Prev: Looking for PS glitch with Tek scope?
Next: RCA chassis ATC010A
From: bz on 3 Jun 2010 05:10
A somewhat interesting case, troubleshooting and 'repair' of an audio amp.

I was presented with a "dead" 'classic' transistorized audio amp.

The owner had checked one of the fuses that was located in the back panel
power socket. There were also two fuses on the main board, also good.

The owner had forgotten to run off the amp and a thunderstorm had come
through the area. He found the amp totally unresponsive when he came home.

Hm, fuse or diode or switch? No visible signs of damage. No smell of smoke.

The amp weighs close to 30 pounds due to the HUGE toroidal shaped power
transformer.

My first step was to try to verify the power circuitry.

I didn't find any AC voltage at the screws of the terminal strip.

Aha, I though, as I started tracing the AC input that came from the back
panel to the power switch and then through a thermal cutout device on the
large heat sink, to the terminal strip. It must be the switch or the
thermal cutout.

Switching the meter to ohms and removing the power cord, I confirmed that
the safety ground was contiguous and that the neutral was also.
But I could not find continuity through the hot side to anything. Must be
the thermal cutout mounted on the transistor heat sink!

I finally cut a small hole in the insulation on the wires leading to the
thermal device. (no needle pointed test leads available)
Hm. Thermal cutout was ok.

Maybe the switch? Pulling the back panel fuse, I confirmed that I had
continuity to the amp end of the fuse holder when the switch was on, but
the hot end of the fuse holder showed NO connection to the adjacent power
plug pin!

Pulling back the 'boot' that covered the back of the panel power plug,
everything looked fine but there was no continuity between the fuse holder
and the strap that tied it to the power plug pin. A few seconds with the
soldering iron and everthing now checks ok!

The solder joint ON the back panel power plug had failed without any
visible sign!

--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.
From: nesesu on 3 Jun 2010 11:18
On Jun 3, 2:10 am, bz <bz+...(a)ch100-5.chem.lsu.edu> wrote:
> A somewhat interesting case, troubleshooting and 'repair' of an audio amp..
>
> I was presented with a "dead" 'classic' transistorized audio amp.
>
> The owner had checked one of the fuses that was located in the back panel
> power socket. There were also two fuses on the main board, also good.
>
> The owner had forgotten to run off the amp and a thunderstorm had come
> through the area. He found the amp totally unresponsive when he came home..
>
> Hm, fuse or diode or switch? No visible signs of damage. No smell of smoke.
>
> The amp weighs close to 30 pounds due to the HUGE toroidal shaped power
> transformer.
>
> My first step was to try to verify the power circuitry.
>
> I didn't find any AC voltage at the screws of the terminal strip.
>
> Aha, I though, as I started tracing the AC input that came from the back
> panel to the power switch and then through a thermal cutout device on the
> large heat sink, to the terminal strip. It must be the switch or the
> thermal cutout.
>
> Switching the meter to ohms and removing the power cord, I confirmed that
> the safety ground was contiguous and that the neutral was also.
> But I could not find continuity through the hot side to anything. Must be
> the thermal cutout mounted on the transistor heat sink!
>
> I finally cut a small hole in the insulation on the wires leading to the
> thermal device. (no needle pointed test leads available)
> Hm. Thermal cutout was ok.
>
> Maybe the switch? Pulling the back panel fuse, I confirmed that I had
> continuity to the amp end of the fuse holder when the switch was on, but
> the hot end of the fuse holder showed NO connection to the adjacent power
> plug pin!
>
> Pulling back the 'boot' that covered the back of the panel power plug,
> everything looked fine but there was no continuity between the fuse holder
> and the strap that tied it to the power plug pin. A few seconds with the
> soldering iron and everthing now checks ok!
>
> The solder joint ON the back panel power plug had failed without any
> visible sign!
>
> --
> bz      73 de N5BZ k
>
> please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
> infinite set.

BZ, it is not uncommon for the plating on the terminal of the power
connector to fail at the interface between the plated metal and the
base metal, especially under solder. The outward appearance seems
perfect, bet when the solder is removed there is typically a black
surface on the base metal that seems to be an excellent insulator.
This same problem also occurs on the pins of TO-3 transstors soldered
into PCBs and on terminal pins staked in to PCBs.
Some of the worst offenders I have come across are tin plating on
brass and tin and/or nickel plating on steel.

Neil S.
 | 
Pages: 1
Prev: Looking for PS glitch with Tek scope?
Next: RCA chassis ATC010A