[ARC5] BC-779 mods

Mon Oct 22 11:20:18 EDT 2012

Super Pro RF mods arent necessary below about 12-14mc but above that it 
helps considerably.

I had a BC-794 thru here not too long ago that had a 6BA6, 6BA6, 6BE6 and 
6C4 front end and very well done about 40-50 years ago by the looks of the 
components below chassis. Sensitivity and AGC action was excellent, on a par 
with a SP-600 once I got thru replacing old caps and out of tolerance 
resistors. I then changed the first RF to a 6GM6 and mixer to a 6BY6 while 
also reducing the 2nd RF gain to barely above unity as is done in the 
HRO-60. That improved SNR above 20mc quite a bit as well as signal handling 
on a good antenna. With the reduced gain image rejection was excellent.

A bit of TC was added as in the SP-400 as well as zener regulating the 
oscillator, mixer and BFO and removing the ubiquitous VR tube. Drift on 10M 
is now on a par with sets such as a NC-183/183D, HQ-140 after a 30 minute 
warmup and very good on the lower ranges.

The audio was a single 6V6 in place of the 6F6 driver which saved a lot of 
current and heat. The driver, output transformers, and PP 6F6's had already 
been removed.
There was quite a bit of distortion so I rewired the driver and output for 
much lower distortion higher fidelity and used a heavier duty transformer in 
place of the one that looked like it came from an AA5.

The early Super Pro's always had excessive gain starting at the RF  and 
right thru the audio. Maybe it was needed in a comm room with lots of noise 
but not at home.

The final step was to again take from the HRO-60 book and gain equalize the 
IF stages to supplement the front end.

The AVC rectifier had been replaced by SS diodes and a 6SN7 product detector 
wired in. It worked fine so I left it alone.

Final testing resulted in performance close to the HRO-60 which had been 
reviewed a few years ago by W1VD who showed that in many areas was the equal 
of or better than a R-390A.

If it was my radio I would have added 2-3 AGC timing selections and 
replicated the SX-28/HQ-180 noise blanker

All in all this 70 year old set was ready to hang with the others on todays 
crowded bands and not take a back seat to most. I used it quite a bit during 
a weekend CW DX contest and slaved to a Kenwood TS-950SD used just as the TX 
and hooked to several very good antennas and it held up well from 
overloading. It never took a backseat in hearing the same DX as others were 
calling plus I had several long runs to add to the pleasure. OTOH a BC-453 
Q5er would have been a help at times (-;

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>; <arc5 at mailman.qth.net>
Sent: Sunday, October 21, 2012 11:49 PM
Subject: Re: [ARC5] BC-779 mods

> On 21 Oct 2012 at 18:45, Richard Knoppow wrote:
>
>>     My main experience with modifications was my BC-779.
>> When I got it I found that the RF section had been modified
>> according to a CQ article.
>
> Yes. I have that article "The Ultimate Super Pro". Ye gods, what an 
> abortion.
> I have a completed example of that thing. I bought it, along with an
> untouched one, from a fellow some time ago.
>
>> About the only really
>> useful mod was to add a voltage regulator to the oscillator
>> and _that_ was not in the article.
>
> Hammarlund some time later recommended soldering a 3 pfd N-750 cap
> directly across the oscillator section of the tuning cap. Made a BIG 
> difference
> in stability.
>
>> Unmodified the receiver
>> works very well other than using rather noisy tubes for RF
>> amps and mixer.
>
> Actually, in my experience with my NIB BC-779 back in the late 1960s,
> changing the detector to a single-triode product detector made a MAJOR
> difference.
>
> That receiver has two RF stages, and there is plenty enough gain to mask
> the mixer and RF amp noise.
>
>>I still have this receiver but I think
>> now it needs all new caps.
>
> Probably does.
>
> I think I have at least 3 BC-779s here. One is complete and untouched,
> another was the unfortunate subject of that CQ mag article I mentioned
> above (complete with hacked out audio section), and the third one, 
> although
> pretty much complete, is really good for nothing but parts...I think. At 
> least it
> is pretty ugly looking...we'll see just how bad it is when I can get to 
> it.
>
> Ken W7EKB
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1427 / Virus Database: 2441/5346 - Release Date: 10/21/12
> 