Amplifier Keying (sniffer) circuits explained
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- Опубліковано 4 вер 2024
- CB amplifiers with automatic keying circuits explained. Normally if your amp ain't keying up, its the keying transistor or the diode in the sniffer circuit is bad. Sure, it can be the relay, resistor or something else, but 9 times out of 10, its the keying transistor or the sniffer diode, often its both. Sure, some amps use 2 transistors and/or 2 diodes in the keying circuit, but same principle. Also note that amps with a preamp or bilateral circuit often use the same transistor for the keyer as they do for the preamp. Common keying transistors used are 2n2222A, 2n3906, and 2n2905a. They are cheap, under $1 and are easy to find. Same with the 1n914 family of small signal diodes. They go for pennies. Best way to test is do the diode test unpowered. Sure a relay coil can go bad or stick, but thats unusual. Usually a sticking relay is again the keying transistor. Most bad relays will not get good connections on the contacts and the relay will key, but the amp will have no output. Bad relay contacts on the input side will show as a high input swr and bad contacts on the output side of the relay will show as the tubes are getting how (cooking) but no power out.
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I like your style brother! Ole Gatekeeper dropping by to thank you for being a teaching man! I am about to start studying AC Boxes in the near future and I can see your channel is a great place to start brother! Love this video!
Glad to help.
Thanks for sharing I got two at home with the same problem.
I hope this helps.
Thanks Doc!!
Welcome!
Man thanks for all the information you share. I have a palamar 300a that only keys up some of the time, im leaning lol👍
replace that keying transistor for starters.
@@tramdr i replaced both the 2n2905a transistors on the board that has the relays on it, still the same
I needed this video, thank you
Glad it was helpful!
So need this Info on my palomar 300a
MAYDAY
First question is, is it not keying at all?
Do you ever work on the old Siltronixs commanche 1011d radio?? Got one in near mint shape with optional frequency counter, it all seems to work but it needs a recap job.
Do you work on them or overhaul them?
Sorry no.
Thanks for the video. Im repairing the preamp and transmit relay transistors in a kris big boomer. Looks like the transistors got hot on the board. Which transistors do you recommend. It had 2, ECG312 in it. Just curious if there are any better than those. Thanks
the 2 ECG312's go to the preamp. For that purpose, it would be very hard to find something better.
What is the little diode value/#
1n4148 or 1n914 extremely common small signal fast switching diodes. 100mhz, 100v around .2a (200ma) There are a few more variants and numbers for them, but kinda like power supply diodes, it really doesn't matter which one you stick in there as long as its a fast one (100mhz) as it is rectifying RF not low frequency ac. Even though they only handle around .2 or .4 amps, thats not important at all either. With power supply circuits, amps is important but in the amp's keying circuit, they are driving the base of the keying transistor, so they are pulling almost no current at all.What does happen is people overdrive the amp and run over 100v's across that diode and short it, and then it will short the keying transistor. Those diodes cost about 10 cents.
There must be low voltage vacuum tubes with 12V heater and a B+ of 12V that would work in that unit.
I have heard of low voltage B+ tubes, but would think that a low voltage tube would not be able to pass enough current to draw a relay in.
@@tramdr Then have the low voltage tube drive a VN66AF VMOS FET.
Sure, but now add a low voltage tube and a Vmosfet to a circuit where a plain Jane transistor will work.
what are your thoughts on using this type of keying circuit on big amplifiers like a 32pill versus direct keying the relay with a foot pedal etc?
Its a lot more difficult to make a dependable keying circuit for any high drive amp than it is for any low drive amp. Low drive just pick off a little bit of the low drive coming in, easy peezy. High drive, with a bunch of watts coming in, now its a lot more challenging to pick off those bigger watts coming in and if you pick off too many, it won't key, and if you pick off too many, you blow the circuit. Also with any big amps there is a thing called Hot Keying. That is the drive is applied before the amp is keyed, which creates arcs with the amps relay. Not really a problem with small to medium amps, a good relay can take it. Lets use a 5kw amp with 500 watts drive in as an example. If you key the amps relay with drive already coming in, that relay is going to switch both the 500 watts drive and the 5kw out at the same time. Those relay contacts ain't going to last long. This is exactly what happens when you either use an auto key circuit without timing, which almost all are, or if you hit the foot switch and the drive is already on. Hot Keyed. If you use a foot switch correctly, you key the amp first, then the Mic. The relay is closing without rf power going in and out of it. Also, same with unkeying. You unkey the Mic or drive first, then you unkey the foot switch.
Nice explanation buddy
Glad you liked it
I'm bigdog911 Dr.
I've already explained it best I can in the videos. If the relay isn't engaging, its probably the 2n2905 keying circuit transistor. If the relay is keying but the amp is not putting out, its probably a bad relay not letting the power either in or out. If your 300A has the flat relays, oh boy, you got a problem. alright.
@@tramdr I have that 3 relay like in video and a fused tube deck