links are not installed) but it functions
as a half-wave voltage doubler when
the links are installed for 110VAC
operation. I also noticed that there
appeared to be a thermistor wired in
series which had burned out.
There were also two optoisolators
on the board. One seemed to be used
to indicate to the control circuit when
the AC input was present. I think this
signal may be important when power
is first applied but I did not trace the
circuit further to find out.
For 110VAC input, the optoisolator
was connected via a 27kW resistor to
the incoming AC line. For 240VAC
input, there was an extra 27kW resistor in series. When the wire link is installed it shorts out the second 27kW
resistor.
But for cost-saving reasons, the manufacturer did not install the second
resistor when factory configured for
110VAC input. This made me question whether the optoisolator may
have been blown when 230VAC was
applied.
I thought at this point I might as well
make the changes required for the unit
to operate from 230VAC, plug it in and
see if it worked. I didn’t have much
to lose; if anything else had failed,
it would probably just blow the fuse
when powered up.
I didn’t have a replacement thermistor but even though a chunk had blown
out the side, the resistance across its
pins didn’t seem too high at 32W. This
was probably higher than a good thermistor but still low enough to allow
the power supply to operate with a
light load. At least I would know if it
still worked.
So I made the changes and replaced
the fuse. I didn’t have the correct slowblow type of fuse on hand so I decided to
use a regular fuse for testing purposes.
I hoped that the inrush current at startup would not blow it. Sometimes, you
have to try your luck!
I put the PCB back into the chassis
and sat it on top of a cardboard box to
ensure that it was insulated from the
top of the workbench. I connected the
mains power cord via an earth leakage
circuit breaker and switched it on at
the power circuit.
The fuse did not blow but there was
a loud crack as a spark shot out of the
side of the thermistor.
Without touching the power supply,
I carefully connected my Fluke multimeter across each of the DC output
connectors. The voltages measured
+5.1V DC, +11.95V DC and -11.96V
DC. Whoopee! All was OK; I guess
one can be lucky sometimes. But I still
needed to replace the thermistor and
put in a proper slow-blow fuse.
On Saturday morning I made a trip
to the local electronics store and fitted the new parts that afternoon. The
power supply passed a no-load voltage
test. Luck was on my side. The fuse had
blown before any damage was done
other than to the fuse and thermistor.
The power Mosfet was clearly OK and
the optoisolator had not blown up with
only one 27kW resistor in circuit.
I made a phone call to my friend to
tell him that it was ready to be picked
up. We decided to have a BBQ on Sunday and he would collect the power
supply at the same time.
I did not hear from him whether the
power supply worked OK when installed into whatever it came from. I
suppose if it had not worked, he would
have let me know straight away. Sometimes no news is good news!
SC
New Rohde & Schwarz oscilloscopes
Rohde & Schwarz have recently introduced the
two-channel RTC1000 series, a compact, lowcost, high-quality digital storage oscilloscope.
It can double as an eight-channel logic analyser,
four-channel pattern generator and a protocol
analyser for I2C, SPI, UART/RS-232, CAN and
LIN; and as a digital voltmeter, component tester,
spectrum analyser and counter. With this eightin-one instrument integration, users get more
value with a minimal footprint on bench space.
For further information contact:
Rohde & Schwarz
Ph: (02) 8874 5100
Email: sales.australia<at>rohde-schwarz.com
Website: www.rohde-schwarz.com/RTC1000
siliconchip.com.au
R&S RTC1000 oscilloscopes are available with
bandwidths from 50MHz to 300MHz. Bandwidth
upgrades are available via software license
all the way to 300MHz and can be purchased
as needed. The maximum sample rate is 2
GSample/s and memory depth is 2 Msamples.
LAN and USB interfaces are standard.
Silicon Chip is expecting to obtain an R7A4000series scope/spectrum analyser (another new
product) for review in the near future.
Celebrating 30 Years
February 2018 63
