Quiet, crackly carbon microphone...

[twocvbloke]

Yesterday, I decided to use my black WE500 to talk to some call centre, and they remarked that we had "a bad line", and I tried the cordless, and it improved the line, so, I'm guessing the carbon mic (made in March 1970 according to the date stamp of "3 26 70") in there is shot...

But, on the plus side, I had bought an electronic microphone from Telephonelines on ebay before christmas, so I have now fitted that, but I thought I'd ask of there is anything I could do to improve the carbon mic, or whether I should just leave it out and stick with the electronic one...

I had read on another forum that some people microwave their carbon microphones for a few seconds, which apparently "roughs up" the grains thus improving them, but, I'm not too keen on doing that...

[LarryInMichigan]

The first things to do with a transmitter are to tap it on a hard surface to try to unpack the carbon and to clean the electrical contacts. 

Larry

[twocvbloke]

The first things to do with a transmitter are to tap it on a hard surface to try to unpack the carbon and to clean the electrical contacts. 

Larry

Yeah, I've done that a few times and it hasn't improved it sadly, and the contacts are shinier than a new penny...

I'm just guessing that it's past it's best now, even though some carbon mics last forever it seems, there are those that fail sooner than others...

[GG]

Don't put them in the microwave.  Metallic objects in a microwave oven can fry the magnetron tube by randomly reflecting microwave energy back to it.  Also, metallic objects can burst into flame or "pop" in a microwave.  Don't risk it. 

Tapping carbon mics on your desk: gently, and if tapping them on their edge, rotate them as you go so they get some agitation in all directions.

Unfortunately there ain't much that can be done once a mic fries.  I've got a pile of mostly European carbon mics that all have excessive static and even a kind of random squealing noise that varies in pitch.  These are useful as "last-ditch" replacements for missing ones but otherwise aren't useful.

Fortunately there is a huge supply of WE T1 and F1 transmitters, and almost as good a supply of AE 41, 81, and 810 transmitters.  So put a masking tape tag on your bad one saying "static" and save it somewhere, but in the meantime replace it with a new one.

[twocvbloke]

Yeah, I didn't want to risk knackering (now there's a good British word for you!!!  ) our nice Samsung microwave, I read it on another forum but am not convinced it's a bright idea... 

The microphone I got to replace it is this one:

http://www.ebay.co.uk/itm/260930962662

I think I originally bought it to put in my BT 286A linesman phone (uses a G-type handset, but has a completely different setup in the mic slot unfortunately), but it works in the WE500s, so, that's in the black one now, and I'm going to securely put the carbon one inside the phone somewhere, cos you never know, someone may be able to restore it in the future...

The only real problem I have with carbon mics is the "hissing" you get if you move the handset about, where the carbon granules move about generating noise, it's why I like acquiring 21A's for my GPO phones, as they vastly improve transmission quality...

I bought a batch of 4 from telephonelines with the one above, 3 apparently buzz in some phones, I've yet to hear them buzz though, then again, I've yet to test them on a live phoneline, though, three of them appear to be GEC units in red A.P. Besson cases (the GEC models should be in a Black case), the fourth being a 2nd-revision A.P. Besson as it should be. The GEC mics needed to be padded inside the red cases as they were loose (wrong case, meaning proo fitting), which could have been the reason for "buzzing", as in resonating to the sound of the user's voice and the case or microphone vibrating at certain frequencies causing a "buzz"...

[GG]

Good idea storing the old mic inside the telephone set it came from; makes it easier to find if you want to put it back in later. 

Carbon mics will normally make a slight noise as the carbon shifts when you move about.  It should stop after the handset isn't being moved, otherwise it's a sign of some trouble.

Without bothering to look it up, I'll assume the 21-A microphone is a dynamic or other type of electronic mic, for example as used on Statesman and Ambassador telephones.  What I've found is that those pick up AC mains hum when they are within a couple of feet of any of the computers on my desk.  So I can't use those phones at my desk for that reason.  Also Australia PMG series 800 sets with DTMF signaling usually have those mics and the same problem.  So it's carbon mics for me at my desk, plus the Panasonic keyphone (KXT-7636) that shows all the line appearances and doesn't have a hum problem. 

That may be what Telephonelines refers to as a "buzz."  There should not be any resonant acoustical effects with any mic in any handset, that are so strong at normal speech volumes as to cause the microphone itself to "buzz" mechanically.

[twocvbloke]

Yeah, the GPO/BT 21A is an electronic microphone, I've used a few and never had any problems with them buzzing, the only time I've heard a buzz is through my GPO phones when they're near my laptop, and not plugged in, which is just plain weird!!! 

The movement noise I got used to, but the quality of the one in my black WE500 was pretty poor despite attempts to improve it, but with the electronic replacement, it's perfect, and louder too (US phones are apparently quieter on UK lines, though I never really noticed cos I'm not on the other end of the line, usually!!! ), the red one will get one of those mics too (and it's currently fitted Carbon will be secured inside the phone too), just to give it the best quality there is for these phones...

[GG]

Re. "louder/quieter" on UK phone lines: have you measured or listened to the audio when calling from one line to another, or are you referring to sidetone?  GPO teles 706 and 746 both have high sidetone in the US, compared to 500 sets, and regardless of the position of the regulator board in the 706.  The exception being a 500 J/K with an unregulated 425-A network, which has sidetone a bit higher than the GPO sets. 

To measure transmission volume across two lines, call from a phone on one line to a phone on the other; hold a receiver up to each ear, with the mic of one near your mouth, and the other receiver held such that the mic is above and behind your head (as far from your mouth as possible).   Talk into the first mic, and then swap the two handsets' positions such that the first one's mic is away from your mouth and the second is near your mouth, and talk into the second mic.  You should be able to tell by ear if they are more than about 2 dB different. 

A useful cross-check on the above is to listen to dial tone in both simultaneously, and if there is an apparent volume difference, switch them between left and right ear to be sure the difference follows the handset rather than being a difference in your hearing.

[twocvbloke]

I never really understood what they meant about US phones being quieter than GPO phones, I'm guessing they just had a bad microphone like I did and blamed it on the differences between british and american phone line power...

I think a lower sidetone is better, it means you speak up more to hear yourself in the earpiece, and thus the person on the other end can hear you clearly too, though to be honest I never understood why there is sidetone there at all, but I guess that's the effect of having a 2-wire phone system...