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Old 23rd Aug 2020, 5:15 am   #1
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Default Codar AT5 TX

Currently in the process of restoring a Codar AT5 TX, and am now at the stage of testing it, would a crystal type microphone be the best type to use?
Ken G6HZG,( VMARS Member)
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Old 23rd Aug 2020, 5:43 am   #2
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Default Re: Codar ATX TX

Hi Ken,

The AT5 was designed for a crystal mic, something like the Acos 45 would have been a popular choice when the AT5 came out. I use an un-amplified Astatic D104 mic on my AT5.

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Old 23rd Aug 2020, 6:14 am   #3
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Default Re: Codar ATX TX

Thanks Roger, better get looking for a Crystal Mike.
Can you still but new ones ?
73`s Ken G6HZG.
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Old 23rd Aug 2020, 7:38 am   #4
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Default Re: Codar AT5 TX

I think you can Ken but beware of really old crystal mics as the crystal material (Rochelle Salt?) absorbs water and then disintegrates. A good test is to see if the mic insert rattles when you shake it! If it does, don't buy it.

A quick eBay search shows some rather pricey new inserts and some well-used mics.

An article I have a copy of, written by Dave/G3UUR, mentions that the 1 megohm resistor from grid to ground on the mike input can be changed for up to 5 Megohm to improve the response, I've not tried that myself. I have a lot of AT5 info if you need anything.

73

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Old 23rd Aug 2020, 8:06 am   #5
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Default Re: Codar AT5 TX

Thanks Roger,
I also had a quick look on ebay, old ones very pricy and I was not aware of the moisture issue.
Many thanks once again,
73`s Ken G6HZG.
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Old 23rd Aug 2020, 2:21 pm   #6
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Default Re: Codar AT5 TX

Electret mics work fine. You’ll need the usual 1.5v cell to drive it, but you won’t be left wondering if a 40+ year old crystal mic is working or not.
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Old 23rd Aug 2020, 3:43 pm   #7
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Default Re: Codar AT5 TX

I too am looking for new crystal mic inserts for my AT5, DX40, DX60 transmitters. They used to be available for a few pounds on eBay, but not any more. As others have said, beware of new old stock, and old, old stock!

(I can supply the text and images for the Practical Wireless AT5 article I wrote a few years ago. It includes all the general AT5 stuff plus a few minor mods and corrections the the published circuit.)

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Old 23rd Aug 2020, 7:44 pm   #8
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Default Re: Codar AT5 TX

I think the electret approach is a good idea, they also seem to work well with transmitters that use carbon mikes. I know of several people who have built them and used the original carbon mic DC supply to run a small amplifier.

In the shack I have a box of 15 to 20 unused mic inserts that I was told were crystal types. They are quite small, about the same diameter as a £2 coin. I shall see if I can test them out on my AT5.

73

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Old 25th Aug 2020, 11:46 am   #9
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Default Re: Codar AT5 TX

I wonder if anyone can identify the mike insert shown in the two attached photos?

They work when connected to my AT5 but the output is somewhat lower than the Astatic D104 un-amplified crystal mic I normally use with that transmitter. The connections look as though they are solder-able or may plug into a socket of some sort. The central pillar is tapped for 8BA (I think). There is no resistance reading across the green pin and ground.

I have several of these inserts, some have a red band in place of the green and seem to have a lower output.

73

Roger/G3VKM
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Old 25th Aug 2020, 12:18 pm   #10
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Default Re: Codar AT5 TX

Possibly Acos Roger. I have some similar ones. Tape recorder mics. They worked well with EF86 or 6BR7 input stages like the Vanguard.
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Old 25th Aug 2020, 1:39 pm   #11
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Default Re: Codar AT5 TX

Hi Jim,

Yes, I can see they might be for something like a 50s/60s tape recorder, quite glitzy! If anyone wants one or two to play with then drop me a PM, postage will be quite cheap.

73

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Old 25th Aug 2020, 11:39 pm   #12
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Default Re: Codar AT5 TX

They are Acos inserts, but I've never been able to find out the part number. But the well-known Acos 43 is 43mm diameter and these are 1" (25mm) which may give a pointer. There was also a type 38.

Mine have the green markings, and they measure around 1.5mV RMS with speech at 6".
I used one, mounted in a block of soft foam, to replace an old 45mm Rion that had suffered from moisture ingress. After a few years it failed (sounded muffled and low level). I don't know if it was the dreaded moisture or if it had been dropped, but I swapped it for another which is still working fine. So it may be worth checking yours by cross-comparison to see if they all sound similar.

The thread is 6BA.

Note that the terminals will not take solder - I wired them up using inserts taken from a small multipin socket.

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Old 26th Aug 2020, 1:25 am   #13
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Default Re: Codar AT5 TX

Quote:
Originally Posted by G3VKM_Roger View Post
I think you can Ken but beware of really old crystal mics as the crystal material (Rochelle Salt?) absorbs water and then disintegrates.
This is an effect that I've never heard of before, and a quick look on Google did not yield much. Can you point me at anything that substantiates the idea that water absorption causes the element to disintegrate?

I have couple of old xtal mics dating back to the 60's; I'm just wondering about how I could do a quick and easy test on them?

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Old 26th Aug 2020, 2:29 am   #14
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Default Re: Codar AT5 TX

Nope, you are indeed right about the Rochelle salt microphones; these salts are deliquescent. It's quite hard to believe they could have used them for manufacture in the first place. The Rochelle salts were eventually phased out in the late 50's when more stable materials were introduced (the first Shure mics in '57). However, I think it is the case that all of these Piezo mics were just referred to as crystal mics, so for vintage mics from certain period of time they might have been made of either the early or later types of materials. Paying good money for such would be a bit of a lucky dip!

B
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Old 26th Aug 2020, 7:45 am   #15
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Default Re: Codar AT5 TX

Thanks Dave, that's very useful info. I might rig up a tester for the inserts and see how much they vary.

Disintegrating crystals, I think the Acos 45 is the worst I've come across for this, the model with the curved body. I've seen several at rallies in recent years and all sounded gritty when shaken. I'll stick to my D104, I think, although it's heavy!

73

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Old 26th Aug 2020, 9:03 am   #16
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Default Re: Codar AT5 TX

Quote:
Originally Posted by G3VKM_Roger View Post
Disintegrating crystals, I think the Acos 45 is the worst I've come across for this, the model with the curved body. I've seen several at rallies in recent years and all sounded gritty when shaken.

Yes, it's a shame as these are nice, ergonomically shaped microphones. My theory is that, being metal, they are more likely to suffer from internal condensation. I actually managed to transplant a good crystal plate into one from another microphone.
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Old 26th Aug 2020, 9:52 am   #17
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Default Re: Codar AT5 TX

In light of the fact that new crystal mic inserts are unavailable, has anyone come up with an alternative? Ideally one with enough output not to need extra amplification and can be made small enough to fit into a crystal microphone housing.

I floated this question with the VMARS group some months ago, but I think in practice the answer is some amplified modern mic with a battery in the handset. Not very original, but time has moved on...
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Old 26th Aug 2020, 11:24 am   #18
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Default Re: Codar AT5 TX

Does anyone know what sort of output level a crystal mic should produce?

I have a vague recollection of a figure of a few mV (<5), but I saw one comment that some old Rochelle mics could push out something like 2V.

A crystal in good condition will be high impedance but if it has succumbed to water ingress you'd have to assume it would exhibit quite low resistance?

B
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Old 26th Aug 2020, 11:56 am   #19
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Default Re: Codar AT5 TX

I've just done a test on my 2 mics. I used a Wayne Kerr LCR bridge, thinking that the low-level voltage it applies might avoid any damage. They measured 2MOhm and 2.6MOhm, and additionally, I could hear the 1kHz applied voltage coming out as a tone from them.

I'll maybe try them on the scope later on, but that test suggests that they are still OK.

B
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Old 26th Aug 2020, 12:57 pm   #20
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Default Re: Codar AT5 TX

What about modifying a condenser mic with a FET output? The impedance of that would be extremely high.....
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