Walt,
If the noise is common to TX and RX on the 351, it could be something common to both parts, such as the VCO, or power supplies.
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Does it vary in level and frequency with higher and lower voltage (ie, 22 and 28V) applied to the set, and is it the same at higher and lower than normal temperatures.
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Although not very scientific, what has worked for me in the past has been to open the set up, and very carefully unplug and re-seat the edge connectors, which could have an intermittency.? It also gives a chance to see if the set has been repaired previously, as sometimes the soldering on the repaired area can be quite rough, and nowhere near as good as the factory soldering.
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It's also worth looking for any loose, missing, or trapped screws, as all the manpacks in the Clansman range seem to have an occasional problem of screws coming adrift and causing mayhem inside!
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Over and above that, it could be worth putting a scope onto the supply rails, especially around sensitive areas, like the VCO's, to see if there is a concurrent spike there, as the tantalum capacitors used 25+ years ago can now start to fail.
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73
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Al
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G8LIT/W4
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