Strange problem with a TDS620A on start-up.

[mwbarth36]

I have been posting problems with the TDS620A scopes and having great results from the responses.  Here is another challenge!  I have a TDS620A (Not the one in the previous post) that came in with the usual problems, with the leaky capacitors.  Start up tests indicated Fail for Acq. Acq. Interface, and SPC.  First thing I did was to re-cap both boards, wash and retry.  The Acq. and Acq. Interface problems were gone.  Further checking showed a couple of the IC's, the O74 op amps were bad, replaced these IC's and SPC error disappeared.

 Here is the problem.  The first power up after setting for 24 hours, the log will display a Fail on the Acq.  If one then runs the Diag. again, it will always Pass.  Even if powered down, and then powered up it will pass.  Can even let it set for hours 4-8, and turn it back on and it will pass.  But, let it set for 24 hours, and it will always Fail on the initial start up.  I have not determined the 'Magic time' between 8 and 24 that is the turning point, but do not see that is useful information.  The next strange feature which should be a clue is in the error log, which always, on the first start up Fail is: digACCal Diag**AC Calibrator Failed - Freq.< 66.67 MHz.  There is a 71 MHz oscillator in this scope which is switched into the input during the testing, and I have monitored that oscillator.  It is 71 MHz, and I can see that it is switched in during the start up test sequence.  There appears to be no difference in frequency, timing of switch in, or anything else, during the first test sequence after setting for 24 hours, and the subsequent test sequence, but only the first one indicates a 'Fail' in the log. 

Other than this the scope operates great, offsets calibrate to zero, and signals all look normal.  May not sound like much of an issue, but am not one to be satisfied without finding the cause, can anyone help, Please!

[Manul]

That's an interesting problem. Could it be NVSRAM?

[shakalnokturn]

No idea whatsoever on why it would take so long to repeat the fail. (Do you collect TDS620's with faults that require  patience?)

Could it be RTC linked? Does the scope keep time after a long period unpowered?

Try confirming that the 71MHz reaches the FISO consistently   during self test.
Next thing I would check is the FISO clocking and temperature regulation although I'm having trouble imagining what could have such a long time constant anywhere in there.

[mwbarth36]

Interesting thought, and I suppose a possibility.  Any ideas as to how to check that out?

[mwbarth36]

The scope appears to keep perfect time.  I did reset time a few days ago, and it is always correct when I fire up the scope.  I monitored the 71 MHz signal.  It is input to the channel 4 preamp, -VIN during test, and is switched in with a FET.  I monitored the 71 MHz prior to the FET at a test point, had better move forward and look at the input during the start up sequence. 

[mwbarth36]

Actually, I do sort of 'collect' 620/640 scopes with problems.  Am a retired electrical engineer, and enjoy repairing these.  One can easily operate these scopes with the case removed, and check anything on either board with no trouble.  And, these are great scopes when they are working.  I find the leaky cap. problem a fairly easy thing to fix.  Have probably fixed 2 dozen 620's in the past two years.

[shakalnokturn]

Any luck tracing input?
I was also thinking of remaining electrolyte in the PLL section that could cause the sampling clock to be off for some time.
If you feed the scope a stable frequency input on higher timebases (the ones where the FISO is involved) do you notice any jitter on the displayed trace?

[mwbarth36]

I am also thinking that the PLL is involved, there is a FET OP Amp in the circuit that was subjected to some acid from one of the capacitors.  Even though I have washed the board, looks like so discoloring remains.  Next step is to replace the OP Amp.