Tek 475 Scope Help???
 

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

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

RB wrote in message
...
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

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

RB wrote in message
...
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

"RB" wrote in message
...


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

"RB" wrote in message
...


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

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.

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.

In article , "RB"
writes:

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???

The difference between a 5500 uF and 6800 uF is not critical in a PS. It's
when you go double or more the original value that you get into trouble with
inrush currents being hard on your diodes and blowing fuses. And ISTR those
big Teks have a relay that brings up the power supplies in sequence for extra
protection.
--Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.

In article , "RB"
writes:

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???

The difference between a 5500 uF and 6800 uF is not critical in a PS. It's
when you go double or more the original value that you get into trouble with
inrush currents being hard on your diodes and blowing fuses. And ISTR those
big Teks have a relay that brings up the power supplies in sequence for extra
protection.
--Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.