I had posted the following back in 2003 about my trials with my P6042. Bob Pease was still with us and my main analyzer was an HP141T! Maybe something in this mess will be of help to you if you are still trying to repair yours.
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My unit came non-working. This is the normal trend. After taking the unit apart I noticed that the tant. caps. used for the main filters in the +/-16V supplies were shot. The more I dug into it, the more I found wrong. It turned out that almost every transistor had failed. The cable going to the probe also was open on three wires and the bulb had failed. It was a question of what was left that was good. If it was not for the wide BW, I would have pitched the unit.
To repair the cable I ended up cutting off about 1/2 a foot from both ends and then testing it. It turns out that the opens were in that area, somewhere. Who cares. Reattachment was difficult work.
With the cable repaired I ended up just doing some basic parts swapping to get the unit running enought to test the hall sensor. I was amazed to find it, and the diff. amp. front end to be in working order. All of the switches tested out fine as well so I decided to go ahead with the repairs.
The following is a list of standard transistors that may be used to replace the original parts.
2N2219; TO-39; Q45,Q51,Q53
2N2905; TO-39; Q42,Q44,Q54
2N3053; TO-39; Q73,Q79
2N3572; TO-18; Q113,Q121,Q123
2N3767; TO-92; Q163
2N3904; TO-92; Q175,Q161
2N3906; TO-92; Q22,Q24,Q156
MPS6515; TO-39; Q167,Q183
MPS6521; TO-92; Q29
All of these parts may be purchased, still, from Mouser (800-346-6873).
Q45 & Q44 are a matched pair and also Q53 & Q54. You may have to buy a few of these to try and get two matched sets.
More transistor problems ................
The P6042 was built back in the late 60's. The common practice at the time was to match transistors and then heat sink them together in order to null out any temperature drift. HP was putting transistors into a single 6 lead package to avoid the problem. The P6042 uses two transistors that were screened by Tek. They only have a house number and are no longer available. Tek's service could not tell me what the original part was which was a surprise. These parts are Q87 and Q96. Both parts are also in a TO-18 package. I plan to try and sort some 2N2222 and 2N2907s and use these for a replacement.
These parts are configured as a voltage follower. The emitter of the NPN is connected to the base of the PNP. So, Vbe must match on both parts.
The first step was to make a test fixture to screen the parts. I used Vishay's TC parts. These are very low temp. coef. and are 1%. I screened these using an HP34401A meter to get down to 0.1% I picked the current based on the normal operating current the parts see in the probe.
The following shows the basic test fixture (with the 2904/6s).
Testing so far has turned out very poor, and I have been unable to get under a mV of offset. I see some major changes when trying different manufactures parts, but running from the same manufacturer, same lot, there is not enough varience to get this close. I am now thinking about changing to a different type part and see if I can get something a bit closer.
I wrote Mr. Robert Pease on the subject who suggested trying some different parts. I procured 40pcs of a 2N3904 and 2N3906s from Motorola and ended up getting some very good results.
A wise comment from Mr. Pease:
"You could ask Fairchild if they sell the 2N3906 and 2N3906 chips in TO-18. I bet if the military wanted to buy 100 of these for $30 each, they could get them just fine,but, how much AUTHENTICITY do you want to pay for?
Best wishes. / rap"
I gave this some thought and decided to stay with the matched set TO92s I came up with. To get around the TO-18 heatsink problem, I ended up just filling the sink with grease and flipping it upside down. It seems stay very stable when I tested the unit with my heat gun.
After changing 3 caps, 13 transistors, 1 fuse, 2 resistors and fixing the cable, the work paid off. This unit must have really had one great SNAFU done to it! From the damage that was done, it appears it had been hit with a VERY LARGE line transient. Or, maybe someone plugged it into 220 without changing the switch from 110. What ever it was, I'm sure some EE said #$^%$ when it happened. Shown is the rebuilt unit with a 50MHz waveform. To get around using the TO-18s, I ended up just filling the sink with grease and flipping it upside down.
Shown is the unit after compensation has been performed. Scale is 10MHz/div, 2bB LOG. My guess is with some very simple modifications you could push this probes 3dB point out to 70MHz.
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