[Hallicrafters] SX-42 advice

[Jeff]

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
>
>