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

[Roy Morgan]

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