Hw-5400 tx signal distortion

[Multi-Volti Devices]

Hello:

Here's what I was told as an RF tech at the Heath factory
in the early 90's (that's 1990's!)

HW-5400 bad signal reports (distorted TX SSB audio or CW sidetone) were
blamed mostly on a defective power amplifier circuit board at the beginning
of the production. Supposedly the circuit board suffered ground loop
problems, or something like that during high current (full power) operation.
I was told that the copper weight was increased to eliminate the distortion.
No revision level change was made, so there was no way to tell what rigs had
the thin copper PA board. While I was there, the RF techs would change the
PA assembly to one built with a known good board.

This was the policy/practice while I was there, but there were a few rigs
that kept coming back with distortion complaints. After very determined
examination, a new clue was discovered. A significant number of people, but
not all, were not using the stock power supply. One was doing some weird
thing like trying to run the rig from a car battery in the basement (rig on
next floor level), with a long cable, 20 feet or so. We didn't find this out
until loaning him a different rig (SB-1400) and he had other problems.

Some people had excessive RF in the shack...running end-fed long wire with a
transmatch for example, or had ground deficiencies.

I think the lucky break came when we had a '5400 and power supply come back
together...alot of the time people usually only sent in the rig, unless the
supply was really screwed up...or sometimes only the supply was sent in
(clock wiring problem, etc)..

This particular rig had tx audio distortion no matter what was
done...eventually it was determined that the problem was only present when
using the matching power supply!

It turns out the receive audio output cable is in a wiring harness with
other signals, and is dressed near some very sensitive circuitry, the mainly
concern being the synthesizer.

When the rx audio cable feeding the speaker jack was carefully picked out of
the wiring harness and rerouted as far away from the synthesizer as we could
get it, the problem cleared.  We installed a length of RG-174 and routed it
around the chassis edges. In radios that came in afterwards, the audio cable
was simply snipped at both ends and a replacement wired in (a lot easier
that picking out of the harness).

I don't know if this made it into the tech bulletins...I seem to recall it
was 'close to the end'. For anyone on the list who may have been at Heath at
the time and has additional info to contribute (although I doubt it...Terry
Perdue was still there, but Engineering wasn't really doing much at that
point related to service procedures.) I don't remember if RTO was still
thereor if anyone else contributed, but it was Kirby who figured it out. I
threw ideas and suggestions at him...but he did the work...I didn't service
'5400's.

I saw similar RF-related problems (but with different symptoms) with the
SB-1400, although it usually just plain oscillated when there was RF
feedback.

For the '5400, try lower power & see if the distortion level changes (but I
think in the case of that one radio, no change was noted, and we finally
tried a different power supply and speaker). Also, if available try
a different power supply, and maybe no speaker, substituting headphones.

See if anything else is wacky, but we did not have repeat complaints from
that point on with '5400's on which we re-routed the audio cable.

Good luck.

Murray

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