Tek 475 Scope Problem

[Michael]

I picked up a Tektronix 475 (not 475A) scope. It works fine, except that
there is no sweep at any speed faster than 1us. Trigger stays locked,
and I can see the vertical working off the screen at the edge, but there
just isn't any sweep. At 1us and below, sweep appears normal.

This problem is there whether I use the A or B (delayed) mode. Seems
like the oscillator just stops. Anybody have any ideas? I'm thinking
maybe it's a switch contact on the sweep time knob, but it's a mess to
try to get to it, so I don't want to take that much stuff apart unless
I'm pretty sure that's where the problem is.

I have the schematic in PDF form, but it's pretty hard to read and is
separated into many sections, so I can't figure out what's going on.
Thanks for any help you can give me.

Michael

 

[Asimov]

"Michael" bravely wrote to "All" (21 Sep 04 00:42:12)
--- on the heady topic of "Tek 475 Scope Problem"

Mi> From: Michael <[email protected]>

Mi> I picked up a Tektronix 475 (not 475A) scope. It works fine, except
Mi> that there is no sweep at any speed faster than 1us. Trigger stays
Mi> locked, and I can see the vertical working off the screen at the edge,
Mi> but there just isn't any sweep. At 1us and below, sweep appears
Mi> normal.
Mi> This problem is there whether I use the A or B (delayed) mode. Seems
Mi> like the oscillator just stops. Anybody have any ideas? I'm thinking
Mi> maybe it's a switch contact on the sweep time knob, but it's a mess to
Mi> try to get to it, so I don't want to take that much stuff apart unless
Mi> I'm pretty sure that's where the problem is.

Mi> I have the schematic in PDF form, but it's pretty hard to read and is
Mi> separated into many sections, so I can't figure out what's going on.
Mi> Thanks for any help you can give me.


This is not specific advice for this problem but from past experience
when an oscillator dies at a higher frequency this means a loss of
gain in the oscillating device. Either a transistor has marginal gain
or a dry decoupling electro in the supply line to the stage, could
cause a loss of loop gain.

A*s*i*m*o*v

.... Electrical Engineers do it with more frequency and less resistance.

 

[Jim Yanik]

I picked up a Tektronix 475 (not 475A) scope. It works fine, except that
there is no sweep at any speed faster than 1us. Trigger stays locked,
and I can see the vertical working off the screen at the edge, but there
just isn't any sweep. At 1us and below, sweep appears normal.

This problem is there whether I use the A or B (delayed) mode. Seems
like the oscillator just stops. Anybody have any ideas? I'm thinking
maybe it's a switch contact on the sweep time knob, but it's a mess to
try to get to it, so I don't want to take that much stuff apart unless
I'm pretty sure that's where the problem is.

I have the schematic in PDF form, but it's pretty hard to read and is
separated into many sections, so I can't figure out what's going on.
Thanks for any help you can give me.

Michael

you need a service manual.

The 475 has THREE different sweep generators,depending on which sweep mode
is used.While working at Tek,I always had to refer back to the manual to
troubleshoot sweep problems.
The circuit description is priceless,VERY useful.

 

[Sam Goldwasser]

"Michael" bravely wrote to "All" (21 Sep 04 00:42:12)
--- on the heady topic of "Tek 475 Scope Problem"

Mi> From: Michael <[email protected]>

Mi> I picked up a Tektronix 475 (not 475A) scope. It works fine, except
Mi> that there is no sweep at any speed faster than 1us. Trigger stays
Mi> locked, and I can see the vertical working off the screen at the edge,
Mi> but there just isn't any sweep. At 1us and below, sweep appears
Mi> normal.
Mi> This problem is there whether I use the A or B (delayed) mode. Seems
Mi> like the oscillator just stops. Anybody have any ideas? I'm thinking
Mi> maybe it's a switch contact on the sweep time knob, but it's a mess to
Mi> try to get to it, so I don't want to take that much stuff apart unless
Mi> I'm pretty sure that's where the problem is.

Mi> I have the schematic in PDF form, but it's pretty hard to read and is
Mi> separated into many sections, so I can't figure out what's going on.
Mi> Thanks for any help you can give me.


This is not specific advice for this problem but from past experience
when an oscillator dies at a higher frequency this means a loss of
gain in the oscillating device. Either a transistor has marginal gain
or a dry decoupling electro in the supply line to the stage, could
cause a loss of loop gain.

With my 485 with somewhat similar symptoms, it was just one of those
internal coax cables that was loose.

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[TekMan]

you need a service manual.

The 475 has THREE different sweep generators,depending on which sweep mode
is used.While working at Tek,I always had to refer back to the manual to
troubleshoot sweep problems.
The circuit description is priceless,VERY useful.

Most likely a problem with the capacitors in the range < 1 mikrosec.
Or switch contacts: come lame or dirty with time.

As Jim proposed: Check the manual section, THERE IS DETAILED section
how to troubleshoot the timebase.

YES, we all know it's hard to "dig" into the manuals. Because we have
to do this every single day )

Be assured that it is worth your efforts - the Tek manual for the 475
(and many other) are brilliant for troubleshoting and understanding
the concept of the circuit.

hth,
Andreas

 

[Jim Yanik]

[email protected] (TekMan) wrote in

Most likely a problem with the capacitors in the range < 1 mikrosec.
Or switch contacts: come lame or dirty with time.

As Jim proposed: Check the manual section, THERE IS DETAILED section
how to troubleshoot the timebase.

Personally,I mostly ignore the troubleshooting algorithms and "advice" that
TEK put in their manuals;there's no substitute for reading the CIRCUIT
DESCRIPTION and knowing how the circuit works in detail.

 

[TekMan]

[email protected] (TekMan) wrote in



Personally,I mostly ignore the troubleshooting algorithms and "advice" that
TEK put in their manuals;there's no substitute for reading the CIRCUIT
DESCRIPTION and knowing how the circuit works in detail.

You are right Jim, that's how all we pro's and ex-pro's of this kind
of work field do.

But, one should not forget: The original poster seems to be not too
experinced in this kind of scope repair (Sorry, was not meant
negative). If you are sitting in front of a faulty scope first time,
then for this kind of work, the troubleshoot alogo's in the tek 475
manual are a big help. Like every good documentation is when you have
a faulty system: It's written for easing the work, and to help repir
technician that don't do this every day to get better, quicker and
more reliably to find the fault.

Of course, like in every electronics circuit: It's fundamental to
"dig" into the schematics and try to understand HOW the circuit does
what it does.

Never forget Jim that we do so for more than 30 years, so that's why
our perspective of "starting the work" is slightly difference.

To the OP: Good luck for the repair.

hth,
Andreas