Not entirely on subject since this is for a transceiver, but figured
this would be the group most likely to generate a useful answer.

I happen to like the older Icom IC-271H and 471H, I have a pair of each
where one is currently the control and one is the in-progress unit.

All the normal stuff has been done, complete teardown and cleaning, all
cold joints, broken traces and iffy stuff dealt with, all new
electrolytics, etc.  On through basic improvments like replacing the
final output jumper from the filter to the rear SO239 with a RG400
jumper into a crimp type N bulkhead to reduce RF in the chassis.

And of course a very careful alignment.

Have been going through everything step by step replacing fets with
newer lower NF units, caps and resistors in critical sections with lower
TC ones.

Anyway, while cleaning up the "APC/ALC" circuit to eliminate overshoot I
found a great deal of noise that caused issues as the APC circuit was
made faster to eliminate overshoot.

Tracked it all down to the -9 supply (nominal, actual -10VDC), a very
old metal can DC to DC that happens to be physically located on the CSS
board but as its own seperate circuit.

It feeds -10VDC to radio and that is used in both the TX APC/ALC to vary
gain of the first amp (3SK74) after the SSB filter or FM modulator (as
the pulldown voltage).

It also feeds into several op-amps in the AGC and APC circuit.

It was down < -45dbm (at xcvr output) but the APC is always running at
least slightly negative causing a multi-mhz wide noise increase.  In FM,
it also AM modulates the carrier at 10 - 15 Khz.

All well under legal limits (90 - 100db below transmitter output) but it
just bothers be and any attempt to make the APC circuit more sensitive
makes it worse.

So first step was attacking the switcher circuit.  Replaced the 100uf
16V Al caps with Panasonic OsCon types, 25V 150uf paralleled with .1uf
ceramics.

This took care of the wideband RF and fixed the multi-mhz wide hump, but
the 10 - 15 Khz (depends on load) AM mod when in FM got better by ~ 10db
but is still there at around -50dbm.

I am looking at disconnecting the entire circuit on the CSS board and
replacing with a modern DC/DC w/ filters on a seperate board.

Initial though was a very low noise isolated output 10V DC/DC converter
that could be rigged as a -10V supply.

Could not find anything with really good specs so am currently thinking
something like a Traco TDN 3-1213WI (200ma 15V) feeding a filter into a
low noise linear reg to -10VDC followed by another filter.

I was hoping someone here might have a better suggestion as to method or
DC/DC converter.  "Long" term stability (seconds and longer) is not
really an issue, it could wander by a few hundred mv and not be an
issue, but any noise is as it will end up coupled into the transmit
chain and AM modulate things...At lower frequencies the circuits in
question actually have a fair bit of gain too.

Just to get a little closer to on-topic, the radio in question actually
has crystal heaters on all XOs and my biggest issue onb that front is a
repeating ~ 4.5Hz jump on a ~ 10.24 Osc that pulls the VCO by ~ 60Hz for
minutes to hours before reverting.

Yes, for best quietness, you definitely should "can it up" in a metal
box, and use feed-through caps for all the I/O, including the commons or
grounds. You have to figure out also where all the currents flow, and
contain the loops. With sufficient L-C filtering on the input and output
(all inside the can along with the converter), you should be able to get
whatever degree of cleanliness is required. The in and out ports should
include common-mode filters (also inside the can). When it's all said
and done, there should be the input power port +/- via feed-throughs,
and the output +/- supply port via feed-throughs.

The can should not carry any current except that which flows through the
common-mode filtering, so don't connect it internally to the either
input or output "common," even though ultimately they are tied
externally. This will help to keep the ripple and displacement current
loops contained within the can. The can should be solidly connected
(mounted) on the system ground chassis, and the feed-through caps will
give the final degree of RF suppression. The I/O ports then can be
connected as required.

Ed

I should mention that the input supply filtering to the DC-DC converter
should have good attenuation at the switching frequency, but not at low
frequencies, so there's no need to get carried away with the size of the
filtering at the converter input. Too much filtering, especially
inductance, can be detrimental, depending on the type of converter used.

If the converter is a simple chopper type without regulation, it will
put out in proportion to input, so line rejection will be poor. If line
ripple is an issue, then common solutions are brute force more filtering
with capacitance, or linear regulation at the input or output.

If a self-regulating type DC-DC converter is used, and uses PWM, it will
usually have pretty good line rejection, so eases the low frequency
filtering requirement. However, since its input resistance is negative
at low frequencies, it affects the choice of input filtering. Avoid
overdoing it with L-C input filters that cut at too low a frequency or
have high Q - it can result in oscillation. Lots of C is usually OK, but
not too much L.

Ed