I bought a $100 bj-300 linear(?) amp and attempted 1 time to hook it up and to put it mildly, it did not go well...AT ALL! I've never seen so much screeching feedback , high swr, ant warning lights going off at once! I'm surprised something didn't explode or at least burst into flames! Lol. Anyway, I was a little traumatized by the experience, but enough time has passed that I'm ready to give it another try. I can't help but think that there is a design flaw causing horrible input/output impedence mismatch, or ? Does any one have any experience (i.e. success running one) with these simple surface mount PCB amps, and/or know of any mods to improve them? I'm running a vintage Uniden pc68xl with adj. dead key, a Stryker sra-10 mag mount and digital swr/pwr meter to monitor everything, and the setup performs flawlessly with 1.0 min-1.3 max swr. I'm attaching a few pictures including the amps schematic. Thanks for any help you can give me!
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Help with Bajoie BJ-300 Chinese amplifier
- Thread starter fastfreddie1269
- Start date
Maybe you got a bad one, it happens. A couple local guys use those amps with no problems at all on 11m ssb. I helped one of them get his set up to work with his radio. Was basically plug and play with no swr issues. Can't remember what his input drive is, but he gets 175 watts on ssb. It will do more, but I set him up so he can just cruise easy with no heat or stress on anything. Two things I've noticed about those amps is the hi/low switch and ssb/am switch do nothing. Just turn it on and talk. They have been using those amps for more than a year with no problems.
It never ceases to amaze me, how these companies throw things together, to make a buck.
The power transistors in your amp, are not designed to operate in the frequency range, you would like. They are UHF parts, for the Motorola UHF radios, in the 450-512 MHz range. I can't recall if it is MICOR, MSR/Mitrek, Maratrac etc. Your amp, was designed around the SD1446, which is a 70 MHz part. The BJ variant, is a copy of the RM Italy, which uses the SD1446. If BJ company, did not make any circuit changes, that amp is not gonna work right.
I believe, BJ company, used M1104's before, which is a VHF (175 MHz) part. I guess they ran out of them. A VHF part, could be used at the low VHF range, but things will need to be considered, like the gain at the low VHF range. Some devices are flatter at the design frequency, then gain increases as the frequency drops, then falls off again. One would need to test, if the datasheet doesn't cover that bandsplit.
In short, I think you are going to have problems with this amp, thanks to BJ company, using whatever they could get their hands on for PA transistors.
SL
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Thanks for your response! I've seen a (foreign language) video on YouTube of a guy swapping an electrolytic cap for one of a different value (see pic w green scribbles), cutting a trace on PCB, removing a surface mount (400 ohm?) resistor, adding I think 2 15 ohm and a 75 ohm resistors (see first pic) and rigging the hi low switch to always be in hi mode (see middle pic). All this was supposedly done to better match amps input impedance to cb radios output. Does any of that sound like it could possibly help? Here's a link to the YouTube video...
Attachments
I doubt it will. The amp in the video, has M1104. This part is the VHF part, used in Maxtrac, Maratrac, GM300 mobiles.
So are you saying that it can't be used as a HF amp? What if the VHF transistors were swapped out for MRF455's (and a bunch of other parts, no doubt!)? Or am I screwed by the Chinese yet again? Lol
A transistor with that many legs is designed for higher frequencies. Having four flat legs for the emitter reduces inductance. This is only necessary above a couple hundred MHz. A transistor meant for HF frequencies won't look like that, and has only two emitter legs. A transistor with good gain at 400-plus MHz will have ten or fifteen times too much gain at 27 MHz. Likewise, a 2SC2879 has decent gain at 27 MHz, but not much at 450 MHz. The problem with too much gain is you can't make it stable. This is why it oscillates and won't settle down. I'd be interested to hear if the YouTube hack actually works as advertised.
Kinda humorous that it looks so much like RM Italy made it, even down to the logo.
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Kinda humorous that it looks so much like RM Italy made it, even down to the logo.
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yes, and SuperLid mentioned. that also.
looks just like a RM Italy KL-300
both have no bias on the RF output transistors,
and both claim to be "linear amplifiers"
maybe adding a feedback network, and bias circuit,
then that little amp can be made to work good.
Negative feedback is all about reducing gain. Maybe that will tame a transistor that has ten times too much gain? Never had to try.
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