Ten Tec service had always told hams, “No matter what is wrong with your Orion
or Orion II, first replace the electrolytic capacitors on the A9 board".
Of course the reason was always assumed to be that Ten Tec had suffered from
the great “Capacitor Plague”, in which a rogue Chinese supplier of electrolytic
capacitors had stolen the "secret formula" for making low ESR electrolytic
capacitors from one of the more reputable suppliers, and had begun making
cheaper capacitors to sell into the market. The “stolen secret formula” was
said to have been a decoy formula which resulted in some pretty awful
capacitors. The situation got so bad it almost broke Dell Computers.
In fact, now that I finally came across an A9 board that could not be repaired
by simply changing all of the capacitors, I found that I needed to analyze the
circuit in order to go forward. I also now understand why it was so essential
that these capacitors be in top shape.
Without any in-depth analysis of the A9 circuit, I had always assumed that this
was a switching supply only for the -20 volt LCD power supply and that the 5
volt supply was linear with Q4 acting to drop part of the 13.8 volts in order
to stay inside the safe operating region for Q5, with the combination of D3 and
D6 establishing the base voltage at Q5 to produce 5 volts at the emitter of Q5.
Well, I freely admit that this simplistic analysis was totally wrong. The 5
volt supply is indeed a switching supply, using the ~20 KHz oscillator in the
-20 volt inverter as the switching signal for the 5 volt supply which consists
of Q4 and Q5 acting as switching amplifiers feeding D4 and D5 to be rectified
and then filtered by C6 and C7. D3 and D6 act to establish a limit to the base
voltage (20 KHz) at Q5 and thus the rectified and filtered 5 volt output at the
emitter of Q5. (You EE folks out there, please excuse my physicist's analysis -
it’s close but maybe not exact.)
This explains to me why Ten Tec kept the A9 board design in the Orion II even
though its display did not need the -20 volt supply. They needed the low heat 5
volt switching supply.
The reason for not using a simpler linear 5 volt supply is simply one of
heating in this portion of the radio. Using a simple 7805 type linear regulator
off the 13.8 volt line means that the 7805 is dropping ~ 7.5 volts and, with
the Orion needing about 2 amps give or take on the 5 volt line, this results in
dissipation of something like 15 watts inside the closed space right behind the
display - not a good thing.
The problem with repairing this design is that, so far as I can tell, it is
very sensitive to the devices used for D3 and D6. I have not been able to
source the exact Zener diode for D6 and I think D3 may not be a 1N4007 as
stated in the schematic. In any case, the 5 volt supply on this board I have
produces ~5 VDC output but it does not regulate at all. It goes from 4.5 volts
to 6.3 volts (even when loaded to 2 amps) as input voltage goes from ~12 volts
to 13.8 volts. I compared with an A9 board that was known to be working
properly and it regulates rather well under the same voltage range and loading.
The bullet proof solution I have come up with is a medical products rated
supply from TRC Electronics. This device is a 40 watt capable supply operating
from the 13.8 volt line in the radio. This supply is fully shielded and
designed for use without any need for EMI suppression. I am shipping this
upgraded supply in a few days to my customer but thought I’d share this if
anyone is interested in this A9 upgrade.
The -20 volt supply will still be operational should you need it for an Orion
I. I would disable it for an Orion II.
This is a significant upgrade in the supply as far as I can tell and produces
less heat than the original A9 design.
I am, as always, open to opposing views…
Gary
W0DVN
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