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Tek p5205 HV differential probe teardown. BTW, what are the red and brown wires?
onlooker:
Well, there is not much to teardown other than showing 2 internal pictures, then ask a few questions and ideas.
As it stands now, a tek p5205 differential probe (100MHz, 1000V, 50X-500X) in working order may cost several hundred dollars even in the second hand market. The 1103 probe power supply will cost a additional several hundred dollars.
The alternative for someone with low budget is to get the ones that occasionally pops up on eBay that had all cables cut and without any accessories. But, it can go about $30~$50 a piece, or may be even lower.
I opened one of such p5205. It looks like it used all off-the-shelf parts. The design also looked simple in contrast to the high original cost. I guess, the cost went into the professional design and the professional specs.
To reconnect the necessary wires, I tried to find the schematics, but, I failed on it. It will be nice if someone can share the schematics. Otherwise, I will continue to understand the wiring from the PCB, do some testing and hopefully not to damage anything.
Edit: bottom picture added.
The back side of the PCB is not very interesting. The 1st picture is the upper side of the PCB. The HV wiring (left side) and the coax wring on the LV (right) side are simple to redo.
The thin Black, Blue, Red and Brown wires are my concerns. I have identified that the Black and Blue wires are the -+15V main power supplies. When the -+15V are on, I can hear a brief beep and LED flash.
More importantly, when one of the HV wire was touched, the LV coax showed the usual distorted mains waveform. This says that the device should be still functional.
Currently I am trying to understand the roll of the Red and Brown wires. According to the instruction manual, it is likely related to input of the offset adjustment voltage. But, why two wires. Anyone know the answer?
The official power supply 1103 also supplies +-5V. But they are not likely used in p5205 since p5205 have circuitry to split/convert the +-15V supplies to a pair of +-5V supplies.
I am also thinking to reduce the attenuation from 50x-500x to 10x-100x since I am really just interested in the differential capability, not the HV capability. Any suggestions?
As to the +-15V power supply, is switch mode supply good enough?
The 2nd picture is with the HV side wired with the wires taken from a old CRT monitor.
MarkL:
One of the wires is probably the input to the scope which tells the scope what attenuation factor to use for the channel, x50 or x500.
Look for a fixed change in resistance to ground when you push the attenuator selector button.
MarkL:
Ok, I popped the lid on a P5205 "compensation box". First off, that's in quotes because there's no compensation in it. The coax goes right into the scope.
Brown goes to the Tekprobe "Data" pin, which is the attenuation sense input (aka "readout" pin). It flips between 690 and 1140 ohms for x500 and x50 settings, but the probe needs to be under power for it to work.
Red goes to the center wiper of a 1-turn 10k pot, and the ends of the pot go to +15 and -15. This is the offset adjustment accessible through the small hole in the compensation box
You are correct on the blue +15V and black -15v.
(Edit: Fix typo.)
onlooker:
Excellent and clear. And thanks MarkL. The next time google will be friend again for this info.
For my power supply, I can now forgo the brown wire. It needs only 4 wires: one for GND, two for the +/-15V rails, and plus a wire for -1V~+1V offset adjustment. A piece of phone cord should work; A coiled cord can even wrap around the coax.
I think there is also a pot on the p5205 PCB that does the offset adjustment during calibration. if I am willing to deface the device to expose the pot, I can further omit the offset adjustment wire to the power supply.
To answer my own question about using switch mode supply, yes, I googled, in its TekVPI to TekProbe adapter, Tek was already using switching power to convert a 12V single rail to +/-15V dual rail. It is only a metter of if I can find one that works noise-wise. It should be no suprise since switching powers are used in powering scopes for some time.
128er:
Hi,
bought four of the P5205 probes a few weeks ago for 50€. Now I'm planning a little powersupply like the Tek 1103.
Almost all Informations are given in this thread. Nevertheless I add my hand drawn schematic to this thread.
IIRC, the probe draws about 100 mA per rail (+15V/-15V).
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