On Oct 31, 5:43 pm, cliff wright wrote:
Andrew VK3BFA wrote:
On Oct 25, 1:27 pm, cliff wright wrote:

Well I've got all my TX problems sorted out fine now.
However I am beginning to think that the receiver is somewhat noisy.
When set up as per manual I am getting about a S3 internal noise level
on sideband.
My unit has had the RF stage modified as per the International Radio
mods and also the balanced mixer. In the next few days I hope to have a
look at this problem too, it is possible that the mixer balance is not
very perfect for example.
It was also fitted with back to back 1N4148 diodes across the RX input
and these might just have been damaged with the tx problem.
Has anyone else had a similar experience with this rig?
Regards Cliff Wright ZL1BDA ex G3NIA.

Remove ALL the mods. Take it back to original - that way, you will be
looking for a fault rather than trying to figure out if its the mods,
faulty mods, something else, etc etc.

The original design engineers had a pretty good idea of how it was
supposed to work, published specifications in the manual, are based on
this. Your battling with "mods" - how can find any fault with
extraneous mods?

Get an established reference point, ie the service manual. Go forward
from there.....

Andrew VK3BFA.

Hi Andrew.
Well I ran it for over a year before making the modifications.
These were basically using more advanced MOSFET's in the rf and balanced
mixer stages with some bias adjustments to suit.
From my measurements (I have a pretty well equipped home electronics
workshop) it did make some improvement to the internal noise floor and
about a 6 to 10 dB improvement in overload capacity.
I must actually beg to differ a little about the original design
engineers. As I pointed out the 820S was produced with a least 5
different 1st mixer source circuits in production and the coupling to
the balanced mixer is not optimum by any means. This doesn't look to me
like a fully optimised design!
BTW I did actually restore things the way they were and it just went
back the way it was, and the noise was still there.
Perhaps I should point out that I have designed and built 10 different
communication receivers in the last 40 years and have owned most of the
well known older commercial units too, in fact in the late 1970s I
worked 200 countries confirmed and got my WAC with a completely homebrew
station running 100 watts to a TT21 final.
Further investigation has confirmed so far that we have a serious local
problem with mains radiated QRN here.
Some experiments the other evening showed that the noise has decided
polarisation characteristics as a 5/8 wave 20 metre vertical produces
a similar noise level to my G5RV but with greater impulsive
characteristics, a bit like auto ignition noise.
The jury is still out but fortunately another local ham works for the
power distribution company and I'm going to get in contact with him to s
share our experiences.
Very dissapointing though as I have a nice elevated site and had great
hopes of getting some good DX.
I'll post anything I find out further.
73's Cliff Wright ZL1BDA ex G3NIA

I stand corrected - it was from my own experience of fixing other
peoples radios where a bizarre collection of "mods" had degraded the
actual performance!. But, as you point out, you have the test gear and
the knowledge to eliminate this.

In the end, its going to be something very simple (isnt it always?)
and possibly related to an error made while doing something else in
there........(been there, done that - still drives me NUTS!!!)

Andrew VK3BFA.