[minima] Re: Anything special about T3's capacitor to ground?
- From: Thomas Sarlandie <thomas@xxxxxxxxxxxxx>
- To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
- Date: Sun, 23 Feb 2014 19:17:07 -0800
Raj,
I have removed and put back the transformer last night. Same results, then
I removed it, rewound it with a thicker magnetic wire and put it back in
place. Unless there is something wrong with my interpretation of the
schema, I think it should be ok. How can I test it?
Right now, I have a 20.004 Mhz signal on both diodes on the transformer
side with 430 mV p-p. That is my BFO frequency so I can safely assume that
the BFO couples in the two secondary windings. Correct?
On the other side (rf pre-amp output) of the diodes, I can see a 20.000 Mhz
signal with 420mV p-p. That is my 30mhz (vfo) - 10mhz.
On the output where the two secondary windings connect and there is this
capacitor to ground, there is not much signal. Nothing I can distinguish,
nothing that looks like my 5 kHz I should be getting there.
Question: Do the three dots on the transformer have any special meaning?
The other transformers do not have those dots. I know they are supposed to
indicate where to put the core but I just wound t3 like I did t1 and t2.
Was that incorrect?
thanks!
thomas
On Sun, Feb 23, 2014 at 3:01 AM, Raj <vu2zap@xxxxxxxxx> wrote:
> Thomas, please check the transformer. Try replace the diodes. Then
> backwards to Q9 and Q10.
>
>
> At 23-02-2014, you wrote:
>
> So I used the AD9850 dds to generate a 10 mhz signal and track that down
> in my minima. I think I found my problem...
>
> LPF, KISS mixer, RF pre-amp and BFO all seem to work.
>
> However, I have absolutely no signal at the output of the BFO mixer. But
> if I remove the 0.1 uF cap to ground, I suddenly have a beautiful signal
> modulated with the difference of the BFO and the IF. If I switch my BFO to
> the alternate frequency (LSB), the modulation frequency also changes.
>
> So I can get a 4.1 or 6.8 kHz (very precisely my BFOs minus 20mhz) but of
> course it still is carried over the 20 mhz. I understand the cap to ground
> is supposed to remove that.
>
>
> *On the schematic, this specific cap (below T3) is marked as a polarized
> component. Should I use some specific type of capacitor here? *
> This is what the output of t3 looks like into my scope, 50 ohm terminated
> - *without* the capacitor.
>
>
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