Microphone A-B Switch Box Problems...

After successfully building a SSTV interface box to go between the computer and CRT SS-6900V7  I wanted an A-B mic switch box.

The idea was so that I didn't have to keep swapping out mic leads from the 6900, when I went from SSTV to speech, etc. 

I built a switch box out of initially plastic and some shielded audio cable, plus sockets and mic plug. I also used a DPDT switch, to change the mic input sockets over.  This would change over both the TX and audio wires. This didn't work well, so I went for an aluminium box inside.

The main mic was okay, but the SSTV interface wouldn't key the radio anymore, unless it was directly plugged into the rig. 

I used shielded cable for the audio and removed the TX wires from the switch and just ran them in parallel. After all this, the SSTV interface still won't key up the radio, even when the switch is thrown. 

Any thoughts?

Noise on the PTT line? (RFI when keyed from common mode or plain old environmental interference maybe from the computer hooked up to it for SSTV? )

The PTT line is active low and directly connected to the main uProcessor. The original mic circuitry having a small inductor in line plus 1nF capacitor to ground and the up/down version of mic has a 10nF to ground on it's PTT line suggesting it's susceptible to such issues.

Might be worth sticking in a decoupling capacitor from the PTT line to ground (1nF, 10nF or even 100nF) in that fancy metal box.

I've seen similar problems when people hook up desk mics with long leads or even mic extension leads to such radios so then either getting noise on their audio or problems when keying up.

If you're getting RF back from your antenna causing the problem then a CMC or common mode choke on your antenna coax may well help. It might not be high enough to give a mic 'kiss' but will definitely upset the uProc in the rig.


Hope that helps.

(Bugger when you get these sort of problems.....worth double checking your wiring again too.)


All the best,
Victor

My first thought was RF causing problems, so I'll put better shielding in. I also thought about decoupling caps too (great minds, etc)...

I'm hoping it's not noise from the computer, as the interface has an optoisolator fitted and both audio feeds (in and out) run through separate audio transformers, to stop ground loops, etc.

You should be able to get it sorted Ian.

You just have to remember that any length of lead can become an unintentional 'antenna' and that common mode loves running down grounds and shields(!)

Opto-isolators from computers sound great but there's still all that electro-magnetic mush emanating from them and I run my laptop as far away from the radio as feasible to prevent such mishaps.
(Plus the odd toroid thrown in for good measure. )

I tried a decoupling cap between tx and ground, but no joy. To eliminate any kind of RF leakage, I put the radio into a dummy load on a short patch lead. I also tried wiring one of the mic sockets straight into the common mic lead, but still no joy. The only way to key the radio using this switch box, was to short the TX to ground, inside the SSTV interface box. I don't if the 5V from the is enough to drive a small relay, instead of the optoisolator?

I've ordered a 5V reed relay from eBay, so hopefully that will solve the problem...

I've tried bypassing the optoisolator with a reed relay, but the result is the same. It keys the radio on its own, but not going through the switch box. Time to strip the box down and start again...

Step by step Ian....

Have you tried keying up by shorting the PTT line at your SSTV interface whilst connected through the switch box? (You know it works direct and the mic keys OK through the box, but how about with the mic unplugged from the switch box?)

If you're anything like me age will have rewarded you by stripping away your keen eye and once dexterous fingers....a slither of solder or single strand of wire can cause all sorts of frustrations!

I wish you well with it.  



(I just spent some frustrating time trying to wind a tiny little poxy trifilar toroid for my latest project.   )

I've shorted the PTT to ground, whilst the interface was connected to the switch box. This did key the radio. I've since this behaviour in some radios before, it's like the optoisolator isn't sinking enough current to key the radio.

Another option would be to have the optoisolator or reed relay trigger a small relay. This would require feeding the interface box with power for the relay.

Problem solved! I rechecked the wiring inside the switch box and I had wired up the mic sockets wrongly.  Luckily, it never damaged the radio and now it works 100%!

Ah, well done there Ian.  

Glad you got sorted.