Plessey PR155
 

Hi,

My PR155 has 50 Hz phase modulation sidebands on the interpolating VFO, when
operating from AC mains. It's clean running from external DC, but the
sidebands return if the mains transformer primary is energised with the
secondary open-circuited. Could it be the magnetic field? Has anyone else
noticed this?

Spectra -
http://www.holmea.demon.co.uk/Misc/AC.png
http://www.holmea.demon.co.uk/Misc/DC.png

TIA
Andrew.

Andrew Holme wrote:

My PR155 has 50 Hz phase modulation sidebands on the interpolating VFO, when
operating from AC mains. It's clean running from external DC, but the
sidebands return if the mains transformer primary is energised with the
secondary open-circuited. Could it be the magnetic field? Has anyone else
noticed this?

Bad supply caps leaving residual 50 Hz on the VFO supply lines. Get out a
meter and I bet you'll see AC noise there.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:
Andrew Holme wrote:

My PR155 has 50 Hz phase modulation sidebands on the interpolating
VFO, when operating from AC mains. It's clean running from external
DC, but the sidebands return if the mains transformer primary is
energised with the secondary open-circuited. Could it be the
magnetic field? Has anyone else noticed this?

Bad supply caps leaving residual 50 Hz on the VFO supply lines. Get
out a meter and I bet you'll see AC noise there.
--scott

Clarification:
There is no ripple on the supply when the set is powered from DC, but I
still get sidebands if I apply AC mains to the transformer primary with the
secondary temporarily disconnected.

Andrew Holme wrote:
Scott Dorsey wrote:
Andrew Holme wrote:

My PR155 has 50 Hz phase modulation sidebands on the interpolating
VFO, when operating from AC mains. It's clean running from external
DC, but the sidebands return if the mains transformer primary is
energised with the secondary open-circuited. Could it be the
magnetic field? Has anyone else noticed this?

Bad supply caps leaving residual 50 Hz on the VFO supply lines. Get
out a meter and I bet you'll see AC noise there.

Clarification:
There is no ripple on the supply when the set is powered from DC, but I
still get sidebands if I apply AC mains to the transformer primary with the
secondary temporarily disconnected.

WHOA! That sounds like magnetic leakage, yeah.

Another possibility might be AC leakage to chassis through a filter
in front of the power transformer.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Andrew Holme" wrote in message
...
Scott Dorsey wrote:
Andrew Holme wrote:

My PR155 has 50 Hz phase modulation sidebands on the interpolating
VFO, when operating from AC mains. It's clean running from external
DC, but the sidebands return if the mains transformer primary is
energised with the secondary open-circuited. Could it be the
magnetic field? Has anyone else noticed this?



This sounds like a it could be a classic case of "tunable hum."

Does adding a good ground to the chassis
eliminate the hum, or lessen it? Are the mains lines
bypassed to the chassis in the radio?

It is not unusual for the mains to become part of the antenna
counterpoise, which under some circumstances can impose
a AC line frequency modulation on the received carrier.

Pete k1zjh

Uncle Peter wrote:
Does adding a good ground to the chassis
eliminate the hum, or lessen it? Are the mains lines
bypassed to the chassis in the radio?

The mains lines are bypassed to the chassis - right behind the power inlet
socket. I probed around this area, after seeing Scott's second post, but
all seemed to be in order.

It is not unusual for the mains to become part of the antenna
counterpoise, which under some circumstances can impose
a AC line frequency modulation on the received carrier.

The phase modulation is on the local oscillator - never mind received
carriers.

Thanks,
Andrew.