DR #26 - NAD 1155 PreAmp Troubleshooting and Repair - Part 2
11 Video Views·Mar 12, 2025
In this video, I complete the work I started on this preamp.
welcome to part 2 of the NAD 11:55
preamp that I've been working on as you
can see I have a whole bunch of new
capacitors in front of me the owner has
agreed to replace all of them which i
think is a good choice that includes the
metal film style capacitor here and
that's going to replace this capacitor C
601 which again this is in the power
supply and it has seen some excessive
heat so I'm pretty sure this hasn't
failed yet but we want to get it
replaced before it does fail after we
replace all of the capacitors will check
the ESR of at least some to see where
they were at we'll also take a closer
look at the board and layout of the
board as well as the schematic to see
how this thing is operating and we'll do
a few tests we'll check the voltages
where we can I believe there's one or
two adjustments for those and of course
we'll do a final sound test before I
send this out and just like that every
single electrolytic in this unit has
been replaced as always I use either nit
chicken or Rubicon 105 C rated caps I
always use caps of identical capacitance
and caps of equal or greater voltage
rating there are no major surprises
during this replacement the back of the
board was very clean before it's still
clean now so there's nothing to show
there the only thing that I think is
worth noting that I found that was
pretty odd and and maybe someone can
tell me that it's similar on other nad
units or maybe even from other
manufacturers but the orientation of all
the electrolytic sizz the same they are
vertical facing this way with respect to
the front the back and even more than
that the negative lead of all of the
electrolytic are facing the back of the
unit which I thought was interesting it
took me about five or six caps before I
noticed that and is actually pretty
helpful the printout of the capacitor
alignment or polarization on the board
sometimes hard to identify and so it was
helpful to to not have to worry about
that I didn't have to remember the
orientation I just remembered that the
negative lead was always facing the back
so that made it pretty easy and was
helpful kind of a stickler it's really
unnecessary but I'm kind of a stickler
for making the caps look really nice so
you can see they're all pretty vertical
with respect to the bottom of the board
before many of them were installed there
were sideways facing all sorts addition
of directions sometimes they were
crooked as in one lead was hanging out
more than the other and so I really take
my time when I install them I try and
make sure they're the same distance say
a matching pair same distance away from
the board and that they're as close to
the board as possible and that the and
that they're aligned vertically if
possible and so that's it for that let's
take a look at the ESR of some of the
caps to see if any of them were bad
here's the evidence of all of my work
I've checked the ESR of most of these
capacitors and believe it or not all of
them are within spec that shouldn't be
too surprising being that there were no
symptoms failure there was no bulging no
leaking from any of the capacitors and
this unit was operating just fine before
I did the replacement but it's still a
great idea to replace these this unit
although I haven't checked the
manufacturers date I assume is 30 to 40
years old or maybe even older and the
capacitors appear to be original as well
they're all Sanyo 85 C rated caps and
they're all the same brand and they all
look to be the same age so I assume
they're their original and you know of
course these are going to dry out over
time and replacing them is only going to
improve the longevity of this unit so
I'm glad the owner went along with that
let's check the ESR of a few of these as
I said I've already checked them but
just to show you what they are or a few
of them anyway this is the biggest
capacitor in the unit this is one of the
smoothing caps and the power supply it's
a thousand micro farad at 35 volts the
spec for this is an ESR no greater than
zero point zero seven ohms and it's
passing just fine at 0.04 we'll look at
a smaller one this is 220 micro farad's
at 16 volts the spec for this is an ESR
no greater than 0.4 ohms and we're
sitting at 0.18 and we'll check one more
we'll look at the ESR of one of the four
blue caps in this unit this is 10
microfarads at 25 volts a spec for this
is an ESR no greater than 6 ohms and
this one is at 1.7 let's take a walk
around the unit and take a look at some
of the circuitry here so we can see how
it operates I'll start with the power
supply section the AC voltage enters the
unit through the back here in the line
cord power transformers here on the left
the secondary of the transformer enters
the board through these three pins the
two outer pins are fused which is never
a bad idea the AC voltage then enters
the bridge rectifier and exits as pulsed
DC the two large capacitors here are
going to smooth that pulse DC into DC
and we'll have something like 32 volts
entering the regulator stage so the
regulator stage is interesting on this
we have two transistors here that are
gonna regulate but we also in addition
to that have this IC IC 601 so all three
of these components are gonna work in
unison to regulate the voltage and it's
gonna output something like plus and
minus 24 volts to the rest of the unit
we have this adjustment pot here to
allow us to adjust to make sure that our
voltage exiting the regulator circuitry
is between 23 and 1/2 and 24 volts and
this is something we'll check in a bit
there's not much else to discuss
relating to this top section of the
board we have the record section of the
circuitry here along with the record
switch we have the photo section of the
circuitry here and we have the listen or
input selection switch here this lower
section of the board is comprised of the
headphone circuitry as well as the
amplifier circuitry the headphone
circuitry is isolated to this part of
the board right here while the amplifier
circuitry is contained a little bit over
here on the right as well as this entire
left section there's not much to say
about either of these sections other
than the amplifier circuitry is
utilizing this op amp this is a jrc
product IC 301 and the output
transistors are way over here on the
right Q 303 and Q 353
the only other thing I wanted to mention
while we're looking at this unit as we
saw in part one I removed cleaned
reinstalled and tested the volume and
balance potentiometers here and I did
mention briefly that the volume
potentiometers contain four pins which
is unusual for a standard potentiometer
although I didn't go into the utility of
that fourth pin the fourth pin as
mentioned by a few viewers in the
comments is utilized in conjunction with
the loudness control circuitry and so
this is the loudness control switch
right here and it utilizes a few of the
capacitors and resistors around it and
when enabled the loudness controlled
boosts the low and high frequencies of
your audio signal specifically this is
useful for when you're listening at low
volumes and low volumes apparently our
ears in our brain tend to attenuate or
decrease the the volume or the loudness
of low and high frequencies and so the
the sound of music and sound in general
at low volumes doesn't sound the same at
at moderate and high levels and so
that's the function of the loudness
control switch in conjunction with the
fourth pin on the volume potentiometer
and this is all fairly new to me I'm not
an audiophile I'm not even really an
into audio that much other than I enjoy
of course music like anybody else and I
also obviously enjoy repairing and
troubleshooting audio equipment and so
the loudness control circuitry is new to
me I had to do a little bit of
Investigation on this and so since it's
new to me and since I like to completely
understand things that are new to me and
I think will do a little bit of an
experiment at the end of this so we can
see how this loudness control switch and
the circuitry affects the signal at low
moderate and high volumes I have the
unit plugged in let's check the DC
output from the regulator section of the
power supply to see where we're at again
we have this adjustment pot here to
adjust the 24 volts if needed the
service manual tells us that we need to
look at the collector of Q 601
and we should be somewhere between 23.5
and 24 volts DC and as you can see we're
at 23 point six and so I don't see any
need to adjust this I now want to test
the loudness control circuitry of this
unit I have a theoretical graph of the
output versus frequency if we look at
the loudness control off state
represented by the solid line we can see
the output of the unit should remain
fairly constant across the frequency
range from above zero to 20 kilohertz if
we look at the loudness control on state
represented by the dashed line as I
mentioned earlier the output at lower
and higher frequencies should be higher
than medium frequencies this is to help
compensate for the attenuation or the
drop in loudness perceived by our ears
and our brain at lower volumes and so I
have a very simple test setup let's go
ahead and take a look at that so we can
see if the theory matches the results I
have my signal generator hooked up to
the input of the preamp we're looking at
the output on the scope this is about
one volt peak-to-peak I want to test
first using the loudness control in the
off position so we should see no change
in the amplitude as we increase our
frequency so I'll start at 100 Hertz and
I'll increase in increments of 100 Hertz
so there's two hundred three hundred
five one kilohertz you can see we
haven't changed our amplitude or output
at all
I'll now increase in increments of 1
kilohertz so here's 1 2 3 5 10 and
that's 20 kilohertz right there you can
see we're still at right around 1 volt
peak-to-peak so indeed with the loudness
control switch in the off position our
output amplitude has not changed at all
no matter where we are on the frequency
spectrum so now let's test with the
loudness control on we'll go back to the
start up conditions at 50 Hertz or
excuse me 100 Hertz and I'll turn the
loudness control on now and you can see
just doing that almost doubled the
output to
very near two volts peak-to-peak so we
can see it's probably working already
so again theoretically at the low end of
the frequency spectrum we should we
should have a high output or a high
amplitude at some point as we increase
across the frequency spectrum the output
should decrease and so let's see if that
happens I'm gonna increase by 100 Hertz
at a time so we're at 100 Hertz let's
increase now to and you can see we're
already dropping and amplitude and
that's 500 Hertz we're already at very
near 1 volt peak-to-peak so indeed we
are dropping in amplitude now I'll
continue to increase again theoretically
we should have nominal output or
amplitude in the middle range of the
frequency spectrum but at some point we
should start to increase our amplitude
again so I'll increase and we should
start to see this output increase we're
at 4 kilohertz right now 5 and it's it's
hard to see while I'm increasing so
slowly so I'm now I'm going to increase
in increments of 1 kilohertz so I'm at 5
kilohertz I'll increase that's 6 7 and
now you can really see the output
amplitude is increasing drastically as
we increase our frequency so this
loudness control circuitry is definitely
doing as we expected the theoretical is
matching the actual that's it for the
nad 1155 preamp I appreciate you
watching we'll see you next time
[Music]