Tektronix 134 Current Probe Amplifier and P6022 repair

[HighVoltage]

I finally bought a Tektronix 134 Current Probe Amplifier and P6022 Current Probe but it is broken.

The Amplifier came without the power supply and a broken BNC connector.
When I power it with 25V DC, it does not work at all.

I know, some of you have a 134 Amplifier and before I start the repair attempt, may be
someone has an idea of where to start.

The manuals and schematics are easily found online.

[orbanp]

Is that the early or late version? - TekWiki has manuals for both.

You could start by checking the internal regulated (+14V) supply.
You could also check/clean the switches.
After that you could check the voltages on the transistors.

Is the probe itself OK?

Good luck, Peter

[HighVoltage]

Based on the manual and the fact that it has the light color, it is a newer version.
But funny, the PCB says "Copyright 1965"

It is extremely well built inside, just amazing.
The 14V rail is bad and is sitting at 8V.

Time to take it further apart.

[HighVoltage]

Probe is fixed, just replaced a few electrolytic capacitors.

[AMR Labs]

I am aware that this is an old thread, but searching on Duckgo for Tektronix 134 repair this is the link that makes the most sense, and probably the only source of information about someone actually having repaired their 134 unit.

Purchased my type 134 amplifier a couple of months back when I saw one going for around $25 on ebay (including original power supply) in anticipation that I would eventually get a matching current probe at some point and at a decent price. There are some crazy prices out there for these type of probes. Also got the hanger kit and the original BNC-F to BNC-M jumper wire both still sealed to complete the set.

So last week finally managed to get a P6021 current probe and tested it with the 134 on my scope by setting it to AC 50mV/div as indicated, to measure the current of about 1.7V 1KHz signal from my audio generator into a 600 ohm (actually 617 ohms) 1/4 watt resistor. The measurements where very close to the expected, perhaps just a tiny bit on the high side.

Verified the signal amplitude from the generator on my HP974A DMM, which indicated 1.71V( rms), so 1.71 / 617 = 2.77mA then converted to Pk-Pk to match the scope reading would be x2.828 = 7.83mA P-P. On the scope I see 4 divisions at 2mA/div scale so just about 8mA. Slightly above the expected reading but still very close.

So far so good, but after reading the 134 instructions I saw that the scope has actually to be set to DC coupling 50mV/div, but when I did that the trace went way off scale. And the reason is that I am getting a constant 5.8V DC output from the 134 amplifier at all times, with or without any signal input.

I checked the schematic and there is C165 a ceramic capacitor of 1uF 25V just between the amplifier output and the BNC output connector, which should completely block any DC voltage. I opened the 134 and everything inside looks undisturbed and original, so I'm guessing C165 must have gotten bad (leaky), but I wonder how this could happen to a ceramic cap. The only thing might be if someone connected a voltage higher than the 25V rating of the capacitor to the output of the 134. I don't think ceramic caps ever go bad just because old age. They are rather very stable.

Anyway, will start doing further checks later this evening. Any comments or suggestion welcome.

[HighVoltage]

$25 is a really good price for these great current probes.

I would start checking all caps and replace what is bad.
Only after that you can start troubleshooting more.

[AMR Labs]

$25 is a really good price for these great current probes.

I would start checking all caps and replace what is bad.
Only after that you can start troubleshooting more.

The $25 was just for the 134 amplifier and power supply. The probe was $175 plus shipping but sent an offer and managed to convince the seller to keep the unneeded terminator and sell it separately, so he took $50 off the selling price. I really was not hoping for much thinking he would tell me to go to hell, but surprisingly sometimes crazy offers do work.

Oh and the amplifier was missing the small knob on the lever selector, had to order from Sphere in Canada, the only one available anywhere and used part. $10, and took almost a month to arrive.

[AMR Labs]

Some pictures.

[AMR Labs]

$25 is a really good price for these great current probes.

I would start checking all caps and replace what is bad.
Only after that you can start troubleshooting more.

BTW did your service manual include a PCB parts layout? I got both manuals available at TekWiki but none has it.
I know is quite a simple circuit, but having the detailed PCB parts layout always is a plus.

[AMR Labs]

Just checked the power supply and voltage is right on the money at 14.17V, with less than 1mV of ripple, so for now going to leave the power supply filter caps alone.
First thing is to pull out C165 and test it.

[AMR Labs]

Problem solved.

The red-white wire that goes to the output connector was switched with the white-brown wire. A quick glance at one of your initial pictures confirmed this.
Now it works with the scope set to DC 50mV/div. But there is slight vertical instability in that the trace is a tiny bit jumpy, it varies about one small division, in this case about 0.2mA. There is also a tiny bit of jitter. Not sure its because I am on the lowest current range of 1mA/div, or one or more of the signal coupling caps might need attention. If so will need to order first from Mouser.

Is yours also a tiny bit jumpy/jittery on the 1mA/div scale? And which caps besides the one of the power supply did you end up replacing?

Test waveform shows 3mA at 100KHz, with 0.65V RMS into 617 ohms, which should be 2.97mA Pk-Pk, so really very close.

[HighVoltage]

Wow, that was an easy fix, congratulations.

Let me hook up my probe to a modern scope and see how much the signal moves.

[HighVoltage]

I have just hooked up my 134 to a AC calibrator at 400 Hz with 50 turn amplifier.
You are correct, at 1 mA/Div setting it is a little jumpy.

[HighVoltage]

Here is a Mask-Test at 1 mA/Div and one at 10 mA/Div

There is clearly a little jumping at 1 mA/Div but absolutely no jumping at 10 mA/Div.

[jonpaul]

I would try control cleaner on the trimpots and adjustments, the pots are ofen scratchy...

Beware of the delicate ferrite cores in P6021/22, and similar.

A single drop of even 100 mm on  a hard surface can chip the ferrite core, that affect LF response ands cannot be fixed.

Jon

[AMR Labs]

I would try control cleaner on the trimpots and adjustments, the pots are ofen scratchy...

Beware of the delicate ferrite cores in P6021/22, and similar.

A single drop of even 100 mm on  a hard surface can chip the ferrite core, that affect LF response ands cannot be fixed.

Jon

Which reminds me how my probe was sent, inside an otherwise empty Small Flat Rate box where it bounced around during shipment.
Fortunately so far no apparent damage to the probe.

About the trimpots, are you referring to the probe or inside the 134 amplifier?

[jonpaul]

Trimpots: FP zero control on 134

j

[AMR Labs]

Trimpots: FP zero control on 134

j

Was going over the 134 schematic. I see total 3 resistor trimmers:

R125/R128 100 ohm, gain adjustment for probe types, and

R154 200K, LF Compensation.

Not sure I'm missing something. Other adjustment all are trimmer capacitors.
Apologies If its something obvious, but don't see the "FP zero control" you mentioned.

[AMR Labs]

Last piece of the kit found.

Ground lead PN 196-3120-01. Last one out of 5 was available brand new, for a song.

[HighVoltage]

Nice find with the original ground clip.

You still have the original BNC connector on the back?
I did remodel mine and installed a new connector into the housing, because the original was broken.

[AMR Labs]

Yes mine has still the whole custom connector. I saw yours was unfortunately missing the plastic cover that rotates to tighten down, and which is unfortunately not available by itself but rather as a complete connector assembly. If no other option, would have done the same you did, and just use a BNC-M to BNC-M cable instead. I don't think its a good idea to clamp the whole amplifier to the scope input connector anyhow, which was probably the main purpose of that still very smartly designed original output connector.

It seems to happen that sometimes these crucial parts find a way to "escape", and are probably hiding out with some socks that disappeared out of a washer dryer.

[AMR Labs]

There seems to be one full PN 131-0319-00 connector assembly available for for the super special bargain offer price of $65 here: ("Save as much as 40%" !!) 

https://www.radwell.com/en-US/Buy/TEKTRONIX/TEKTRONIX/131-0319-00

NO relation to the website.

[AMR Labs]

HOLD THE PHONE!

Found one on ebay in the UK, and price is $12.34 (two are currently available), and $5.55 shipping (to the US). Its the full 131-0319-00 connector assembly still in sealed bag.

https://www.ebay.com/itm/233946892332

Seller also has some other somewhat rare connectors, but his listing is really not showing everything on the initial ebay search until you open it. Got to it through Duckgo.
And only found the above when I started looking for that other funny connector that is on the power supply output, which might be a good idea to have as a spare.

I ordered one of each Output and DC connector, total about $27 shipped. There is still one 131-0319-00 BNC assembly left, in case you want to order it and one day get your 134 back to original.

[HighVoltage]

Interesting, that you are finding unobtainium parts at ease.
Very nice.

However, I think I prefer the new BNC connector, the way I have installed it on my 134.

[AMR Labs]

Interesting, that you are finding unobtainium parts at ease.
Very nice.

However, I think I prefer the new BNC connector, the way I have installed it on my 134.

Of course makes sense, in your place I'd probably done the same. Just purchased the two spare connectors because they where so low cost to be almost "unbotanium", and I'm kind of always trying to future proof my stuff (specially the older one) by getting probable spares for wear and tear while they are still available at a decent cost, or at all.

But if that one for $65 where the only option, then of course a $10 BNC female would do just as well.

Funny how nowadays we are still using 30-50 year old stuff that was never really designed having in mind that it would still be used after all those years. There's over engineering (extra quality) in it that really makes a big difference. Those where the times where quality really did matter, and no self respecting company would even consider adding build-in obsolescence as your competition would eat you alive.

At a friends grandmother's house they have an Oster blender from the 50s or 60s (cream colored base) that is still working and being used almost daily. It can crush ice with same ease as a banana. The motor inside is so big and heavy you can hardly hold the base with only one hand.