siliconchip.com.au Australia’s electronics magazine May 2019 67
SCchannel speaker. I dug around in my
filing system and found the circuit di-
agram I used last time I fixed this am-
plifier, about six years ago.
The amplifier is built like a brick
outhouse with 2mm steel plate used
for the whole chassis and covers; not
a bit of plastic in sight. I removed the
nine screws holding the top cover in
place and opened it up. I then identi-
fied the phono/select board assembly
which sits vertically and plugs into
another PCB behind the front panel.
I connected my CRO (cathode ray
oscilloscope) to the two phono input
channels and I could see both wave-
forms from my iPod. This was diffi-
cult as the vertical phono board has
the input selector switch shaft run-
ning along the whole length. I then
identified the output terminals from
the phono preamplifier and connect-
ed up my CRO. The left channel was
OK but nothing was coming out of the
right section.
The circuit showed 6.8μF coupling
capacitors connecting the output
transistors to the selector switch, so I
hooked up scope up to the driven side
of the capacitors and found a good sig-
nal on both channels.
I then had to try to get the board free
enough to replace the faulty capacitor;
I did not want to remove it completely
as this involved desoldering about 10
wires. After removing the aluminium
front panel and another 10 screws, I
was able to move the board enough to
get to the capacitor.
Replacing the right channel cou-
pling cap resulted in audio from both
speakers. I decided to replace the left
channel’s coupling cap as well, to be
on the safe side; after all, the amp is 50
years old! I wonder if a new amplifier
purchased today would last that long.
My friend had also told me that
the power-on indicator lamp was not
working, so I checked that and found
the globe (28V 40mA) was blown. I
searched in my container of small
globes and found a 6V 40 mA bulb
that looked like it came out of a tel-
ecom switchboard or equipment rack.
The amplifier circuit showed a 390W
2W resistor in series with the globe, so
after doing a bit of maths, I determined
that a 1kW 1.6W resistor should allow
me to use the 6V globe instead. I found
a 1kW 3W wirewound resistor about
the same physical size as the original
and mounted it on the opposite side of
the power PCB, to allow more air flow.
A bit of fiddling with the new globe
had it mounted correctly and working.
I left the amp running for a few hours
and found no sign of the replacement
resistor overheating. Since it was all
working, I put it back together, took it
to my friend’s house and connected
everything back up.
While I didn’t ask for any payment, I
was promised a bottle of scotch for my
efforts; that’s what you call a bonus!Ducted vacuum repair
G. H., of Littlehampton, SA has had
problems with two different vacuum
systems and both of them involved
Earth leakage faults. He managed to
solve both...
Our house is about 20 years old.
It has a weatherproof double power
point on the back wall of the garage for
garden appliances. About a year ago,
plugging in and switching on the gar-
den vacuum via the right-hand socket
caused it to trip the Earth leakage de-
tector in our main circuit board.
Oddly, after resetting the RCD, it
worked fine. And I also found that us-
ing the left-hand socket never causedthe RCD to trip. The fault ended up
being a mass of spider webs embed-
ded in the back of the power point. I
cleaned it out thoroughly and sealed
the gap in the wall around the power
point, to keep spiders out.
We also have a ducted vacuum
cleaner inside our house which has
worked well for many years. The main
unit is in the garage, so it produces
very little noise and no smell of musk.
Then, a few months ago, as my wife
pushed the cleaning hose into the wall
socket, all the power to our house went
off. The pipe has a metal ring which
connects two terminals inside the wall
socket, turning the unit on automati-
cally. The switching is all done at low
voltage for safety.
I unplugged the motor unit in the
garage and checked the fuse box. The
RCD had tripped again, presumably
due to excessive Earth leakage. Re-
setting it restored power to the house.
I plugged the motor unit in and care-
fully switched on the power. The hose
must have been left in the wall socket,
so it sprang to life. We continued to use
it without any problem until the other
day, when the RCD tripped again. This
time, I restored the power but left the
motor unit unplugged.
I first measured the resistance
from the low-voltage switching wires
to Earth, Neutral and Active. I was
pleased to see that all readings were
open-circuit, so that part was safe. The
low-voltage supply comes from a 12V
transformer, which powers a relay to
switch the Active conductor.
But the resistance between the Neu-
tral and Earth pins of the vacuum mo-
tor was less than I expected. I meas-
ured the resistance between the Earth
pin on the internal circuit board and
both the Active and Neutral connec-
tions again; it was too low.
I disconnected the motor from the
main board. Measuring the resistance
from the motor power wires to the
Earthed casing of the motor also gave
low resistance readings.
So I began dismantling the motor
carefully and measured as I proceed-
ed. Eventually, I concluded that this
leakage was due to carbon deposits
which had come from the brushes. I
thoroughly cleaned the carbon brush
holders, then dried them before reas-
sembly. All of the resistance readings
were open circuit, as they should be,
and since re-assembly, the vacuum has
not missed a beat.