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Old 18th Feb 2021, 12:47 am   #1
Techman
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Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 4,985
Default Yaesu FT-950 power drop and no low frequency output

I've got this Yaesu FT-950 that I said I'd look at for someone. I didn't make any promises and I did say that I probably wouldn't even remove the covers unless I had a good idea of what was wrong with it. I know very little about these sets and can really only apply normal radio theory to it, so thought I'd pick some possible brains before delving in - that's if I even risk delving in at all.

It's been expanded and has been used mainly on CB 27MHz FM, but not exclusively so. The reported fault was that it drops off in power over a short time of use, and in fact 'as found' when I got it home and connected it up to a Bird watt meter and dummy load, it was on 27MHz and outputting around 8 watts as it had been left set by its owner when last used. After leaving it switched on for half an hour the power dropped to around 3 watts where it remained stable. When switched off for a while I found that it again outputted 8 watts which gradually dropped off over the first half an hour, checking regularly, regardless of whether transmitting or just receiving, so this tends to rule out a problem with the RF output stage going down as the set warms up. The carrier can be wound back up from the setting in the menu - as an example...the carrier can be set to say 25 watts after the first half an hour, but after being switched off for a while it can then be switched back on only to find the carrier dangerously outputting (depending on circumstances) at 50 watts. This can be lived with so long as you know about it, although not ideal.

It was then that I discovered other faults that the owner was not aware of. There's virtually no AM carrier, probably 5 watts from cold with everything set flat out, dropping off over time to hardly anything, as with FM, but it does 'swing' to around 40 watts on a whistle into the mic. Then I discovered that there's no output on any modes on 1.8 and 3.5MHz - it doesn't really start transmitting anything until 7MHz. A bit of 'on-line' research found a discussion from over a decade ago with a similar fault on this model, but it ended up not going anywhere with no outcome...perhaps it was this same radio doing the rounds even back then - I don't know the age of this set.

Any ideas anyone, or should I just give it a wide berth and give it back to the chap without taking the lid off?
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