Electronics > Repair
Tektronix TCP202 current probe repair - Schematic and suggestions needed
AndersJ:
Thanks Giaime,
I cannot tell for sure how the SOT-23 next to the MC1458 is marked.
It looks like it could be GJ1 or GJ or perhaps 6J1 or 6J,
Can you try to see what it says, perhaps using a magnifying glass.
I have not been able to locate a SMD part marked GJ1.
I also tried 6J1 but cannot find that either.
The closest one I found is 6J which is a N-ch fet 2N4391
or
a some sort of resistor network.
/Anders
Giaime:
Hi!
1AM is easy, should be MMBT3904.
The other one looks to me 6J1, could be MMBF4391 but it's not 100% guaranteed.
vtp:
From my notes on TCP202 disassembly and reverse engineering for repair:
6J = MMBF4391
1AM = MMBT3904
26 = MMBD1203
Both JFETs are used during degauss. One is to isolate OP27 from the power amplifier (1/2 1458, BCP68/69), the other appears to enable degaussing oscillator (1/2 1458).
lukier:
--- Quote from: vtp on July 11, 2018, 04:13:41 pm ---From my notes on TCP202 disassembly and reverse engineering for repair:
--- End quote ---
Would you mind sharing some of this info? Some pictures from the back, maybe simplified schematics?
I'm asking, because I don't like my AM503/AM503B amplifiers and I think it is silly to replicate scope frontend (ranges, AC/DC) and 50 Ohm amplifier there when there is one in the scope. Don't know why Tektronix went for it even in TCP300A (although allegedly there they've stopped using custom hybrid ICs).
So I always wanted to somehow mod my A6302 probe to be more like TCP202 (although with LeCroy's interface preferably), but there is very little online on these devices and signal conditioning.
I know very briefly how the DC probe works, studied old AM503 schematics a bit, but I'm not confident enough to risk damaging my A6302 :)
vtp:
--- Quote from: lukier on July 11, 2018, 04:59:53 pm ---Would you mind sharing some of this info?
--- End quote ---
My notes are four pages written and hand drawn schematics and I do not have scans of those. Not that they would do much good as the whole thing is a bit scattered around those pages.
But the operating principle is similar to other current probes. There is the head with its HALL-element and coil in the same magnetic circuit. HALL-element signal is differential input to the OP27 with 1k60s as inputs and 20k0s to GND/output. Then it goes trough 475R to one JFET that can disconnect the OP27 during degauss.
If the sensed signal is allowed to continue it goes to input of 1/2 1458 which has PCB68/69 discrete output stage driving the coil in the probe head. This is used to compensate the field sensed with HALL-element. The other end of the head coil is filtered and brought out then directly to the probe output BNC-connector (at the bottom board). So the coil is in series with the PA and probe output. Gain of the PA looks to be set with 10k0/2k00 resistor divider and it is non-inverting. OP27 output is brought to the 1458 inverting input trough the said 475R and the JFET.
The rest of the probe electronics are degaussing oscillator and power supplies. It uses +-15V and +-5V from the tekprobe interface.
TCP202 head differs from others regarding few parameters, the coil DC-resistance is higher at about 6.5 ohms to make driving it with a smaller amplifier easier. A6302 and TCP312 coil DC-resistances seem to be around 2 ohms. These are not very accurate measurements, they were taken with a Fluke 87 from the element.
...
To the OP, I do not quite understand how someone has managed to blow up those two JFETs and the 1458. The probe head may have touched live high voltage wire but even then it should have been limited to high current going trough ground shields. Unless there has been a breakdown in the head itself when you are looking at about €1200 repair (flat exchange fee from Tek - or at least was several years ago) due to unobtainium head.
It might be a good idea to check the head.
If you look at the head element from the connector end the pin out should look like
--- Code: ---_____
| |
| |
|OOO| pins top row
|OOO| pins bottom row
-----
--- End code ---
The pins being HALL2, HALL1, +3V on the top row and COIL, -3V, COIL on the bottom row from left to right. That +-3V is voltage in A6302, TCP202 may have different hall operating voltage but I will leave it like that here.
Resistive measurement between pins:
+3V - -3V = 276 ohm
+3V - HALL1 = 183 ohm
+3V - HALL2 = 179 ohm
-3V - HALL1 = 189 ohm
-3V - HALL2 = 182 ohm
HALL1 - HALL2 = 110 ohm
COIL - COIL = 6.5 ohm
Yours may differ from these several tens of ohms. If something reads hundreds of ohms different or around kohm or open then you have a damaged head.
The same way it is easy to check quickly if an A6302 is ok as those signals are all brought to the cable connector.
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