[Hallicrafters] SX-42 advice

[Jeff]

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



On 3/12/2018 10:53 PM, Jeff wrote:
> Hi All -
>
> I'm rebuilding a SX-42 for my father -- was his first receiver. Making 
> fine progress, but run into a couple of issues that no doubt have been 
> solved by someone.
>
> First, is there any way to replace the wax paper caps that live 
> underneath the bandswitch - that appear to have braid for leads -- 
> WITHOUT removing the bandswitch?  I've gotten all of them except for 
> the two that I can see.
>
> Second, it's deaf as a post at the moment.  Resistor and capacitor 
> replacement are making a difference each time I test my work, but 
> still very poor.  But the real issue is terrible distortion on signals 
> using the BFO -- not present in AM.   The S-meter also behaves 
> erratically as well -- going to try another 7A4.
>
> Any advice on tracking down the BFO fault -- or any special tools or 
> skills on those buried capacitors?
>
> Thanks!
>
> Jeff
>
> WB3JIH
>
>
> ______________________________________________________________
> Hallicrafters mailing list
> Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Hallicrafters at mailman.qth.net
>
> List Administrator: Duane Fischer, W8DBF
> ** For Assistance: dfischer at usol.com **
>
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html