On 1/18/2013 12:44 PM, John Fields wrote:
Hello,

I have a Tektronix 465 which is perfect in every way except that the
horizontal trigger has gone belly up and the trace free-runs no matter
what.

Clue, please?

Thanks,

The 465 was one of those products sold to government agencies and large
corporations in great numbers.
If you bring out a new model number, you open yourself up to competitive
bidding.
If you change what's under the hood and keep the model number...not so much.
The 465 underwent many changes over its lifetime.
Can't say how many of those were in the trigger circuit.
Any service manual can help...you can download one...but
one matching the serial number is best.

The trigger is a bunch of switches and amplifiers driving a tunnel diode
or two.
Pretty easy to follow the signal with another scope.
If I had to bet, I'd bet on a TD being bad.

I did have a 453 that had trigger problems on some ranges caused by a
leaky timing cap
in the trigger holdoff circuit.

You should be able to determine something from the relative behavior
of the A and B triggers.
My 465 is in the attic, so I can't try it, but there's a trigger view
function that's picked up after the switching at the first amplifier.

If you find that it is a bad TD, you'll be tempted to sacrifice the
B-trigger to fix the A-trigger. Depending on the TD package,
there's significant risk of busting all the TD's. Mess with the TD's
as a last resort.

I'm sure this hasn't happened to you, but I've chased my tail many times
only to find that some pot switch had been inadvertently switched
off the detent.

I'm also assuming that you have personally seen the working trigger and
it broke later.
That's different from, "I bought this thing on ebay..." where someone
may have already harvested the TD's.