The AM Forum

Hello every one.
I have recently purchased a Hallicrafters SX28. The set is in quite good condition cosmetically but has suffered a catastrophy below the chassis.
The mains transformer is U.S. as are the LF choke, and output transformer.
All the PSU electrolytics have exploded and the Rectifier tube went with them.
Using an external PSU and output stage the set is quite lively with the exception of the 3.00 to 5.8 Mc/s band, It looks as though a previous owner has disturbed the F.P. Fixed Pad and disconnected the trimmer for this band.
I have spent several hours attempting to bring the calibration of this band back into line but with no success, the error can be as much as ½ a meg  from end to end.
Is there a set up procedure for re setting this Fixed padder ?
Regards,
John Swingewood G4CVU

Whew!  About the power supply, it looks like someone plugged 220 volts into 115 volt primary transformer windings.

As for alignment:
Perhaps the padding replacement padding capacitor is mis-marked or not the same as the manufacturer's original value. 

Is it 1/2 meg off on high freq. side of band or low side? That may be a good hint for a lot of experienced fellows on this board. They may be able to help you but may ask more details. 

normally the inductance takes care of the hi side and the trimmer handles the low end of each band or vice versa depending on he design. Talk to Slab, He'll know.

John,

The HFO should track ~455 Kcs above the marked frequency (455 is approximate, it depends upon the exact frequency of the xtal filter used in the IF and assumes IF is aligned to the xtal filter properly).  I would first make sure that whatever the previous owner did in the oscillator section is corrected and then use an external receiver to calibrate the oscillator section (listen for the signal in the external receiver at ~455 above the dial frequency).  The trimmer cap should be used to set the high end of the range and the adjustment in the coil sets the low frequency end of the range (Cg high and S3 low for the range of interest).

Make sure that no one went wild bending plates in the oscillator section of the tuning capacitor.  It has been a few years since I have been in my SX-28 but as I recall there are sub-sections in each of the main sections of the tuning capacitors and only certain sub-sections are used on certain bands.

Good luck!
Rodger WQ9E

P.S. John,

All you need to do now is to figure out how to get the Johnson Viking 1 that Ellen listed in the for sale section across the ocean.  The SX-28/Viking 1 make a great looking vintage pair!

Rodger WQ9E