Re: TDS 380 won't trigger


Gary Appel
 

Thank you Siggi.

The gentleman on EEVBlog has a 'scope that reported exactly the same errors as I have, so it sounds like it may very well be the same problem. I tried squirting a little freeze mist on the acquisition memory. It didn't "fix" the problem, but the trace certainly responded. With a little freeze mist it tried to come back to the same value as when the trace was rolling.

After spending some more time looking at the 'scope today, it does appear to be triggering. I'm not sure if that has changed since last night, or if I was not setting the trigger correctly last night. As you suggested, I set the trigger to AC and with the level at the center line it triggered on Ch1, Ch2, Ext, and line.

At low sweep speed, where the trace rolls across the screen, the 'scope is displaying a waveform, and it makes sense that they wouldn't be using the acquisition memory for that, although there is an substantial offset and calibration error in that mode. With a 0 offset and 0 position ground lies about two divisions above the center line. Using the measure menu to measure the level, it does not agree with the volts / division. So there is likely a second problem.

Most of the IC's have a '95 date code. The Dallas chip has a '96 date code. It is certainly on borrowed time. I have seen a number of references to replacing the Dallas chip, after first copying the contents. It's probably time for that as well.

I do have an analog 'scope as well - a 2465A, that is also likely on borrowed time. I suspect the Dallas chip in that 'scope is probably just about as old.

Thank you for the link to the technical manual. I have a copy of the service manual, which says very little, and has no schematics. The technical manual looks like it will be a big help.

It sounds like my first task should be to replace that acquisition memory. Meanwhile, in my impatience, I just ordered a replacement 'scope. It's only 100 MHz, but now that I'm not earning a living with my equipment, I'm not sure I need a 400 MHz digital 'scope any more.

Thanks for your assistance, I will post if I make some progress.

Gary Appel

-----Original Message-----
From: Siggi
Sent: Saturday, June 23, 2018 12:56 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] TDS 380 won't trigger

Hey Gary,

I've never been inside one of those, so I don't know much about them -
alas. Your problems do sound very much like what one gentleman went through
on the EEVBlog <
https://www.eevblog.com/forum/repair/pls-help-me-repair-tektronix-tds-380/>
and apparently fixed by replacing the acquisition RAM.
Unfortunately there isn't much information available about these that I'm
aware of. From reading between the lines of the service manual <
http://w140.com/tektronix-tds380-tech-ref-schematics.pdf>, my guess would
be that these scopes are CCD based like the 2430/2440 scopes, so you may be
able to glean a lot of background from their service manuals.

The CCD-based scopes have two acquisition modes, where "fast" one stores
the entire acquisition in the CCD, and reads it out on a trigger event. The
"slow" mode is only used in roll (IIRC), and it's possible that the
acquisition memory is out of the loop there. Have you tried the roll mode
with a suitably slow signal?

Do you have a second scope to diagnose the sick one?
When you say "won't trigger", you mean that the trigger light does not turn
on, or the "signal" rolls?
Note that if the acquisition system is out, then autoset won't work, so
you'll have to fiddle with the triggering controls to get sensible
settings. Also if the sample memory is out, the "signal" will roll at most
or all time base settings...
Have you tried the ext trigger input, does that work?

Note that by your description, this very much could be dodgy acquisition
RAM, except that the triggering is analog, and should be working even if
the acquisition RAM is barfed - hence the suggestion to give the triggering
controls a really through fiddle.
AC coupled input, with NORMAL trigger at mid-level should work, if any
triggering works.

Siggi

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