Tektronix AM503: Baseline Offset Digression

One of my Tek AM503 current probe amplifiers (SN B064098) suffered from DC offsets in the AC / GND / DC modes, to the extent that zeroing the GND (more formally known as “CAL DC LEVEL”) offset wouldn’t keep the other two baselines on the scope screen. Kibitizing with another AM503 owner with a different problem clued me to apply a change made in later units: replace the 1 kΩ resistor at R220 with a 470 kΩ resistor to reduce the source impedance changes between the switch positions:

AM503 - R220 change
AM503 – R220 change

For the record, R220 sits parallel to the attenuator shield above and to the right of Q230 (in the black clip-on heatsink):

Tek AM503 - R220 detail
Tek AM503 – R220 detail

The new resistor somewhat reduced the offset problem, but also dramatically increased the noise level I’d been studiously ignoring, to the point where the AM503 output was unusable:

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

The rule of thumb is that it’s always a connector or, perhaps, a similar metallic contact in the signal path. The AM503 has a breathtakingly aggressive switched attenuator covering the 94 dB range from 1 mA/div to 50 A/div:

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

The switches are cam-driven bifurcated gold-plated spring fingers contacting gold-plated PCB pads under that aluminum shield:

Tek AM503 - Attenuator Contacts - detail
Tek AM503 – Attenuator Contacts – detail

The spring-loaded thing to the right is R206, the first 50 Ω 2× attenuator in the form of thin-film elements fired on a ceramic substrate. The two switches put C218 into the signal path in AC mode.

You (well, I) clean the fingers by very gently pulling a strip of lens cleaner moistened with isopropyl alcohol through the closed contacts:

Tek AM503 - Attenuator Contact Cleaning
Tek AM503 – Attenuator Contact Cleaning

The pale blue cylinder is the attenuator cam roller extending across the PCB behind the front-panel knob. The two switches bypass C218 in DC mode and connect R220 to ground in GND mode.

Clean gold-on-gold contacts are about as good as it gets and those things looked absolutely pristine. After wiping the contact connecting R220 to ground had no effect, it finally penetrated my thick skull that the problem wasn’t in the attenuator contacts and had to be downstream in the amplifier and filter chain.

Reseating all the cable connectors and jostling the (socketed!) semiconductors also had no effect.

Could one of the semiconductors have gone flaky after four decades?

More tomorrow. Spoiler: yup.