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TDS 380 won't trigger
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My TDS-380 quit triggering today, along with some other odd behavior. It appears to still collect data and display it - if I set the sweep slow enough to have the trace sweep across the screen, I can see a low frequency square wave. But with a 100 mV ptop input, the TDS-380 reads more like 65 mV ptop. My immediate suspicion, without looking any further, is that the Dallas chip has failed - accounting for the calibration error. Has anyone had experience with a failed Dallas chip? Might it cause the trigger to fail?
Thanks, Gary Appel |
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Hey Gary,
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the NVRAM is checksummed, and if the NVRAM fails, the scope should display an error at power-up. If the scope is not displaying an error, then more likely this is due to SMD electrolytic capacitor leakage that has started shorting things and/or eroding traces. See e.g. this < > accounting of capacitor and other problems in 400-series scopes. Siggi On Sat, 23 Jun 2018 at 03:36 Gary Appel <garyappel@...> wrote:
My TDS-380 quit triggering today, along with some other odd behavior. It |
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Thanks Siggi,
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I wrote that e-mail just before I turned off the computer last night. I should have added information about error messages. On power-up I get the following messages: Pass -- Powerup Checks Fail++Cal Initialization Diagnostics gives me the following messages: Acquisition: Fail Calibration: Fail CPU: Pass Display: Pass Front Panel: Pass The Error Log has the following failures, consistently: DiagAcd_acqMemData DiagAcd_acqMinPat DiagAcd_acqMemAddr DiagAcd_tbcDataRdback DiagAcd_fastFisoAddr DiagAcd_fisoShortPipe DiagAcd_fisoCell DiagAcd_peakDetectAcq I don't see anything referring to a checksum or NVRAM failure. Looking at the acquisition board, the only electrolytic caps I see appear to be at the analog inputs. This is the first time I've opened this 'scope up, and I'm amazed - it's mostly empty space! Even the CRT is tiny! Any thoughts would be appreciated. Gary Appel -----Original Message-----
From: Siggi Sent: Saturday, June 23, 2018 7:09 AM To: [email protected] Subject: Re: [TekScopes] TDS 380 won't trigger Hey Gary, the NVRAM is checksummed, and if the NVRAM fails, the scope should display an error at power-up. If the scope is not displaying an error, then more likely this is due to SMD electrolytic capacitor leakage that has started shorting things and/or eroding traces. See e.g. this < > accounting of capacitor and other problems in 400-series scopes. Siggi |
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Some additional information.
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Along with an error in level there is a positive offset of around two divisions. The 'scope will trigger only on line. At low sample speeds where the trace sweeps across the screen I can see a 10 Hz square wave. When I speed up the sample rate and trigger on line, it occasionally captures the input waveform, but mostly clips in the positive direction, with a clipping warning on the screen. I believe these positive values are independent of what range I am on. I did check the supply voltages to the main board, all are okay. Thanks for any thoughts, Gary Appel On Sat, 23 Jun 2018 at 03:36 Gary Appel <garyappel@...> wrote:
My TDS-380 quit triggering today, along with some other odd behavior. It |
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Hey Gary,
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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 < > 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 < >, 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 On Sat, 23 Jun 2018 at 17:42 Gary Appel <garyappel@...> wrote:
Some additional information. |
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Thank you Siggi.
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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: [email protected] 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 < > 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 < >, 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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On Sat, 23 Jun 2018 at 20:45 Gary Appel <garyappel@...> wrote:
Thank you Siggi. At low sweep speed, where the trace rolls across the screen, the 'scope isIf 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 toI'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 cake. 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 onPiffle - 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'sGood luck! |
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It's alive!
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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 To: [email protected] Subject: Re: [TekScopes] TDS 380 won't trigger On Sat, 23 Jun 2018 at 20:45 Gary Appel <garyappel@...> wrote: Thank you Siggi. At low sweep speed, where the trace rolls across the screen, the 'scope isIf 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 toI'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 cake. 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 onPiffle - 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'sGood luck! |
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On Fri, 29 Jun 2018 at 00:19 Gary Appel <garyappel@...> wrote:
It's alive!Excellent - congrats on the fix. |
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