GPO/BT Transmitter 21A mix-up...

[twocvbloke]

Okay, the other day I was using the Green 746, and noticed there was an excessive amount of earth-hum on the line, initially I put it down to the fact that our phoneline has been having problems lately (crackling, ADSL disconnecting, no connection, etc.), but I tried the phone on the 616 yesterday and found that it was also having earth-hum on there, and the 616 is not currently connected to the line, and eventually it dawned on me...

Last year, I bought a lot of 4x 21A transmitters, one was good, the other three apparently had earth-hum problems, so, I threw those into a box until I needed them, and, it seems I picked up one of the bad ones when I swapped the green 746's transmitter out (the old 16A worked, but had excessive crackling), so, I gathered them all up, popped them open, and hey presto, I found the problem, three of them are early GEC circuits in red A.P. Besson cases (The GEC models were in Black cases), padded with some kitchen roll to hold them steady, and the early GEC circuits were prone to picking up earth hum and strong RF interference, later versions had extra shielding to protect them and were marked with "S" on the case...

So, I stuck the proper A.P. Besson 21A in the phone, and the hum was gone on both the 616 and the phoneline, so problem solved...

Now, to figure out how to sort out the other three so they're properly shielded...  

EDIT:
Some more info on the 21A transmitter:
http://www.britishtelephones.com/trans21.htm

[twocvbloke]

I love redneck engineering...

Looking at the picture of the various 21A transmitters in the link above, you can see that the 2nd version of the GEC 21A has a metal shield around the microphone, just as they have on the rear of the PCB, so, the gears of thought squealed into life, and I got to work on making up a home-made shield with some foam padding for insulation, aluminium foil to form the metal shield, and masking tape to hold it all in place, and you know what, it worked!!! 

I folded the foil over the rear shield so it had electrical continuity between, taped it all down, trimmed off the excess round the microphone, hooked it up in my cream 746to test it, and no earth hum, tried it in the green one, no hum, so, yep, it worked...

And as a comparison, I tried the other two transmitters in both phones, and they did produce the humming, so, yep, works perfectly, even if it's a bit rough around the edges...