Re: LF: Softrock RX divider IC problem, unexpected frequency out

[M0FMT <peteg8fmt@gmail.com>]

Hi

Despite Andy Talbot’s (who I respect highly and feel unworthy making this comment) comment which I am sure will solve the issue. However my experience with several of these kits in the V6.2 format has never been an issue at 136khz.

This will not help other than give confidence that the Softrock Tayloe detector is a robust design. These kits in my experience are excellent. If you are getting frequency variation it will be the Xtal oscillator, nothing else can cause it, however I have never experienced it myself, the divider a dual flip flop correctly fitted will work reliably. The fact you are getting out of band signals (Strong BC) is probably because there is no LO to the multiplex chip it is just working as a nonlinear detector (a diode) being over loaded by the BC signal.

The important thing is to get the Oscillator working reliably ensure the buffer amp is delivering a TTL level to the connected flip/flop input, check with a scope. If all is well the problem is the flip/flop check it is getting the correct Vcc. If no divide by 4 at the input to the multiplex , check with scope,   then the flip/flop has a problem. My guess the chip is fine it is the quality of the fitting (no criticism intended it’s not easy) . If you have done all of the above then I would recommend you remover the flip/flop chip. If you have a spare new one then fit it after you have fastidiously cleaned the pad. Your output to the multiplex will be divide by 4 of the xtal frequency and the two LO feeds will be phase shifted by 90 degrees.

Good luck it will work fine but go through the steps above and confirm LO with a scope at each step.

73 petefmt

On Fri, Jan 26, 2018 at 3:17 PM, Andy Talbot <andy.g4jnt@gmail.com> wrote:

The linearly biassed gates used to buffer the input of the Softrock LO fail at low frequencies, breaking up into spurious oscillations at the threshold changeover, not operating the divider properly  and hence giving an apparent change in freq  I had this problem  when I was using a "Finningley Dongle" driven by an external DDS source.   The solution was to add my own Schmitt trigger input buffer before letting it get to the 74HC74 device.   That way  it would work down to below 10kHz RF input.

Andy  G4JNT

On 26 January 2018 at 15:06, David Bowman <davidg0mrf@gmail.com> wrote:

Hi Chris.

It may be worth going back to the oscillator and making sure the frequency there is what you expect.

Could be that the divider is working fine, but the input to it is shifting around.

73

David  G0MRF

On Fri, Jan 26, 2018 at 1:45 PM, Chris Wilson <dead.fets@gmail.com> wrote:

Hello  LF and VLF folk!

I have built a couple of Softrock Lite II IF frequency receivers,
normally they would be used to view another receiver's IF on a
bandscope, but popularly they are also used on low and medium
frequencies as stand alone SDR receivers. The MF one works fine. Th LF
one was intermittent, then failed with high current draw. I isolated
it to a faulty or shorted divider chip. The original one was marked
78C49NM. I couldn't find one of those, so plumped for one from RS
Components here in the UK, a CD74AC74M96G4, RS part number 662-6939
The oscillator worked on the bench and from an xtal frequency of 461.5
kHz gave the expected divide by 4 output of 115.364 or very close.
Then suddenly it started receiving broadcast stations and the divided
outputs Q and /Q were on 153.814 KHz! There they remain. Fiddling I
briefly saw a return to 115.36 Khz, but it was not for long. All
voltages to it's pins are correct at 5V. I am not sure if it's a
faulty divider, or a bad connection or short.

Xtal case is grounded and seems stable. R16 in the oscillator changed
on advice from 22.1k to 10k as they apparently tend to stop running
with 21.1k there. I attach the schematic. Wrong IC? Details of the
divider's operation are at http://www.wb5rvz.org/softrock_lite_ii/03_div

The new IC details are at https://uk.rs-online.com/web/p/flip-flop-ics/6626939/

I see these frequencies on these pins:

Pin 3 : 464.444 kHz

Pin 11 : 464.444

Pin 12 : 153.814

Pin 8 : 153.814

Pin 2 : 307.625

Pin 2 : 307.625

Is this a random IC fault or does it have some mathematical
correlation? My maths is pitiful!

Not sure what to do now, I have another IC but before risking pad
damage thought it prudent to ask, many thanks.

Pins 14 and 2 on the mixer IC show good, 90 degree out of phase square
waveforms of normal amplitude on a scope, but at 153.814 KHz....

Schematic is at http://www.gatesgarth.com/schematic.jpg



--
Best regards,
 Chris                          mailto:dead.fets@gmail.com