Tek 475 Scope Help???

[RB]

Hi....recently I picked up an old Tek 475 200Mhz scope not working
I managed to get a full service manual for it but have had no luck getting
it running.

The +50 Volt rail (which I belive all other rails are derived from) is
perfect, as is the 110V, +15, +5 rails.

The negative rails are out, the -8Volt rail reads -6 volts but the -15 Volt
rail is way out at -23 volts.

Im not an expert with this stuff but like to have a go, it would be a shame
to chuck it out.

Its really annoying me because the schematic for the -15 volt supply "looks"
so simple, Ive checked caps, all resistors, transistors and even replaced
the IC (MC1458) with no change at all.

The CRT works fine, lots of brightness, the vertical amps both appear fine
but the timebase is werid, some (lower) ranges wont display a trace at all
and the higher ranges show multiple horizontal line (with no input) and the
trace "curves" to the left hand side of the screen.

If I apply the cal signal input I can see the square wave but triggering is
erratic, reminds me of my first "KnightKit" scope I got when I was a kid
that used a thyratron in the timebase.

Info/parts are a little scarce in this part of the world (Australia)

Does anyone have any hints/suggestions????
Thanks

 

[Steve]

Check the filter caps in the power supply. You've got a terrific scope.
With a little patience and a few caps you'll be up and running.

The erratic sweep could be caused bt dirty contacts on the sweep switch.

Steve

 

[Jim Yanik]

Hi....recently I picked up an old Tek 475 200Mhz scope not working
I managed to get a full service manual for it but have had no luck
getting it running.

The +50 Volt rail (which I belive all other rails are derived from)
is perfect, as is the 110V, +15, +5 rails.

The negative rails are out, the -8Volt rail reads -6 volts but the -15
Volt rail is way out at -23 volts.

Im not an expert with this stuff but like to have a go, it would be a
shame to chuck it out.

Its really annoying me because the schematic for the -15 volt supply
"looks" so simple, Ive checked caps, all resistors, transistors and
even replaced the IC (MC1458) with no change at all.

The CRT works fine, lots of brightness, the vertical amps both appear
fine but the timebase is werid, some (lower) ranges wont display a
trace at all and the higher ranges show multiple horizontal line (with
no input) and the trace "curves" to the left hand side of the screen.

If I apply the cal signal input I can see the square wave but
triggering is erratic, reminds me of my first "KnightKit" scope I got
when I was a kid that used a thyratron in the timebase.

Info/parts are a little scarce in this part of the world (Australia)

Does anyone have any hints/suggestions????
Thanks

Well,read the circuit description,find the current limit(foldback) parts
for the -8V supply,and check to see if the supply is in foldback.If that's
the case,then the problem may not be in the PS itself,but elsewhere.
I've seen where the series pass xstr B-E v-drop increased and triggered
foldback;replacing the series pass(chassis-mounted) xstr fixed the
problem.Or a bad part in another circuit could put it into foldback.Like a
bad tantalum decoupling cap for another circuit.

Then the unregulated supply for the -8 could have a bad bridge rectifier or
electrolytic filter cap,so there is not enough unreg V for the -8 to
regulate.I've seen the PS filter caps leak electrolyte all over the top of
the motherboard,some corroding right thru the PCB traces.

Gotta have the PS regulated voltages correct before you can worry about the
rest of the scope,but you know that!

 

[Harvey White]

Hi....recently I picked up an old Tek 475 200Mhz scope not working
I managed to get a full service manual for it but have had no luck getting
it running.

You're better off than most.

The +50 Volt rail (which I belive all other rails are derived from) is
perfect, as is the 110V, +15, +5 rails.

That's a plus.

The negative rails are out, the -8Volt rail reads -6 volts but the -15 Volt
rail is way out at -23 volts.

That is not good, sounds like regulator failure. The -8 to -6 could
be too much load on the -8.

Im not an expert with this stuff but like to have a go, it would be a shame
to chuck it out.

Its really annoying me because the schematic for the -15 volt supply "looks"
so simple, Ive checked caps, all resistors, transistors and even replaced
the IC (MC1458) with no change at all.

If it looks that simple, it might be, but what's the reference for the
-8? If it's too much draw, then there's a problem. The -15 might
have used the -8 for a reference. You'd want to check that.

The CRT works fine, lots of brightness, the vertical amps both appear fine
but the timebase is werid, some (lower) ranges wont display a trace at all
and the higher ranges show multiple horizontal line (with no input) and the
trace "curves" to the left hand side of the screen.

First fix power supplies....

If I apply the cal signal input I can see the square wave but triggering is
erratic, reminds me of my first "KnightKit" scope I got when I was a kid
that used a thyratron in the timebase.

Info/parts are a little scarce in this part of the world (Australia)

Power supplies in this kind of equipment are critical, because a lot
of the timing references and current sources may work off the
negative supplies.

Trace out the -15 volt supply, and see that there's a feedback from
the power supply output voltage to one part of the op amp. That same
op amp may have some sort of reference, which has to be exact. The op
amp is acting like a comparator, giving you a difference signal
between the reference and the supply output. That goes back to the
pass transistor. Trace it out, then look to see what the voltages
are, and what they should be...
Best I can suggest... if you are lucky, they even have some circuit
theory....

Harvey

 

[RB]

Thanks to Steve, Jim and Harvey for all the suggestions.
Ive spent a few hours on it today and have got both rails working within
specs.

-15 Volt rail was a crook 1458 Op Amp (some time was wasted trying to
desolder it from the top of the PCB till I realised it was in a socket!

-8 Volt rail was a dud electro.

All working now, triggering perfect upto nearly 400Mhz, only fault is trace
has some 50Hz "wobble" in it, Id say maybe replace all electro's in psu????

Are the values of these electros important????? ie, across the -8Volt rail
Tek have used a 5500uF, at the moment Ive got a 6800uF tacked across the
bottom of the board, Id never be able to find a 5500 cap but could probably
source a 5600 locally...is it that critical???

Thanks again

 

[Mike Nowlen]

Are the values of these electros important????? ie, across the -8Volt rail

Tek have used a 5500uF, at the moment Ive got a 6800uF tacked across the
bottom of the board, Id never be able to find a 5500 cap but could probably
source a 5600 locally...is it that critical???

6800uF is plenty close enough.

 

[Harvey White]

Thanks to Steve, Jim and Harvey for all the suggestions.
Ive spent a few hours on it today and have got both rails working within
specs.

Good. Glad to hear it.

-15 Volt rail was a crook 1458 Op Amp (some time was wasted trying to
desolder it from the top of the PCB till I realised it was in a socket!

Don't you love sockets?

Hint for removing bad chips... Cut off the pins flush with the board,
then the solder sucker only needs to get out the tiny little piece in
the board. Alternative, cut off the chip body, leaving the lead out,
then pull gently with needlenose pliers while heating the connection.

-8 Volt rail was a dud electro.

All working now, triggering perfect upto nearly 400Mhz, only fault is trace
has some 50Hz "wobble" in it, Id say maybe replace all electro's in psu????

Probably so. You might even be able to use the scope to troubleshoot
itself.

Are the values of these electros important????? ie, across the -8Volt rail
Tek have used a 5500uF, at the moment Ive got a 6800uF tacked across the
bottom of the board, Id never be able to find a 5500 cap but could probably
source a 5600 locally...is it that critical???

Most of the electrolytics are in the range of about +/- 20%. I
haven't seen precision ones. So the 6800 would be ok, I'd say, unless
there's some sort of charge/surge issue in the power supply. That's
roughly within the 20%. 5600 would be just fine. but do not go below
the voltage indicated. Don't go too terribly far above it either,
since most electrolytics need a certain percentage of the rated
voltage to work properly. (15 volt cap on a 5 volt circuit is better
than a 50 volt electrolytic).

Harvey