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THE AM BULLETIN BOARD => Technical Forum => Topic started by: AJ1G on May 14, 2018, 07:29:38 AM

Title: SX-101 MK 1 Mysteries
Post by: AJ1G on May 14, 2018, 07:29:38 AM
While checking out a Hallicrafters SX-101 over the weekend, it initially came up and ran very well.  However, after several hours of use it developed a very odd frequency instability consisting of a low frequency cyclic variation of received CW and signals of something less than 100 Hz, with a nominal period of 1 second.  Also detectable on the internal calibration signal.  Seems to vary in frequency deviation from band to band.  Seems to be associated with the first conversion oscillator. Oddly enough, although it is very evident on every other band switch position including 160 meters, it is NOT present on 10 meters.  10 meters in the short term does not show any evidence of the 1 Hz period frequency oscillation, but does show some long  term slow drift.  I am going to do the obvious things like look for any periodic variation of DC voltages, clean band switch contacts with deoxit, and check and sub in a replacement, according to the 101 MK 1 schematic from BAMA, 12AU7A 1st heterodyne oscillator and cathode follower (provides 1st LO output for potential use of transmitter frequency control).

And therin lies the second mystery.  Where the subject 12AU7A dual triode is supposed to be according to the manual (the chassis tube label is unreadable/missing) is a 12BY7A sharp cutoff pentode.  Whatís up with that?  A Halli backit design change either factory or owner installed?  Or a design mod developed by someone and implemented by individuals after publication in ham magazines of the day?

Odd to say the least.  Other than this wierd low frequency FM/SBE effect, the radio works very well so far.

Title: Re: SX-101 MK 1 Mysteries
Post by: AJ1G on May 14, 2018, 07:59:01 AM
Just figured out the second mystery.  Checked the BAMA 101 MK III manual out.  The set here apparently is not a MK 1, itís at least a MK III.  Couldnít open the BAMA MK II manual which is in a wierd file format.

The MK III indeed uses the 12BY7A as the first conversion LO, they apparently also dispensed with the cathode follower buffered 1st LO output function as well. Also explains why the S meter adjust pot on the set here is on the rear panel where the LO output jack would be on the MK 1.

Now off to try to determine MK type by set S/N if possible.

Title: Re: SX-101 MK 1 Mysteries
Post by: AJ1G on May 14, 2018, 07:48:18 PM
Mystery solved....funky 12BY7....rock steady with a replacement temporarily robbed from the HT-32.  Iíll send Tron the funky one for another SBE VFO!

Title: Re: SX-101 MK 1 Mysteries
Post by: nq5t on May 14, 2018, 11:02:07 PM
Couldnít open the BAMA MK II manual which is in a wierd file format.

You can find DJVU desktop viewers at under the downloads&resources link.

Title: Re: SX-101 MK 1 Mysteries
Post by: K9DXL on May 15, 2018, 11:34:42 AM
On your SX-101 MKIII, the 12BY7 is on all the time the receiver is plugged in, a 1958 solution to achieve rock-solid stability.  Operating with the filament on but no B+ eventually causes "cathode poisoning" or "cathode stripping" the end result being what you see. The '101 is a great receiver, but I have it plugged into a power strip that I turn on a few hours before operating, such as a scheduled net. Or leave it plugged in and stock up on 12BY7's!

Title: Re: SX-101 MK 1 Mysteries
Post by: AJ1G on May 15, 2018, 08:39:32 PM
Thanks for the insight.  Iíve been wondering what the physics was behind what was going on in the circuit to setup the wierd slow seemingly nearly sinusoidal frequency modulation. I once had a bad chirp on the CW note of a TCS that was eliminated simply by replacing the 12A6 master oscillator tube.

Title: Re: SX-101 MK 1 Mysteries
Post by: MikeKE0ZUinkcmo on May 15, 2018, 11:56:01 PM
...Couldnít open the BAMA MK II manual which is in a wierd file format...
It's available at the bottom of BAMA's home page. The DJVU encoding format has been around for many years and its popularity is second only to the PDF format I'd suspect.

Title: Re: SX-101 MK 1 Mysteries
Post by: AJ1G on May 16, 2018, 05:06:18 AM
Thanks re the other file format.  I came across a .pdf MK2 manual elsewhere, was able to verify the one here is a MK3 because the MK2 still had the 12AU7 1st conversion LO of the MK 1 vs the 12BY7.  There is a MARK stamped on the chassis forward of the calibrator sub chassis, but the number is no longer visible.  Itís not a 101A because it still covers 160 and 11 meters.

Was interesting when checking out the 11/10 meter band range the other day that while 11 was full of caw mon good buddies,  10 was totally empty. My conclusion from that observation is that hams know we are in a solar minimum, so they KNOW 10 will have no propagation, but the CBers just keep trucking because they donít have any place else to go.

Title: Re: SX-101 MK 1 Mysteries
Post by: WB6NVH on May 16, 2018, 11:02:44 AM
I have the same problem and I assume everyone else with this receiver does as well - - put a new 12BY7 in and some months later it's no good thanks to the filament being lit 24/7.  Thus as above, I have to shut power off with a power strip and just turn that on when I am using the receiver.  I have always wondered why these didn't start coming back to the dealers for this when they were new.

There was an article about making a solid state 12BY7 for Drake gear back in an old issue of Ham Radio Magazine and I was thinking of doing that for the SX-101, however there doesn't seem to be that much drift when you don't leave the 12BY7 filament powered anyway.

Title: Re: SX-101 MK 1 Mysteries
Post by: kt4ae on May 23, 2018, 05:31:45 PM
I finally got tired of replacing the 12BY7A's. Cost aside, each one is different and once you find a good one, you don't want to waste it.  So, I wired the separate filament transformer through the power switch. At the same time, I disconnected that chassis heater which is on all the time. Frankly, I didn't find that it took any longer to stabilize afterwards than it did before, about twenty minutes.

Harry, KT4AE
Watkinsville, Georgia
AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands