sx-28 restoration ongoing

[belizando@gmail.com]

Thanks to Phil Nelson and many others for great internet
documentation and numerous usenet posts regarding the SX-28. I
followed Phils RF deck instructions and re-capped my radio, which is
currently still on the bench. I also have a sx-28 parts radio which
has been invaluable
for various mechanical repairs. Even after re-alignment, the radio
still needs help, however. It has a mod using a 6sn7 where the 6j5
should be, along with an internal switch. I'm guessing it is a product
detector, and I'm having so much trouble with this radio with AVC
problems and intermittent sensitivity, and s-meter issues, that I've
decided to restore the original detector with a 6j5 and try to rebuild
the section using the schematic and the parts deck as a guide. It is
an intimidating undertaking, though. The schematic shows no pin
numbers for the tubes, and the wire re-tracing is tough, since their
color coding is long gone. I'm considering buying another working
radio to use as a guide, and then when I finally have two working
sx-28's, I'll sell one of them off. However, I'm sure there are enough
production variances to not completely rely on the copy. After
numerous successful restorations on other radios, this one's been an
abyss that I seem to be falling into far deeper. Any old articles on
product detector mods for the sx-28 would be great to read to assist
in un-modding this thing!
Thanks!
Ben

[jakdedert]

wrote:
> Thanks to Phil Nelson and many others for great internet
> documentation and numerous usenet posts regarding the SX-28. I
> followed Phils RF deck instructions and re-capped my radio, which is
> currently still on the bench. I also have a sx-28 parts radio which
> has been invaluable
> for various mechanical repairs. Even after re-alignment, the radio
> still needs help, however. It has a mod using a 6sn7 where the 6j5
> should be, along with an internal switch. I'm guessing it is a product
> detector, and I'm having so much trouble with this radio with AVC
> problems and intermittent sensitivity, and s-meter issues, that I've
> decided to restore the original detector with a 6j5 and try to rebuild
> the section using the schematic and the parts deck as a guide. It is
> an intimidating undertaking, though. The schematic shows no pin
> numbers for the tubes, and the wire re-tracing is tough, since their
> color coding is long gone. I'm considering buying another working
> radio to use as a guide, and then when I finally have two working
> sx-28's, I'll sell one of them off. However, I'm sure there are enough
> production variances to not completely rely on the copy. After
> numerous successful restorations on other radios, this one's been an
> abyss that I seem to be falling into far deeper. Any old articles on
> product detector mods for the sx-28 would be great to read to assist
> in un-modding this thing!
> Thanks!
> Ben

I've never attempted something so complex as a complete rebuild of an
SX-28; but I'd suggest you go through the schematic with a tube manual
and mark all those pin numbers as a first step.....

jak

[gb[_2_]]

> wrote in message
...
> Thanks to Phil Nelson and many others for great internet
> documentation and numerous usenet posts regarding the SX-28. I
> followed Phils RF deck instructions and re-capped my radio, which is
> currently still on the bench. I also have a sx-28 parts radio which
> has been invaluable for various mechanical repairs.
[snip]
> Any old articles on product detector mods for the SX-28 would
> be great to read to assist in un-modding this thing!
>
> Thanks!
> Ben

Ben -

Bill Orr' Radio Handbooks use to have an artcile and schematic for using a
6SN7 as a product detector in 1950s and 1960s vintage receivers (that use
simple diode detectors).

Additional SX-28 resources (amny with photos)
http://xoomer.alice.it/egirland/sx28/sx28home.html

John's SX-28 Rebuild / Restoration -- from bare chassis on up !!!!
http://www.schmitzhouse.com/Johns_Electronics_18.htm

w9gb

[Chuck Harris]

gb wrote:
> > wrote in message
> ...
>> Thanks to Phil Nelson and many others for great internet
>> documentation and numerous usenet posts regarding the SX-28. I
>> followed Phils RF deck instructions and re-capped my radio, which is
>> currently still on the bench. I also have a sx-28 parts radio which
>> has been invaluable for various mechanical repairs.
> [snip]
>> Any old articles on product detector mods for the SX-28 would
>> be great to read to assist in un-modding this thing!
>>
>> Thanks!
>> Ben
>
> Ben -
>
> Bill Orr' Radio Handbooks use to have an artcile and schematic for using a
> 6SN7 as a product detector in 1950s and 1960s vintage receivers (that use
> simple diode detectors).

The later Radio Handbooks had the same detector, but they used a 12AU7
instead of the 6SN7.

Don't assume that the person doing the modification did not know what
he was doing. Adding a product detector is generally a great improvement
for a radio that is used for CW or SSB reception.

What I would do, is trace out a schematic for the area around the 6SN7,
and see what was done. It might be a great improvement to a mediocre
(by todays standards) receiver.

-Chuck

[belizando@gmail.com]

thanks for your help. I found the documentation the previous owner
did, and he used half of a 6sn7 in replacement of the 6j5 bfo tube and
labeled it an "inf. imp. detector. what would be the advantage of
doing this?
Also, Johns sx-28 restoration (and others) are the most incredible
restorations i have ever seen documented. Thanks for the links!
I've got a working sx-28 on loan to help in comparing/restoring mine.

[Chuck Harris]

wrote:
> thanks for your help. I found the documentation the previous owner
> did, and he used half of a 6sn7 in replacement of the 6j5 bfo tube and
> labeled it an "inf. imp. detector. what would be the advantage of
> doing this?

The original detector in the SX-28 was what is known as a "plate-detector".
They have an advantage in short-wave radios in that they provide a lower
load to the final IF transformer, and only minimally affect the Q, and
hence the bandwidth. The stage stays sharper, improving adjacent signal
rejection.

Plate detectors are nasty things because they introduce high distortion
both on low amplitude signals, and on high modulation signals. They are
suitable for shortwave voice quality signals, but terrible for music.

The so-called "infinite-impedance" detector, also known as a "reflex-detector"
attempts to correct the problems of the plate-detector by adding some gain,
and negative feedback. The added gain even further reduces the loading of
the detector on the final IF stage, and the negative feedback improves the
linearity of the rectification function that does the demodulation of the
AM signal. In other words, it improves the distortion on both the low and
high volume signals.

The original owner must have been offended by the poor audio quality of
the SX-28, and wanted to listen to his short-wave music broadcasts in
high-fidelity.

-Chuck

[belizando@gmail.com]

Amazing! Thanks for the information. I am going to restore the section
back to its original condition.
Ben

[belizando@gmail.com]

I have managed to keep the infinite impedance detector, and make it
switchable between it and the plate detector.
The 6sn7 was configured to use 1 section for the bfo and the other
section for the new detector, which plucks the signal from the if
transformer. After that odyssey and replacing some wrong-value
resistors, also found the oscillator fixed pads were monkeyed with,
and the band six pad had a broken solder joint. Looking forward to the
next re-alignment, also with all NOS tubes. Have made a lot of
progress, thanks for your help.
Ben