[Premium-Rx] WJ-8615D Issues.

[Bob Betts]

Hi again David:
As you may recall our previous discussion concerning switchers, I was never happy until the hum + hash was down to single digit mV envelope levels. The few that I have used in receiving equipment were cured with either a Pi ot Tee filter. You may want to add a choke inline with the DC line that has the added electrolytic. If there is any improvement, you might try the above mentioned 3-element filter. I think I used about 10 (or so) uH coil with suitable conductor size for the current flow...maybe 20 or 22 Ga. The current supply demands of that radio are not terribly great, so maybe 4700 uF on both sides of an inductor would be sufficient...just thinking out loud.
 
Also, I suspect that with induction motors, like the fan's field coil, that any noise on the DC line can be re-radiated -- like a loop antenna. I had some problems with radiated noise from the switcher itself, but also found that moving some of the cable harnesses (antennas) changed the character of the problem. Actually, just my hand capacitance on the wires affected the unwanted re-radiation.
 
Have a go at it...you're almost there !
 
Best,
Bob, N1KPR

http://www.bobsamerica.com
http://www.youtube.com/n1kpr


________________________________
From: David Schofield <davesc50 at tiscali.co.uk>
To: Premium-Rx at mailman.qth.net 
Sent: Wednesday, August 7, 2013 4:40 PM
Subject: Re: [Premium-Rx] WJ-8615D Issues.


Hi Again,

For anyone interested in the "solution" to the SSB hum issue, I discovered the following;

The hum was due to ac ripple being superimposed on the 10.7Mhz reference signal to the SSB converter.
It looks like my replacement PSU isn't quite as smooth as the original one used by WJ.
An extra 4700uF across the +15v rail removed about 85% of the problem, the remaining hum however has kept me guessing....
The only "cure" I have found so far is to disconnect the fan.
Somehow the noise on the supply rail is being induced by the fan, in fact if I physically slow the fan down, the pitch of the hum changes....
I have tried all sorts of things to filter it out, but can not remove it 100%
As the noise is only very faint now, and was only ever audible on SSB, I have come up with a compromise solution.
I am hardly ever going to use this Rx on HF, or SSB, so have utilised an existing unused hole on the rear of the unit designed for the ac mians voltage selector switch, (unused as both the original switch mode PSU fitted by WJ, and my supply are both dual voltage), and installed a fan on / off switch.
This means that I can switch off the cooling fan if I want to listen to crystal clear SSB, and leave it running for the remainder of the time.
Whilst this is not an ideal solution, it works OK for me, is fully reversible, and I don't see it causing any major cooling problems, as the Rx is not rack mounted and has large rubber feet fitted to the base allowing for plenty of air flow.
The PSU I used is also nearly double the rating of the original one and runs a lot cooler.

I'm still open to suggestions, if anyone out there knows how to prevent induced noise from a fan getying back into a power supply. I have tried ferrite beads, filter caps Etc, but can't clear it up.
I suspect that the problem still lies with my replacement power supply as opposed to the receiver design, but as I never had it working with the original supply, cant prove or disprove this.

Anyway, it works fine now.
Thanks to everyone who replied to the original mail with lots of usefull info as well.

Cheers,
Dave. S.David Schofield <davesc50 at tiscali.co.uk> wrote:Hi All,

I've just acquired a WJ-8615D which to the most part is working very well indeed.
When it first arrived the PSU had failed spectacularly, and after a couple of atempts at rebuilding it I decided it was beyond salvation....
To that end I brought a more up to date, better spec'd PSU and made up a set of adaptor leads, and a plate to fit the new module so an original unit could be fitted should I ever find one, or actually succesfully fix the original.

The receiver is now functioning well, apart from the fact that the SSB demodulator has a loud hum of approx 50Hz superimposed on the output.
Before I start to investigate this, I was wondering if their are any known issues that can cause this. I suspect a bad earth, or possibly a failed decoupling capacitor on the SSB module. The new PSU seems to be performing 100% ok, so I think this is a secondary fault.
SSB seems to be working OK underneath the hum, so it would be nice to get it working 100% ok.

The other thing is that this unit is fitted with the optional oreselector, which appears to be making it pretty deaf, although it does seem to be doing it"s job and improving intermodulation Etc. especially on HF.
If I bypass the preselector the gain is far better, but full of harmonics Etc, is this normal for these receivers?
If it is I can live with it, I have plenty of other "hotter" receivers, and would rather have reduced gain than spurious images, but though I'd ask in case their are any known issues in this area either.

Thanks in advance,
Dave. S.
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