[Hallicrafters] SX-42 advice

Wed Mar 28 22:45:39 EDT 2018

Well list -- I don't know if there's anyone out there that can help me, 
but this has really got me stumped.  In search of the elusive SX-42, 
here's what I've been able to establish:

First, shotgunned every single component in the circuit, INCLUDING 
replacing the tube socket.

Disassembled switch 3 (Function switch) and cleaned and inspected all 
contacts and wafers.  Replaced R64, R66, and R72 (R64 was 1.7M, but 
others were in tolerance.  Replaced them anyways.)

Replaced C102, C103, and C104 (BFO can).

Replaced C99, C100, C104 (Bypass caps)

Replaced R69-71.

Disassembled shroud on C105, inspected and cleaned rotor and plates.  
Ensured ground connections were solid.

Took voltage readings on 7A4.  Plate = 148 v, Cathode 28 v. Checked with 
11M DMM and my TS-505 vtvm.

Now I have 48 volts on the GRID!  And if I disconnect R71 (47K to grid), 
I can get BFO action.

Thinking to eliminate as much internal wiring as possible, I lifted one 
end of R70 and R71 (ends going to switch 3AA) and shorted them directly 
together.  BFO killed and grid voltage was back at 48 volts.

Tried several 7A4s, all behave the same.

A part of me is thinking, "just eliminate R71", but obviously it was 
there for a reason.  But I've been staring at this schematic now for 
four days, and I just can't understand where the high grid voltage is 
coming from.  I suppose it is possible that I have installed a bad "new" 
capacitor, but I don't even know where to begin looking at this point.

Really looking for some expert advice here.  This receiver is working 
very nicely now, and after repairing stuff around the bandswitch, I 
refuse to admit defeat by a simple triode oscillator.

Hoping someone points out my stupid error.

Jeff

WB3JIH

On 3/25/2018 5:09 PM, Richard Knoppow wrote:
>    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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