The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Tag: Repairs

If it used to work, it can work again

Tektronix AM503: Balance Pot Bushing

One of the Tekronix AM503 current probe amplifiers arrived without the panel bushing for the Balance trim pot. Back in the day, you could presumably order part number 350-0301-02 and have it delivered (most likely) by your local Tek representative:

Balance pot panel bushing – Tek part listing

Those days are over.

A few minutes produced a doodle with pretty-close measurements:

Balance pot panel bushing – dimension doodle

The as-built bushing turned out just a smidge too long, so make yours a scant eighth of an inch. Maybe the Tek dimension is the overall length?

An SLA resin printer might crank out such a thing, but it’s well below the looks-good / fits-well resolution limit of an ordinary fused-filament printer.

Applying the mini-lathe to a 1/4 inch white acrylic rod produced a reasonable facsimile:

Tek AM503 Balance pot bushing – front

The side view:

Tek AM503 Balance pot bushing – side

Acrylic is definitely the wrong material for the job, but it came readily to hand while pondering the Shelf o’ Rods. Acetal would be better, as you could easily trim off the aforementioned excess length with a knife.

All’s well that ends well:

Tek AM503 Balance pot bushing – installed

A dab of white acrylic adhesive around the raw opening holds the bushing in place and it looks good enough to me.

The motivation for this boils down to having the bushing center the pot twiddler required to set the balance, which I must do every time I fire up the amps, even after waiting for the half-hour required to stabilize them at their operating temperature.

2022-01-27
Bafang Battery Charge Port: Autopsy

My friends in Raleigh sent a small box with the various tools I made, along with the scorched Bafang battery charge port. As it turned out, none of the tools were useful and the real fix required opening the battery cover enough to remove and replace the charge connector.

A view looking straight into the connector, with the side contact on the top of the image:

Bafang Battery Charge Port – damage overview

Gutting the connector shows why my homebrew shell drill wasn’t going to work:

Bafang Battery Charge Port – components

There’s not much left of the central pin: the nugget hanging on its side is much larger than I expected. Most of the pin melted into that nugget, with a bonus droplet on the near side.

The rectangular chunk (upper right) is the switch terminal, with the tab from the side contact (on the right) welded to it.

Fortunately, none of the mayhem (including a few small sparks during the connector replacement) damaged the battery management circuitry or triggered a shutdown, so the reset tool wasn’t needed.

It’ll make a great 3D printing show-n-tell exhibit, in the unlikely event I ever do an in-person talk …

2022-01-26
Verizon FiOS Battery Replacement

Being that type of guy, a red LED glowing in the far corner of the basement attracts my attention:

Verizon FiOS – replace battery

Back in the day, Verizon didn’t make it obvious that the customer is responsible for replacing the battery keeping the ONT alive during power failures. I expect VZ would eventually let me know the battery was dead, remind me I was on the hook for the replacement, then offer to send a tech around with a Genuine VZ Battery to maintain reliable service.

It’s an ordinary 12 V 8 A·hr sealed lead acid battery and, yes, it’s been in there for a while:

Verizon FiOS – OEM battery date

However, being that type of guy, I just happen to have a box of not-dead-yet SLA batteries waiting for recycling:

UPS SLA 2021-10-22

They’re three years younger than the VZ battery …

2022-01-25
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

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

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

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!

2022-01-21
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

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

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

A closer look:

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!

2022-01-20
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

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

Tek AM503 B075593 – C155

Which had stopped being a capacitor some time ago:

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

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

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.

2022-01-19
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

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

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

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

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.

2022-01-18