■ FIGURE 4
and get cable or Direc TV?
says that their service
manuals are available only
to “authorized service
dealers.” Do you know of
some service like Sam’s
PhotoFact of years past
where I might get service
info for this item?
AA service manual
is available from
htm. It will cost $10 US.
A1) Digital TV does require a
better signal to noise ratio.
The phase lock loop in the
analog TV will stay locked
even when down in the noise but the
digital converter can’t tell the difference
between noise and signal, so has to
have signal well above the noise.
+- 10MV OFFSET
2 and 3) I would not expect digital
TV to work from rabbit ears unless
the transmitter was very close. I
recommend a roof mounted antenna
with amplifier and rotator. If that is
not feasible, look into a wide band
helical antenna that will fit inside.
ANot directly because the
input offsets will be different.
Two op-amps on the same
chip have the same offset,
so you can do it there, but these are
separate chips. Figure 4 is a way to do
it. You will lose a maximum of 0.3 volts
of output swing, but the current capacity
will be double. In the typical case,
you could get away with no offset
trim, but you should always design
for the worst case ( 10 mV offset).
QI have a Zenith (now LG)
DLP Projection TV model
Z44SZ80 that no longer will
power on (after a power
surge). Before sending it to the recycle
bin, I would like to try to fix it, but LG
QI purchased a DataStream
TV converter box from
RadioShack. With rabbit
ears, a preamp, and a VHS
in series I get 10 channels across the
VHF and UHF bands. I connected the
converter just after the antenna and
also tried just before the TV. Only
one station came in and did so very
well in the latter configuration. The
other channels said weak signal.
Eliminating the preamp and VHS
player did not work either.
1) Does digital TV require a stronger
2) Do I need to update my antenna?
3) Do you have a better
recommendation for installation?
3) Should I return the converter
and buy another brand?
4) Or, should I scrap the converter
4) Cable will provide more channels
and options, and will be more
reliable than the antenna, but costs
more. The choice is up to you.
QI am a subscriber to Nuts &
Volts and I LOVE your Q&A
column. I work in a failure
analysis lab; the reason I am
writing is that we will soon be getting
in an old battery charger (see Figure
5). The charger is designed to charge
a bank of large lead-acid batteries.
The batteries are in parallel and have
a float voltage of about 36 volts.
I understand the purpose of
T2, L2, D1, and D2 — it is a simple
full wave rectifier. But what is the
purpose of T1, C1, and L1? Why are
they doing that? Any ideas?
— Michael Craft, Saint Paris, OH
In the Q&A section of the August
‘08 issue, the schematic in Figure 6
and Figure 8 show a relay as the
collector load of a 2N2222 transistor.
As you know, when the transistor
turns off, the energy stored in the
inductance of the relay coil will
cause a voltage spike. Since there is
no snubber or diode across the coil,
the only way for the inductance to
discharge is by causing the collector-base junction to conduct as a zener.
Maybe there is not enough
24 October 2008
energy in the relay coil to destroy
the transistor, but I would think
repeatedly causing collector-base
breakdown would have some
negative impact on reliability.
— Jim Stewart
There should be a diode placed
in parallel with RLY1 with its cathode
connected closest to the positive
supply and the anode placed at the
other end of the relay coil. The diode
should not be switched out of the
circuit by any of the control wiring.
With no diode present, the back
EMF of the field collapse when
power is removed from the relay
will cause the transistor to fail and
control of the relay will be lost.
A diode such as a 1N4004
should do the trick.
— Mike Flood
Response: In my defense, the
diode is not needed in Figure 6
because the solar cell turns the
transistor off very slowly, so LdI/d T
will be small. In Figure 8, I added a
switch that will turn the transistor
off rapidly and the diode is needed.
Mea culpa once again.