Repair worklog : Old rusty HP 3458A

[pelule]

Yepp, I fully agree.
Buying potential damaged tools is like playing in a lottery. Sometimes you are lucky and sometimes you don't.
If the repair is not just fun it it self (like I love to dig into old analogue circuits) and more the way to save money, you may have bad experiances after all.
But I have seen already peoply buying used stuff at EBAY for higher price than the new one. Thus...

[Sbampato12]

But I have seen already peoply buying used stuff at EBAY for higher price than the new one. Thus...

I could never understand that... always find somethings like this on ebay or on locals websites...

[Smith]

Buying potential damaged tools is like playing in a lottery. Sometimes you are lucky and sometimes you don't.

Most people are probably interested after seeing so much 'succesfull' repairs on this forum. But I still wonder how much of these repairs fail due to funds or unobtanium parts and don't make it to the forum.

I know not every gear I have tried to repair succeeded. Maybe 75% did. Especially with scopes it's a wild guess.

Its nice to see this 3458A being brought back to live. Fortunately TiN seems to have the funds to do so, I think most people would have sold it again after seeing how much is needed to repair the thing.

[TiN]

I hope to have funds , hah. It's approaching "don't bother" region at pretty fast speed.

On a serious note, I am looking at it as way to learn something new, as way to create good content, be it repair worklog or teardown. If project end up with functioning machine, well it's another piece of gear to be used for no reason () and likely some investment, as these things will be around next decade or two, no worries.

Little bird whispered to me, that HPAK confirmed order for parts and they have everything in stock

[Smith]

Does the 8.5digit  interfere with your needs regarding the KX project?

By the way should we expect any progress on your KX project coming soon?

[TiN]

8.5 DMM does not solve signal generation task, which is project targeted for.
I'm doing some ADC/DAC evaluation currently, hope to post some updates relatively soon.

[Smith]

8.5 DMM does not solve signal generation task, which is project targeted for.
I'm doing some ADC/DAC evaluation currently, hope to post some updates relatively soon.

I meant your desired signal stability and repeatability to test (obviously not to calibrate) 8.5 digit meters.

Can't wait to see some updates, it's a verry interesting project.

[pelule]

That's the right way, TiN. Take it for the fun, not for the money.
If you save some money with your "hobby", it will be the cream.
I have done a lot of repairs of "vintage" tools so far (the oldest one is from ~1907 and operating).
I prefer tools that have been a mile stone in technology and which seem to be worth to be alive. I try to keep them as close as possible to the origin. But my first rule is to have fun, it's a hobby.
During the years I had to find ways to overcome unavailable parts. Thus I learned a lot on that clever "old style" solutions, about some basic I was not aware of and last but not least, on cleaning techniques.
There is just one critical point: the space required becomes more and more in conflict with my Girl.

[OldSchoolTechCorner]

Same reason I took one, more for hobby and restoration. Same reason I restore reel to reel decks and other vintage electronics on the side. You will always get some that don't see the point, as they just look at it for profit and don't care about anything else and to me that sad.

[TiN]

Rear bezel from 4gte.com have arrived. 

[deadlylover]

Good luck with the repair. ^^

You probably already know this, but Keysight offers 2 levels of calibration for the 3458A. There's the cheap less expensive one that's around ~$700 and then there's the standards lab cal at ~$2600 or so. IIRC, they had uncertainties on the 10VDC range of 3.6ppm and 0.2ppm respectively.

I do think the standards lab cal is slightly overkill for mere mortals like us... 

[TiN]

Since I wait for new transformer to arrive, how can we test inguard supply to confirm it's OK?

Glad you ask, I have a solution.  From previous Keithley Model 2001 repair projects I have one extra power transformer, which happen to have same voltage outputs, as HP one, just at lower power rating. But since we connect only inguard supply A4 board, we don't need full power to check LDO's operation. So let's replace blown zeners and test A4 PCBA with properly wired Keithley mains TR-280 transformer. Never know which parts could ever come handy, eh? 

But first, replaced original CR11,CR12 with fresh zeners (got 50pcs from eBay, just few dollars with free shipping).



Cleaned board both sides, everything else look intact and good.



Now can connect input IEC plug socket to transformer's primary (since I need 110VAC, I used WHITE + BLACK wires).



Keithley used same type of 5-pin connector, but with different pinout. Secondary connector need rewiring as on photo above. I powered transformer separately and measured VAC voltages on secondary, without A4 board connected, to make sure all voltages correct.



If you see schematics of A4 board above, you may noticed MH1,MH2,MH3 connections. These are connected directly to guard frame in 3458A's chassis, which is acting as mecca star point for GND potential. Since I testing board separately, not mounted to chassis, I needed to connect these MH* points together with copper wire. This will be our ground point for measurements as well.

Apply power to transformer, and if nothing smokes, test output DC voltage.



There are nice labels near test points, telling us voltage test points location.




All voltages are OK, also 0.325VAC 60Hz signal to read mains frequency is OK too. 

Repair of A4 inguard power supply now complete.

I also got used bezel off eBay in good condition, to save some money on this project, as it's just mechanical part to keep meter in one piece.
New one on bottom side, old busted rear bezel is on top.



Original unit's bezel had three out of four screw mounts destroyed, so it was not holding well.



Stay tuned... 

[Towger]

Since I wait for new transformer to arrive, how can we test inguard supply to confirm it's OK?

I tried much the same, bypassing the PSU on a faulty monitor a couple of weeks ago. Unfortunately it did not have good results 

Keep up the good work.

[TiN]

Beware doing such if your PSU is switching, as minimum load requirement not unusual for them, meaning that they will not work or even get damaged by operation without loading. Lot of SMPS have external feedback networks, which could go wacky if there is no connection to load. 

[TiN]

Some folks tend to say, that old equipment was designed much better than recent one, system and service wise... Well, I had zero problems tearing unit to pieces within 15 minutes and not even looking into service manual. Well done.

Then the hard part came - scrubbing steel chassis with alcohol from rust. God, it's messy. Gone thru two packs of 3M Scotchbrite already.
It does look cleaner, but some narrow areas still rusty. Photos later.

Also one thing came up to my memory, reading explosion stories here about EMI filters. I even found that thread, exploded 3458A. And yes, your guess is right, my 3458A having exactly same filter, Schaffner FN 323-3/05, made in first week of 1995. Going to order new Delta filter from Digikey ASAP

And DS1220Y-200+ showed up, fresh and nice, 6th week 2014.
DS1230's, transformer, mechanical parts and A9 LTZ1000 reference board should arrive anytime next week.

[TiN]

Made an order on Digikey for fan, caps to recap A6 outguard power supply board (call me paranoid, but I don't trust 20 year old electrolyte capacitors.  ).



Only gotcha is that there was no axial type C8 8300uF 35V capacitor, so I got usual one of similar size instead, will bodge something up. 

Also one thing catch my eye :



See anything suspicious?
Hint-hint : P3 connector goes directly to fan...

[Macbeth]

Overdriving a 12V FAN?

ETA: U2 must be a 7812 surely. 7815 on a 15V supply rail. Nah...

[TiN]

I see someone reading, great job

It does look like HP do overvoltage fan, unless they using 7815 in uncommon way by having low input voltage, perhaps?
I will definately take a measurement after new transformer installation.

CLIP's BOM does say it's 7815 as well:



Also this reply another possible question if fan run variable speed. No, it does not, 3458A is not quite a home appliance instrument, so FAN is always on max the moment you turn instrument on.



This LDO originally have little cute heatsink clip on it as well, I removed it for sake of marking photo.

Let's take a look on fan itself, with my favourite way : destructive analysis. Fan impeller is covered in gunk and barely moves by hand, so I don't care about it now.



It's made by PAPST, decent company with good reputation (as far as I know), sized 60x60x25mm and made in Germany. Let's crack it open, shall we?



Eek, all those metal rusty particles.. It have dual ball-bearing suspension, original PAPST design. Rather neat fan it was.

Hint for fan replacements : save the wire. I usually desolder wire from old fan and replace it on a new fan, it's quicker for me than looking for same type of connector and adjusting wire length..

Couple extra photos for you, volt-nuts..



Just look at that 8051 microcontroller, preprogrammed by Intel itself (laser-coded 3458-85502 mark) in 1993. It runs front panel interface code and handles speaker (by resetting 555 timer nearby ).



Big serious L296HT fella is main +5V regulator, which powers all A5 outguard controller board and rest of +5V consumers around. Brown PCB module is simple DC-DC/AC converter for VFD, similar to ones used in Keithley gear. It supply around 60 VDC to display glass segments and VAC voltage for filaments.

And now messy part : cleaning steel chassis frame. Tomorrow will be more cleaning, and hopefully moar pics.

[Macbeth]

I'm a little confused with the supply rail. I'm guessing CR3 is a 30V zener (not seen that symbol before), so it could be used as a 30V regulator or else some kind of over volt clamp on a lower voltage unregulated supply which would be about 15V looking at J15. That is too low for the 7815 drop out. Maybe it's a typo and it's something like 25V unregulated?

[TiN]

I'll measure actual voltages when get all assembled.

Desoldered calibration NVRAM, DS1220, guess what, it's still seems OK.



Not that it matters with last cal date in 1991, but still, interesting to know that even 25 year old NVRAM could be still storing all data.
Will replace it anyway, not to tease luck.

[OldSchoolTechCorner]

I'll measure actual voltages when get all assembled.

Desoldered calibration NVRAM, DS1220, guess what, it's still seems OK.



Not that it matters with last cal date in 1991, but still, interesting to know that even 25 year old NVRAM could be still storing all data.
Will replace it anyway, not to tease luck.

Quite surprise battery held up that long. Could be because it's well sealed.

[Dr. Frank]

Hi TiN,
Save the content of the 2kx8 NVSRAM and copy to new one.

If these are the correct constants, fitting to  your DC board (7.15xxxV and 40.0xxxkOhm), and maybe also for the AC board, you may check drift, and perhaps do not need to recalibrate AC high frequency constants, only 10V and 10k, for your purpose.
Frank

[Theboel]

Hi TiN,
Save the content of the 2kx8 NVSRAM and copy to new one.

If these are the correct constants, fitting to  your DC board (7.15xxxV and 40.0xxxkOhm), and maybe also for the AC board, you may check drift, and perhaps do not need to recalibrate AC high frequency constants, only 10V and 10k, for your purpose.
Frank

If TIN replace the reference board are the calibration data is still useful ?

[TheSteve]

Hi TiN,
Save the content of the 2kx8 NVSRAM and copy to new one.

If these are the correct constants, fitting to  your DC board (7.15xxxV and 40.0xxxkOhm), and maybe also for the AC board, you may check drift, and perhaps do not need to recalibrate AC high frequency constants, only 10V and 10k, for your purpose.
Frank

If TIN replace the reference board are the calibration data is still useful ?

I would say it still would be useful. The reference boards should be close enough he will know if the unit appears to be working 100% even if off a little, with no cal data at all it may be very tough to know if it even works properly.

[HighVoltage]

Really interesting thread to follow you on the restoration process.
Will be very interesting to see, how many hours you have put in to the project at the end.
I am looking forward to your next steps.