Folding Step Stool Re-seating

The top step of a folding step stool we’ve been (ab)using forever finally wore out, mostly because it was covered in vinyl and intended as a seat. We always used it as a step, despite knowing you should never stand on the top rung of a ladder: “Do not stand on or above this level”.

I tossed the ripped vinyl and warped particle board, cut a random chunk of wood-textured paneling (which Came With The House™) to fit, match-drilled four holes, and it looks OK:

Folding step stool - reseated
Folding step stool – reseated

The original seat / step / whatever used press-fit studs with a flat flange covered by the vinyl, but I just slammed 10-32 tee nuts into the paneling:

Folding step stool - tee nut installed
Folding step stool – tee nut installed

That’s a ring of low-strength threadlock around the inside of the nut; I do not expect the screws to come out ever again.

I cut the screws to length with a Dremel cutoff wheel using a slightly shortened tee nut as a fixture:

Folding step stool - screw shortening fixture
Folding step stool – screw shortening fixture

Not visible: the vacuum hose clamped to the vise sucking up all the abrasive + metal dust.

Good for an hour of Quality Shop Time™ on a cold winter morning!

Tektronix AM503: Special Adapter and Failed BNC Bullet

The Tektronix AM503 manual specifies a Special Adapter to inject a signal directly into the input connector in place of the A6302 Hall probe:

Tektronix AM503 Special Adapter
Tektronix AM503 Special Adapter

The intricate Amphenol plug might still be available at some phenomenal cost, but I’m willing to just jam a pair of wires into the AM593 connector and be done with it.

I combined a pigtail BNC sporting a male connector, two 51 Ω resistors in parallel, two snippets of 18 AWG wire (an exact match for the 40 mil connector pins!) with the ends filed smooth, and some heatshrink tubing to make a roughly equivalent adapter:

Tek AM503 - Crude Special Adapter
Tek AM503 – Crude Special Adapter

Because the pigtail didn’t quite reach the function generator, I joined it to a longer cable with a BNC bullet, whereupon a slight tug ripped the guts out of the bullet:

BNC Bullet - failed
BNC Bullet – failed

A closer look:

BNC Bullet - parts
BNC Bullet – parts

The center hole comes into play with their equally craptastic BNC tee connectors.

Comparing this bullet with others from the same eBay lot shows the outer shell didn’t get quite enough crimp around the metal ring. Because it’s not an electrical connection, I eased some epoxy onto the internal shoulder where that ring seats, then slid the guts back in place.

Yak shaving in full effect!

Tektronix AM503: Recapping

It turns out that the Tektronix AM503 with the 4 MHz oscillation (B075593) had a 2 MHz oscillation after I replaced C165:

Tek AM503 - corroded capacitor
Tek AM503 – corroded capacitor

So, despite it not showing any leakage or damage, I replaced C155:

Tek AM503 B075593 - C155
Tek AM503 B075593 – C155

Which had stopped being a capacitor some time ago:

Tek AM503 B075593 - C155 measurement
Tek AM503 B075593 – C155 measurement

I also replaced C165 with a newer capacitor.

Again, having the hood up, I pulled C452 and C462 from the ±19.3 V supplies:

Tek AM503 B075593 - C452 C462
Tek AM503 B075593 – C452 C462

Despite the 1987 date code, they seemed to be in fine shape, but I replaced them anyway. The new caps have a 50 V rating, not the original 63 V: only a factor of two headroom.

The four new capacitors in their new home:

Tek AM503 B075593 - replaced caps
Tek AM503 B075593 – replaced caps

The power supply voltages looked clean before and look clean now.

The AM503 still has the mysterious 4 MHz oscillation, so the capacitors weren’t the problem. Even though the amp is still sick, I feel better.

Tektronix AM503: Bring the Noise

Popping a 2N5911 dual JFET into the noisy Tektronix AM503 (B064098) eliminated both the noise and the offset problems:

Tek AM503 - SDS2304 cal - 1 mA-div
Tek AM503 – SDS2304 cal – 1 mA-div

The test signal (yellow) comes from the scope’s calibrator output into a 2320 Ω resistor, so the AM503 calibration is about right: 0.6 mA ≅ 1.5 V/2320 Ω.

Just to maintain historical accuracy in the two AM503 amps in the TM502 mainframe on the Electronics Workbench, I transplanted the good (not noisy) OEM Tek Q230 (from SN B075593) into the previously noisy-and-offset-prone AM503, which now works fine. I now have a pair of works-pretty-good AM503 amps, one not-so-good AM503 in the to-be-fixed lookaside buffer, plus a defunct Q230 dual JFET.

That third amp (B075593, now with the NOS 2N5911) has a nasty noise problem:

Tek AM503 B075593 - SDS2304 cal - 1 mA-div
Tek AM503 B075593 – SDS2304 cal – 1 mA-div

The barely visible yellow trace is the same calibrator signal as before, but the output is a howling 4.2 MHz (!) sine wave. The oscillation amplitude responds to the AM503 front panel gain control, making it possible to see what’s going on:

Tek AM503 B075593 - 4 MHz oscillation
Tek AM503 B075593 – 4 MHz oscillation

Flipping the front panel switch to limit the AM503 bandwidth to 5 MHz shaves off the fur:

Tek AM503 B075593 - 4 MHz osc - 5 MHz BW
Tek AM503 B075593 – 4 MHz osc – 5 MHz BW

Disconnecting the probe or unplugging P220 kills the oscillation, as does setting the front panel switch to CAL/DC LEVEL, which means it’s an internal feedback problem.

It’s trivially easy to construct an amplifier circuit that becomes an oscillator at the slightest provocation, but this puppy had been working dependably for somebody else during the three decades (!) before I bought it and continued for a few years after that, so the overall circuit topology is known-good.

Shooting this one will require more pondering, as the obvious first step of replacing the power supply’s electrolytic caps had no effect.

Amazon Laminator Wiring

Someone with a jammed Amazon laminator inadvertently dislodged the switch wiring, so I took a few more pictures to help. Note: I see absolutely no reason to assume any two laminators will have the same wire colors, but the overall functions should be the same.

The top set of three switch terminals control the overall power to the laminator:

Amazon Laminator - switch wiring
Amazon Laminator – switch wiring

The center terminal comes from the unmarked (no ridges) wire in the line cord. The two outer terminals are connected together with a short jumper from the terminal nearest the motor, with a longer black wire to the wire nut binding other black wires.

The bottom set of terminals select the temperature:

Amazon Laminator - switch bottom contacts
Amazon Laminator – switch bottom contacts

The white wire on the center terminal goes to the wire nut holding the other white wires and a black wire (!) going to the middle of the three thermostats on the extrusion. The black and blue wires on the outer switch terminals go to the thermostats on the aluminum extrusion to the heater.

Verily, it is written: There’s nothing like a good new problem to take one’s mind off all one’s old problems.

Tektronix AM503: Q230 Dual JFET Replacement

Some suggested 151-1032-00 replacements obviously won’t work, such as Tekwiki’s 2N5397 single JFET. Bonding a pair into a single heatsink might suffice, but two separate cans generally aren’t identical enough for the purpose.

Curiously, Tekwiki also lists the 2N5911 as a 151-1032-00 replacement, which (being an actual dual JFET) looks more promising. This agrees with another cross-reference, although the “Sim[ilar] to” suggests considerable caution.

The 2N5911 pinout, as taken from its datasheet:

2N5911 Dual JFET pinout
2N5911 Dual JFET pinout

The actual Tek 151-1032-00 can in its heatsink, oriented with the tab at the top (just visible to the right of the heatsink fin):

Tek 151-1032-00 - top view
Tek 151-1032-00 – top view

Testing one side (with the tab on the left):

Tek 151-1032-00 test side A
Tek 151-1032-00 test side A

And the other side (tab still on the left):

Tek 151-1032-00 test side B
Tek 151-1032-00 test side B

A picture being worth a kiloword:

Tek 151-1032-00 - measured pinout
Tek 151-1032-00 – measured pinout

The drain and source over on the left side seem to be swapped compared to the 2N5911, although both gates are on the proper pins. This being a JFET, the source and drain may be electrically identical and it’s possible the tester labelled them backwards. The only way to be sure Tek wasn’t tragically clever is to poke around the PCB to figure out which pins connect to which other components.

So take a picture of the component neighborhood around the Q230 sockets:

PXL_20220105_210538214
PXL_20220105_210538214

Overlay it with a similar picture of the solder side, suitably reversed / recolored / transformed to match:

Tek AM503 - 151-1032-00 area - X-ray traces
Tek AM503 – 151-1032-00 area – X-ray traces

The copper-side traces aren’t complete, as the red coloring marks only traces under the soldermask and omits bare solder-coated traces. Some traces on the component side run invisibly under parts. If I were doing it for money, not love, I’d pay more attention to the details.

Devote some time to tracing the traces and labeling the parts:

Tek AM503 - 151-1032-00 area - part IDs
Tek AM503 – 151-1032-00 area – part IDs

Then doodle out the actual connections:

Tek 151-1032-00 - part connections
Tek 151-1032-00 – part connections

R246 shows Q230B lives in the left side of the can, because it’s connected between the B gate and B source pins, and confirms the tester swapped the B source and B drain pins. Whew!

R236 connects the B drain and the A source, confirming the pinout matches the 2N5911.

Comfortingly, the A side gate goes to all those other parts as it should.

So a 2N5911 will drop right into the Q230 socket with the proper pins going to the proper places. Whether it’s electrically Close Enough™ to the Tek spec, whatever it might have been, remains to be seen, but a good transistor circuit won’t depend too much on the actual transistor parameters.

Tektronix AM503: Noisy Q230 Dual JFET

The fact that changing R220 also changed the noise should have pinpointed the noise source, but such things are always more obvious in retrospect than in real time running. This post should help me start the next debugging spree a bit further up the learning curve.

The AM503 signal path includes a pair of … unique … differential amplifier ICs made by Tektronix back in the early days of integrated circuitry:

Tek AM503 - U370 U350 detail
Tek AM503 – U370 U350 detail

The picture has the signal flowing right-to-left through U350 and U370, starting with the Q310 dual NPN in the metal can and the Q315/325 PNP pair (both over on the right side near the cable).

The schematic goes left-to-right:

AM503 - Current Probe Amplifier Schematic - Output Amp - Diag 3
AM503 – Current Probe Amplifier Schematic – Output Amp – Diag 3

This AM503 PCB does not have the change that split Q310 into two separate transistors, as shown in the upper left of the schematic.

The single-ended input signal comes from Q230 at the output of the attenuator on the previous schematic page:

AM503 - Current Probe Amplifier Schematic - Attenuator - Diag 2
AM503 – Current Probe Amplifier Schematic – Attenuator – Diag 2

Here, U350 mashes the signal together with the DC offset control and makes it differential, U370 filters it, then Q390 produces a single-ended signal for the scope output. The architecture makes sense when you realize the AM503 started out as a mainframe oscilloscope plugin and became a TM500 plugin as the result of a mid-flight product pivot; think of it as the hardware version of technical debt.

Anyhow, the differential output of U370 shows the noise across pins 6 and 8 (yellow and magenta):

Tek AM503 - diff output U370.6-U370.8
Tek AM503 – diff output U370.6-U370.8

Again in retrospect, pins 9 and 5 would have been a better choice.

The white line is the difference between the two pins and resembled the scope output in the bottom trace well enough to satisfy me.

The differential input to U350 on pins 16 and 14 also shows a distinct similarity to the output noise:

Tek AM503 - diff input U350.16-U350.14
Tek AM503 – diff input U350.16-U350.14

It’s essentially impossible to snap a scope probe around those IC pins, but merely extraordinarily difficult to securely grab the tails of the pin sockets extending beyond the solder side of the PCB.

Finally, a look across R317, the emitter resistor between the halves of Q310:

Tek AM503 - diff input - R317
Tek AM503 – diff input – R317

That was enough to finally convince me the problem lay upstream of Q310.

Ruling out the DC level pot required balancing another AM503 atop this one to plug its cable into the PCB, which showed same output noise.

Hat tip to Sherlock Holmes:

“When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.”

https://www.goodreads.com/quotes/1196-when-you-have-eliminated-all-which-is-impossible-then-whatever

Because Q230 is socketed, I pulled it out and popped it into another AM503, whereupon the noise followed the transistor:

Tek AM503 - three amps - GND
Tek AM503 – three amps – GND

All three AM503 amps are set to GND / DC LEVEL CAL. The cyan trance is the formerly noisy amp (now with a good Q230), the magenta trace is the formerly good amp (now with the bad Q230) , and the green trace is the best of the three AM503 amps (untouched, for well and good reason) in my collection.

So I must replace a four decade old dual JFET sporting Tek part number 151-1032-00. Such things are available, but in rather grisly as-is condition or as New Old Stock collectibles.

Perhaps a middle ground would suffice? A couple of those should arrive in a while, but it’s not clear they’re a drop-in replacement.