[minima] SOLVED! Re: Re: Anything special about T3's capacitor to ground?
- From: Thomas Sarlandie <thomas@xxxxxxxxxxxxx>
- To: "minima@xxxxxxxxxxxxx" <minima@xxxxxxxxxxxxx>
- Date: Thu, 27 Feb 2014 00:48:58 -0800
Hello everyone!
It was a little past midnight and there was plenty solder smoke in the
shack when I finally heard my minima for the first time ...
Special credit to Mark who sent the decisive last message which led to
victory. Thanks to everyone who shared comment and tips. The international
brotherhood of electronic wizards has once again lived to its reputation!
After Mark last messages, I decided to build a new mixer on a separate
board, starting from scratch. I used new diodes that I re-matched. Picked a
new transformer and checked it's core AL value (thanks Mvs Sarma for
suggesting this; it was ok). Used a drill to prepare the trifilar wires,
carefully placed the three windings, etc - gave it lots of love.
Then I used a little piece of coax to drive the bfo signal from my main
board and used my DDS to provide an IF at 20 Mhz. With that complete new
setup on a new board, I still had the same problem: with the capacitor to
ground there was no signal out of the mixer.
At this point the only thing that could be bad was the BFO so I rebuilt
another BFO from scratch on my new board.
With the new BFO, I finally got a working setup. The audio signal was there.
I then looked very carefully at the BFO on my main board and found my
problem in 30 seconds. The 100pF capacitor that is supposed to be between
the base and the emitter was between the base and the collector. I moved it
and it fixed my problem. (And then considered whipping myself with a
coaxial cable ...)
I have fixed a lot of software bugs but I think with this bug I have earned
my first home-brewing scar ;)
I would have found the problem earlier if I had suspected the BFO was the
problem. I assumed it was ok because I was getting a decent signal out but
I was wrong.
Anyway, I am now listening to 20m and 40m. It's pretty much dead now but
just plugging the antenna in the minima, I can hear a large difference in
noise level. I have birdies, am stations and it finally sounds like a
receiver. I am confident there will be qsos going on when I wake up
tomorrow.
I hope everyone will sleep better knowing that the excellent schematic of
Mr Farhan was not the problem - I take full responsibility!
Thanks again everyone. Each evening spent on this radio is a ton of new
stuff learnt - next steps, the transmit stages.
73!
thomas
On Wed, Feb 26, 2014 at 7:23 AM, Mark G0MGX <mark.g0mgx@xxxxxxxxx> wrote:
> Hi Gang
>
> I'm back home and have just wound a 8 turn trifilar transformer on an
> FT37-43 and coupled it up to the output of a 20MHz crystal oscillator I had
> lying about.
>
> Here's the setup:
>
>
>
> This is the signal into the BFO port (yuck - but 20MHz):
>
>
>
> My analogue signal generator (
> http://g0mgx.blogspot.co.uk/2013/10/so-hows-sig-gen.html) is connected to
> the RF port:
>
>
>
> Then I see kind of crap out of the audio port but if I alter the fine tune
> on the sig gen I can see what looks like an audio signal:
>
>
>
> There is a load of RF in this signal also, this is 0 to 150MHz:
>
>
>
> and if I zoom in on the 20MHz, we can see the mix of the two RF signals,
> this is 50KHz either side of the 1st peak above at 20MHz - the signal
> closest to the centre and the largest amplitude is the BFO:
>
>
>
> 6.56 of EMRFD is probably the best reference to understand whats in these
> signals.
>
> Mark
> G0MGX
>
>
>
> On 26/02/2014 12:19, Mvs Sarma wrote:
>
> Thomas,
> please see whether the core qualifies for the AL value specified for
> FT37-43. Wind 10Turns and measure inductance. it would be AL /10. as per
> www.kitsandparts.com datasheet is 350 with 20% Tol
>
>
>
> On Wed, Feb 26, 2014 at 1:29 PM, Thomas Sarlandie <thomas@xxxxxxxxxxxxx>wrote:
>
>> Mark,
>>
>> If you have a chance, I would be very interested in reading what is the
>> signal strength you measure here at the output of that transformer. I might
>> be looking at a problem that does not exist - maybe my sensitivity problem
>> is somewhere else.
>>
>> Mvs Sarma: Thanks for the tips. I have tried smaller capacitance
>> without any success and I have looked hard for solder bridge. I might try
>> building a new mixer out of circuit to make sure I start from scratch.
>>
>> 73,
>> thomas
>>
>>
>> On Mon, Feb 24, 2014 at 9:25 AM, Thomas Sarlandie
>> <thomas@xxxxxxxxxxxxx>wrote:
>>
>>> Mark,
>>>
>>> I am getting confused too ;)
>>>
>>> The radio is in RX. I believe my audio amp works ( I can receive AM
>>> broadcast by putting my finger on the input wire ). I can also receive my
>>> 3kHz tone even with my capacitor in the circuit. My problem is I don't
>>> receive any other signals. It's like the receiver is completely deaf except
>>> to very strong signals applied to the input.
>>>
>>> When you say it's normal for the input of the audio amp to be weak,
>>> how weak are we talking? Micro-volts? Milli-volts? Should I be able to see
>>> something on the oscilloscope? Maybe the very high impedance output is
>>> normal.
>>>
>>> If it is, then my sensitivity problem might be somewhere else. How
>>> could I test that?
>>>
>>> Can you confirm that plugging an antenna on the receiver makes a
>>> difference in the level of noise you can listen to? Plugging an antenna on
>>> mine makes absolutely no difference to the noise strength.
>>>
>>> thanks again for your replies!
>>> thomas
>>>
>>>
>>> On Mon, Feb 24, 2014 at 1:41 AM, Mark G0MGX <mark.g0mgx@xxxxxxxxx>wrote:
>>>
>>>> Hi Thomas
>>>>
>>>> I think I am getting confused (nothing new there). If the radio is in
>>>> TX then the point where your scope is connected is where we would inject
>>>> audio to mix with and modulate the BFO signal. If we are in RX (that's my
>>>> assumption) then we would only expect to see the injected signal being
>>>> modulated i.e. an audio signal and very weak one too - most of the gain in
>>>> the Minima is at audio. Is the audio amp working (does it buzz when you
>>>> stick your finger on the input)? I am not sure what you should expect to
>>>> see at that point? The audio tone there will also alter and be totally
>>>> dependent on the tuning of the LO and hence the output of the mix.
>>>>
>>>> If your scribbles are correct then the transformer looks correct to
>>>> me. Without checking the signals on mine I can't help much more until I get
>>>> home (Thursday)....
>>>>
>>>> Mark (just boarding in Abu Dhabi & bound for London)
>>>>
>>>>
>>>> On 24 February 2014 08:48, Mvs Sarma <mvssarma@xxxxxxxxx> wrote:
>>>>
>>>>> thomas,
>>>>> even a cheaperTA7358 and perhaps LKA1168 or so ( a compatible) would
>>>>> also work, I suppose
>>>>>
>>>>>
>>>>> On Mon, Feb 24, 2014 at 1:29 PM, Thomas Sarlandie <
>>>>> thomas@xxxxxxxxxxxxx> wrote:
>>>>>
>>>>>> Raj: Yes I have all the dots on the same side. I have checked several
>>>>>> times now. Unless I am really reading something backwards I think I have
>>>>>> that covered.
>>>>>>
>>>>>> Farhan: I don't but I will look into that. Would a ne602 work too?
>>>>>>
>>>>>> It's really raging to see my 4KhZ signal get out of the mixer but
>>>>>> not be able to tame it into the audio pre-amp because the output
>>>>>> impedance
>>>>>> is so high. What could explain this? I will take any theory ;)
>>>>>>
>>>>>> 73,
>>>>>> thomas
>>>>>>
>>>>>>
>>>>>> On Sun, Feb 23, 2014 at 11:07 PM, <farhanbox@xxxxxxxxx> wrote:
>>>>>>
>>>>>>> Thomas, if you have an ade-1, just drop it into the circuit.
>>>>>>> - f
>>>>>>> Sent from BlackBerry® on Airtel
>>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: Raj <vu2zap@xxxxxxxxx>
>>>>>>> Sender: minima-bounce@xxxxxxxxxxxxx
>>>>>>> Date: Mon, 24 Feb 2014 12:29:53
>>>>>>> To: <minima@xxxxxxxxxxxxx>
>>>>>>> Reply-To: minima@xxxxxxxxxxxxx
>>>>>>> Subject: [minima] Re: Anything special about T3's capacitor to
>>>>>>> ground?
>>>>>>>
>>>>>>> Thomas, please check all three DOTS are from the same end of the
>>>>>>> transformer.
>>>>>>> Most important that same winding start and end is correct.
>>>>>>>
>>>>>>> At 24-02-2014, you wrote:
>>>>>>> >Thanks Mark. That's how I have been doing it. I was not sure
>>>>>>> because T1 & T2 do not have those dots. Is it because phasing is not
>>>>>>> important on them?
>>>>>>> >
>>>>>>> >I have just rewound a new T3 using a new roll of #32 AWG wire on a
>>>>>>> new FT37-43. I get the exact same problem. So here is a picture of what
>>>>>>> I
>>>>>>> am doing with some labels as to where the wires go. I hope I am doing
>>>>>>> something very stupid and someone will be able to point it out:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Regards
>>>>> Sarma
>>>>>
>>>>>
>>>>
>>>>
>>>
>>
>
>
> --
> Regards
> Sarma
>
>
>
>
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