On the SX-42:? Osterman’s book indicates that there were several circuit changes during the production years of this model.? One of the back stories appears to be what happened with FM radio.? From 1936-46, FM was in the 42 – 50 Mhz band.? However, the FCC required FM stations to move to the new frequencies of 88 – 108 MHz by 1948.? In response, Hallicrafters added a sixth band, to bring the coverage up to 110 MHz.? What’s really confusing is that the model number stayed exactly the same despite all the circuit changes.

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73 – Steve, KW4H

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Jacques; Holy Moly! as we used to say, no wonder we? are in no-mans’ land.

Continuing the SX-42 diversion…I saw some of the detailed changes sheets on the BAMA site ?too, and they seem to address much of this.

It seems clear that at some point there were significant changes beginning at the secondary of the 3^rd IF transformer and ongoing, and the major rev must be adding that switch so that both the AM and FM go thru the first 7H7, whatever it is really doing.

Wonder why the change/changes?? Makes one think the original circuit did not have enough amplification on AM. ??

I am still trying to make sense of the circuit; is ?the 7H7 really amplifying or what?

Getting back to Sterves Original question about why one tube, not the other: One wonders Why use a Loctal when most are octals? The S-47 uses a 6SG7 in the same spot? but is called ??“FM 3^RD IF AMP –AM DET” and it is produced at the same time.

How is your band switch doing?

Don VA3DRL

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Hi Don,

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The different produced versions of the SX-42 made me scratch my head big time…

Let’s say that in the first versions (according to the schematic 89D210) the first 7H7 is identified as a “1^st 濒颈尘颈迟别谤”.

Probably because it is used only for the 10.7 MHz IF signal and then feeds the 2^nd 7H7 tube identified as the “2^nd limiter and 10.7 MHz AM detector”.

The 455 kHz signal from the 3^rd IF transformer is not coupled to the 1^st 7H7 but applied to half of a 6H6 used as a detector, the other half of the same tube supporting the Noise Limiter function.

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In the late production versions (according to the 89D257 schematic) both the 455 kHz and 10.7 MHz signals are applied to the first 7H7 tube grid, and it’s designation was changed to “3^rd 滨贵”.

Both 455 kHz and 10.7 MHz AM signals are then “detected” by the second 7H7 tube (control grid to cathode) and the right part of the 6H6, used as a detector in the first versions, is completely disconnected.

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And A LOT of components values and circuitry topology changed between the first and the last SX-42 made.

The one I restored last spring was something of a mix between the first and the last versions already (stamped May 12, 1947) and was more or less conform to the XXXX production run, so I decided to rebuild it to be compliant to the schematic 89D257E.

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Let’s figure here that there was 20 engineering changes applied successively to the production runs X, XX, XXX, XXXX and 1, updating the related schematics from 89D210 to 89D210K.

Then the 89D257 come, with more engineering changes creating versions from A to G.

The 89D257F required a change to the SW 1 (the Band switch), sections K and KK, and this is why I had not been able to implement this one.

But the Rev. G change, which added a 6.8 ohms resistor (R108) in series with the 6H6 noise limiter heater was doable.

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At the end, I do not know if my own “version” of the SX-42 work better (or worse) than any other…

But, at least, it works on all bands and modes !

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Jacques ?and the gang

Do you/we believe that an early version of the schematic was much the same as later versions BUT the old drawing was in error by calling the 7H7 a Limiter, when all the time it was an IF amp?

don