[Kenwood] Kenwood TS-950SD Blowing 10 Amp Fuse on the Final Board

[Martin Sole]

I'm sure others will chime in with their ideas, here's my thinking. I have a
950SD on the bench at the moment that had a not so different problem and
requires both final transistors and drivers. Think the pre driver is okay.
It is important to understand that there are 2 very different PA designs
used in the 950 series, the original 950 S and SD use bipolar transistors in
the output stage whereas the later 950 SDX uses FET's. The basic amplifier
arrangement is similar but of course much of the supporting circuitry is
changed.

On the 950 SD the 10 A fuse feeds only the final PA transistors and control
circuitry for the fan and high temperature shutdown. Most likely failed
final transistors will be the cause.

To start out testing the unit you should hinge the PA module out and replace
the two short RCA cables with longer ones that will reach. These are not
actually necessary for testing though. With the fuse removed measure the
resistance from the fuse output (right hand side when viewed from the front)
to ground. This should be several hundred ohms I believe. In any case you
should get a feel for what the current draw might be.

Powering up the radio with the fuse removed you should be able to measure
the voltage at the input to the fuse. It should be 48 volts +/- a little. I
have seen 2 cases now where the series pass transistor Q8 has failed putting
70+ volts on the PA transistors. I suspect this has been at least once the
cause of transistor failure.

Hope this gets you started

Regards
Martin, HS0ZED


-----Original Message-----
From: kenwood-bounces at mailman.qth.net
[mailto:kenwood-bounces at mailman.qth.net] On Behalf Of Durl Grant
Sent: 28 February 2011 07:51
To: kenwood at mailman.qth.net
Cc: Wenty Bethune
Subject: [Kenwood] Kenwood TS-950SD Blowing 10 Amp Fuse on the Final Board

Hi Group,

I am posting this on behalf of my friend Wenty 6Y5IC (his computer is
currently down). He has 2 - TS-950SD's and they are both blowing the 10 Amp
Fuse on the Finals Board.  The radios will power on and stay on but will not
transmit. He has replaced the fuse on several occasions , but when he try to
TX there is no output, so the fuse is either blowing when he powers up or
the second he goes into transmit.  When he checks the fuse is always blown.
The other 950SD does the identical thing.  It's ironic that he has 2 -
950SD's with the same identical problem.

Could it be that the Output Transistors (Finals), are shorted, or is
something else on the Finals Board is shorted that's causing the fuse to
blow?

If anyone has experienced this problem I would appreciate hearing how you
resolved it.  l looked at quite a lot of the Threads in the Archives but I
didn't find anything that relates to this problem.  Of course I didn't read
all the archives, and there's always the chance that I missed it on those
that I did read, which was quite a lot.

Your assistance with this issue would be very much appreciated.  Thank you.

73,

Durl, ZF1DG
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