Crystal Filter

The crystal filter unit came from a parted HRO-7.  It is essentially the same circuit as that of a late model HRO-5TA1 with a 6-position selectivity switch and variable phasing control; earlier HROs (and FB-7 superhet) had a variable capacitor for selectivity control instead of discretely switched selectivity.

As with the BFO and IF cans, the crystal filter unit was disassembled to replace the original leads with cloth covered wire.  Those exiting below the chassis Continue reading …..

Chassis – Cans, Shielding, etc.

I acquired some sheet aluminum panels cut to size from a local welding shop for RF shielding, two(2) pieces measuring 5×4.5 in. have a 1/2 in. lip at one end, bent at a right angle, to attach to a larger 14×4.5 in. panel that runs along the middle of the chassis behind the RF stages.  While I contemplated using a lighter steel material, the heavier aluminum will help to stiffen the chassis a bit.

I started painting these panels, one side at a time on consecutive days.  The pieces were lying flat atop small pieces of wood on a covered table on the backyard porch.  However, I discovered an unforeseen problem when gathering the pieces to bring into the house before dark on the second day:

The mist created by the paint stream had coated the underside that was previously painted, causing a milkish appearance where the panel was not setting on blocks.  Continue reading …..

Chassis – Mount Tube Sockets / Variable Caps

Yesterday the mounting hardware arrived.  I had ordered a number of different length 6-32 zinc-plated machine screws ranging from 3/16 in. up to 5/8 in. from McMaster Carr.   While BoltDepot was cheaper for the same machine screws, McMaster had narrow 6-32 zinc-plated nuts and 6-32 Black-Oxide truss head machine screws (for the front panel) that BoltDepot did not carry.   The narrow nuts (1/4 in O.D.) work better for mounting certain parts than standard 5/16 in. O.D. nuts.  After factoring in shipping, it was more economical to just get everything from McMaster.

The tube sockets were mounted using 1/2 in. 6-32 screws, #6 external tooth washers, flat washers, and standard 5/16 in. O.D. nuts.  The National SW-3 Continue reading …..