[Hallicrafters] SX-101 Mk III Overload / Desense

Sat Dec 4 12:58:26 EST 2010

Hi Carl, 

Thanks for the note and the ideas here. 

I pulled and tested both tubes on my Hickok (mutual conductance) tester - both tested good. I swapped out a couple of other 6BA6's for V-4 with no effect. Will try a different 6CB6 when I can round one up. 

The 15k screen resistors tested 15.6k and 15.8k so they look OK. The 47k resistors tested 61k and 54k. They are probably fine, but I replaced both anyway since I already had them clipped. No change. 

Your note motivated me to make another complete round of voltage measurements.

V-1 and V-4 are on-spec except the cathode voltage is high for both: 2.7 versus 1.6 for V-1 and 1.7 versus 1.1 for V-4. Not sure if this is a cause or an effect. Cathode resistors were good on both. I followed this upstream to the RF gain pot which runs smoothly 0 to 10.1k. R-60 looks OK as well at 360k versus 330k. Cathode voltage runs up smoothly as the RF gain is decreased, but I have not been able to get it below the values noted above. 

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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