REPAIRED : 2465B with auto test stopped on "ADD" led

[Joseph Bozzer]

My problem :
On power, the auto test start but stop when it arrive at the "ADD" led lit .... and screen remain totally black

Fixing :
1) I measured the voltage on pins 14 and 15 of U2101 (DAC) and find them very near 0v .. it was not normal because we have a simple voltage divisor powered by a 10v
2) I tested the R2010 to R2016 resistors value .. all was ok
3) I tested the CONTINUITY of the tracks between them
    bingo!! .. I discovered, under a little oxydation bubble on the track near R2016, a break on it
4) after fixing it, the auto test gone until end and the screen lit

Start state :
 

Track break discover :


Repair :


Fix schema and verified voltages :


Result :

[Zucca]

Congrats!

[Kwakerman]

Good effort, I had similar fault caused by open R2013.  You need to get rid of / replace the 4 SMD caps on that board as they leak and cause issues (may even be the cause of your probs). Well documented issue that causes greater damage the longer they are left.

[jonpaul]

Hello typical of leaky SMD lytics, have seen similar many times.

DO NOT assume its done, the electrolyte is corrosive, it creeps and spreads into the internal layers and flows across the board!

ALL SMD lytics need to be removed, board thoroughly inspected and carefully cleaned, then and replace the lytics with new SMD or better radials with bent leads.

Just the ramblings of an old retired EE

GOOD LUCK!

Jon

[Joseph Bozzer]

Yes ... all 4 capacitors installed have serious oxidation on their pins .. it's acid oxydation able to gnaw the copper of the tracks
Next work : replace urgently the 4 SMD condensers by new models with leads

[Kwakerman]

Just to avoid any confusion it is the 4 metal can ones that need replacing, the 3 in your picture and one at the top right corner of the board.

[Joseph Bozzer]

HELP!
I begin to desolder the condensers and I don't remember the polarity of this condenser





I tried to find it on the schema but I discovered that I don't have the schema corresponding at my board  ... can you help me?

[BravoV]

Not sure if this helps, as mine is newer version with tantalum cap, instead electrolyte.

You can also probe for continuity test for GND, as this is logic board.

[Joseph Bozzer]

Thanks Bravo V!

Another question : I liked your picture with partial zoom ... how do you do that?

[Joseph Bozzer]

I just finished to change the 4 SMS capacitors by 4 capacitors with leads
warning : It's not easy to take off the old and the tracks are very fragile

I'm glad that the defect fixed above, made me see the danger of these capacitors and allowed me to intervene in time because this breakdown was easy to discover but I am not sure that cuts elsewhere would not have been a nightmare to solve

That said, I don't understand that a large house like Tektronix has chosen such poor quality capacitors?

[jonpaul]

Dear Joseph: I see you are in Belgium, I have good friends in Louvain and Tienen.

1. A5: Tant Cap polarity in photo: - towards corner of board, + towards center of board

2. Why choose...

The TEK designers  in late 1980s  used mostly TH and A5  SMD was just beginning to go to production. In fact earlier A5 boards were thru hole, so this was one of the FIRST TEK SMD boards!

The components were still in new development especially ELKO SMD and tants. The cap mfg did not qualify all the materials for use with certain SMD board assembly, solder profle and flux as these were still evolving. The seals at the case><lead was susceptible.

Finally neither the capacitor mfg nor TEK could envisage these scopes would still be working 30-40 years later! Normally consumer gear life is 5-10 yrs max, instruments 20 yes and industrial contreol, telecom, etc 40 yrs (Central Office)

The temperature cycling and improper SMD solder/cleaning practice probably caused the seals to break and start to corrode.


I must says that this is only one of a few issues on the Methuselahs!

 The other is the TH aluminum EKLO caps on A2, A3 power supply boards: Many go bad over time, and have low C or high ESR. In addition the RIFA X and Y line filter caps crack and burn especially on 240 V use.

For long life and reliable service, a rework  suggestion:

a. "recap" PSU A3, A2
b. Replace Dallas NVRAM with new socketed or the DIY magnetic FRAM
c. As you see replace all SMD lytics on A5

Afterwards a full CAL is recommended.

Notice that replacement of NVRAM Dallas DS1227 will mean to read data on existing version and these 28 pin TH are especially hard to unsolder without damages.

GOOD LUCK

Jon in Paris

PS:  https://groups.io/g/TekScopes  dedicated to TEK,  moderated with some ex TEK EE and techs

Just the ramblings of an old retired EE

[jrene]

Hello All,

Thanks a lot for this post, it help me to fix my own 2465B, a 10K resistor broken on the DAC reference circuit.

Best Regards