Hi Don,

?

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.

?

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.

?

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.

?

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 !

?

?

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