[Vintage-Audio] Scott Amp Goes Pop!

Sat Aug 31 21:57:00 2002

Rudy, 	

It is possible that the rectifier tube did receive enough natural (ins pite of
excellent packing) vibration during transit to cause an internal short. However,
I may be in error here, but because the tube had only a maximum of three seconds
of power I doubt it had adequate time to even be a factor in this equation. I
suspect the resistor itself simply failed without outside assistance. 	

There are enough technically gifted people on this list to know for sure if the
time power was present was sufficient for a shorted rectifier tube to cause this
resistor to explode.	

Gentlemen?	

Duane W8DBF	

----------
From: Rudy Rutenber <[email protected]>
To: [email protected]
Subject: Re: [Vintage-Audio] Scott Amp Goes Pop!
Date: Saturday, August 31, 2002 9:30 PM

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" <[email protected]>
To: <[email protected]>
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
> [email protected]
>
>
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