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This is only a preview of the January 2016 issue of Silicon Chip. You can view 39 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Raspberry Pi Temperature/Humidity/Pressure Monitor Pt.1":
Items relevant to "Valve Stereo Preamplifier For HiFi Systems":
Items relevant to "High Visibility 6-Digit LED GPS Clock, Pt.2":
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Valve Preamplifier
. . . continued from page 35
Make sure nothing conductive is
near the PCB and it isn’t close to the
edge of your bench. Then, keeping clear
of the assembly, plug the power supply
into mains. Within about one second
of power being applied, the HT voltage
should reach 285V or thereabouts and
stabilise, with the green and red LEDs
lit. Either way, switch off power and
wait for it to discharge to a safe level
(below 40V) before continuing.
If there’s a fault, once the HT rail has
discharged, check component placement and orientation as well as solder
joint integrity.
Assuming all is well, connect regular probes to your DMM but leave it
on the 300V (or higher) range. Power
the board back up and measure the
voltage between pins 4 and 5 on both
valve sockets (see Fig.6). You should
get a reading close to 12.6V. Now check
the voltages at the other pins relative to
GND. You should get ~285V for pins 1
and 6 and close to 0V for pins 2, 3, 7 and
8. Pin 9 is not connected to anything.
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You can now switch the power off
and push the two valves into their sockets. They will be stiff, especially if this
is the first time the sockets have been
used. You may find it easier to gently
rock them in. While you can in theory install the valves with HT voltage
present, it’s much safer to wait for it
to decay first.
With the valves in place, power back
up and check the HT voltage, using the
test pads in the centre of the board. It
should rise to around 270V at first and
then slowly decay to around 250-260V
as the valves warm up and their operating current builds.
In the unlikely event that the HT
supply remains above 280V and there
are no board or valve faults, this may
be because component variations are
causing the supply to deliver more current than it’s designed to. The simple
solution is to reduce the value of the
150pF capacitor to 120pF. This will increase the switchmode frequency and
reduce the duty cycle and should bring
the HT back in line. If you need to do
this, don’t forget to wait for LED2 to
go out before working on the board.
Finally, perform a live signal test.
Switch off, wait for LED2 to go out and
connect a signal source to CON1/CON2
and an amplifier to CON3/CON4. Next,
turn the volume right down, power on
and wait 30 seconds or so for voltages
to stabilise. Then press play on the
signal source and slowly advance the
volume until you hear clean, undistorted sound.
If the sound is distorted or missing,
switch off and carefully check the component values around each valve socket as well as the solder joints.
Putting it in the case
That’s all for this month. In the sec-
Advertising Index
Altronics.................................. 72-75
Digi-Key Electronics....................... 5
Emona Instruments...................... 65
Front Panel Express....................... 9
Hare & Forbes.......................... OBC
Icom Australia.............................. 17
Jaycar .............................. IFC,45-52
KCS Trade Pty Ltd.......................... 3
Keith Rippon ................................ 95
LD Electronics.............................. 95
LEDsales...................................... 95
Master Instruments...................... 95
Ocean Controls.............................. 6
Radio & Hobbies DVD.................. 62
Sesame Electronics..................... 95
Silicon Chip Binders................ 64,96
Silicon Chip Online Shop............. 86
Silicon Chip Subscriptions......... IBC
Silvertone Electronics.................... 7
Tendzone...................................... 11
Tronixlabs.................................. 8,95
ond and final article next month, we’ll
go over the details of how to put together the custom laser-cut case and
SC
fit the PCB inside it.
WARNING!
SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such
projects should be considered dangerous or even lethal if not used safely.
Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or
high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you
are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone
be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine.
Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability
for projects which are used in such a way as to infringe relevant government regulations and by-laws.
Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the
Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable.
96 Silicon Chip
siliconchip.com.au
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