Re: TDS 684A needs recapping?


Ray
 

I got a tds694A, it had some ripple, super imposed on the waveforms so I recapped the power supply.The 684 and 694 are very similar, the powersupply is the same as far as I knowAnyways the ps electrolytic caps are all through hole.There is about 30 +- a few. 2 where bad. The rest I changed out to get better esr and have peace of mind.Get good quality, 105 deg rated and 5000 hr or better rating.Things got better but I wound up changing all tantalums on the aquisition board as well. Then I changed the tantalums on the output of the 3.3 v dc dc converter.The converter creates a lot of ripple on 3.3 and i am actually looking to replace it with a better, modern eqivalent.Your mileage will differ since the aquisition board is different.Another area will be the display unit.There are lot of caps in there as wellBtw I used a 45 years old weller wtcp solder iron to do all the work. Small tip for the smd tanatalums.Just use a needle nose to lightly pull on the smd caps body then heat one side and switch back and forth a few times and it comes right off. The through hole caps where actually the hardest to get out. Had to use solder sucker and or just steady heat and pull on thr cap and wiggle them out.RaySent from my T-Mobile 4G LTE device------ Original message------From: Rogerio ODate: Sat, Sep 12, 2020 09:07To: TekScopes@groups.io;Cc: Subject:[TekScopes] TDS 684A needs recapping?Hi,
I have bought a tds 684A at Ebay some years ago and it was working ok.
As soon as I received it I opened it up to check if there was some indication of leaking capacitors.
It seems it was assembled in 1994.
Since I could not find any, I closed the instrument.
Everything went well for some years but the instrument started to show some "flicker" on the display from time to time.
Also the data shown is wrong, so it seems I will have to change the NVRAM.
I have already posted a question about possible causes of this flicker and it was suggested that I checked the power supply (and/or all the caps in the unit).
Since I am afraid of damaging the boards replacing the SMD capacitors I have uploaded some pictures of the boards to get an advice about the need to replace them since all solder joint (in my opinion) show no indication of corrosion.
The album is "TDS 684A needs recapping?"
Would you please check the photos and give some advice about the need of replacement?
Concerning the NVRAM, I have read conflicting information about loosing the calibration if I replace it.
What is the current NVRAM model that I can buy?
There is a person selling a FRAM replacent for these scopes but the price is a little high for me ($100).
Any advice on this issue?
Thank you all.
Roger

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