P6046/Repairs
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 270 kΩ, 1/10 W carbon composition resistor and it will eventually drift high so the 10.5 V 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.