DX-60A power supply and repair notes

[Christopher J Galbraith]

Hi gang,

Thanks to N3CW for the DX-60A schematic, I was able to compare the DX-60,
DX-60A, and DX-60B power supplies.  As suspected, the DX-60A has one less
R-C section compared to the earlier and later models, causing excessive 120
Hz on the PA and modulator B+.  Even with new capacitors, the hum makes a
visible envelope on the CW and unmodulated AM waveforms (viewed on an
oscilloscope).  When demodulated on a local receiver, the hum is very
objectionable on AM.  

To improve this, one can just add an additional R-C section to the power
supply.  I did this, and it improved the B+ ripple from over 7% to under 2%,
and reduced the demodulated hum by roughly 10 dB.  There is still some
noticeable 120 Hz hum with a very strong signal, but it is improved to what
I would expect with a stock DX-60 or DX-60B.  For anyone interested, here is
a pdf describing the DX-60x power supply differences and the modification to
the DX-60A:

http://tinyurl.com/2xmm7j

To completely eliminate the hum I would add a 3-5 H, 250 mA choke in place
of the 100 Ohm resistor (it would likely need to be placed on the top of the
chassis, though.)  I did also measure the ripple on the low-level audio
stages and it is far below 2%, so I suspect that adding additional filtering
there would not reduce the hum further.

In addition to this mod, I would also recommend replacing the following
components:

1. All electrolytics (stock values)
2. All plastic capacitors in the audio section (increase to 0.005 or 0.01 uF
for lower frequency response)
3. All 2W carbon comp resistors as they tend to change value (all of mine
had by over 25%), check the other carbon comps, too, and replace if they
have drifted out of spec.
4. 8.2 Ohm R13 (cathode current monitoring resistor), which is probably
cracked or changed in value--use a 1 or 2W metal film type

Finally, the function switch is a known problem on these transmitters.  I
installed a series "snubber" made of 2.2 kOhm and 0.005 uF, across the HV
switch contacts.  This prevents "break" arcing, but I still experience
significant arcing on "make".  On making contact, the switched contact feeds
a large capacitor (which is nearly a short-circuit to ground at first), so
current is not at all limited.  I suspect this is why the function switches
of DX-40s and some other similar rigs that use choke input (and thus current
limited at start-up) power supplies do not have the same function switch
failures--there is much less "make" arcing, and they use a series R-C
snubber to reduce "break" arcing.  After all of this, I have decided to
install a relay to replace the HV contacts on the function switch, and use
the function switch contacts (4 and 5) to switch 6.3 VAC to the relay
coil--I found this idea by searching the Heath archives, and I think it is a
great solution to save the function switch from eventual destruction.

For other DX-60 mods, also see the great technical section at the AM Window:

http://www.amwindow.org/tech/htm/mods.htm

73, Chris KA8WFC

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