[TMC] R5007/FRR-502 Tuning Unit Info

[Tom Brent]

In June of 2004 there was a lengthy discussion on this list regarding a
"different" tube line-up in these receivers and I described changes to the
receiver subchassis in my posting a few weeks ago. This was a modification
to some of the standard units in service with the Royal Canadian Air Force
and there is a similar story to tell about the tuning units.

TMC manufactured 8 different tuning units to cover the frequency range of 50
KHz to 32 MHz. With one exception, they all utilized 6AG5 tubes for the HFO
and both RF amps: TN-5014/FRR-502 (16-32 MHz) substituted a trio of 6AK5's.
It should also be noted that the 3 lowest frequency units (50-100, 100-200
and 200-400 KHz) have a 4 position crystal filter. Like the receivers with
the tube variation, some tuning units employed with the RCAF were modified
and use a 5842 and 6BZ6 for the first and second RF stage, respectively. If
you have a tube manual handy, take a look at the specifications for the
5842. It is a very high performance tube and explains why additional
shielding  is installed across the 1st RF amp tube socket in these modified
units. The 5842 is also treasured by the glass audio crowd and that
explains, at least in part, why they are so pricey.

These receivers are fun to play with; they are easy to work on and provide
good performance for general listening. They are not without their
shortcomings however. The frequency readout is abysmal but thankfully this
is easily solved in this day and age by an outboard frequency display such
as the ones available in kit form from AADE for a very reasonable price. A
BNC connection is already on the back panel, ready for you hook it up. The
other big problem is the tuning rate. Remember, these receivers were not
designed for band scanning, they were set up on one frequency and locked
there for days or weeks. For our use however, the tuning is "touchy", to say
the least. There are a number of ways to get around this. A small DC power
supply (perhaps tapping off the filament power) could be used to control the
reactance tube for its intended purpose of remote frequency control. At
least one of our group has added varactor tuning. Perhaps the easiest method
however, involves a modification to the crystal tuning control on the front
panel. If you do not use crystal control for the HFO, this 50 pf. variable
can be connected in parallel with the main tuning capacitor to provide fine
tuning. On the 2 - 4 MHz. tuning unit for example, disconnect the lead from
the ungrounded side of C532 (crystal tuning). Solder 1 lead of a 5 pf.
capacitor to C532. Solder the other lead to the ungrounded side of C527. Set
C532 to mid-range and do a realignment.

Although I'm lucky enough to own 19 of the tuning units, I do not have any
of TN-276 (FFRD-3) covering 200-400KHz or TN-277 (FFRD-3M) covering 485-515
KHz. To make the family complete, I would be willing to trade  my extra HF
units for the LF ones I am missing.

Happy tinkering!!!