Re: 465 oscilloscope problem


Siggi
 

On Sat, 2 Apr 2016 at 20:25 'Tewell, Kevin' tewell.ka@pg.com [TekScopes] <
TekScopes@yahoogroups.com> wrote:

OK here is what I have starting over :) I got a 465 scope and the power
would come on and that was it. So I took the cover off and found that the
fuse was bad, I put a new one in and turned it on and immediately blew the
fuse on the outside. So I checked the test points and +55 was ı.9 ohms and
I was told to check the C1534 from the antique radio forum person so I did
and I thought it was bad and took it off. And that is all I have done to
the scope. And from what I hear so far was a mistake :( So where should I
go from here besides ordering a new part for C1534. Thanks for the help.
Hey Kevin,

welcome to the group.
It sounds like you're in a little over your head with the troubleshooting
ATM, but not to worry - that's where we all start. I think you'll find it's
great fun and super-rewarding to go through the process, and then to
ultimately fix your scope :).
The thing to do is to be methodical and to work through the issues one at a
time until you have a working scope.

It sounds like you have a short or a near-short on the 55V rail. No matter
what else may be the matter with your wonderful old scope, it doesn't make
sense to power it up again until you've found and cleared the short.
If you haven't already, download (or buy) a service manual for your scope.
There's a free one at BAMA <http://bama.edebris.com/manuals/tek/465(2)/>,
and Artek Manuals will for sure have a high-quality scan available for a
nominal price.
You may also want to download this document <
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf>;, as it's
a great general guide to fixing oscilloscopes of your 465's vintage.

There are many ways to find a short, but without knowing what sort of test
gear you have at hand, it's hard to advice. Do you have a digitial
multimeter - which make and brand? Do you have a lab power supply, one
that's current limited? Do you have a second scope at hand?
(I've come to understand that the right number of scopes to possess is "one
more").

It sounds like you've removed one or two caps from the scope already. Did
that clear the short, or do you still measure 1.9Ohm across the 55V supply?
How about when you measure in the opposite direction - do you measure the
same value?

If you read through the service manual at BAMA: <
http://bama.edebris.com/manuals/tek/465(2)/>, on page 243 there's an
isolation procedure for the 55V power supply.
By isolating the supply, you can find whether the fault is in the supply
(e.g. a bridge rectifier or a filter cap), or in one of the consumers.
The steps in the isolation procedure are to
1. "Pull P300 from the interface board",
2. "Unsolder the dummy resistor near J-1",
3. "Pull P115 from the front of the interface board",
4. "Remove the trigger board."

In your shoes, I'd do those steps in the order 1, 3, 4, 2, and measure the
resistance of the 55V rail to ground at each step. If ever the resistance
increases greatly after one of those steps, you've now isolated the fault
to a sub-assembly.
If it never changes, you'll be looking at a fault in the power supply.

Does that make sense?

Note that I don't have a 465, and I'm a bit of a n00b myself. I can hold
your hand through the troubleshooting process. I won't, however, be much
help in replacement component selection, nor in figuring expected values of
measurements. Thankfully we have plenty of folks in the group that are
experts when it comes down to detail like that - but first we need to
narrow down and isolate the fault.

Siggi


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