The AM Forum

I noticed the KWM-1 applies 275 volts thru the microphone contact to ground for keying the transmitter. That is a lot of voltage on a D-104 microphone switch. I guess old large likely high voltage cord is not replaceable with new coiled mic cord due to voltage. Should I be concerned? Should I place an arc suppressor on the mic contact?

169 views and no opinions.

Yes, I too was hoping someone with Collins experience would jump in.

Maybe there is a reluctance in modifying anything Collins?

Are you 100% sure, the KWS-1 has 275v on the Microphone PTT switch?

Lots of simple relay, or transistor switch designs out there. The D-Lab PTT board

has been making inroads into many boat anchor class transmitters.

I have a Gonset GSB-201 linear amplifier that puts about -130v on the keying contacts, and even holds a charge when just sitting there. Ouch!

Jim
Wd5JKO

The 250 volts is fed through voltage divider R-118 and R-119. There should be considerably less than 250 volts at the mic.

I bet when you close the relay the voltage is a lot less.

With it floating (meaning, relay not energized), there is nothing to bring thr voltage divider down.

When you actuate the relay, the coil resistance drops the 275 volts down to a 'realistic' (radio shack pun there) level.  At least, most tube rigs I worked on for CBers, etc back in the day operated just like this.

--Shane
WP2ASS / ex KD6VXI

275 volts unkeyed. 0 volts keyed. I guess it would be safe with entire mic base grounded. I place an arc suppressor on the mic contact.

A better question might be what is the available current at the key…if a person is the pathway at 275 volts to ground, is it a tingle or an ouch or v-fib?  Not my radio, but if it was, there’s no way I’d leave 275v on the mic connector. But, I also have to clarify that maintaining collectibility or originality or so-called resale value for an old ham radio means nothing to me, so I don’t worry if I have to drill a hole in it or rework things.

Ed

The mic base was grounded in the D104, depending on the configuration. Other mics where considered double insulated....  Meaning you where not able to come in contact with any voltages unless you where really trying to.

If the mic jack pins are recessed, you really don't have anything to worry about unless you like doing stupid things like putting paperclips in microphone Jacks.

If it's the style with the pins sticking out, then yeah, that's dangerous.

Safety was more up to the operator vs the manufacturer back then.....  Lol

--Shane
WP2ASS / ex KD6VXI

'If it's the style with the pins sticking out, then yeah, that's dangerous'. What does this mean?

I had a B&W 5100B for a while—AC line cord with ring-lugs to the threaded studs of the inline filters, sticking out the back of the chassis and entirely exposed. A modern-day inspector from the Consumer Product Safety Commission would cough up a hairball and die right on the spot. Safety was up to the operator: just don't stick your hands back there, that's all.

As WA5VGO correctly noted, in a properly working KWS-1 there is no dangerous voltage across the microphone PTT contacts.

Relay K-101's coil is in the plate circuit of the relay control tube which is one section of a 12AT7.  Normally the tube is biased off so the relay isn't closed.  The 12AT7 cathode is fed from a 22K resistor from the 275 volt bus and it has a 680 ohm resistor to ground so around 8 volts of cathode bias is provided via this voltage divider from the 275 volt line.  The PTT line connects to the 12AT7 cathode.  When the PTT switch is closed, the cathode is shorted to ground removing the bias and the relay is activated via plate current for the 12AT7 flowing through the relay coil.

Unless the 680 ohm resistor is open or has been removed, there is nothing close to 275 volts on the PTT terminal.  At worst, the mic PTT switch sees the voltage developed at the junction of the 22K and 680 ohm resistors which is very little voltage unless Ohm's law has been repealed since I took my extra exam many years ago :(

Rodger WQ9E