DR #21 - B&K Precision 1477 Oscilloscope Tear Down & Tune Up
1 Video View·Mar 12, 2025
In this video, I completely tune up my one and only B&K 1477 scope. Specfically, I remove all 3 PCBs, replace all electrolytic capacitors, replace thermal paste between transistors and heat-sinks, clean all variable resistors, and finally test the scope.
hey everyone welcome to duality repair
I'm gonna be looking at my own B&K
precision 1477 oscilloscope
this scope has served me well for the
better part of two years however I don't
have any information on the maintenance
history and I've never worked on it
myself so I thought I'd take this time
to give it a tune-up if I can it's been
functioning pretty well lately I think I
only see one minor problem so I'm on
channel a here and I'm hooked up to the
calibration trace really the only
problem is just a little bit of noise at
the counter-clockwise end of travel you
can see a little bit of noise there as I
move it the counterclockwise end is
pretty smooth and in it I don't feel any
resistance or roughness as I'm moving
the knob it's just the electrical noise
there so I think that'll just need a
little bit of contact cleaner on that
knob and I'll do the same for the
channel B's position knob although it
doesn't show quite that much noise
that's pretty much the only problem that
I see that triggers pretty well and you
know the traces are both really nice and
clean and everything else looks pretty
good on them the thing I want to do will
have the unit open first of all it's
just out of curiosity take a look around
see check the layout of this I've never
worked on one of these before I do have
the service manual so I'll be able to do
some pretty in-depth work if I need to
I'll inspect all of the boards for any
signs of leaked or bulging capacitors
and I'll spot check some of them for
their ESR to see if any of them need to
be replaced if it looks like they're all
original or they're all really old I'm a
recap the entire unit anyway just to
keep this thing running as this is my
only scope and I want it to last as long
as it possibly can
so next I'm going to open it up and
we'll take a look around
and here we are inside the scope this
thing is as close to immaculate as I
think it could possibly be I hardly see
any dust bunnies all of the board's look
to be in tremendous shape I don't see
any signs of anybody tampering or
removing replacing any components on
here so far I'll have to take a deeper
dive into each board as we get to them
but so far it looks to be in really good
so hats off to all of the previous
owners however many there may have been
for taking such good care of this thing
let's take a quick look around before we
dive into each individual board so
obviously we have the CRT here with this
black shielding covering it and then we
just have three boards so we have the
power supply board on the back with
obviously the main transformer here on
the side here we have the sweep board
and then on the bottom the vertical
amplification board so we'll work on
each board individually I'll start with
the power supply board I originally
thought this was going to be a real big
pain to get this thing out because not
only do I have to remove it from this
frame here which wouldn't have been too
bad on its own but I also think I would
have had to remove this transformer as
well as remove this bottom cover in
order to clear the end of the CRT you
can see the end of the CRT ends right
here so there's about two inches of
clearance that I would need it didn't
really seem feasible but luckily it
looks like this whole frame that this
board is mounted to will separate from
the chassis independently there's just a
few screws on this side a few screws on
the other and probably a few on the
bottom this whole thing should separate
after of course they disconnect all of
the connectors coming from the two
boards in the front and see some of the
wires from the bottom here they all roll
out to the power supply board so let's
disconnect there and then I should be
able to separate it before we can work
on it okay it wasn't too difficult to
remove the power supply board from the
unit let's take a look around and make a
few notes on the general condition of
the board before we look at the
circuitry we're looking at the
components and the solder joints I was
able to confirm that all of these
components are original which is
astounding based on the age of this unit
it's it's 30 plus years old just
astounding another note one thing I was
originally worried about was this glue
here you can see the glue on the bottoms
of the capacitors I was worried about
this because some glue over time
especially over several decades can can
become conductive but I verified with a
multimeter that this is not conductive
so that's not a problem
and then another note is inspected all
of these solder joints here and they're
all in really good shape I don't see any
signs of cracks they're not even dull
they're pretty shiny so I don't think
I'm going to have to touch these up at
all which is good news to be honest even
though it's good practice if I try to
touch all of these up I'd probably end
up causing a problem that wasn't there
to begin with so I'm gonna leave those
alone now the individual circuitry
itself there's three basic sections of
this so the first section is on the left
half of the board and this is the power
supply section it's going to supply most
of the voltages for the board and the
unit with the exception of one or two
CRT voltages this upper right quadrant
here is the vertical amplification
section it's going to amplify the
vertical signal before it's sent to the
vertical deflection plates in the CRT
and this bottom right quadrant outlined
by this white line on the board here is
a DC to DC converter you can see the
transformer here the function of this is
to supply high voltage to the CRT so we
can walk through a little bit of the
circuitry individually we'll start in
the power supply section on the left we
have the power coming in from the mains
here it's going to go to the power
switch in the front through this
connector and come back into this into
the board through this connector it'll
go through the fuse here into the
transformer from this connector it'll
come out of the transformer at this
connector or it'll enter the board now
you can see there's three bridge
rectifiers here there's three different
taps on the transformer so there's three
separate rails coming from the
transformer you can see up here we have
an assortment of electrolytic this is
going to filter the voltages from each
individual rail
we have several transistors around the
board here around this section and a
couple down here these are gonna
regulate the voltages we have this
package here
I'm really glad this is functioning
because I don't know if they make this
package anymore this is a dual op-amp I
am used to the eight pin dip so I'm
really glad that that's functioning I
don't know
they make a an op-amp what's a package
like that anymore so it's good that
that's functioning so that's the power
supply section obviously most concern is
going to be the electrolytic capacitors
those are all going to be replaced as
they are all original and they're 85 C
rated anyway so they should be replaced
we move on to the upper quadrant again
that was the vertical amplification
section really the only thing to note
here is you can see the four transistors
here that are mounted as the heat sinks
these are going to be the amplifiers of
the vertical signal which is why they're
monitor the heat sinks they have a high
supply voltage and they're gonna have a
lot of current running through them and
so they need to be kept cool to to
prolong their life and then we'll move
on down to the bottom quadrant here this
again is the DC to DC converter which is
going to supply the high voltage to the
CRT so we have this inductor and
capacitor here this is gonna be part of
an oscillator circuit which is going to
drive this transistor here and they
can't see it it's just the heatsink
it's a transistor mounted to that
heatsink now you can't see it there
either and it's going to drive this
transformer the primary the transformer
and then the secondary obviously it's
going to supply the high voltage to the
CRT so with the board in such good shape
you might ask well why do we need to do
anything with it if any broke don't fix
it right well with a board this age it's
always a good idea to replace
electrolytic
even if they test well now they could
fail at any moment and so I did measure
the ESR of all these capacitors I
believe there are 11 most of them are in
spec a few of them are near the spec and
two of them are failing so this one
right here is failing fairly badly this
is the filter capacitor for the 195 volt
rail so not surprising that it's failing
and then this odd one down here and I
say odd because it's not the radial
style like this that's the standard
style but it's axial so it's got a pin
on either end this one's also failing
really badly the the spec is something
like 20 ohms and it's measuring
something like 80 ohms so really badly
these two absolutely need to be replaced
since I'm replacing them and
the age of the board and since I don't
want to come back in here I'm gonna
replace all of them so the caps are on
order they'll be here soon and the only
other thing I really wanted to do as far
as preventive maintenance wise is to
remove the thermal compound between each
of the transistors and the heat sinks
and reapply new thermal compound the
reason again due to age the thermal
compound you can see the white on there
and you're not gonna be able to tell
what I'm doing but it's really brittle
it's no longer you know it's no longer
liquid at all it's it's it's really hard
and it's not gonna maintain its normal
conductivity much longer if it is even
now so it's really important to keep the
transistors that are mounted to heat
sinks cool so it'll be important to
remove that thermal compound and reapply
new thermal compound so I will be back
once I have the new capacitors installed
and I have all of the heat sinks cleaned
and new thermal compound reapplied I
reapplied the thermal paste on all of
the six transistors that are mounted to
heat sinks I have the new capacitors in
you can see the old ones are in the back
and the new ones are in the front two
things to note obviously you can see the
size difference on the new ones they're
all significantly smaller with the
exception of the radial one over here
and maybe this thousand microfarads not
too much smaller than the old one but
other than that look at the look at the
difference in this biggest one here so
again this is a 47 micro farad a 250
volt and this is an exact equal 47 micro
farad or 250 volts higher temperature
rating even 105 C vs 85 so much smaller
it's amazing what the new capacitors can
do and then ESR so again I mentioned two
of them failed this is the one that
should have been 0.6 or less 0.6 ohms or
less you can see that is reading 2
that's the same as what it was reading
in circuit about 2 ohms and a little
disappointed to see that the new one is
reading better but it's still actually
out of spec which is what's
disappointing and I got two of these on
the same value and they're both reading
that something like 1.6 1.8 so slightly
better but still out of spec
no still gonna replace it since it's
brand new and the other one the radio
one this one again the spec is 20 no
greater than 20 ohms and it's reading
really high 78 ohms and at least this
new radio one is reading well in spec at
least much better than that 120 ohms
this is a higher voltage rating this is
the old one is rated at 250 volts the
new one is rated at 350 volts they
couldn't find a 250 volt radio at 1
microfarad so that's what I had to go
with but so here are the new capacitors
I'm going to get them installed and I'm
gonna reinstall the board and I'll be
back
all right it was a bit of a hassle but I
was able to remove the sweep board from
the unit as you can see I had to remove
this front panel here as well as the CRT
screen and I had to remove all of these
nuts as well there were not some almost
all of these knobs just to get this
board out I had to desolder one of the
potentiometers since the way it was
mounted and the chassis would not have
come loose otherwise so I decided it and
removed it from the board and here it is
so let's take a look around again this
looks to be in good condition all the
solder joints look to be unharmed no
cracks which is good and the capacitors
are all original again and they're 85c
so these will all be replaced and I will
check the ESR of all of those and
compare them to the new ones just to see
how much of an improvement we can expect
but just the capacitor replacement alone
so we can take a look around the board a
little bit obviously again this is the
sweep board so it's among other things
responsible for controlling the sweep
timing and the sweep signal to the CRT
the horizontal deflection plates so we
only have four connectors on here so
this connector over here on the left
these pins here is the power input so
this is going to be a power input from
the power supply this adjacent connector
here is kind of interesting so this
connector looks like it goes to the
vertical amplification board and it
actually supplies voltage or power from
this board to that board so it's kind of
a throughput so we have power supply
coming in to this connector and then we
have more power coming out from this
connector to the vertical amplification
board interesting we have this connector
up top which is going to share the clock
signal between the vertical
amplification board and this board and
the connector down here on the bottom
which is going to share the signal so
channel a channel B and the ground
signals between again the vertical
amplification board in this board we
have some familiar components this
component right here in the middle is
going to be the sweep generator it's a
flip-flop a whole bunch of transistors
all of these transistors are going to be
responsible for amplifying the sweep
signal with the final transistors here
these biggest ones are gonna be the
final outputs or the final amplification
and then we have these two posts here
right here and right here these are
gonna be the posts that send the signal
directly to the CRT horizontal
deflection plates and we have a few
switches and knobs so these knobs or
switches right here this top one is the
synch switch so it's gonna you can
select between positive and negative
slope and positive and negative video
syncing and then the bottom one is going
to be your source selector so source or
channel a channel B or the external and
then of course the top is the sweep and
so that's the one I'm really gonna want
to focus on and we have the one on the
bottom as well or the back and that's
the triggering switch or the triggering
pot and that's it so we obviously have a
lot of potentiometers here for
adjustments which we'll go through if we
can if I have the equipment wants them
complete as far as maintenance as I said
I'm gonna replace all of the capacitors
the other thing I'm gonna want to do is
take a look at all of these switches and
and dials here so these two types of
switches I've never had any experience
with I'm gonna look at them more closely
to see if there's anything I can do to
clean them regress them whatever I don't
think there is and if there is I don't
think it's necessary but I'll take a
look at that and I'll do that if
necessary the trigger pot on the back
I'll spray a little contact cleaner in
there and work it back and forth and
clean that out and then the sweep knob
in the front here so it's got that pot
in the very back I'll do the same thing
I'll pour some or spray some contact
cleaner in there and work it back and
forth that's with this front knob
actually I remove the knob but that
works the back pot there and then the
actual selector switch you can see it's
kind of got two adjacent boards here and
I was thinking of spraying contact
cleaner on those as well but that's it
would be overkill these are just too
exposed it would make a mess it's
unnecessary so I think what I want to do
if you see all these things work is you
have these contacts here
that lead down to the board and you have
this wheel in the front and the wheel is
what's turning you can see it moving in
there and it's difficult to get the
light on it but the contacts are making
tracks or grooves on that wheel and
there's a little bit of noise when I
change this so I think if I spray some
contact cleaner on a q-tip and then work
it on that wheel and then work the wheel
around a bunch of times and clean it off
and maybe repeat and do this for the
front and the back
I think that'll clean that up pretty
well and I'll be able to remove any
noise that was there so that's it for
the sweep board I'm gonna get to work
I'll replace the components I'll clean
the switches that I can and we'll get it
installed and I'll probably want to test
it before I move on to the vertical
amplification work the work on the sweep
board is just about complete I say just
about because there was an oversight on
my part when reading this capacitor here
see - OH - I did order a 1 microfarad 50
volt electrolytic but it's actually a
non polarized or bipolar capacitor so I
have to wait for that capacitor to come
in I have ordered it so before I can
finish this board otherwise you can see
I've recapped it with all new 105 C
rated energy con direct replacements and
they went in no problem and measure the
ESR of all the old ones and compared
them to all the new ones and some of
them not all of them were out of spec
and all the new ones look really good so
I'm happy about that
I was able to clean the sweep knob
pretty good again we discussed that
wheel earlier I just ran a q-tip soaked
with contact cleaner along that wheel
and I worked it back and forth and I did
that several times for this side and I
also notice that there's a wheel on the
back side as well it's kind of hard to
see there so I did the front and back
for both and then I sprayed some contact
cleaner in this pot here work that back
and forth and as well as this pot work
that back and forth as well
so once I get this new non-polarised
capacitor in I'll install that and then
this board will be ready to install
while I'm waiting for that I'm going to
move on to the next board the final
board which is the vertical
amplification board now I have removed
it obviously this one was really easy
thankfully to remove once you remove all
of the nuts from the front faceplate
there's just the two tabs that you have
to remove from the back and then this
board slides right out really nice
so let's go over this board quick from
left to right mm-hmm we have channel a
position knob again this is one of the
things they definitely want to focus on
it's fairly noisy at least at one end so
I'm gonna want to clean that pretty well
you can see there's two potentiometers
here this front one is the position knob
this back one is actually the DC balance
for channel II and you rotate that by
that Center pin there and you just use a
flat head now that's an actual part of
the adjustment procedure so I don't want
to touch that so I'm just gonna want to
clean this front pot really well and
I'll work that back and forth I think
the best way to do it is to spray a
little cleaner in from the bottom you
can see there's a good opening in the
bottom and then work it back and forth
and of course I'll do the same for
channel B's it's got the same setup the
position pot is in the front and the DC
balances in the back I'm only going to
mess with the position pots you can see
the inputs here so you have channel A's
input here and Channel B's here and you
can see they come right here into the
board into this input capacitor irie
soldered those inputs they were a little
flimsy looking so I restarted them up
pretty well they look a lot better a lot
more stable and robust then we have the
selection switches here so this is the
let's see DC AC ground switches or
levers they call them so there's one for
channel a and channel B the coupling
levers so again I'm not going to touch
these I looked at the same type of lever
on the sweep board there's no noise on
these and I don't think it's worth
trying to clean those so I'm going to
leave those alone I'll do the same on
these boards or this board I'll leave
that alone and the next we have the
volts per division knobs for channel a
and channel B respectively these again
have the same type of selection switches
as the other one it's got the wheels
these have three boards each or three
wheels each the wheels are a lot smaller
but I'm going to clean all three of
those using the same method with the
q-tip and the contact cleaner and work
it back and forth several times and then
I'll clean the pots back here these are
two pots adjusted by these front knobs
here so I'll clean those up really well
and then we have the same type of lever
selection switch or whatever in the
center here and this is for the what's
this for oh the mode selection so
channel a or channel V or channel a and
B plus minus again I'm gonna leave that
alone so I'm gonna work these two V ours
here and I'll work these two selection
switches here and then of course I'll
replace all the electrolytic there's
only seven on this board so the least
invasive work will be on this board you
can see it's pretty symmetrical we have
two on either side here we have two up
here and another right here so I'll
replace all seven of those I have those
so I'll get those replaced
I can look at the layout of the board a
little bit so again we have our inputs
here and here and we have our volts per
division selection switches here we have
at NU ater for channel a here and
attenuator for channel B here and when
we have theirs inside here there's just
two ICS IC 101 and IC 102 those are
going to be to handle the selection so
if you want to display channel a or
channel B or both that's going to be
done with these two I season here we
have some transistors back here you can
see several of them and these are going
to be pre amplification for that
vertical signal and we have this this is
actually an IC here so this is the
output IC this is the final part of the
path before the signal is sent out
through this connector here to the power
supply board where we saw it has the
output section where
final amplification will take place
before it's sent to the CRT so that's
about it I'm gonna get to work I'll
replace the capacitors I'll clean these
potentiometers and these switches here
and this board will be ready to go back
in once I get it back in I will finish
the sweep board work I'll install that
and then we'll be ready to test ok I'm
finally done with all of the work on the
scope I have the scope back and it's
home take a look at all of the
capacitors I've replaced now let's take
a quick look at the performance results
so I have both channel a and channel B
hooked up to the calibration port and if
I move channel ease position up and down
you can see there's no noise at all on
channel a and there's no interference
with channel B and I can do the same
with channel B's position knob again no
noise and no interference with channel a
so that's much improved the other thing
that may be difficult to tell on camera
but I can certainly tell based on
experience with using the scope is that
the traces are a lot more focused so
that's a positive result as well I
actually noticed that after I replace
the capacitors in the power supply
section so I'm really happy with the
results of this I think I've definitely
prolong the life of my scope and I look
forward to using it for many more years
and in many videos to come so thanks for
watching we'll see on the next one