Recapping a Panasonic RF 930

[watchmaker]

I treated myself to this radio as a kid, $40 USD in 1970, back when a dollar was still worth 80 cents. (Over $300 USD today dollars).  Had it for at least 2 decades (wife remembers it) but lost track of it.  Had the best sound and remember listening to Elton John on a NYC FM station while I was 90 miles away.

Anyway, somehow ran across it on eBay and they now sell for about $20.  OK, mono, but I got rid of all my stereo stuff when we moved.  Not even a TV.

The issue was 60 Hz hum with volume all the way down while listening with earbuds that was not present on Batt.   Cord reversal did not help.  OK.  Replaced the PSU caps

Still great sound although my ears are far from what they used to be (no one used hearing pro when shooting or using chain saws).

There were two things that made me take a second look.  First, Panasonic used the chassis as the high side and sent the negative end to the ckt board.

Question:  Why did Panasonic elect to do this?  I thought it was convention to send the negative to the chassis.  See attached schematic.  S3 is the selector switch (set to Batt) and S2 is the power switch (set to Off).  The positive terminal of the batt pack is sent directly to local ground (chassis).

The other thing is look at the board!  Can you imagine hand soldering and aligning this on a production line??  Very tight fits.  And although it uses AFC, it does a great job on selectivity and sensitivity.

[m k]

Time was different, salaries also.
(VW Golf came '74)

Maybe they were still learning away from the old ways.
Or maybe they learned that it's better that way.

[watchmaker]

OK, there is no RF advantage.  THANKS!

I am still intrigued by the positive polarity of the ground.  I was a 2 year electronics vocational ed student in high school in the late 60s and even that era we were taught the ground reference is negative.  The US Navy used the same convention even though the USN did (and still does!) teach electron flow.

I may collect some other Panasonic schematics (pre and post 1970) to see when they came to their senses.

We visited my daughter this weekend and her husband is an EE/patent attorney whose clients include SkyWorks.  Like you, he was at a loss as to why anyone make that choice.

And I am still processing what the production looked like hand soldering all those components onto such a compact board.  I have two of these radios and the soldering is not well controlled. And all the final slug positions on the IF cans are marked.  Working there must have been Hell.

[m k]

Maybe it has nothing to do with electronics, maybe it's economics.
No need to invent a wheel, no matter the shape of the old one.

[andy3055]

Is it possible that the +ve ground was used because of the PNP transistors they used at the time? This was very common those days.
As for the hum, check or replace C54, 55 and 58.