UK Vintage Radio Repair and Restoration Discussion Forum

UK Vintage Radio Repair and Restoration Discussion Forum (
-   Vintage Test Gear and Workshop Equipment (
-   -   Tektronix P6201 FET Probe repair (

Craig Sawyers 22nd Apr 2021 4:03 pm

Re: Tektronix P6201 FET Probe repair
The input JFETs in the P6201 are unselected. Semiconductors ending with 00 and 01 have no selection criteria.

In the parts list Tek used to mark selected parts with an asterix.

Anyway the original 151-1023-00 was specced by Tek to have an Idss range from 5-15mA/V and Vp(max) of 6V. So the usual bog standard JFET spread.

The later fitted MMBF4416 also ends in -00 again strongly suggesting that it is an unselected part.


ronbryan 22nd Apr 2021 5:05 pm

Re: Tektronix P6201 FET Probe repair
Thanks Craig.


G0HZU_JMR 22nd Apr 2021 6:14 pm

Re: Tektronix P6201 FET Probe repair

I don't have enough FETs to be choosey about Idss values, but it would be useful to know if Jeremy's higher voltage 4416A FETs also have similar Idss to mine and if the operating performance at the higher 10mA drain current would be inferior to the original 5mA for any reason.
I don't think it would be a good thing to reduce the source resistor from 2k down to 1k. Ideally this resistor should emulate a current source because it helps the AC waveform at the source to track the amplitude of the AC waveform presented at the gate. This is good for reducing the input capacitance. The 2k resistance should perform better than 1k in this respect. The 4p7 cap across the source and gate is probably there to help combat negative resistance at the gate.

I suspect you can fit a 2N4416 with Idss of 10mA because the circuit will still run very close to the target 5mA through this JFET.

ronbryan 26th Apr 2021 5:22 pm

Re: Tektronix P6201 FET Probe repair
2 Attachment(s)
I've managed to fit the MMBF4416 in place of the original round FET. It's the black rectangle at the pointed end of the pcb. The drain tab is extended with a bit of stripped 30AWG wire-wrap wire.

After fitting the FET, the gate resistance to ground was found to be only 2M, instead of 10M. It seemed that the 4.7pF gate-source capacitor C120 was leaking, but when I removed it, I saw there was a thin black line beneath it which was conductive. Fortunately, a quick scrape with a scalpel removed the leakage path.

I experimented with the value of the source resistor for Q100, but quickly found that although reducing it to 1k did reduce the source voltage to the specified 0.05V, this caused too much current to flow through the 1/8W 160 ohm collector load for Q120 and Q130. I ended up with the original 2k source resistor and a source voltage of 0.85V to gnd, which raised the emitter voltage of Q130 to 2.29V to gnd. Even then, the 160 ohm resistor R170 is run at its maximum rating, so not ideal.

I had a further look at the P6201 parts list and found that later serial numbers of the probe are fitted with 0.4W SFR16 Philips/Vishay metal film resistors with the same 3.6mm body size instead of the original BB carbon types.

A quick test of the pcb with a scope and a 10MHz sinewave input showed that the AC amplifier circuit was working, but instead of being near unity gain it was down by 50%. The output co-ax matching resistor, a tiny wire-ended 50 ohm 'pellet' resistor R190 tacked across 2.2pF peaking capacitor C160 had fallen to bits, leaving just a 1k trimming resistor R185 in parallel as the cable matching component. This 1k, when loaded by my 10x scope probe impedance (at 10MHz), was causing the signal loss.

Without a replacement 'pellet' resistor or anything like it small enough, I compromised and removed the 2.2pF SM speed-up capacitor and soldered a 1/8W 51 ohm C3 metal oxide resistor in the space created.

That finally got the AC amplifier working and I replaced the pcb in the probe body and confirmed it worked when connected to the DC-LF amplifier and DC-HF amplifier in the box at the scope end.

There is some adjustment to be done at the 'box' end to correct a 90mV trace offset when the probe is switched to DC coupling, but this is covered in the manual, so hopefully will be straightforward. I don't have the fast rise pulse generator, sampling scope, precision termiator etc for proper calibration, so I will see how I get on with it as it is.

I've attached a photo of the tiny wire-ended 'pellet' resistor (with the green dot) before it fell apart and would be interested if anyone has used these 50A10L parts, apparently made by CTS Corp in the USA.


G0HZU_JMR 27th Apr 2021 9:26 pm

Re: Tektronix P6201 FET Probe repair
The technology of the components is certainly quite varied in that probe. I'm going to guess that Q130 (PNP BJT) in your schematic is biased at something like 13mA. I'd therefore guess that the impedance at the emitter will be 0.026/Ic = 0.026/0.013 which is about 2 ohms. In reality it will probably be a bit higher than this but it may mean that the ideal value for the R190 resistor is in the order of 47R as indicated on the schematic. This will sum with the 2R emitter impedance to get close to an ideal broadband 50R here.

I suppose you could use a pair of 0603 SMD resistors in parallel here of values 150R || 68R to get close to 47R for R190.

Radio Wrangler 27th Apr 2021 9:55 pm

Re: Tektronix P6201 FET Probe repair
46.4 Ohms in the E96 series (for 1% parts)


ronbryan 1st May 2021 9:12 am

Re: Tektronix P6201 FET Probe repair
1 Attachment(s)
The original reverse terminator network on my probe was a 50 ohm pellet paralled by a 1k BB resistor trimmer and a 2.2pF 0805 capacitor, which pre-dated the version shown in the circuit diagram in post #1. I have replaced that network with just a small wire ended 51 ohm Electrosil C3 metal oxide resistor, the nearest tiny component I had.

Without the correct termination, as calculated by Jeremy above, I had a look at the termination glitch generated from the reflection, which appears 20ns from the front edge according to the manual. The photo is taken using a Tek 465 100MHz scope connnected to the P6201, 50ns sweep, so one small division on the graticule is 20ns. With this setup, it is nowhere near fast enough to catch any spikes, but it does show a slugged response a little after the transition. Maybe this would have been improved if I could have refitted the 2.2pF speedup cap and trimmed the 51 ohm back to 47 ohms, but it is not easy to add tiny parts in such a small space. (I probably should have looked at the 50MHz pulse generator output direct via a terminated co-ax cable, to check the waveform, but ...)

I corrected the 85mV output offset when DC coupled, by adjustment of a couple of pots in the DC-LF section in the scope end box, but the probe needs at least a half hour warm-up before drift is more or less stabilised.


All times are GMT. The time now is 6:07 pm.

Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.