Forgot to mention the AGC circuit. It is part of the IF and worth to take a
look at, including the switch.
From: Korhonen Esa.P (NET/Oulu)
Sent: 15 November, 2002 09:59
Subject: Re: FT901 Noise
Some thoughts from among falling snow flakes...
Given that the noise generates an S9 reading, we can exclude all audio related
circuits. Different VFO didn't help either, so it can be excluded as well. I
would exclude the front end as well, since it has band specific filtering and
the noise is across all bands (I assume). The front end can not be blown if the
receiver chain basically works and you can hear a signal that is stronger than
What do we have left? IF stages, local oscillators and mixers. Mixers probably
are ok since the RX chain works otherwise. So, we have come down to locals and
IF. The locals are crystal oscillators and most likely can not produce anything
like this, and it would have to be all of them if the problem is across all
bands. Voila, the only thing we have left are the IF stages. A problem there
sure would be able to generate a reading on the S-meter and this kind of noise.
Wiley KF6IIU suggested cold solder joints. That is a very good suggestion so go
ahead and look at bad joints in the IF circuits. The IF amplifier transistors
probably are ok since the RX chain works, but besides a bad solder joint there
may be a faulty component near the transistor that makes it oscillate.
> > From: "David" <firstname.lastname@example.org>
> > To: <Yaesu@contesting.com>
> > Date: Tue, 12 Nov 2002 19:33:41 -0500
> > Subject: [Yaesu] FT901 Noise
> > Hello and thanks for reading I have a FT-901DM With some sort of
> > self =
> > generated noise.
> > To be more specific the transmit works the recieve works, So long as
> > you =
> > recieve a signal That
> > Is over S9. constant. And of course no antenna hooked up With the
> > lack =
> > of a better description I would have to say a white noise of sorts.
> > I =
> > tried bypassing the vfo for the outboard vfo but it is still there.
> > My problem I guess is were do I start on trynig to troubleshoot this
> > =
> > problem. I do not have a service manual but do have th schematic for
> > it. =
> > Also I did remove the cards and clean the edge connectors, to no
> > avail. =
> > The S Meter reads S9 with the white noise and I do not hear any kind
> > of =
> > Hum just white noise and a signal that does not exist.
> > Thanks David KR4OW
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