[Milsurplus] [ARC5] ATD Transmitter Longwave Tuning Unit Repair

[Mike Morrow]

I wrote:

>> CRR-47207 TUs should have PA PLATE SWITCH position 4
>> (360-450 kHz) wired to a higher inductance tap than than
>> the tap for position 3 (295-360 kHz).  That's what the
>> CRR-47207 schematic shows, and that is how my #317 is
>> wired.

Dave wrote:

> Agreed.  I think it must be a factory wiring error.

No, I'm saying the opposite: There is NO wiring error in your 47207 nor my 47207.

PA PLATE SWITCH J is wired  to the tapped PA coil as follows:

Pos 1 200-240 kHz - Max inductance + Max capacitance, 2000 pF
Pos 2 240-295 kHz - Inductance 1 tap below max + Max capacitance, 2000 pF
Pos 3 295-360 kHz - Inductance 3 taps below max + Max capacitance, 2000 pF
Pos 4 360-450 kHz - Inductance 2 taps below max + Half capacitance, 1000 pF
Pos 5 450-540 kHz - Inductance 4 taps below max + Half capacitance, 1000 pF

It may seem odd that the higher frequency position 4 uses more inductance than the lower frequency band 3.  Going from switch J position 4 to position 3 not only uses a different coil tap, but also the PA capacitance is doubled.  The doubled capacitance requires less inductance in position 3 to resonate 295-360 kHz than position 4 with half capacitance requires to resonate 360-450 kHz.  So...it is 100 percent technically correct that the coil taps for position 4 and 3 are wired to switch J as our 47207s are, exactly the schematic shows.  FWIW, the four other TUs except one also use non-sequential coil tap wiring configuration as PA capacitance is switched in or out.

So...NO wiring errors are anywhere. :-)

>> Have you tried the CRR-47206 antenna coil?

> Yes; both stand-alone for my 630 Mtr digital station 
> and with the ATD on Longwave. It does work, but introduces
> some signal attenuation due to circulating currents between
> the internal coil and its grounded metal case.  I need to
> do a real comparison to my unshielded variometer.

The ATD is a super kludge when operating 200-540 kHz.  The external antenna loading coil connects to the transmitter output circutry using the one ATD antenna connection.  When the ATD antenna relay transfers the antenna to the receiver, the 47206 loading coil goes with it.  That's guaranteed degradation of reception.  The ATD manual, section 2-6, points this out and suggests that the loading coil tap selector be manually switched to position 10 to short out the entire loading coil when receiving!  I'm pretty sure that the simple counterproductive presence of the loading coil inductance in the receiver's antenna path is what is reducing your receiver sensitivity, and not any effect of the loading coil case.

Operation of the ATD with any of the other four TUs may take place with local or remote control.  But to operate the ATD on 200 to 540 kHz and the loading coil with attached trailing wire, the operator must manually remove the fixed antenna from the ATD antenna connection.  Then, a lead from the loading coil must be connected there.  Even after the ATD has been all tuned up on, the receiver won't be happy with that loading coil in its path unless you select load coil tap 10 to short the load coil during reception.  Meanwhile, the ATD is inoperable on its other three channels because of the external load coil connected to its antenna terminal.  This is really a less than mediocre design.

The Collins ATC or AN/ART-13 uses a dedicated connection to its external LF/MF PA tank like the CU-32/ART-13A.  There's no operator swapping of leads at the antenna terminal to enable LF/MF operation.  Even better, the T-47 MF/HF antenna always winds up connected through the CU-32 to the MF/HF antenna on key-up conditions even when the T-47 is set to transmit LF/MF when the key is down.  Thus, the receiver connected to the T-47 never ever has its antenna path degraded by the LF/MF tank in the CU-32.  (The operator never has to manually bypass CU-32 reactances while receiving, the CU-32 does that automatically.)  The T-47 may be immediately selected from LF/MF to any MF/HF channel with no operator antenna lead swaps.  The Bendix ATD is a clown show in comparison to the Collins ATC and T-47.

73
Mike / KK5F