[Vintage-Audio] Scott Amp Goes Pop!

[Rudy Rutenber]

Duane,
I am quite sure the rectifier tube is shorted.  If you have not fixed it
yet.  I would just replace the resistor and the tube, then bring up the
power slowly with a variac.  Sorry I am not in MI.
Rudy

----- Original Message -----
From: "Duane Fischer, W8DBF" <dfischer@usol.com>
To: <vintage-audio@mailman.qth.net>
Sent: Saturday, August 31, 2002 5:05 PM
Subject: [Vintage-Audio] Scott Amp Goes Pop!


>
>
> Hi All,
>
> I am in Glint, MI Genesee Township. I have a H. Scott LK-72b stereo
amplifier.
> It was just recapped and completely gone through by a friend in CA. He
burned
> her in for four or five days with no problems.
>
> It was very well packed, padded and cushioned when UPS made the transit.
No
> problems there. All of the tubes were shipped in a separate box.
>
> Everything appeared fine. I plugged the four 7591a into the proper sockets
being
> certain to match the correct tube to the correct socket. The bias was
already
> sset. I plugged the rectifier tube in. I checked all other tubes to be
sure they
> were properly seated. Everything was as it should be.
>
> I plugged the H. Scott LT-110 FM tuner into the unswitched power outlet on
the
> back of the amplifier. I did not turn the tuner on! I connected the
speakers,
> being certain that the jumpers were set for 8 ohms. Speaker polarity was
> correct.
>
> I connected the FM tuner output to the FM tuner input on the amplifier
input
> side. I connected an antenna to the FM tuner.
>
> I then plugged the amplifier into the wall outlet. I turned it on.
>
> I heard a soft hum immediately from the right rear of the chassis. The
amplifier
> and tuner have no cases, by the way. The big power transformer is located
here.
> This hum was NOT from the speakers, some six feet away. It was immediate,
no
> warmup time involved.
>
> About two seconds after I turned the amplifier on, I heard a sharp pop,
heard a
> sizzling sound and instantly shut the power off. (I still had my left hand
on
> the on/off switch). I was bent over the amplifier.  I smelled burne
components
> at this time.
>
> I immediately disconnected the line cord, unplugged the tuner, unplugged
the
> tuner input cable from the amplifier and disconnected the speakers.
Although I
> smelled no smoke at this time, I still carried it outside and set it on
the
> concrete sidewalk just in case.
>
> The fuse did not blow.
>
> The only visible damage was a large resistor to the right of the rectifier
tube.
> The side was blown right out of it.
>
> As far as I know, there was insufficient time for B+ to be present, as the
> rectifier tube had not even warmed up. This blew in no more than two
seconds
> after power was turned on.
>
> I need somebody in MI with experience who is willing to examine the
amplifier,
> determine the problem and repair it. I have the service manual and
schematic.
>
> If you are willing and able, please contact me by e-mail. Thanks!
>
> Duane Fischer, W8DBF
> dfischer@usol.com
>
>
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