siliconchip.com.au Australia’s electronics magazine July 2019 109
I previously wrote in to ask wheth-
er I could use a 1N4148 diode in-
stead of a BAV21 for D2, the diode
across the VAS transistors (Q7 & Q8)
in the lower-power version of the
SC200 amplifier module (January-
March 2017; siliconchip.com.au/
Series/308).
You pointed out the IN4148’s re-
verse voltage rating was (just) in-
sufficient, even with the ±42V DC
rails from a 30-0-30V transformer. I
decided to risk it anyway.
Could you explain what function
D2 performs in the SC200 design?
After building the SC200 mod-
ules, I decided to build your Ultra
low noise remote controlled ste-
reo preamp (March & April 2019;
siliconchip.com.au/Series/333).
Having just finished the preamp,
I noticed that the right channel was
intermittent at full volume. I also
noticed that the output of the left
channel SC200 module was low in
volume and had distorted treble.
I feared that this might be due to
my use of the 1N4148 diodes in placeof the BAV21s. I thought I might have
damaged the input section or VAS
transistors.
But it turns out that I had only
installed a 500mA fuse in the nega-
tive rail for the left channel, which
unsurprisingly had blown (the oth-
er three fuses were the correct 5A
values). Should I use 4A rated fus-
es instead, given that I am running
the modules from a lower supply
voltage?
The fault with the other channel
turned out to be a faulty motorised
pot in the preamp. The ground-
side pin to the right channel track
wasn’t riveted properly on the phe-
nolic board, and any pressure on the
preamp PCB caused an open circuit-
earth on the pot.
I used curved pliers to squeeze
the rivet and stop the end of the pot
track going open circuit. So far so
good; it would be a pain to have to
replace the motorised pot.- D2 speeds up recovery from clip-
ping, which improves amplifier sta-
bility when it’s driven hard and also
Role of BAV21 diodes and fuse ratings in the SC200 amplifier
reduces distortion under those con-
ditions. We explained this in more
detail in the January 2013 issue
(siliconchip.com.au/Article/1322).
That article was on the Ultra-LD
Mk.3 amplifier but it applies to the
SC200 too.
The amplifier will work if you
leave them out; many of our earlier
designs lacked this diode, but it’s bet-
ter to have it in case you ever drive
the amplifier module into clipping.
The DC fuse ratings are not that
critical. You certainly could use 4A
fuses with the lower supply voltages
and they may provide slightly better
protection. In reality, they are only
likely to blow if the output transis-
tors go short circuit and in that case,
the instantaneous current will be
well above 10A so they should blow
pretty fast either way.
Thanks for explaining the interest-
ing potentiometer fault. That is not
one we’ve encountered before, but
the phenolic boards used in many
pots are quite fragile compared to
fibreglass (FR4) boards.free contact between the hull and
transducer will do the job. We speci-
fied silicone grease because it won’t
dry out or otherwise go bad over time.
Most other types of thermal grease
should also be suitable.
Reluctor not triggering
ignition system
I built your High-energy Ignition
System for Cars (November-December
2012; siliconchip.com.au/Series/18)
from a Jaycar kit, Cat KC5513. But I
can’t get it to work.
I intended to install the system on
a Honda CB125S engine. I was restor-
ing one and I had actually built/re-
stored a second, more powerful ver-
sion (stroked).
These engines originally came with
points ignition, but later models had
reluctor triggers with capacitor dis-
charge ignition (CDI). All the parts
were interchangeable, so I built the
unit in the reluctor-triggered version
and installed a reluctor trigger in my
new engine.
I also converted the 6V system to
12V (by rewinding the stator and in-
stalling a modern voltage regulator).
Amazingly, everything worked and
this new engine started right up. But
almost as amazingly, I could not get
the engine to run above idle and spent
hours trying to diagnose it.
I decided to simplify things and re-
turn to the original, stock engine which
only had about 40 hours of operation
on it. So I pulled the Kettering ignition
out and installed the reluctor trigger
that had been in the second engine.
Now the ignition system will not de-
liver a spark.
I immediately tried the “diagnostic
mode” (using LK2) in-situ and it fires
the spark plug merrily at a fixed fre-
quency. This suggests to me that the
trigger input is suspect. I tried chang-
ing the trigger polarity using LK3 but
that didn’t help. I also installed a tran-
sistorised timing light to check for
missing sparks, but no luck. It is not
sparking at all.
I connected my digital Fluke mul-
timeter set to AC volts to the reluctor
trigger outputs and kicked the engine
over. The meter shows a signal from
the reluctor. It reads about 0.4V AC
while the engine is spinning. It is amomentary transient event, so the
peak voltage is likely higher. Can you
help me get it working? (G. N., Min-
nesota, USA)- Your reluctor output voltage does
seem rather low. Typically, you should
get about 30V AC at high RPM, reduc-
ing to around 2V AC at low RPM. This
should be measurable with a multime-
ter, although at low RPM your meas-
urement would be an averaged value,
so possibly lower than expected. An
oscilloscope is a better way to observe
the waveform.
Also try measuring the reluctor re-
sistance. Maybe it is open circuit. It
should be around 1kW-10kW. Or it
could have a shorted turn, preventing
it from producing sufficient output, or
the reluctor gap is too large. Check if
this gap can be reduced. Also, check
if the reluctor has one side connected
to chassis.
If you have a mains transformer or
AC plugpack which produces 5-12V
AC, this voltage can be used to check
if the Ignition System is working by us-
ing its output as a fake reluctor signal.
Check that the trigger voltage at TP
TRIG (the collector of Q2) changes