[Hallicrafters] Need Help With SX-101 USB Problem

Thu Jul 16 01:14:08 EDT 2015

I was referring to the USB sideband crystal.

I don't own an SX-101 but just took a look at the schematic on BAMA. The sideband crystals are 1600kc & 1700kc. The 1700kc crystal is the one needed for USB reception. You could tune those frequencies on an AM band radio and verify if the USB crystal is actually running or not when you lose the USB function. I was thinking that the crystal may be not starting back up after you switch the sideband selection, due to perhaps some component changes over time and/or crystal aging.

The V12 12AT7 (Second Conversion Osc.) dual-triode uses one half of the tube for the USB crystal oscillator and the other half for the LSB crystal osc. In other words the USB and LSB oscillators are two completely separate circuits, so one might easily work when the other doesn't. (Rather odd, that is, I would have thought they'd just swap crystals and use only one triode for the osc.) So, perhaps you should try a couple other 12AT7 tubes. It may be that the tube half for the USB is weak.

If that doesn't help, verify that all the components associated with pins 2, 3, 4 of V12. According to the schematic, those are the half of the tube used by the 1700kc crystal. See if any resistor values changed a lot & maybe replace the cathode resistor bypass cap at pin 3.

T9 is the coil/transformer for tuning up the USB & LSB oscillators. It may be that the USB side needs to be peaked up. The schematic says the lower slug in the T9 can is the one that tunes the 1700kc osc. Or perhaps there is a fault in the tuned circuit parts in the can if it won't peak up.

This step is for the not-too-weak... swap the two crystals (temporarily) and see if the baulky crystal startup moves with the 1700kc crystal or if it stays on the same side of the tube circuit. If nothing else helps, this would verify that its the fault of an aging crystal.

Hope this makes sense. Perhaps others will have better suggestions for testing the oscillator.

Wayne

 WB4OGM

-----Original Message-----
From: richard <ramnaz at peoplepc.com>
To: hwhall <hwhall at compuserve.com>
Sent: Wed, Jul 15, 2015 10:23 pm
Subject: Re: [Hallicrafters] Need Help With SX-101 USB Problem

Hello Wayne.
How would I know & are you talking about the Crystal in the tube
in the back left side??
Thanks again for your note.
 Richard

-----Original
Message-----
>From: hwhall at compuserve.com
>Sent: Jul 15, 2015 9:19 PM
>To:
mark.k3msb at gmail.com, ramnaz at peoplepc.com
>Cc:
Hallicrafters at mailman.qth.net
>Subject: Re: [Hallicrafters] Need Help With
SX-101 USB Problem
>
>I get the impression the problem might be related to a
crystal that has become sluggish, and needs a lot of excitation to get
started.
> 
>
> Wayne
>WB4OGM
>
> 
>
>-----Original Message-----
>From:
Mark K3MSB <mark.k3msb at gmail.com>
>To: richard <ramnaz at peoplepc.com>
>Cc: List
Hallicrafters <Hallicrafters at mailman.qth.net>
>Sent: Wed, Jul 15, 2015 5:33
pm
>Subject: Re: [Hallicrafters] Need Help With SX-101 USB Problem
>
>
>Hi
Richard
>
>I have an SX-101A and have exactly the same problem
you're
>experiencing.
>
>I've had my receiver for about 15 years and it
started to do
>this about 5
>years ago.  I swapped the USB crystal with the
one in my SX-111
>and the
>problem went away.
>
>It came back last winter.  
USB works fine when
>the radio is turned on,
>but cuts out after a while.  
Switching between USB and
>LSB sometimes
>helps;  mostly not.
>
>I recapped
my receiver several years ago
>and also did the cleaning with
>DeOxit.   
Obviously, that didn't help.
>
>I
>think the only way to fix the problem is
with an alignment as K9AXN
>mentioned. 
>Parts go out of tolerance after 50
years, and I'm not
>surprised part of the
>converter needs tweaked.     I
suspect heating of a
>component and the resultant
>change in value is what is
happening.   What
>you could do it try some freeze
>spray, but the offending
part may be in the
>converter subchassis which would
>make it rather difficult
to get to.
>
>I don't know if you need an entire
>alignment or not;  it's
been many moons
>since I cracked the manual.
>
>73 Mark
>K3MSB
>
>
>
>On
Wed, Jul 15, 2015 at 5:07 PM, richard <ramnaz at peoplepc.com>
>wrote:
>
>>
Hello.
>> Have a SX-101 that  when I go from USB to LSB & then want
>to go
back to
>> USB it is Dead but if I pull the 12AT7 tube out & put it back
>in
then the
>> receive in USB comes back on or if I turn the on & off many
times
>then the
>> USB receive comes back on yet the LSB never goes out.. Any
body have
>this
>> problem & if you fixed it can you tell me how? I have even
put a small
>fan
>> inside & that does nothing .
>> Thanks Much
>> 
Richard
>>
>> Richard
>Morris
>>
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Richard Morris