P6046

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Tektronix P6046 Differential Probe and Amplifier

The Tektronix P6046 is a differential probe and amplifier. It was initially intended for use with a 1A5 and attaches through a special multi-pin connector. A separate amplifier was available as an option to use the probe on a 50 Ω input scope set at 10 mV/Div.

With a 1A5, it has a 45 MHz bandwidth. The raw bandwidth of the probe and amplifier is 100 MHz. Note that the probe itself acts as either a 1× or 10× attenuation probe dependent on a signal voltage on pin D of the connector. When used with the amplifier, 10× mode is used in the 200, 100, and 50 mV/DIV settings and 1× mode for 20, 10, 5, 2, and 1 mV/DIV.

Operation

Ground the probe! Seriously. Sticker the probe "Ground Me!" The two probe tips are (in DC mode) directly connected to the gates of a pair of JFETs. Use the shorting plugs on the tips when not in use. I also recommend stickering the probe amplifier with the ATTEN BAL setup proceedure from section 2-6 of the manual.

Repair Issues

Pay attention to the maintenance section of the manual about the operation of the P6046 outside its housing. The bottom section of housing ties the circuit grounds together. The schematic diagram calls out four different grounds: The cable (using the three line earth symbol), the front posts (triangle-1), the middle mounting screw (triangle-2) and the rear posts (triangle-3). When operating the probe without the top housing, reattach a front and rear post screw to hold the board down to the lower housing. When operating the probe with no housings, tie a front and a rear post together with a jumper. The center circuit board mounting screw ground is only used for the high frequency CMRR adjustments and none of those will work without both housings in place.

The input FETs must be a matched pair. A possible replacement is the 2N4416A in a metal TO-72 package with the substrate pin provided as in the factory part 153-0559-00. It is pretty expensive; currently (2014) it's about $6 US dollars each. Another much cheaper replacement is the 2N3819 in a plastic TO-92 package. The matching criteria should be Idss.

If the "DRAIN VOLTS" adjustment step (R125) fails, check and replace R124. It is a 270K, 1/10W carbon composition resistor and it will eventually drift high so the 10.5V drain voltage can not be reached. The calibration section is not clear about the settings for this measurement, but the schematic diagram shows the voltage measured with 200 mV/DIV set on the amplifier.

Calibration

The Tektronix P-6046 is excellent, and usually quite inexpensive because nobody knows what it is. Make sure it works when you buy it, because these probes are extraordinarily tricky to trim up for CMRR after repair. -- Jim Williams, The Art of Analog Circuit Design

Newer versions of the probe have plastic plugs that allow access to the following adjustments

  • R209 - 42 MHz CM balance
  • C209 - 18 MHz CM balance (and drain voltage measurement)
  • C245 - 50 MHz CM balance
  • R155 - Gain

The full calibration shield has access to all ten adjustments. You will need to make this by drilling holes in a housing. The full shield is needed to recalibrate a probe after replacing the input fets. Some used probes will be found with an access hole drilled for R235 (GAIN SWITCHING BALANCE) since this is adjusted to remove the trace shift when the probe switches from 1× to 10× mode.

To calibrate the amplifier, first check the power supply voltages. They are floating. Do not check with respect to ground! When adjusting, watch for exposed line voltage in the power supply, especially the fuse holder. The power supply must be adjusted with everything connected together. Note that you can check the voltages from the back of the amplifier output board.

Manuals

There are three versions of the manual

  • 070-0756-00 - (C) 1968
  • 070-7129-00 - (C) 1988
  • 070-7129-01 - (C) 1992

The 1968 version assumes you have a 1A5 for absolute gain calibration of the probe without the amplifier.

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