On Aug 30, 1:05*am, John Byrns wrote:
In article
,
*Patrick Turner wrote:
That is an impressive tubed stereo receiver. If it is a 1960 model
then does it not predate the addoption of the Zenith-GE multiplexing
for stereo info using a 38kHz sub-carrier?
I searched for a free online available schematic for the S-8000 which
this receiever seems to be. Probably it is after 1961 and with the
addopted standard MPX stereo decoding circuits of the time. Anyone
bidding for this would be able to check this out because you have a
picture of the front cover of the owner manual.
I could not find a free schematic although someone was able to sell a
service manual for just about all Sherwoods from those ancient times.
The top of chassis view clearly shows the presence of a post June 1961
Pilot Tone Stereo decoder.
These old sets usually develop a number of faults over the years. It
happens also with Scotts and Fishers et all. But they do respond to
having all dud tubes and R&C parts replaced along with some upgrade to
electrolytic cap values, and some possible revisions to output tube
biasing methods.
What revisions do you recommend to the output tube biasing methods? *
Most of these old receivers originally used a "fixed bias" scheme.
I cannot see the full schematic of the receiver in question.
But many such things were seriously compromised by very poor bean
counter inspired biasing of OP tubes.
It was common practice to use the heater filaments of the phono
stages for the common cathode resistance for all 4 output tubes so
that you could have Ek at every OP tube at +25V at a total Ia = 300mA,
if there were 4 x 12AX7. Trouble occurs when OP tubes drift and don't
all bias the same with Ia variations of +/- 70% as I have seen in bad
cases.
Almost none of the well known brands used separate individual RC
cathode biasing for each OP tube which was the most user friendly
method. None I know of ever had separate trim-pots to set the fixed
bias which most owners got wrong causing smoke and burned out parts.
As I have so often said, much of the design methods used in 1960
receivers was bloody crook ****e, and they always used a minimum cost
design method which meant poor lowest common denominator performance.
Patrick Turner.
--
Regards,
John Byrns
Surf my web pages at, *http://fmamradios.com/