[Hallicrafters] SX-42 - trouble shooting weak and intermittent performance.

[DJ via Hallicrafters]

Hi,

Yeah, I caught that "extra" from the digest but it was too late, anyway thanks...

Did the PHONO input thing... audio is working!

I guess its off to the RF and or the IF. These schematics are horrible.

I'll see what else I can accomplish today.

Keep the ideas coming. Anybody have voltage specs for the tube pins?

Best Regards,
Douglas

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On Sat, 7/25/15, Roy Morgan <k1lky68 at gmail.com> wrote:

 Subject: Re: [Hallicrafters] SX-42 - trouble shooting weak and intermittent performance.
 To: "DJ" <fourzulufive at yahoo.com>
 Cc: hallicrafters at mailman.qth.net
 Date: Saturday, July 25, 2015, 5:27 PM
 
 
 On Jul
 25, 2015, at 6:55 AM, DJ via Hallicrafters <hallicrafters at mailman.qth.net>
 wrote:
 
 > ...I broke out
 the SX-62 and the SX-42 and fired them up…. The 42 is not
 exactly dead but is experiencing some problems.
 
 Douglas,
 
 My guess is that the SX-42 suffers from one
 fault, since you did a complete job of overhaul earlier.
 
 > ...I have my trusty
 Heathkit VTVM so I should be able to hone in on the problem
 and get her back up to speed.
 
 I think so, too.  The VTVM is perhaps the
 single most valuable test item.
 
 > 1. Audio intermittent and weak. Audio is
 about 1/2 to a 1/4 of what it should be when normal.
 Sometime the audio just just comes on but never at full
 
 Here is an odd idea:  It
 seems that you may not have a signal generator, audio or
 RF.  Run a (shielded) wire from the top of the volume
 control in the SX-62 over to the top of the volume control
 in the SX-42.  Pull the previous tube in the SX-42 and see
 if you can get similar audio from both radios, fed from the
 working SX-62.  (Memory is faint: do either of these radios
 have a “Phono input”?  If so, that could be used as
 audio input in the SX-42.)  This will tell you if the
 trouble in the SX-42 is in the audio section or earlier in
 the radio.
 
 > 2. Weak
 reception/ insensitivity.
 
 > 3. S/FM meter appears to not be
 working.
 
 These two combined
 hint at a trouble in the RF or IF section.
 
 > So far I've replaced
 all the tubes with the ones from the 62 and back again.
 These all seem to be fine.
 
 A very good starting step.
 
 > ...the 7A4 has been replaced with a 7G7
 and modified accordingly. The radio works with this and I
 left it alone rather than change it back.
 
 I see from your website that
 the 7A4 triode was the BFO tube, and the 7G7 pentode would
 produce more gain and perhaps more reliable operation.  If
 it used to work as is, just see if it is working still. 
 You report that this stage also is involved in the S meter
 in FM mode.  Maybe some one on the list will have info on
 that modification to help sort it out.
  
 
 PS: Your post to the list
 contained a number of messages. It’s good to  edit out
 the parts of a Digest you receive, or any message you reply
 to, that don’t relate to the topic at hand to avoid
 overloading mail systems of folks who have limited band
 width.
 
 Roy
 
 Roy Morgan
 k1lky68 at gmail.com
 K1LKY Since 1958