[Hallicrafters] SX-42 advice

[Richard Knoppow]

    This is an example of where having a known properly working 
unit would be useful for comparison measurements.
    What type of meter are you using? I have not looked at the 
handbook to see what they specified but if they generated the 
values using a standard VOM and you are using an electronic meter 
its quite possible the voltages you are getting are much higher. 
This is only something to check.
     Distortion when using a BFO is usually not a result of the 
BFO waveform being distorted but something happening in the 
detector. Normally, for CW and SSB on conventional diode 
detectors the level of the IF signal must be quite low. Probably 
you know that. If the BFO injection level is lower than normal it 
may require very low signal levels into the detector.
    It was common in pre-SSB receivers, i.e., those using 
conventional diode detectors for CW, to limit the BFO level into 
the detector to where it did not tend to desensitize the 
receiver. Some receivers had a BFO level adjust meant to be set 
for the best signal to noise ratio. This was also to keep the BFO 
from operating the AVC in receivers where the AVC is not disabled 
in CW. Product detectors are far superior but don't generate AVC.

On 3/25/2018 12:33 PM, Jeff wrote:
> Thank you to everyone that replied to my questions.  I picked up 
> a couple of the appropriate tools, and got all of the capacitors 
> (including all the bandswitch ones) replaced.  I also now have 
> decent sensitivity and the S-meter seems tamed for the moment.
> 
> In going after the distortion on the BFO though, I have run up 
> against a problem that is really frustrating me.  I pulled the 
> BFO coil and replaced all of the mica caps in it with modern 
> stuff, redid the wires (as they were all a uniform brown color 
> now) so I could read them and reinstalled.
> 
> The problem is that according to all of the documents, I should 
> have around 17 volts on the grid of the 7A4 -- I have over 40. 
> Same with the Cathode -- same voltage.  Plate voltage is 145.
> 
> I have no BFO action on the radio now at all.  A 10X scope probe 
> shows no signal, but a 1X probe shows a strong sine wave.  I run 
> the output of the BFO (before the 7pf capacitor into pin 5 of the 
> 6H6) into the frequency counter and I can tune the slug to 455 Kc 
> with no problem.  In fact, if the scope probe is connected to the 
> 7pf capacitor, I get BFO action on the radio...
> 
> There was someone who indicated the same problem on a ARF thread, 
> but his solution was to just put in a bigger capacitor on the BFO 
> output.  I suppose I could do that, but it doesn't sound right 
> for some reason.
> 
> I have the original manual and schematic, but there are no 
> voltage charts.  I'm going by the SAMS and SX-62 manuals.  I 
> don't think these voltages are normal though, and likely explains 
> why I'm having trouble. Tried multiple 7A4s as well -- same deal.
> 
> Anyone have any suggestion as to what might be causing this, or 
> where to begin looking?
> 
> Thanks!
> 
> Jeff


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL