Tek485 HV/LV supply failure repaired - advice wanted on tantalum cap replacement

[Lorenzo_1]

My Tek485 failed recently with at first a fuzzy display about 25mm square in the centre of the screen and then total disappearance of the beam.  Balance node indicated a possible short on the HV supply, and I still had good low voltage supplies.  Shortly after, the inverter shut down and wouldn't start, cycling at about 10Hz. After disconnecting the input to the HV transformer (per manual) inverter still wouldn't start and balance node indicated a problem with one of the LV supplies.  So perhaps two problems.

A check of the LV outputs from the power transformer indicated that it was OK but there was no -15V signal on the downstream side of the diode bridge from the +/-15V supply.  Not too surprising to find a dead 100uF tantalum capacitor (C2019 on the power board), as I've had these fail before.  Replaced it temporarily with an electrolytic and the LV supplies came back to life and the inverter started.   However, reconnecting the HV input led to inverter cycling again (at about 3Hz this time) and indicated an HV short.  While it was cycling, a resistor in the autofocus control system started to smoke. Replacing the resistor didn't fix the problem.

Decided to check the basics of the -3kV supply on the horizontal amp board (while it was off!).  Turned out there was an open HV fast rectifier diode (CR1656) tied to the overheating resistor (circuit extract attached showing the miscreants).  Replacing the diode brought back the inverter and CRT display, but there was very low intensity on A&B sweeps unless the intensity was wound up high.  Focus was also poor.  Replaced the resistor a second time and all came good - crisp display, good focus and normal intensity control.  Good to have this fine machine back to life again. 

I'm inclined now to bite the bullet and refurbish this scope more thoroughly by replacing all the tantalums and some of the other vulnerable bits.  Seems likely the tantalums will continue to fail progressively.  I'm inclined to just replace like with like but perhaps I should substitute a more modern cap type for the tantalums or, alternatively, fit higher voltage tantalums? Would appreciate advice on what, if anything, might be suitable tantalum replacements or whether like-for-like is a sufficient strategy - and any pros/cons.  I'd hope to not throw the calibration too far out of whack - none of the minor repairs I've done so far have thrown it off. It's pretty impossible for me to calibrate it fully here or send it south for calibration at a viable price. 

[tautech]

My advice would be to replace any bead tantalums with at least 35V or better still 50V versions.
They're often used for both local bulk capacitance and decoupling but suffer from designers using them too close to their max voltage ratings. Modern units are much better but it's still a good idea to not subject them to more than ~ 2/3 of their max rated voltage.
For the high value ones (that are really just local bulk capacitance) add a ceramic 0.1, 0.22, 0.33, 0.47uF in series parallel with the same value electrolytic to take care of any high speed stuff.

[uncle_bob]

Hi

The one thing *not* to do -- do not replace the tantalum caps wholesale with aluminum electrolytic caps. Their high frequency performance is very different and that's not good in a high frequency gizmo like a scope.

When picking alternate caps, stay with parts that will run at least 1/3 of their rated voltage in use. There is a weird issue with all types of polar caps. Their reliability does not go up  past that point because other effects start to come into the picture. Since cost goes up as you buy bigger and bigger caps, the bang for the buck factor is not in your favor. (Yes bang for the buck could have two meanings in this case. If you have ever had a tantalum let loose, you know what I mean).

Bob

[CJay]

Hi

The one thing *not* to do -- do not replace the tantalum caps wholesale with aluminum electrolytic caps. Their high frequency performance is very different and that's not good in a high frequency gizmo like a scope.

Useful information filed away fro future reference.

When picking alternate caps, stay with parts that will run at least 1/3 of their rated voltage in use. There is a weird issue with all types of polar caps. Their reliability does not go up  past that point because other effects start to come into the picture. Since cost goes up as you buy bigger and bigger caps, the bang for the buck factor is not in your favor. (Yes bang for the buck could have two meanings in this case. If you have ever had a tantalum let loose, you know what I mean).

Bob

Seconded, a colleague had a PSU that was squealing but it was a mechanical squeal if that makes sense, like a loose ferrite in a transformer or similar, he isolated the problem by prodding and poking to a tantalum capacitor, then he demonstrated the fault to me by showing how if he squeezed the cap between his fingers the noise changed.

Unfortunately the cap didn't like being squeezed and let go while he was holding it.

[tggzzz]

See https://www.eevblog.com/forum/repair/tek-485-repair-and-restoration-prologue/msg803728/#msg803728

Generic advice seems to be:

• for switching power supplies, use Nichicon PW series
• if you have to replace a (low ESR) tantalum with an aluminium electrolytic, higher rated voltage capacitors have lower ESR
• as others have said don't replace a 15V tant bead capacitor on a 13V rail with another 15V capacitor. (And yes, there is a 13V rail in the 485, but it isn't on the PSU board. Start at the calibrator circuit, and work away from there)

And don't forget the Yahoo TekScopes group.

[Lorenzo_1]

Thanks all - that's great advice.  I'll have to spend a bit of time working out what's needed.  Appreciate the help.

[David Hess]

Tektronix was somewhat lackluster in voltage derating solid tantalum capacitors and perhaps for a good reason.  Epoxy dipped solid tantalum capacitors were new at the time and one of their selling points was the lack of a need for voltage derating; this proved to be optimistic.

Almost all of the failures I have seen were associated with a lack of voltage derating so when I find a bad one, I replace all of the similar solid tantalum capacitors instead of every solid tantalum capacitor in the instrument with parts which are significantly voltage derated.

Aluminum electrolytic capacitors can generally be used as replacements however they require 2 to 4 times the capacitance to get an equivalently low ESR at high frequencies.

[Lorenzo_1]

Finally got all the parts together to do this job and tackled it this week - successfully.  I followed tautech's and others' advice and replaced tantalums with higher voltage-rated versions of the same.  I also replaced all the HV caps on the transformer and inverter boards as C4757P did for his Tek485 refurbishment and changed over the last few HV diodes while I was in there.  Good news is it went fine and it started up first go - and kept  going, which is more to the point!   I have just finished a home-grown calibration to check performance - it came up very well with just a few minor tweaks. Decided to rely on my remarkably well-performing 3325A function generator (which EEVBloggers also helped me fix) and a calibrated 34401A multimeter - not NIST standard but certainly good enough for my purposes. I do like repairing this old gear that you can get at without a microscope!

Not having a stock of caps to draw from and with no local supplier, it was a challenge compiling an order. Made up a spreadsheet from the manual to help me pull a list together.  I've attached it here in case others in the same spot find it useful (can't guarantee it's totally accurate but it worked for me).  Took 2-3 months to finally get all the bits together - it's not helpful when Australian suppliers say they have stock but then order/backorder from UK and elsewhere with substantial lead times!   Attached a couple of photos of the HV/transformer board and the cathode supply refitted and the Inverter board with a fresh crop of caps sprouting nicely.

I do have one unresolved issue.  There are three 15uF 50V tantalums in the power supply that I couldn't find higher voltage replacements for. They're C2011, 2012 & 2013 in the manual.   Not sure whether to leave them in place and trust they won't blow (a reasonable chance) or replace with electrolytics and parallel small ceramics.  My inclination is to leave well alone, but suggestions appreciated.  Schematic shows C2011 bridges the +25V and +5.5V supplies from the main transformer (U1960) and C2012 and 2013 bridge the +25V supply line for the vertical amp to earth (see schematic attached). 

Hoping now for a few years of untroubled usage from this old CRO.  Thanks for all the advice - not sure I'd have been game to try this without it.

[tggzzz]

Did you spot the 15V tant beads running at 13V?

I had C911 (diagram power distribution 17) toast R965 when it went short circuit. There are a few other tants on that 13V rail as well.

[Gyro]

I guess the remaining question is why a company like Tek, in it's heyday, didn't bother to de-rate such components on expensive test gear? It's almost as though they inbued bead Tants with some mystical powers..    25V ones wouldn't have taken up significantly more board space. I'm sure they must have allowed greater headroom on electrolytics.

I wonder if they used higher rated parts on their M variant military models.

[David Hess]

I guess the remaining question is why a company like Tek, in it's heyday, didn't bother to de-rate such components on expensive test gear? It's almost as though they inbued bead Tants with some mystical powers..    25V ones wouldn't have taken up significantly more board space. I'm sure they must have allowed greater headroom on electrolytics.

I wonder if they used higher rated parts on their M variant military models.

When solid tantalum capacitors first became available, manufacturers advertised the lack of needing to voltage derate them as one of their advantages and in specific applications, this is sort of the case.  Their major problem is stress fractures in the field induced by temperature and perhaps humidity cycling which is why hermetically sealed solid and wet tantalum capacitors are much more reliable than epoxy dipped and molded solid tantalum capacitors.

[tautech]

Finally got all the parts together to do this job and tackled it this week - successfully.  I followed tautech's and others' advice and replaced tantalums with higher voltage-rated versions of the same. ........

I do have one unresolved issue.  There are three 15uF 50V tantalums in the power supply that I couldn't find higher voltage replacements for. They're C2011, 2012 & 2013 in the manual.   Not sure whether to leave them in place and trust they won't blow (a reasonable chance) or replace with electrolytics and parallel small ceramics.  My inclination is to leave well alone, but suggestions appreciated.  Schematic shows C2011 bridges the +25V and +5.5V supplies from the main transformer (U1960) and C2012 and 2013 bridge the +25V supply line for the vertical amp to earth (see schematic attached). 

It's part of a voltage doubler to provide the 25V rail and 2012 & 2013 are the smoothing caps for the doubler.
As they're 50V rated they should be fine, leave them be for now but if you must  replace with the same, there's plenty of voltage headroom in the design here. 

Nice work, don't you love it when a plan comes together and all goes well. 

I personally wouldn't muck with the calibration unless there was damn good reason to and after some basic checks with voltages sources and a few different waveform something seems wrong.
If you can get access to a known accurate scope and can run them side by side with a tee'ed signal source as a sanity check. Sure it's nice to know that your instrument is "right" but providing all the precise inputs to do so is no trivial task.

[CJay]

Thank you for the Excel sheet, I *think* it's for the same 'scope I have here, when I get a free day I'll recap mine too and that list will be really useful.

[Lorenzo_1]

Thanks for the feedback.  I did replace the several 15V tantalums with 25v rated versions so hopefully they'll survive a bit better.  I think the advice to leave well alone is sound, as everything looks to be performing spot on at present.  Looks like getting a commercial calibration done would set me back ca $450, and can't be done locally in any case, and hire of a calibrator for a day or two ca $1500, so I can't see either option being worth the candle for my purposes.    I'm not using the scope for any high precision work and I have an HP54201A that provides a cross-check for lower bandwidth signals and passes all it's internal self-calibration checks. The Tek 485 A/B delayed sweep was a bit out of sync but a small tweak has it performing by the book.  Last task will be to clean the switches on the Ch 2 vertical attenuator - they're a bit intermittent.  Did Channel 1 a while back using Deoxit etc and it's behaving perfectly.   This is a bit of a weak feature on this scope I think - have cleaned them a couple times with IPA over the years but that seems to provide only temporary relief.   Don't like fiddling with those little leaf switches more than absolutely necessary.

The cap list is from a July 1988 revision of the service manual.  I did find a couple of caps fitted (seemingly original) that didn't match what was in the manual so replaced them with like caps just higher voltage.  Might be worth checking your board as you might end up ordering a few parts you can't use.