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Tektronix TDS220 repair

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vishaldotgupta:
in my board i have four transistors like q151, q141, q110  i could not see the fourth one on your board

please confirm the voltage on these three or four as the case may be.  also confirm the part nos and smd marks.  i could not verify from the pics

I found some fried smd resistors on my unit.  after replacing them, the logic ic's start to get some voltage of around 2.5V

The error signal vanished on start up, probe compensation starts from 5 khz and then becomes 1khz just before self test

however when attempting self cal, it fails, oscilloscope now shows waveform but voltage reading is not correct

need to do some more reverse engg,  but now i am very close to getting it fixed up

sbeube02:
If I remember correctly, the 3 transistors are for +5V digital IC, +3.3V analog IC and -3.3V analog IC.
I will try to do the measurements as soon as I will be home and send you the piece of schematic I have drawn.

vishaldotgupta:
yes, you are right.  The collector of these transistor are connected to the PSU.  let me have the voltage on base and emitter.
do you have a fourth transistor as well

Also check if ADG321 C/D is gets somewhat hot.  mine is getting hot and the measured signal is bit noisy.  keeping finger on the ic increases the noise.  as usual factory calibration during power up self test gets passed but self calibration fails

i am not sure if the noise is occurring because the sheild has been removed or it ADG321 ic has gone bad

sbeube02:
Did some measurement today.
Pin 1 to 3 from left to right.
Q151 marking "CE" --> BCP69T1
1: -3.3V
2: -3.96V
3: -2.61V

Q141 marking "P1F" --> PZT2222
1: 3.52V
2: 4.92V
3: 2.78V

Q110 marking "P1F" --> PZT2222
1: 4.9V
2: 4.92V
3: 4.16V

I also checked voltage on U110 LM358:
1: 4.9V
2: 0V
3: 2.48V
4: 0V
5: 2.48V
6: 2.48V
7: 3.51V
8: 4.92V

Power supply on logic ICs:
U240: 4.92V
U360: 4.92V
U440: 4.32V

Negative power supply 5V rail is a little bit lower than expected with -4V instead of mini -4.35V

JFJ:
I found this thread while searching, unsuccessfully, for a supplier of the 4-ground pin BNC connectors that are pictured in Reply #3, above. Can anybody tell me the name of the manufacturer of those connectors?

The 2-ground pin type that were used on earlier versions of the TDS220/210 main board, are made by AMP/TE Connectivity, but I can't find the type pictured above in their catalogue.


Hello sbeube02,

Did you have any success with the repair of your TDS220?

If not, then a way forward would be to replace the LMV358 dual op-amp (U110), as your voltage measurements indicate that one of its inverting inputs (pin 2) is shorted to GND:

--- Quote from: sbeube02 on March 30, 2016, 04:19:24 pm ---I also checked voltage on U110 LM358:
1: 4.9V
2: 0V
3: 2.48V
4: 0V
5: 2.48V
6: 2.48V
7: 3.51V
8: 4.92V

--- End quote ---
That could be due to R224 being a short circuit. However, given the low voltage and extremely low current that R224 is subjected to, it's far more likely that it's the op-amp that has failed.

If the op-amp was functioning correctly, the voltage at the inverting input would be same as the reference voltage that's applied to the non-inverting input (pin 3). That makes it quite simple to calculate what the output voltage at the emitter of Q110 should be. I couldn't make out the values of R222 and R224 from your schematic, so I sketched out the relevant part of the circuit from my TDS210 main board:



Using the 2.48V reference voltage that you measured (at pins 3 and 5), and the resistor values from my rough sketch, the voltage at the emitter of Q110 should be:



The second op-amp in U110 controls the emitter voltage of Q141:



Your measured Q141 emitter voltage (2.78V) can be confirmed to be correct using the same calculation as above:

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