Subject: ICOM R7000 SSB Mod


ICOM ICR7000
Poor audio quality and intelligibility on SSB.

My R7000 performed well on SSB initially, but progressively worsened over
the months and years until SSB became almost unintelligible, particularly on
the higher frequency VCO.

VCO PHASE NOISE
Investigation of this fault revealed that the problem was caused by very
high phase noise on the PLL sub-assembly VCOs.  The bandwidth of the noise
on the lower frequency VCO was 5KHz, and on the higher frequency VCO was
8KHz.

I found very high noise levels on the supply rails feeding the VCOs.  The
voltage out of the regulator IC1 was clean, but the switching transistors Q8
and Q14 had excessive voltage drop across them, allowing noise modulation of
the supply voltages to the VCOs and other stages on the PLL board.  The
voltage drop across Q8 was 300mV, and across Q14 was over 400mV.

CAUTION, The modifications listed below involve the replacement of small and
fragile components.  Do not attempt these modifications unless you have
experience working with this type of componentry and have the correct tools
and a temperature controlled soldering iron.

I replaced the 2.2K switching transistor bias resistors R37 and R57 with 1K
resistors. This reduced the voltage drop across the switching transistors to
well under 200mV, and achieved a significant reduction in VCO phase noise.
As an added bonus I also noted an improvement in general receiver
performance.

To further reduce the noise on the VCO supply voltages, I made the following
modifications:
1. Replace the electrolytic capacitors C6 and C17 in the VCO enclosure with
220uF 10V capacitors (This part of the modification has been published by
others).
2. Replace C178 and C179 to the rear of the VCO enclosure with 470uF 10V
capacitors.
3. Install 33ohm resistors in the wire leads carrying the VCO supply
voltages from the switching transistors to the VCO filter. (From L73 to Q8,
and from L56 to Q14).

The result of this modification is a dramatic improvement in SSB
intelligibility and quality.  The CW note changes from wideband noise that
sounds a bit like a tone, to a tone with some residual low frequency noise
modulation, again a dramatic improvement.

G Wiseman, VK5EU.