[Hallicrafters] SX-42 advice

Sun Mar 25 17:09:48 EDT 2018

    I understand about "dragging". Compare the voltages with your 
meter. OTOH, since your other receiver does not behave the same 
way it is already telling you there is a problem. FWIW 
comparative measurements are one of the most powerful trouble 
shooting tools.

On 3/25/2018 12:55 PM, Jeff wrote:
> Richard -
> 
> Understood.  I actually have another working SX-42 here in the 
> shack -- I've just been resistant to the idea of dragging IT out 
> and onto the bench.  They ain't small!
> 
> I'm doing my voltage checks with a TS-505D/U VTVM...
> 
> I'm going to look at the detector circuit next.  Was just baffled 
> by the voltages at the BFO.
> 
> 
> Jeff
> 
> 
> 
> On 3/25/2018 3:46 PM, Richard Knoppow wrote:
>> 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
>>
>>
> 
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL