar 2018, 9:59 pm

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,

cooperman · 14th Mar 2018, 9:38 am

Would anyone care to guess on how many turns of wire to start off with?

Andrew2 · 14th Mar 2018, 10:28 am

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.

cooperman · 16th Mar 2018, 11:44 am

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?

paulsherwin · 16th Mar 2018, 2:34 pm

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

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.

cooperman · 16th Mar 2018, 9:07 pm

No lockfits - just ITT BF241 in plastic D shaped enclosure with standard wire leads.

13th Mar 2018, 9:21 pm	#1
cooperman Pentode Join Date: Feb 2018 Location: Cheshire, UK. Posts: 121	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
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	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
cooperman Pentode Join Date: Feb 2018 Location: Cheshire, UK. Posts: 121	Re: Armstrong 625 AM Aerial Would anyone care to guess on how many turns of wire to start off with?

14th Mar 2018, 10:28 am	#4
Andrew2 Nonode Join Date: Mar 2010 Location: Dukinfield, Cheshire, UK. Posts: 2,037	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
cooperman Pentode Join Date: Feb 2018 Location: Cheshire, UK. Posts: 121	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?

16th Mar 2018, 2:34 pm	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	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 Guest Posts: n/a	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.

16th Mar 2018, 9:07 pm	#8
cooperman Pentode Join Date: Feb 2018 Location: Cheshire, UK. Posts: 121	Re: Armstrong 625 AM Aerial No lockfits - just ITT BF241 in plastic D shaped enclosure with standard wire leads.