[Hallicrafters] SX-42 schematics.

[Chuck McGregor]

At 06:13 PM 2/17/2003 -0800, you wrote:
>   I'm (trying) to restore two SX-42s and am having a little difficulty getting the tube voltages correct and find the resistances at the tube pins are not correct either.
>My SX-42 have serial numbers HA-73528 and HA-76643 and I'm using schematics from SAMS (dated 8/48) and BAMA.
>   My problem is that the two schematics do not agree and in fact I think the SAMS has an error in that supply voltage is not applied to the first limiter (7H7) plate.  As a consequence the S-meter does not respond correctly and probably other effects occur too.
>   I think the s/n are probably a latter series (but I'm not sure).  
>Does anyone have a schematic other than the BAMA and the 8/48 SAMS that might resolve some of my problems.  In advance, thanks.
>Skip Magnuson


Skip-
I'm about halfway done with my SX-42.  I'm finding many of the 1/2 watt composition resistors read from 120% to 180% of their marked value, and many have marking colors distorted by overheating.  Not only the waxed paper caps but many apparently installed in the 1950's-60's are leaky.  
I have schematics from BAMA,  Ryders, and Sams. 

Looking at the schematic from Ryders it appears to me that the plate voltage  path for the 7H7 is thru R55, past C87 bypass to ground, past R54 which feeds the 7H7 screen, thru R45 to  Switch 1, deck AA - where it is tied to the +250volt dc buss only in switch positions 5 and 6, corresponding to the two top bands.   If C87 has shorted to ground, these switch contacts may be burned.

Looking at the Sams schematic, first I note that the 7H7 plate resistor is 10K rather than 33K;  the screen resistor is 47K rather than 100K ,  the B+ to the 7H7 does not appear to be switched but rather supplied from one end of the S-meter pot.  But, most important, there is no wire connecting anything to the junction between part 181 the power supply choke and  part 179, the dropping resistor for the low regulated b+ buss.  Apparently MISSING is a connection from the right hand end of the S-meter pot to the +250v unreg buss at the junction of the power supply choke and the dropping resistor (179).

Now, squinting at the poorly digitized schematic from the Halli manual (BAMA), the resistor values are the same as the Sams schematic.  The connection between one end of the S-meter pot and the power supply is present, and the B+ to the 7H7 is unswitched.

Also, in the Ryders schematic, the second diode in the 6H6 noise limiter is the AM detector, while in the Sams and BAMA schematics the first 7H7 apparently is acting as a linear 3rd IF in the lower bands with the second 7H7 acting as the AM detector, and the second diode of the 6H6 unused.

So it looks like the Ryder's schematic is from one version of the SX-42,  the Sams and BAMA schematics are from a distinctly separate version, and the Sams schematic seems to have a critical wire missing.  Don't know which version is earliest.

Just for a check, I took my flukemeter over to my belly-up SX-42 and checked.  On mine, the second half of the 6H6 is used, and the B+ to the 7H7 is switched, and there is no switching in the cathode circuit of the 7H7, so the Ryders schematic matches my radio.  BTW my serial number is HA79493 .

Hope these rambling are useful to you.

--Chuck N7RHU