[Hallicrafters] SX-101 Mk III Overload / Desense

[Andrew, Jim - BD]

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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