ranges, I only get 5mA or so test current for both. It should be 6.3mA for
63V and 10mA for 100V. What should
I do? (A. J., via email)
● The test voltages at those settings
could be incorrect, or the step-up circuit isn’t able to supply enough power.
Check the open-circuit voltage with
a multimeter to see if 63V and 100V
are being generated. If not, check the
divider resistors at pin 5 of IC1 for
correct values.
If the voltages are correct, then with
the multimeter connected to the battery ground and the positive test terminal, short the test terminals. The
voltage shouldn’t drop. If it does, perhaps the windings on transformer T1
are incorrect or the insulation on the
Advertising Index
Altronics.................................23-26
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Emona Instruments.................. IBC
Jaycar............................. IFC, 51-54
Keith Rippon Kit Assembly....... 103
Lazer Security........................... 103
LD Electronics........................... 103
LEDsales................................... 103
Microchip Technology.............OBC
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PCBWay......................................... 7
PMD Way................................... 103
SC Bridge Rectifiers.................... 83
Silicon Chip PDFs on USB......... 50
Silicon Chip Shop................ 67, 77
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The Loudspeaker Kit.com............ 9
Used Gear.................................. 103
wires has not been properly scraped
off where it is soldered to the PCB.
How to add a volume
control to any amplifier
My question is around fitting a
potentiometer for volume control on
the Hummingbird Amplifier (December 2021; siliconchip.au/Article/
15126). I bought a single kit of the
Hummingbird Amplifier from Altronics (Cat K5158). I have been through
the instructions and read the last two
pages about setting it up. I just want
to set it up and run it with one channel of audio from ±15V DC.
I bought a 30V centre-tapped transformer (Jaycar MM2005). I have built
the Universal Power Supply board and
I’m going to run it with the ±15V DC
configuration.
Where do I fit a potentiometer to
control volume? I couldn’t see any
mention of this in the instructions. Do
I need to buy a separate volume control module? (E. M., Hawthorn, Vic)
● Referring to the accompanying
circuit:
1. Connect the input signal ground
to the ground of CON2.
2. Connect the input signal conductor to the clockwise end of the potentiometer track (‘B’) via a series capacitor. With the pot shaft facing you and
the pins down, this will be the righthand pin.
3. Connect the pot wiper (middle
pin, ‘W’) to the signal input terminal
on CON2.
4. Connect the remaining pot pin
(‘A’) to the signal ground (either at
CON2 or the input connector, whichever is convenient).
The value required for the series
capacitor depends on the potentiometer value. For example, if using a
10kW potentiometer, use a minimum
of 2.2µF (we’ve shown it as 10µF in the
circuit, which allows a non-polarised
electrolytic to be used). You could use
a 2.2µF greencap or similar.
You can get away with lower values
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Silicon Chip
SMD markings are
inconsistent
I bought the SC6988 SMD kit for the
Discrete Ideal Bridge Rectifier (September 2024 issue; siliconchip.au/
Article/16580). I got a bit stuck trying to identify the numerous SOT-23
parts. The confusing aspect is that if
you search for a BC856, for example,
you might find a package code of “3D”,
depending on the variant, not “9AC”.
By searching on the package codes,
I think I have correctly identified the
parts – it would be useful for others
to have these codes – providing you
continue to provide the same variants
in the kit.
Qty Code Device
2
1D
MMBTA42
2
2D
MMBTA92
4
9AC BC856CMTF
4
1C
BC847C
4
Y2
BZT52C12/BZX84C12
Is that correct? (I. T., Duncraig, WA)
● The codes you list look correct.
It is unfortunate that the same part
from different manufacturers can have
different markings. Each part should
ideally have a standard marking for a
given package. The BC84x we supply
is actually a BC846C and it is indeed
marked 1C.
We have BC856Cs marked both 15S
and 9AC; obviously, the latter is what
we used for your kit. Y2 is the code
listed in the Fairchild data sheet for
the BZX84C12. Our supplier lists the
MMBTA42 as 1D (the ST Micro data
sheet says it can also be A42) and
MMBTA92 can be 2D or A92.
SC
You can
add a
volume
control
to most
amplifier
modules
like this.
Wagner Electronics..................... 10
104
if the pot resistance is higher, but don’t
make it too high, or it will introduce
noise. 5-20kW is ideal for a low-noise
amplifier, although value at the lower
end of that range will load the signal source more. Most modern signal
sources shouldn’t be bothered by that.
If any of this requires significant
cable runs, use shielded cable. You
can use the shield to make the ground
connections.
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