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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 17th Feb 2021, 11:47 pm   #1
Techman
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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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Old 18th Feb 2021, 12:35 am   #2
SiriusHardware
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Default Re: Yaesu FT-950 power drop and no low frequency output

Do you have a link to a diagram? I think you need to know whether the set controls the power output by varying the PA stage supply voltage or by changing the size of the RF signal being passed to the input of the PA stage.

If it's an all-mode set then presumably the PA stage is linear - although you say the power declines whether the TX is used or not, maybe there is a problem with the biasing which may well be constantly applied whether the unit is in TX mode or not.

The lack of TX below 7MHz may be a different problem altogether.
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Old 18th Feb 2021, 12:59 am   #3
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Default Re: Yaesu FT-950 power drop and no low frequency output

Yes, I had read somewhere about this constantly applied biasing.

As far as I can tell, SSB doesn't seem to be affected and seems to stay constant, so it seems to be a carrier fault.

Copies of the service and user manual found on the 'net below, plus a couple of pictures of the actual set in question.

Actually, I've just found that the two manuals are too large to upload, but they're out there somewhere on the 'net, so I'll try to find where I found them originally.

Edit: This looks like it:-
http://www.radiomanual.info/schemi/Y...T-950_serv.pdf
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Last edited by Techman; 18th Feb 2021 at 1:19 am.
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Old 18th Feb 2021, 2:14 am   #4
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Default Re: Yaesu FT-950 power drop and no low frequency output

The forum rules specifically ban illegal equipment for CB use, so we had better end this here as it seems like it would go back into such use.

The restoration of modified radios back to their original (and therefore legal) purpose would have been OK

David


Update: It's going back into amateur service. The owner bought it in a got-at state. He's working his way through the RAE, so we can re-open the thread.
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