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Cleaning silver with electricity
By harnessing the power of electrons, silver cleaning can be quick,
easy and efficient.
The most popular way to clean
tarnished silver and silver-plated
objects is using a paste containing
ammonia and a mild abrasive. But it
gradually removes the silver, which
is particularly bad for silver-plated
items. Another method uses acidified
thiourea, which tends to leave a yellowish residue.
Alternatively, the silver object can
be placed in direct contact with an aluminium pot or a sheet of aluminium
foil, and immersed in a hot solution
of sodium bicarbonate, setting up an
electrolytic cell. While this method
works quite well, it is fairly slow and
works best with small objects.
This method can be sped up enormously and made much more efficient
by the simple application of an electric
current, as explained in the video at
https://youtu.be/57iwtmT4LNQ
Tarnish is silver sulphide, generated
over time by hydrogen sulphide in the
air, or perhaps from contact with egg
yolks. The reaction is:
4Ag + 2H2S + O2 → 2Ag2S + 2H2O
The reaction can be reversed by
electrons, with a reduction potential
of -0.69V. In a mildly alkaline solution,
the sulphide remains ionised and soluble in water:
Ag2S + 2e− → 2Ag + S2−
The reduction of aluminium ions
involves a potential of -1.66V:
Al3+ + 3e− → Al
This reaction is also reversible
under certain conditions. When these
metals are in contact with each other
in a mildly alkaline solution, an electrolytic cell is set up.
The difference in reduction potential facilitates oxidation of aluminium
and liberation of electrons to silver sulphide, forcing its reduction to metallic
silver. The sulphide ions travel to the
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Silicon Chip
aluminium, which is converted into
aluminium sulphide.
The reaction can be greatly sped up
with the help of a 6V battery. As before,
the metals are placed in a solution of
hot sodium bicarbonate and table salt,
but the aluminium foil and the silver
are not in contact with each other.
The silver is connected to the negative terminal, and the aluminium is
connected to the positive.
The battery facilitates the movement
of electrons out of the aluminium and
into the silver. The result is dramatic.
Within just a few seconds, the blackened silver turns shiny.
The beauty of this method is that
the silver sulphide is converted back
to metallic silver and redeposited on
the item. Since silver ions are positively charged, they can’t go anywhere
else, and there is no risk of damage to
the silver.
But, beware – if you get the polarity mixed up, your precious ancestral
silver will be rapidly stripped!
The practical procedure is very
simple. Fill a non-metallic bucket or
dish with very hot tap water. Add a
tablespoon of salt and a tablespoon of
sodium bicarbonate (baking soda) and
stir to dissolve.
Place a sheet of aluminium foil overhanging the side and connect it to the
battery’s positive terminal. Then connect the silver item to the battery’s
negative terminal and dunk it in the
liquid – keep the connection point
dry and above the liquid. Fizzing will
start, and within seconds, the tarnish
will disappear, and the silver will be
restored.
When you are satisfied with the
result, remove the silver item and
invert it so that the half that was outside the liquid is now immersed. Then
attach the negative end of the battery
to the top of the silver item above the
liquid. The remaining part will be
cleaned in seconds.
Australia’s electronics magazine
Remove the item and give it a gentle
rubdown with a damp cloth. Rinse it
thoroughly with tap water to make sure
that there is no residual salt. That’s all
there is to it. Silver cleaning need no
longer be drudgery!
Jim Goding,
Princes Hill, Vic.
Pong in a picture frame
Congratulations to Dr Hugo Holden
on his Silicon Chip article on Arcade
Mini Pong (June 2021; siliconchip.
com.au/Article/14884) and on a neat
board design. It’s a very nice and compact layout – at first glance, I thought
he had switched to SMD chips to make
it that small.
Come to think of it, one could do
that to take this even further. It would
enable a discrete Pong PCB somewhere
around the size of a postcard. With
Chinese manufacturers like JLCPCB
offering very affordable SMD assembly
in small lots, a batch of pocket-sized
Pong boards would be quite feasible.
Hugo Holden’s circuit analysis has
been invaluable for me when I got
started with my “framed Pong” project. It took me a while to get hold of an
original Pong board, and I started my
upscaler FPGA development before I
had an actual board, based on his documentation.
The detailed analysis also gave me
the confidence to buy a non-working
Pong board, since I had all the information needed to fix it. Thank you very
much for sharing your knowledge!
I wrote up my project at www.
e-basteln.de/arcade/pong/pong/
Jürgen Müller,
Hamburg, Germany.
DAC chip has a fault in one channel
I built the USB SuperCodec by Phil
Prosser published in the August to
October 2020 issues (siliconchip.com.
au/Series/349), and while it generally
worked, I found that the performance
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of one channel was much worse than
the other, with a much higher noise
floor and a distortion figure 20-30dB
higher.
With further analysis, I discovered
that it was one DAC channel that was
not up to scratch. To investigate, I disconnected the four 240W resistors from
pins 19, 20, 23 & 24 of IC9 to the op
amps and swapped the channels over.
The high distortion remained with the
same DAC channel, ruling out the op
amps or associated components as
the culprits.
So it looked like I had a faulty DAC
chip. Not having the tools or skills to
remove the SMD, I sent the board over
to Phil Prosser, who graciously offered
to swap the chip for a new one.
That fixed it. Phil pointed out that
a static discharge on one of the output
pins could have damaged the faulty
channel. Thanks to Phil for fixing it.
Stephen Gordon,
Thurgoona, NSW.
DAB+ radio updates
On May 18th, the ABC/SBS digital radio signal quality was adjusted.
ABC Classic has been increased from
80kbit/s to 120kbit/s.
After all the initial sound quality
complaints, it will be interesting to
see if anyone notices. ABC Jazz has
been increased slightly to 88kbit/s,
which is not likely to make a noticeable difference.
The losers are ABC country, triple j
and triple j Unearthed, reduced from
80kbit/s to 72kbit/s. This is interesting because triple j’s younger audience
should have better hearing! ABC Kids
Listen has also been reduced from 72
to 64kbit/s.
The ABC local radio for each capital
city is transmitted in 64kbit/s stereo,
despite their AM transmitter radiating
in mono except for Darwin, which is in
FM stereo. SBS is unchanged, meaning that Chill and Pop Asia are still
72kbit/s stereo.
All these channels use HE-AAC V2
compression. There is now an even
more efficient compression called xHE
AAC, but current DAB+ receivers cannot handle it yet.
A Government DAB+ transmitter
carries 18 programs. The commercial/
community DAB+ transmitters are carrying up to 32 programs each.
There is an opportunity for regional
areas to roll out DRM+ using the vacant
analog TV channels 0-2, that can cover
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Silicon Chip
a much larger area than DAB+, which
uses higher-frequency channels,
increasing line-of-sight losses by 12
times. This could replace many AM
and FM broadcasts.
One DRM+ transmitter can carry
the pair of programs from commercial broadcasters, and there is now a
six-channel modulator that can transmit the 18 ABC/SBS programs using a
single antenna.
2.8 million DAB+ receivers have
been sold to date in Australia, and
77% of new cars have DAB+ receivers.
Alan Hughes,
Hamersley, WA.
Secondary circuit breaker
recommended with variacs
Dr Hugo Holden’s Variac-Based
Mains Regulator article (May 2021;
siliconchip.com.au/Article/14856) is
a great idea. I’ve had a commercial
version protecting my small collection
of vintage electronic equipment for 15
years, and it has been invaluable.
We live a long way up a country
road, where electricity is supplied
by the proverbial “thin piece of wet
string”, meaning that the mains voltage varies widely (far outside the nominal statutory limits) depending on
farm loads, domestic cooking times
and so on.
Modern appliances with switchmode power supplies take it all in
their stride, but it is a different story
for the old-time stuff. As just one familiar example, until the advent of LED
lamps, globe life in our home was very
short indeed!
However, I have a recommendation for anyone who decides to build
one. If the variac of choice does not
already have one as an OEM fitting
(they usually don’t, in my experience),
I suggest a thermal overload should be
inserted in the secondary between the
variac and the outlet socket, rated at
the nameplate current of the variac.
It is very easy to damage a variac
by inadvertently overloading it, and
a simple thermal circuit breaker or
overload will prevent much heartache.
They are readily available, small in
size, moderately priced, and do a better
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job than a secondary fuse because they
will trip on small sustained overloads,
which can damage the carbon brush.
A photo is shown below of a typical
thermal overload breaker, stocked by a
reputable NZ supplier (mytools.co.nz).
John Reid,
Tauranga, New Zealand.
Eliminating transformers in the IMSC
I was interested in reading the recent
advice you gave to a correspondent
who wanted to run the Silicon Chip
Induction Motor Speed Controller
(IMSC) from 115V AC (April 2021,
Ask Silicon Chip, p110). I have built
a couple of them to run my pool filter
and bench drill (both from 230V), and
they work very well.
I have made some modifications to
the circuit for improved performance
that would also enable 115V AC operation. It does require some repackaging of the unit and is a project for
the experienced constructor, but the
results are beneficial.
I replaced both the transformerbased high-side and low-side 15V
and 12V DC power supplies entirely
with a couple of 12V 1A switchmode
supplies from eBay (intended for the
LED Christmas lights market). Many
sellers have them. These supplies are
enclosed, small in size and have a wide
input AC voltage range, operating from
90V AC to over 240V AC.
They allow the unit as a whole to run
from 115V AC since the high voltage
part of the circuit will be unaffected, as
noted in your advice. However, as you
mention, the low voltage protection
circuitry will require modification. I
could have used a 5V DC switchmode
supply for the cold side, but chose 12V
to standardise on one DC supply type.
This means I had to add a small flag
heatsink to REG1.
I changed the output voltage of the
high-side supply to the recommended
15V by replacing just one resistor in
the switchmode supply. By coincidence, I saw later (after I had traced
out the circuit) that Oatley Electronics
had posted the procedure to change the
output voltage of this generic power
supply on its website.
Notably, instead of powering the
switchmode supplies directly from
230V AC, I used the mains rectified
325V DC bus that powers the 3-phase
VFD chip. The same AC input terminals to the switchmode can be used,
and they are polarity independent.
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This modification has significant advantages for me.
Since the switchmode supplies operate down to below
100V, when power is switched off to the unit, the control
circuitry remains powered along with the VFD chip, so
the unit will continue to drive the load until the DC bus
drops below 100V, or until the undervoltage protection
kicks in (I don’t know which happens first).
When powering a 1500W pool pump, this process takes
less than a second. Even when the motor stops drawing
its load current, the switchmode supplies continue to discharge the DC power supply capacitors until the DC rail
is well below 100V.
This means that the load resistors across the high voltage caps for safety are not required, so a major source of
heating in the enclosure is eliminated.
I was surprised to note another benefit. Builders of the
IMSC will have noticed that when the power to the original unit is disconnected, the motor being driven stops
completely with a jerk. Another reader reported this phenomenon some time ago and asked a question as to why
this happens.
The answer proposed that the integral protection diodes
in the VFD chip were shorting the motor and acting like
a dynamic brake. I haven’t tested this or other theories
(I don’t have the HV test equipment or the courage) but,
when the switchmode DC power supplies are introduced,
this effect disappears, and the motor runs down smoothly.
I think the IMSC is one of the most useful projects presented by Silicon Chip. An industrial electronics-scale
project was a welcome addition to the project stable. It’s
a shame that packaged kits are no longer available for this
and, for that matter, most Silicon Chip construction projects – an unfortunate sign of these ‘maker’ times.
David Hainsworth, Westlake, Qld.
More on software for 3D printers
Thanks for your advice on software for 3D printers
(Ask Silicon Chip, June 2021, p108). I tried OpenSCAD
(https://openscad.org/) initially, and that worked so well
that I didn’t explore alternatives.
It is easy to use, with a very good Wiki-style user manual. It is one of few software products that didn’t provoke
me to put fists through computer screens. I noticed three
quirks, but these are more amusing than annoying.
It is a programming language, not an interactive pointand-click tool. Although that might seem odd in modern
times, it leaves a record of what I’ve done and allows me
to leave comments for myself. So if I don’t get my clever
gadget perfect the first time, which I didn’t, I can read
what I’ve done and fix it.
It resembles languages like LISP more than procedural
languages like BASIC or C, and everything felt back-tofront for a while. A bit like the German language it is,
with verbs at the end of the sentences. That encouraged
me to define my gadget using modules so that each module, Module N, had a pattern like:
Use Module N-1
Add these bits
Remove these other bits
Once I got the hang of it, I produced modules that
were quite pleasing and reliable. Although its documentation mentions variables, it also warns that they aren’t
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variables, but more like constants. For a proper programming language, this would create huge problems, but for
doing what OpenSCAD does, it is no worse than a mildly
irritating curiosity.
I sent my clever design to KAD3D, and they turned it
into a gadget that looks like the gadget I drew.
Keith Anderson, Kingston, Tas.
Historical articles enjoyed
First off, let me thank you for the effort that you have
put into the magazine over the years. Good work! Your
article on the humble three-pin Aussie plug and socket was
very interesting, and someone had done a lot of research
into it; a great read (September 2020; www.siliconchip.
com.au/Article/14573).
Same with the articles on the VCR and its not so humble beginnings (March-June 2021; siliconchip.com.au/
Series/359). Coming from the electronics service industry, now retired and living in Tasmania, that article series
brought back a whole lot of memories from a bygone era,
including my studies at the local TAFE.
Would it be possible to publish an article on the Compact Disc, DVDs, LaserDisc etc and the problems and formats that this medium provides? I find that type of article
interesting and a good read.
I re-read one of your articles from May 2019 (p104) on
The History of Stromberg Carlson, and Admiral Television. This was a blast from the past. My family had one
of these Admiral televisions (Imperial 800), and I can still
remember the set being delivered around 1958/9. I was
just a little kid at the time.
This set was still going up until my parents purchased
a colour television; whatever happened to that set, I do
not know! I remember that when Channel 0 started transmission (later to become Channel 10), these and a lot of
other sets needed the “0” biscuit modified to receive the
new channel.
The other time the set was repaired was when the flyback transformer failed. That, the horizontal output valve
(6DQ6) and damper diode (6AX4) were all replaced during
a house call.
When I started working, I was lucky enough to land
an apprenticeship as a radio and television trainee. I had
not long turned 15 at the time, and I worked in the television reconditioning section. Many of these Admiral TVs
were traded in at the time, and a lot were passed over as
being too hard to work on due to the “new” printed circuit boards they used.
I think that was before solder wick and solder suckers.
The standard iron at the time was the old scope 300W iron,
with no temperature control, and using one on printed
boards was a real learning curve. Too much heat and the
copper track would lift off the board. One soon learned.
Television techs of that era enjoyed point-to-point wiring, and the ease with which those sets could be repaired.
How soon that was to pass! I had a soft spot for our own
Admiral set and soon took to several of the Admiral sets
that were traded in.
One Admiral set I remember also had a radiogram in the
bottom of the cabinet. This pulled out like a drawer in a
cupboard. I remember that the radiogram had push-pull
output valves (possibly 6AQ6s) in the audio output stage.
Stephen Gorin, Mildura, Vic.
SC
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