[Hallicrafters] SX-101 Mk III Overload / Desense

Sat Dec 4 14:00:12 EST 2010

Hi Jim,
	I suspect that grid voltage on the mixer is your problem.  Don't have 
the schem handy, but at 0 volts on the grid, it's running wide open, 
needs that -14 bias.  Check into why it's not -14.  Is there a leaky 
coupling cap from the previous stage, has the AVC to grid resistor 
opened up?  look in that area.  Just picked up the 101A skem, if the mk 
iii's same, ck R8, 1meg, and C20, 51pf.  You don't say which grid, pin 1 
or 7, if pin 7, ck the coupling cap from the 12BY7, C42.
	The 101A manual says grid 1, pin 1 should be -1.5v, & grid 3, pin 7 0v. 
  That's with AVC on & sensitivity at 10, max. gain.
73,
Al, W8UT
www.boatanchors.org
www.hammarlund.info

"There is nothing -- absolutely nothing -- half so much
worth doing as simply messing about in boats"
Ratty, to Mole

On 12/4/2010 12:58 PM, Andrew, Jim - BD wrote:
> Hi Carl,

> The only other significant anomaly I found was grid voltage on V-2 the 1st Converter Oscillator is 0 versus -14.5 VDC spec. It stayed on 0 for all bands. The 12BY7 tested OK, but I swapped it out anyway and de-ox'ed the pin sockets as well. I thought about a carbon path, but I cleaned the area socket top and bottom and can't see anything unusual. Seems as well that this would create a problem during all phases of reception if it were the issue.
>
> I have to keep reminding myself that I am only having this problem because the receiver is such a red-hot performer.
>
> 73,
>
> Jim, K5UI
>
>
> -----Original Message-----
> From: hallicrafters-bounces at mailman.qth.net [mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of Carl
> Sent: Friday, December 03, 2010 6:59 PM
> To: Andrew, Jim - BD; Ian; hallicrafters at mailman.qth.net
> Subject: Re: [Hallicrafters] SX-101 Mk III Overload / Desense
>
> Id say its symptomatic of a tube in the AVC path with leakage that is
> driving the grid positive with a strong signal. This only gives you 2
> choices, V-1 and V-4 and with V-1 being a sharp cutoff tube it doesnt work
> worth a damn anyway.
>
> Also check the resistors in both screen circuits.
>
> Carl
> KM1H
>
>
> ----- Original Message -----
> From: "Andrew, Jim - BD"<Jim.Andrew at conocophillips.com>
> To: "Ian"<ianwebb5 at comcast.net>;<hallicrafters at mailman.qth.net>
> Sent: Friday, December 03, 2010 4:58 PM
> Subject: Re: [Hallicrafters] SX-101 Mk III Overload / Desense
>
>
>> Hello Ian,
>>
>> Thanks much for the note and the thought on this.
>>
>> Well, I should have been more clear on the description of the AVC
>> performance - should say it works as I would normally expect based on
>> on/off performance with actual signals.
>>
>> AGC plate voltage is -0.2VDC under recommended test conditions. (Manual
>> says this should be -2.0.) It goes to -2.0 or less if I plug an antenna
>> in, then back to -0.2 VDC if I tune across a strong signal when signal
>> blanks out. Neither SSB selector/pitch nor the s-meter sensitivity pot
>> have an effect when the signal is blanked.
>>
>> R-90, the 220k resistor at the plate measures 252k which doesn't seem
>> outrageous. Heading further upstream, R12 a 100k resistor which couples to
>> T1 measures 97k. I had previously peaked T1 per the manual and it is still
>> near peak. Heading down another branch, R-1 a 1M resistor on the RF Amp
>> grid measures 1.05M.
>>
>> Hmmm ...
>>
>> Jim
>>
>>
>>
>>
>>
>> -----Original Message-----
>> From: Ian [mailto:ianwebb5 at comcast.net]
>> Sent: Thursday, December 02, 2010 7:54 PM
>> To: Andrew, Jim - BD; hallicrafters at mailman.qth.net
>> Subject: RE: [Hallicrafters] SX-101 Mk III Overload / Desense
>>
>> Jim,
>>
>> Well, wouldn't the with and without AVC mean that the "with AVG" isn't
>> really working????
>>
>> What did you find when you measured voltages in the AGC circuitry?
>>
>> Ian, K6SDE
>>
>>
>>
>>
>>> -----Original Message-----
>>> From: hallicrafters-bounces at mailman.qth.net [mailto:hallicrafters-
>>> bounces at mailman.qth.net] On Behalf Of Andrew, Jim - BD
>>> Sent: Thursday, December 02, 2010 5:42 PM
>>> To: 'hallicrafters at mailman.qth.net'
>>> Subject: [Hallicrafters] SX-101 Mk III Overload / Desense
>>>
>>> Hello All,
>>>
>>> I have been restoring an SX-101 Mark III. The good news - this is a hot
>>> receiver, looks great, and with a small exception everything works. The
>>> bad news - when I tune through a strong signal ( like the crystal
>>> calibrator output) something overloads in the receiver and the signal
>>> goes to near zero. I can recover the signal by simply tuning away (10-20
>>> kHz) or by turning down the RF gain. The problem is noticeably worse on
>>> LSB than USB even though I recalibrated the BFO to 50 kHz and signal
>>> strengths are similar for both sidebands.
>>
>>> At first I suspected an AVC problem, but the event happens both with and
>>> without the AVC engaged. It sounds precisely like what happens during
>>> alignment of the 1600 and 1700 kHz 2nd Converter Oscillators so I have
>>> gone through that procedure several times, only to find I have
>>> aggravated the situation.  Adjusting the crystal activity slug while the
>>> signal is muted does not seem to help.
>>>
>>> Have only peaked the 50.5 kHz IF by ear - wondering if that might have a
>>> role in this. I have a replacement 12AT7 2nd Converter Oscillator tube
>>> on order, but in order to properly troubleshoot I would appreciate
>>> comments anyone might have on the circuit theory, either in this stage
>>> or elsewhere, that might cause such a problem.
>>>
>>> Thanks in advance for any thoughts or similar experiences you might have
>>> had.
>>>
>>> Two steps forward, one back. At least the tubes keep the shack warm.
>>>
>>> 73,
>>>
>>> Jim, K5UI
>>>
>>>
>>>
>>>
>>>
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