Re: TDS 380 won't trigger

Gary Appel

It's alive!

The parts arrived today.

I replaced the acquisition RAM with a MB8464 - not the original part, but it appears that the original part was unobtainium. The 'scope now displays traces correctly at all sweep rates, and the offset and calibration are again correct.

Thanks again for pointing me to the information on EEblog, and the technical manual.

Gary Appel

-----Original Message-----
From: Siggi
Sent: Sunday, June 24, 2018 6:32 AM
Subject: Re: [TekScopes] TDS 380 won't trigger

On Sat, 23 Jun 2018 at 20:45 Gary Appel <garyappel@...> wrote:

Thank you Siggi.

The gentleman on EEVBlog has a 'scope that reported exactly the same
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
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
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
Ch2, Ext, and line.

Cool - it all sounds consistent with RAM failure then.

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
the level, it does not agree with the volts / division. So there is likely
second problem.
If you've tried to run signal path compensation after the acquisition
system wonked out, this could be working as intended. The acquisition
system will have been used to measure the DC offset to compensate, and it
could have measured a large "ghost" offset.

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'm not sure it's worth the effort to preserve the memory contents of the
NVRAM, as the calibration procedure for this scope looks like a piece of
All you need is some DC and a "fast rise" (<10ns) pulse generator. This is
pretty typical for the CCD-based scopes, it was easy as pie to calibrate my
2430 using what I already had around the lab, plus some spit and bubblegum.

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.
Piffle - my 2467 has a keeper with an '85 date code in it, and it's still
going strong :). Of course I'm set to calibrate it, should the keeper give
up the ghost, so I'm not too bothered - I'm quite curious to see how long
the battery will last.

It sounds like my first task should be to replace that acquisition memory.
Yups, sounds like.

Meanwhile, in my impatience, I just ordered a replacement 'scope. It's
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.
Good luck!

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