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Probe identify problem on 7A16 plug-in (Mainframe 7603)


Attilio
 

Hello everybody,
I have a problem of probe identify on a 7A16A plug-in inserted on a 7603 mainframe, in X1 mode it works, but in X10 mode the indications on the ranges from 5 mV to 500 mV are identical to those from 1 V to 5 V (in practice readout always indicates 10V, 20V and 50V).
I have seen that the probe for position X10 inserts a resistance of about 12 K towards ground on the probe identify line. I tried to simulate the resistance between the BNC ring and ground and it does the same defect, to get a correct indication I had to raise the resistance to 120 K (with 68 K I have the indication X100).
I replaced the Q620 darlington transistor on the 7A16A "Readout" board but the defect persists.
I need help to solve it, thanks.

-- Cheers
Attilio


Ozan
 

Hi Attilio,
My comments based on looking at the schematic
http://bama.edebris.com/download/tek/7a16a/tek-7a16a.pdf (the one on w140 has readout section cut off and R732 voltage is wrong)
Without any connection to readout, voltage at the base of Q620 (Darlington) should be ~ -15V
With 11k to ground at readout ring base voltage should be about -12.5V. Readout circuit decides 10x mode when ~ 100uA is flowing through R622 (1.27V across it).

If you are seeing much higher voltage than -12.5V with 11k then either R621 (2.05k-ohm) has a problem (open circuit, value drifted, cold solder) or it is not getting -15V to its bottom terminal.

This is the most likely explanation. If the voltage at the base is ~ -12.5V and you still see the issue, let us know the voltages at base, emitter, collector of Q620 and voltage at the terminals of R622 with 11k at the readout pin.

Ozan


Attilio
 

Thanks Ozan,
i checked the readout board and found that the components on the base of the darlington Q620 are different from the schematic in the manual.
in series at the base of Q620 there is a 470 ohm resistor and the R621 is 22.2 K with a parallel capacitor marked 420 BP, the rest of the circuit is as per the manual diagram (R622 is 12.7 K ). The readout board has P / N 670-2310-07 which I have no documentation of.
Can I change the 22.2K resistor to a 2K resistor (R621) and leave the rest of the circuit unchanged ?

Thank you.
--Cheers
Attilio


Ozan
 

i checked the readout board and found that the components on the base of the
darlington Q620 are different from the schematic in the manual.
in series at the base of Q620 there is a 470 ohm resistor and the R621 is 22.2
K with a parallel capacitor marked 420 BP, the rest of the circuit is as per
the manual diagram (R622 is 12.7 K ). The readout board has P / N 670-2310-07
which I have no documentation of.
Can I change the 22.2K resistor to a 2K resistor (R621) and leave the rest of
the circuit unchanged ?
Are other components (R720, R718, CR740) in readout section on sheet <1> matching the schematic?

If everything else is the same R621 of 22.2k instead of 2.05k is the reason for your observation. Perhaps it was designed for a different probe coding system, or someone made a modification. My expectation is changing it to 2.05k (or 2k) will recognize 11k readout resistance as a 10x probe as you want. You can try by adding a 2.2k resistor in parallel with R621 (effectively making it 2k) temporarily instead of desoldering and if it works do the permanent change by replacing R621 with a 2k resistor.

Series 470 ohm at the base of Q620 and parallel cap to R621 has little effect.

Ozan


Attilio
 

Yes, the rest of the board is the same as the diagram. It would seem that this reading card was designed for specific probes, it doesn't seem to have been modified after production, I don't know what the final dash 07 consists of.
However I replaced the 22.2K resistor with two selected 1K series resistors to get 2.05K and everything works perfectly.

Thanks Ozan for the help!

--Cheers
Attilio


Ozan
 

Yes, the rest of the board is the same as the diagram. It would seem that this
reading card was designed for specific probes, it doesn't seem to have been
modified after production, I don't know what the final dash 07 consists of.
However I replaced the 22.2K resistor with two selected 1K series resistors to
get 2.05K and everything works perfectly.

Thanks Ozan for the help!
Good news and congratulations for the successful modification.
Ozan