A strange Clansman handset fault. - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 24th Jun 2013, 12:01 pm

At last week's Newbury radio-rally I bought (for £3) a brand-new-in-bag Clansman handset.
Plugged it into my PRC320 and was surprised to find the radio went straight to 'transmit' and there was an audio-squeal.
Investigation showed the press-to-talk bar didn't do anything, and when you pressed it you couldn't hear the microswitches changing over.

To cut a long story short - the PTT bar on these handsets is waterproofed from the two microswitches by a rubber membrane. On this brand-new handset the membrane was somewhat distorted and stiff and rather than just flexing when the PTT bar pressed it, it would 'pop' in - and had enough hysteresis to stay popped-in and holding the TX microswitch closed when the PTT bar was released. There was enough pressure to partially-depress the audio-switch; I am presuming there was some sort of contact rectification going on which detected stray RF and fed it back into the mic amp stages hence the whistling.

Removing the rubber membrane and doing some gentle stretching seems to have gained it enough elasticity to let it work properly.

[these handsets have *two* microswitches as part of the PTT: one does the usual ground-this-line-to-transmit duty, the other connects the output from the microphone to the audio-stages of the transmitter. Why the latter? Remember that you can have numerous handsets/headsets simultaneously wired to a single radio: you don't want to make _every_ microphone go live and relay battle/vehicle noises when _anybody_ presses the PTT].

--G6Tanuki.

QQVO6/40 · 24th Jun 2013, 1:27 pm

In Australia we have a product called "Armourall". It is used as a rubber and vinyl restorer for use on the interior of cars, dashboards, door liners and other such stuff. I have found the same thing on a Clansman handset that I have been using on a PRC-352. The rubber was as tight as a drum. You have already mentioned the 2 microswitches. They have to be adjusted to operate correctly as I have had a couple of handsets where the transmit switch would operate but the microphone switch would not no matter how hard it was pressed.
The idea of separate switching for PTT and audio is not so important when using a handset but these sets were used for other things as well and separate functions were necessary. A device for sending high speed Morse code was sometimes used and it was a bit like a small tape recorder where you recorded your message at normal speed and then put it to air at a much faster speed of up to 300 words per minute. Bit hard for an enemy operator to copy. These units needed the set to be put into transmit fist and then send the audio signal.
I hope this helps a bit.

Cheers, Robert.

G6Tanuki · 24th Jun 2013, 4:36 pm

Yes, we have "Armor-all" here too - I've used it on plastic car bumpers to good effect.

In the case of this particular handset I just stretched and twisted the little rubber diaphragm in my hands for about five minutes to get some flex back into it, then reassembled it with a dab of the same clear Dow Corning silicone dielectric grease I use to pack antenna/coaxial junction boxes.

So far it seems to be working fine.

--G7Tanuki

24th Jun 2013, 12:01 pm	#1
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	A strange Clansman handset fault. At last week's Newbury radio-rally I bought (for £3) a brand-new-in-bag Clansman handset. Plugged it into my PRC320 and was surprised to find the radio went straight to 'transmit' and there was an audio-squeal. Investigation showed the press-to-talk bar didn't do anything, and when you pressed it you couldn't hear the microswitches changing over. To cut a long story short - the PTT bar on these handsets is waterproofed from the two microswitches by a rubber membrane. On this brand-new handset the membrane was somewhat distorted and stiff and rather than just flexing when the PTT bar pressed it, it would 'pop' in - and had enough hysteresis to stay popped-in and holding the TX microswitch closed when the PTT bar was released. There was enough pressure to partially-depress the audio-switch; I am presuming there was some sort of contact rectification going on which detected stray RF and fed it back into the mic amp stages hence the whistling. Removing the rubber membrane and doing some gentle stretching seems to have gained it enough elasticity to let it work properly. [these handsets have two microswitches as part of the PTT: one does the usual ground-this-line-to-transmit duty, the other connects the output from the microphone to the audio-stages of the transmitter. Why the latter? Remember that you can have numerous handsets/headsets simultaneously wired to a single radio: you don't want to make _every_ microphone go live and relay battle/vehicle noises when _anybody_ presses the PTT]. --G6Tanuki.

24th Jun 2013, 1:27 pm	#2
QQVO6/40 Hexode Join Date: May 2009 Location: Newcastle, Australia Posts: 321	Re: A strange Clansman handset fault. In Australia we have a product called "Armourall". It is used as a rubber and vinyl restorer for use on the interior of cars, dashboards, door liners and other such stuff. I have found the same thing on a Clansman handset that I have been using on a PRC-352. The rubber was as tight as a drum. You have already mentioned the 2 microswitches. They have to be adjusted to operate correctly as I have had a couple of handsets where the transmit switch would operate but the microphone switch would not no matter how hard it was pressed. The idea of separate switching for PTT and audio is not so important when using a handset but these sets were used for other things as well and separate functions were necessary. A device for sending high speed Morse code was sometimes used and it was a bit like a small tape recorder where you recorded your message at normal speed and then put it to air at a much faster speed of up to 300 words per minute. Bit hard for an enemy operator to copy. These units needed the set to be put into transmit fist and then send the audio signal. I hope this helps a bit. Cheers, Robert.

24th Jun 2013, 4:36 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: A strange Clansman handset fault. Yes, we have "Armor-all" here too - I've used it on plastic car bumpers to good effect. In the case of this particular handset I just stretched and twisted the little rubber diaphragm in my hands for about five minutes to get some flex back into it, then reassembled it with a dab of the same clear Dow Corning silicone dielectric grease I use to pack antenna/coaxial junction boxes. So far it seems to be working fine. --G7Tanuki