EMRFD Message Archive 825

Message Date From Subject
825 2007-06-03 20:49:08 wbdmxbaileybobbut... sa612 circuit
I am having a weird problem.

this is my first time using a 612 chip for a direct conversion
receiver.

I built the mrx-40 mini receiver to start with and am having a
problem with the oscillator.

if I place my scope probe on pin 7 of the chip, I see a nice strong
signal at 7.040 and can hear a cw qso in progress, and the vxo tuns
nicely.
if I place the probe on pin 6 there is nothing at all.
the only time I hear a signal is with the scope probe on pin 7,
remove it and the sig disappears.
I modified the parts to the same as used in the 40er circuit and it
still the same problem.

am I correct in thinking that I should see the xtal signal on both
pins 6 and 7 if it is working correctly ?

I have removed parts and tested them and replaced them several times
and still am having the same problem

it acts like the scope probe is adding something to the circuit to
make it oscillate.
it is in direct position.
any suggestions ?
826 2007-06-03 23:18:18 DSNman@comcast.ne... Re: sa612 circuit
Attachments :
    What are the values of the 2 capacitors? What is the value of the resistor from pin 7 to ground? Try 100 pf for the 2 capacitors and 27K for the resistor. Sounds like your adding capacitance from pin 7 to ground with the scope probe.

    --
    Loren Moline
    WA7SKT

    -------------- Original message ----------------------
    From: "wbdmxbaileybobbuttercup" <wb7dmx@hotmail.com>
    > I am having a weird problem.
    >
    > this is my first time using a 612 chip for a direct conversion
    > receiver.
    >
    > I built the mrx-40 mini receiver to start with and am having a
    > problem with the oscillator.
    >
    > if I place my scope probe on pin 7 of the chip, I see a nice strong
    > signal at 7.040 and can hear a cw qso in progress, and the vxo tuns
    > nicely.
    > if I place the probe on pin 6 there is nothing at all.
    > the only time I hear a signal is with the scope probe on pin 7,
    > remove it and the sig disappears.
    > I modified the parts to the same as used in the 40er circuit and it
    > still the same problem.
    >
    > am I correct in thinking that I should see the xtal signal on both
    > pins 6 and 7 if it is working correctly ?
    >
    > I have removed parts and tested them and replaced them several times
    > and still am having the same problem
    >
    > it acts like the scope probe is adding something to the circuit to
    > make it oscillate.
    > it is in direct position.
    > any suggestions ?
    >
    >
    >
    827 2007-06-04 07:33:09 wbdmxbaileybobbut... Re: sa612 circuit
    829 2007-06-04 11:41:04 DSNman@comcast.ne... Re: sa612 circuit
    856 2007-06-13 07:01:01 wbdmxbaileybobbut... sa612 circuit
    I thought I posted this message but can't find it so I am posting it
    again.

    I built the sa612 chip oscillator circuit as shown in the data sheet
    and it is working fine.
    but I have a question about how it works.
    I can recive a good strong sig on my other reciver, so I know the
    oscillator is working.
    I get a nice scope sig on pin 7, but no sig on pin 6.
    also a good signal on the other side of the xtal.
    I would expect to see a good sig on both pins 6 and pin 7 and nothing
    on the other side of the xtal (coil side)

    am I correct in what I am thinking or do I not understand how it works ?
    858 2007-06-13 07:43:24 Arv Evans Re: sa612 circuit
    The SA612 oscillator section is just a conventional transistor.  Check the datasheet at:
             . 
    Pin 7 connects to the emitter and is the feedback path.  Pin-6 connects to the base.

    From your description it sounds like the scope input may be loading down and stopping oscillations when connected to pin-6, but works OK when you monitor the lower impedance signal on pin-7.  A low-capacitance  high-impedance scope probe might be needed to see the pin-6 signal.

    Arv - K7HKL
    _._

    wbdmxbaileybobbuttercup wrote:

    I thought I posted this message but can't find it so I am posting it
    again.

    I built the sa612 chip oscillator circuit as shown in the data sheet
    and it is working fine.
    but I have a question about how it works.
    I can recive a good strong sig on my other reciver, so I know the
    oscillator is working.
    I get a nice scope sig on pin 7, but no sig on pin 6.
    also a good signal on the other side of the xtal.
    I would expect to see a good sig on both pins 6 and pin 7 and nothing
    on the other side of the xtal (coil side)

    am I correct in what I am thinking or do I not understand how it works ?

    859 2007-06-13 17:26:52 wbdmxbaileybobbut... Re: sa612 circuit
    860 2007-06-13 17:39:26 Arv Evans Re: sa612 circuit
    Your HP Lo-Capacitance probes should work fine.  It is possible that your oscillator is right on the edge of non-oscillating and the scope probe loads it just enough to stop it when connected to pin-6.  Increasing the feedback cap a bit (the cap between pin 6 and pin 7) could increase the amount of feedback and possibly make the oscillations stronger, but this might also affect stability.

    Another indicator might be to measure the DC voltages on pin 7 versus that on pin 6.  It should be around 0.6 volts difference on a high impedance (11 megohm input) voltmeter.

    A simple diode detector connected to the same DC voltmeter could show if there is actually RF on both pin-6 and pin-7.

    If your converter is working, why the concern about not seeing signals
    861 2007-06-13 17:48:54 wbdmxbaileybobbut... Re: sa612 circuit
    862 2007-06-13 19:05:59 Arv Evans Re: sa612 circuit
    Touch one of the input pins on the LM386 to see if you hear hum.  If yes, then the LM-386 is probably OK.  Change out the SA612 for the one in your simplified circuit.

    An alternative is to keep the SA612 only circuit intact and  connect it's output to your computer sound card for the audio section.  Then download & install some SDR (Software Defined Radio) programs and enjoy.

    Arv
    _._

    wbdmxbaileybobbuttercup wrote:

    863 2007-06-13 19:20:17 wbdmxbaileybobbut... Re: sa612 circuit
    864 2007-06-13 22:36:00 Arv Evans Re: sa612 circuit
    A commercial SDR system would have the receiver controlled by your PC, but if you can live with manually tuning in your stations, then all you need for a basic SDR receiver is a direct conversi