Don't chuck those dead dead BT 21A electronic transmitters (red microphones) - UK Vintage Radio Repair and Restoration Discussion Forum

Nickthedentist · 2nd Jun 2015, 1:20 pm

Hello everyone,

Phone collectors will know how valuable the red "21A" transmitters are, both for replacing the carbon mics in the 700-series 'phones (their original purpose), and also for fitting into 200- and 300-series instruments, which results in much improved transmission quality (i.e. the person you're phoning can hear you properly!).

But they're getting scarcer, and often die for no apparent reason.

The cause of this is failure of the 14-pin DIL IC which they contain, a Ferranti ZN-470AE-3 in the ones I've seen. I presume this happens because of voltage spikes on the line. Of course, these are NLA and have been for many years.

Or have they?! Have a look at good old Amazon: http://www.amazon.com/utsource-10pcs...anti+zn470ae-3

Ordered mine a couple of weeks ago, and they've just arrived. No customs charge if you order a pack of 5 or 10. They certainly look real enough, have a date code of 1985 and were delivered in a stout box, with antistatic packaging inside.

I will report back once I've fitted one.

Nick.

mark pirate · 2nd Jun 2015, 1:38 pm

That is handy to know, I have four 700 series phones. Will be interesting to see if you have complete success!
Seems like a good price for 10.

Mark

Dave Moll · 2nd Jun 2015, 2:05 pm

Thanks for that info. If I have a failure (I've obviously been lucky so far), they could be just the job

G4XWDJim · 2nd Jun 2015, 2:53 pm

Thanks for the info Nick. I use these microphone inserts instead of carbon mics on wartime radio transmitters. They produce much better quality audio at greater output which helps the modulator no end.

I only ever use them on a few volts, are they meant to be working on somewhere near 50 like the rest of the telephone system. How do I know which way round to connect them. The back is blank.

Another question - what's the difference between the red ones and the blue ones. I've got a blue one with 21/A on the back. It packed up some years ago. Should this one be fixable in the same way.

Jim

G6Tanuki · 2nd Jun 2015, 3:36 pm

The inserts are not polarised; in fact, inside is a bridge rectifier to convert whatever polarity you apply to something the internal circuit [basically an electret mic and a transistor] can handle.

Nickthedentist · 2nd Jun 2015, 4:32 pm

Datasheet here. The 21A seems to use the circuit illustrated with minimal/no mods.

G6Tanuki · 2nd Jun 2015, 6:40 pm

Another use for these: wire one in series with a "green with white stripe" magnetic earpiece, a 4.5V battery and a Morse-key.

Put the 21A and the earpiece insert closely facing each-other so the sound-holes of one face the sound-holes on the other.

You now have a serviceable Morse practice-oscillator: this approach got a number of BT techs up to the old 12WPM "Class-A" ARE standard.

Nickthedentist · 7th Jun 2015, 11:47 am

Quote:

Originally Posted by G4XWDJim

Another question - what's the difference between the red ones and the blue ones. I've got a blue one with 21/A on the back. It packed up some years ago. Should this one be fixable in the same way.

Jim,

It seems the ones I have most experience ones are the ones made by A.P. Besson. I can't say whether the other manufacturers' ones are similar inside. The Rathdown one apparently contains a moving coil insert instead of the electret ones the othes use, and the text implies it uses discrete transistors: http://www.britishtelephones.com/trans21.htm

I've also seen black/blue ones by AKG(!)

bobbyball · 7th Jun 2015, 1:09 pm

I have found that the red ones are less susceptible to interference. For example I have a 710 phone on the wall in the kitchen and there is a dimmer switch for the lights nearby. I had one of the blue "21A" inserts in this and it was always affected by the dimmer switch. Changing to the red version the interference is much less, and the sound quality better. No doubt due to the electret insert.

Takes me back to my days at Ferranti...

ThePillenwerfer · 7th Jun 2015, 1:53 pm

Somebody on here traced the circuits out. I can't find the thread but here are the diagrams and photos of one of the ones made from discretes.

- Joe

Dave Moll · 7th Jun 2015, 3:26 pm

The most significant difference between the red and blue insets as far as I'm concerned is that with the red ones the outer (and indeed the inner) case can be removed to enable the innards to fit into a restricted space.

N.B. If the inner case is removed to lay the circuit bare, it may be necessary to wrap it in some insulating tape to avoid unwanted electrical contact.

Nickthedentist · 7th Jun 2015, 8:53 pm

The red AP Besson ones can be fitted into 200-/300-series handsets simply by cutting a notch in the red rear casing - you don't need to resort to dispensing with the rear section, or even just fitting the black inner section and loads of foam.

See here: https://www.vintage-radio.net/forum/...ad.php?t=34859

and a pic of Russell_w_b's implementation of it: https://www.vintage-radio.net/forum/...7&d=1241260063

And thanks for the PDF, Joe.

Nick.

2nd Jun 2015, 1:20 pm	#1
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,853	Don't chuck those dead dead BT 21A electronic transmitters (red microphones) Hello everyone, Phone collectors will know how valuable the red "21A" transmitters are, both for replacing the carbon mics in the 700-series 'phones (their original purpose), and also for fitting into 200- and 300-series instruments, which results in much improved transmission quality (i.e. the person you're phoning can hear you properly!). But they're getting scarcer, and often die for no apparent reason. The cause of this is failure of the 14-pin DIL IC which they contain, a Ferranti ZN-470AE-3 in the ones I've seen. I presume this happens because of voltage spikes on the line. Of course, these are NLA and have been for many years. Or have they?! Have a look at good old Amazon: http://www.amazon.com/utsource-10pcs...anti+zn470ae-3 Ordered mine a couple of weeks ago, and they've just arrived. No customs charge if you order a pack of 5 or 10. They certainly look real enough, have a date code of 1985 and were delivered in a stout box, with antistatic packaging inside. I will report back once I've fitted one. Nick.

2nd Jun 2015, 1:38 pm	#2
mark pirate Dekatron Join Date: Mar 2008 Location: Worthing, West Sussex, UK Posts: 5,185	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) That is handy to know, I have four 700 series phones. Will be interesting to see if you have complete success! Seems like a good price for 10. Mark

2nd Jun 2015, 2:05 pm	#3
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,129	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) Thanks for that info. If I have a failure (I've obviously been lucky so far), they could be just the job __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

2nd Jun 2015, 2:53 pm	#4
G4XWDJim Octode Join Date: Oct 2008 Location: Littlehampton, West Sussex, UK. Posts: 1,465	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) Thanks for the info Nick. I use these microphone inserts instead of carbon mics on wartime radio transmitters. They produce much better quality audio at greater output which helps the modulator no end. I only ever use them on a few volts, are they meant to be working on somewhere near 50 like the rest of the telephone system. How do I know which way round to connect them. The back is blank. Another question - what's the difference between the red ones and the blue ones. I've got a blue one with 21/A on the back. It packed up some years ago. Should this one be fixable in the same way. Jim

2nd Jun 2015, 3:36 pm	#5
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) The inserts are not polarised; in fact, inside is a bridge rectifier to convert whatever polarity you apply to something the internal circuit [basically an electret mic and a transistor] can handle.

2nd Jun 2015, 6:40 pm	#7
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) Another use for these: wire one in series with a "green with white stripe" magnetic earpiece, a 4.5V battery and a Morse-key. Put the 21A and the earpiece insert closely facing each-other so the sound-holes of one face the sound-holes on the other. You now have a serviceable Morse practice-oscillator: this approach got a number of BT techs up to the old 12WPM "Class-A" ARE standard.

7th Jun 2015, 1:09 pm	#9
bobbyball Octode Join Date: Jul 2011 Location: Stockport, Greater Manchester Posts: 1,215	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) I have found that the red ones are less susceptible to interference. For example I have a 710 phone on the wall in the kitchen and there is a dimmer switch for the lights nearby. I had one of the blue "21A" inserts in this and it was always affected by the dimmer switch. Changing to the red version the interference is much less, and the sound quality better. No doubt due to the electret insert. Takes me back to my days at Ferranti... __________________ Robert

7th Jun 2015, 3:26 pm	#11
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,129	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) The most significant difference between the red and blue insets as far as I'm concerned is that with the red ones the outer (and indeed the inner) case can be removed to enable the innards to fit into a restricted space. N.B. If the inner case is removed to lay the circuit bare, it may be necessary to wrap it in some insulating tape to avoid unwanted electrical contact. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

7th Jun 2015, 8:53 pm	#12
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,853	Re: Don't chuck those dead dead BT 21A electronic transmitters (red microphones) The red AP Besson ones can be fitted into 200-/300-series handsets simply by cutting a notch in the red rear casing - you don't need to resort to dispensing with the rear section, or even just fitting the black inner section and loads of foam. See here: https://www.vintage-radio.net/forum/...ad.php?t=34859 and a pic of Russell_w_b's implementation of it: https://www.vintage-radio.net/forum/...7&d=1241260063 And thanks for the PDF, Joe. Nick.