-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Browning high voltage on mike pin
- Thread starter Crambone
- Start date
Shadetree Mechanic
Delaware Base Station 808
Mostly because I don’t like the ideas of B+ voltage running into my metal mike. Call it a pacemaker issue if we must.
The radio feeds about 300 Volts DC to a 22k 10-Watt resistor, and then into the relay coil. When the mike is keyed, the resistor gets 2/3 of this voltage, the relay coil the other one-third. The relay coil says 110 Volts DC, but the shock hazard is closer to 300 Volts.
Any time the relay in that transmitter must be replaced, the job begins with replacing the socket. No relays to fit the old socket have been made for decades. The (now) industry-standard 'ice-cube' size relay has the rows of pins spaced differently. The socket is not that hard to come by, but the wiring order underneath is different. Every wire (except two) gets moved to a different socket pin. The two coil pins are in the same place.
Barkett will sell you the relay and socket. It fits in the same chassis hole, thankfully. The same socket shows up at surplus outfits like All Electronics, Marlin P. Jones and others.
What we do when this is called for is to use a 12-Volt DC relay. The 12-Volt relay has been made by literally dozens of second-source vendors. Tends to be cheaper than the 110-Volt DC version.
I reasoned that if we're going to the trouble to change the socket, adding a source of 12 Volts DC for the new relay is not a big increase to the cost of the job. The original selling point 30 years ago was to tell the customer he could get a new 12-Volt relay from RatShack if he ever needed one.
Not so relevant these days.
Two 470uf 25-Volt electrolytics and two rectifier diodes are used in a half-wave voltage doubler circuit that takes the radio's 6.3 VAC heater power to get about 16 Volts DC with no load. This fits on the spare lugs of the tie strip just inboard from the relay feeding the coil's 'hot' side pin of the new relay socket. The other coil pin gets a PNP transistor with the emitter on the relay socket, the collector grounded and the base connected to the yellow wire leading to the mike socket. Doesn't have to be a big one, the PN2907A used to key the Texas Star linears works fine.
You could just hook the wire from the mike socket directly to the 'cold' side of the relay coil, but adding the transistor reduces the risk of adding AC 'hum' to the transmit audio.
This does decisively eliminate the shock hazard.
I should look for pics I can post for this procedure next time I'm at the shop. Pretty sure we have some.
Never have worked out a way to fix the shock hazard using the old relay.
73
Any time the relay in that transmitter must be replaced, the job begins with replacing the socket. No relays to fit the old socket have been made for decades. The (now) industry-standard 'ice-cube' size relay has the rows of pins spaced differently. The socket is not that hard to come by, but the wiring order underneath is different. Every wire (except two) gets moved to a different socket pin. The two coil pins are in the same place.
Barkett will sell you the relay and socket. It fits in the same chassis hole, thankfully. The same socket shows up at surplus outfits like All Electronics, Marlin P. Jones and others.
What we do when this is called for is to use a 12-Volt DC relay. The 12-Volt relay has been made by literally dozens of second-source vendors. Tends to be cheaper than the 110-Volt DC version.
I reasoned that if we're going to the trouble to change the socket, adding a source of 12 Volts DC for the new relay is not a big increase to the cost of the job. The original selling point 30 years ago was to tell the customer he could get a new 12-Volt relay from RatShack if he ever needed one.
Not so relevant these days.
Two 470uf 25-Volt electrolytics and two rectifier diodes are used in a half-wave voltage doubler circuit that takes the radio's 6.3 VAC heater power to get about 16 Volts DC with no load. This fits on the spare lugs of the tie strip just inboard from the relay feeding the coil's 'hot' side pin of the new relay socket. The other coil pin gets a PNP transistor with the emitter on the relay socket, the collector grounded and the base connected to the yellow wire leading to the mike socket. Doesn't have to be a big one, the PN2907A used to key the Texas Star linears works fine.
You could just hook the wire from the mike socket directly to the 'cold' side of the relay coil, but adding the transistor reduces the risk of adding AC 'hum' to the transmit audio.
This does decisively eliminate the shock hazard.
I should look for pics I can post for this procedure next time I'm at the shop. Pretty sure we have some.
Never have worked out a way to fix the shock hazard using the old relay.
73
Okay, so this picture kinda sucks. Didn't light it terribly well.
This is not the completed hookup, but shows the 12-Volt coil, power and keying-transistor detail before the rest of the wires are installed and block a clear view of the 12-Volt mod details.
The electrolytic caps in this pic are 1000uf 25 Volt, but we have found that 470 uf works just fine. They're smaller and take up less space.
The axial cap at the upper right has 6.3 Volts AC feeding into its negative lead. That wire is not yet installed as you see it here.
The shunt diode has its cathode (banded end) connected to the positive lead of the axial cap, the other end is grounded.
The series diode is strung from the positive end of the axial cap to the right-hand 'hot' coil lug of the relay socket. The radial filter cap has its positive lead on this lug of the socket and the negative lead grounded.
A third diode is connected in parallel with the relay coil to suppress the reverse-polarity "inductive spike" pulse that will come shooting out of the coil when you unkey. This could be unhealthy for the keying transistor if it were not suppressed with this diode.
The PN2907A keying transistor has its emitter lead on the left-hand 'cold' relay-coil lug. The collector is grounded. The center base lead has a 220-ohm 1/4-Watt resistor between the transistor lead and the yellow wire from the mike socket. We have since found that you can leave this out and connect the transistor's base directly to the yellow wire.
One detail that's not terrribly clear is a pair of .001uf disc capacitors. Any voltage rating 25 Volts or higher should be fine. Each goes to one of the sockets coil pins, the other side to the ground lug between the two coil lugs. This serves to prevent RF from leaking from the relay's antenna circuit into the coil circuit. Might not be necessary, but it's just conservative RF-design practice.
But that's the gist of it.
73
This is not the completed hookup, but shows the 12-Volt coil, power and keying-transistor detail before the rest of the wires are installed and block a clear view of the 12-Volt mod details.
The electrolytic caps in this pic are 1000uf 25 Volt, but we have found that 470 uf works just fine. They're smaller and take up less space.
The axial cap at the upper right has 6.3 Volts AC feeding into its negative lead. That wire is not yet installed as you see it here.
The shunt diode has its cathode (banded end) connected to the positive lead of the axial cap, the other end is grounded.
The series diode is strung from the positive end of the axial cap to the right-hand 'hot' coil lug of the relay socket. The radial filter cap has its positive lead on this lug of the socket and the negative lead grounded.
A third diode is connected in parallel with the relay coil to suppress the reverse-polarity "inductive spike" pulse that will come shooting out of the coil when you unkey. This could be unhealthy for the keying transistor if it were not suppressed with this diode.
The PN2907A keying transistor has its emitter lead on the left-hand 'cold' relay-coil lug. The collector is grounded. The center base lead has a 220-ohm 1/4-Watt resistor between the transistor lead and the yellow wire from the mike socket. We have since found that you can leave this out and connect the transistor's base directly to the yellow wire.
One detail that's not terrribly clear is a pair of .001uf disc capacitors. Any voltage rating 25 Volts or higher should be fine. Each goes to one of the sockets coil pins, the other side to the ground lug between the two coil lugs. This serves to prevent RF from leaking from the relay's antenna circuit into the coil circuit. Might not be necessary, but it's just conservative RF-design practice.
But that's the gist of it.
73
Last edited:
Thanks guys this really helps a ton! I was hocked I was actually able to get the Browning working as it should in the first place. If it wasn’t for guys like you willing to share your knowledge I would be hanging myself! I truly appreciate all the help and shared knowledge.