Date: Fri, 26 Jan 2018 16:23:55 +0000 (GMT)
From: DAVE PICK <daveyxm@virginmedia.com>
To: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Message-ID: <1397698645.2553258.1516983835152.JavaMail.open-xchange@oxbe7.tb.ukmail.iss.as9143.net>
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Subject: Re: LF: Softrock RX divider IC problem, unexpected frequency out
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Chris

You seem to have made a divide by three rather than four...

I'm not sure what's causing that but make sure any unused inputs are tied up or
down as appropriate.

You have measured the frequency of the oscillator but is it a good square-wave
on 464?

Something isn't quite right with the oscillator if its 3kHz off..

If it's a bit wobbly it may be mis-triggering. A capacitor change may be needed
around the oscillator part in order to keep the low-frequency crystal happy.

Dave

> 
>     On 26 January 2018 at 13:45 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
> 
> 

>
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    <meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3DUTF-8=
"/>
 </head><body><p>Chris<br></p><p>You seem to have made a divide by three ra=
ther than four...<br></p><p>I&#39;m not sure what&#39;s causing that but ma=
ke sure any unused inputs are tied up or down as appropriate.<br></p><p>You=
 have measured the frequency of the oscillator but is it a good square-wave=
 on 464?<br></p><p>Something isn&#39;t quite right with the oscillator if i=
ts 3kHz off..<br></p><p>If it&#39;s a bit wobbly it may be mis-triggering. =
A capacitor change may be needed around the oscillator part in order to kee=
p the low-frequency crystal happy.<br></p><p>Dave<br></p><blockquote type=
=3D"cite"><p>On 26 January 2018 at 13:45 Chris Wilson &#60;dead.fets@gmail.=
com&#62; wrote:<br><br><br><br><br>Hello LF and VLF folk!<br><br>I have bui=
lt a couple of Softrock Lite II IF frequency receivers,<br>normally they wo=
uld be used to view another receiver&#39;s IF on a<br>bandscope, but popula=
rly they are also used on low and medium<br>frequencies as stand alone SDR =
receivers. The MF one works fine. Th LF<br>one was intermittent, then faile=
d with high current draw. I isolated<br>it to a faulty or shorted divider c=
hip. The original one was marked<br>78C49NM. I couldn&#39;t find one of tho=
se, so plumped for one from RS<br>Components here in the UK, a CD74AC74M96G=
4, RS part number 662-6939<br>The oscillator worked on the bench and from a=
n xtal frequency of 461.5<br>kHz gave the expected divide by 4 output of 11=
5.364 or very close.<br>Then suddenly it started receiving broadcast statio=
ns and the divided<br>outputs Q and /Q were on 153.814 KHz! There they rema=
in. Fiddling I<br>briefly saw a return to 115.36 Khz, but it was not for lo=
ng. All<br>voltages to it&#39;s pins are correct at 5V. I am not sure if it=
&#39;s a<br>faulty divider, or a bad connection or short.<br><br>Xtal case =
is grounded and seems stable. R16 in the oscillator changed<br>on advice fr=
om 22.1k to 10k as they apparently tend to stop running<br>with 21.1k there=
. I attach the schematic. Wrong IC? Details of the<br>divider&#39;s operati=
on are at http://www.wb5rvz.org/softrock_lite_ii/03_div<br><br>The new IC d=
etails are at https://uk.rs-online.com/web/p/flip-flop-ics/6626939/<br><br>=
I see these frequencies on these pins:<br><br>Pin 3 : 464.444 kHz<br><br>Pi=
n 11 : 464.444<br><br>Pin 12 : 153.814<br><br>Pin 8 : 153.814<br><br>Pin 2 =
: 307.625<br><br>Pin 2 : 307.625<br><br>Is this a random IC fault or does i=
t have some mathematical<br>correlation? My maths is pitiful!<br><br>Not su=
re what to do now, I have another IC but before risking pad<br>damage thoug=
ht it prudent to ask, many thanks.<br><br>Pins 14 and 2 on the mixer IC sho=
w good, 90 degree out of phase square<br>waveforms of normal amplitude on a=
 scope, but at 153.814 KHz....<br><br>Schematic is at http://www.gatesgarth=
.com/schematic.jpg<br><br> <br><br>-- <br>Best regards,<br> Chris mailto:de=
ad.fets@gmail.com<br><br></p></blockquote><p>&#62;</p></body></html>
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