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Got a homebrew 40m cw transceiver I got from a rally. It chucks out a very nice spurious signal about 30 - 60 kc high (it changes) - the space between primary and spurious increases with freq and would seem to close on each other around 6.5 MHz ish.

Any thoughts on cause or cure? Its a nice bit of kit too...metal case, proper job Jackson capacitor, audio cw freq readout (at about 50wpm!!)

D

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David. G4YVM


Sat Jul 22, 2017 9:39 am
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 question on spurii 
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Do you have the circuit for it? That might give a clue as to what is causing it, it might be too much drive to the mixer if it's a hetrodyne type system.

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Sat Jul 22, 2017 10:03 am
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Paul
No, ive got nothing thats the problem. I cant even see a design note on the board. Quite frustrating

D

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Sat Jul 22, 2017 1:27 pm
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Can you take a reasonable photo of the board? Either myself or someone else might be able to identify the components on the board and take a guess at what sort of circuit is involved.

I must admit that I had a similar problem on my old home brew gear, on 80m I had a sprog do the same thing as you describe, it was because the third harmonic of 80m was coming out close to the 10.7MHz IF I was using and the amateur down the road was hearing the mixing product. All I had to do was put a 3db attenuator pad in the oscillator port of the ring mixer and that cured it.

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Sat Jul 22, 2017 3:08 pm
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Here you go Paul.

It is very neatly made so I find it difficult to imagine the builder got it wrong. Having said that, the cw readout of frequency has developed a fault: it always used to be too loud (maybe he was deaf) and a tad on the fast side, but useable. I turned it on today for the first time in maybe two years and the speed is now around 50wpm...totally mental. So maybe something has failed. Dunno.


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David. G4YVM
Sat Jul 22, 2017 4:23 pm
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That is as I thought, it looks like there are four diodes just to the south east of the power transistor, that could be the ring mixer. There are four crystals in there, can you see the frequency of them? If you can, list them from left to right and the remaining one at the bottom right hand corner of the board.

I suspect that two of those crystals are the lattice filter and they will give a clue as to the IF frequency, with a bit of simple maths it might be possible to work out the sproggy's frequency.

I don't suppose you posses an oscilloscope, it would be very easy to check the levels to make sure the ring mixer isn't being over driven.

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Sat Jul 22, 2017 4:44 pm
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I do have a scope actually, as part of my self training and i try to use it a fair bit. Its old and im not so hot with it...but i have one!

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Sat Jul 22, 2017 4:46 pm
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Theyre all4.0 mhz xtals

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David. G4YVM


Sat Jul 22, 2017 4:49 pm
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OK, the IF is probably 4MHz, that will probably put the VFO on the LF side of 7MHz at 3-3.5MHz or thereabouts. The other thing I tried to work out is the harmonics from the VFO falling near 11MHz which is on the high side of 7MHz by 4MHz, but couldn't see anything of any significance.

The only thing I can think of at the moment is the mixer stage is being driven too hard and causing non-linearity somewhere, because 60kHz is not making sense at the moment, unless someone else sees something I've missed.

If you can work out where the mixer is and check the oscillator level against the spec sheet if it's an IC or if it's a ring mixer lets us know what level it is, there might be some clue.

Sorry it's all if's and but's at the moment.

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Sat Jul 22, 2017 7:17 pm
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Thanks Paul

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Sat Jul 22, 2017 7:48 pm
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