In January 2024, I tracked down a pair of vintage Threshold Stasis 3 power amplifiers from a seller down in KwaZulu-Natal. The serial numbers—H8109641 and H8107584—trace their build date directly to October 1981, placing them squarely in Nelson Pass’s golden era of Stasis design.
Both Chassis arrived with a grim diagnosis: completely non-functional and written off as blown. Given the demands of ongoing work and other commitments, the pair was carefully packed away in storage, where they waited patiently for well over a year.
By April 2026, the bench was finally cleared to make room for them, and work commenced on the first unit, H8109641. What follows is a step-by-step photographic record of its teardown, troubleshooting, and painstaking restoration to its original condition.
Pulling the covers off H8109641 revealed a grim reality: years of exposure to the humid, salt-laden coastal air of KwaZulu-Natal had taken a severe toll. Corrosion had crept into virtually every corner of the chassis. Surface rust was eating away at the fasteners, the power transformer was visibly rusting, and a crust of green and white corrosion clung to the circuit boards and component leads. Even the anodised aluminium panels showed unmistakable signs of galvanic attack and pitting.
On the electrical side, things looked just as dire—all five rail and protection fuses were blown, suggesting catastrophic failure. On close inspection, I noticed some of the 0.68Ohm emitter resistors in the output stages were cracked open. Any lingering hope of a simple repair or targeted component swap vanished immediately. Bringing this classic Stasis 3 back to life would require a comprehensive, ground-up rebuild.
We completely disassembled the chassis and stripped down the power amplifier modules.
And so began the painstaking process of rebuilding and deep cleaning every part of the amplifier.
Sandblasting and Acid dipping of the internal heat conductor plates:
Ultrasonic cleaning of the PCB with isopropanol at 50'C after Ice blasting with CO2
I performed a curve-tracer test on all the power transistors and removed three blown transistors from the left channel and four from the right channel. I also replaced the input and driver semiconductors on both channels. Here is a cross-reference of the transistors alot of them are now becoming obsolete and it took some scratching to get two sets together:
Full part No Equivalent Found in
Amp
MPSA42 A42
MPS6571 NTE123AP
MPS6571
2N4250 NTE159
2N4250
MPSA92 A92
2SA2592 MJE182 2SA2592
2SA1112 MJE172
2SA1112
MJ15025G CLASSA6 / STASIS PNP
MJ15024G STASIS NPN
Sometimes I wonder what must happen to an amplifier to blow 60% of its Semiconductors and power resistors?
I also replaced all the 5W power resistors (0.68R, 30R, 33R), all the capacitors (including the electrolytic caps on the power supply), the fuse holders, all internal wiring, speaker terminals, and RCA Plugs.
This is the Circuit diagram I could find:
The reassembly:
With the electronics done, focus shifted to the metalwork. Decades of coastal oxidation had left the exterior surfaces looking weathered and dull, so we opted for a full mechanical and chemical reset. We stripped every aluminium chassis panel, bracket, and heatsink of its original anodised coating in an acid bath, returning the metal to a clean, bare substrate.
Stripping and re-anodising is easily the best way to return vintage aluminium to a flawless, factory-fresh finish—but it introduces a major headache: every trace of the original silkscreened lettering and graphics is destroyed in the process. Re-creating traditional silkscreens for a low-volume restoration is notoriously tedious and prone to wear, so we opted for a modern alternative. Rather than screen printing, we took the freshly re-anodised panels straight to the CO2 laser. By laser-engraving the original legends directly into the new anodic layer, the markings are razor-sharp, perfectly aligned, and absolutely permanent.
This is the newly anodised and laser-engraved rear plate:
The Ice-Blsated and painted, restored power Transformer mounted on the newly anodised baseplate with new rubber mountings
Another issue was the main power-supply capacitor bank. Electrolytic capacitor technology has come a long way since the original late-70s/early-80s designs—modern 2026-spec reservoir caps pack vastly superior ripple ratings and lower ESR, but they come in substantially smaller can sizes. Dropping them straight into the chassis wasn't possible; the original capacitor clamp mounting holes gave us an opportunity to avoid drilling new holes in the baseplate, since we were adamant about preserving the original layout and wiring geometry without drilling new holes into the baseplate.
To solve the size problem, we CNC machined a platform mounting. This bolts directly into the original factory capacitor clamp mounting holes on the baseplate, mounting the new, compact cans. This allowed the modern capacitors to sit at the exact centerline, height, and orientation as the original layout, keeping the original busbar and maintaining a clean, original aesthetic
Getting the hue on the grab handles and VU meter bezel right was one of the trickiest cosmetic challenges of the entire build. The anodiser assured us we’d get a rich, vintage gold, but the parts came back looking fluorescent yellow—so bad I won’t even inflict a photo on you.
After plenty of head-scratching and research, we abandoned standard dye anodising and used a chemical conversion instead. We went with an electroless dichromate (Alodine / chromate conversion) passivation process directly on the prepared aluminium. The result was perfect: it yielded that subtle, warm, authentic iridescent gold tone we were chasing, hitting the factory look spot-on.
.Final assembly took slightly longer than planned, largely because of the hardware. The external machine screws and decorative chassis fasteners had gathered noticeable surface rust, but aggressive wire-wheeling or harsh acid pickling would have instantly stripped away their original black passivation. We wanted to preserve that factory look without resorting to mismatched modern replacements.
After running the problem through an AI assistant to explore non-destructive rust removers, we turned to Evapo-Rust. The selective chelating formula worked exceptionally well—it dissolved the iron oxide down to bare, clean metal without attacking the underlying base material or stripping the original blackened finish. With the fasteners de-rusted, lightly oiled, and looking period-correct, the final mechanical assembly could finally come together.
My sincere thanks go to Nelson Pass and the original team at Threshold Audio for engineering such an exceptional amplifier. Hearing this Stasis 3 come alive after decades of neglect is absolute proof of just how timeless that original circuit topology truly is.
The rebuilt amplifier has now taken its rightful place in the listening room, paired with a recently restored Threshold SL-10 preamplifier and driving a set of reference KEF Model 4 floorstanders. The system synergy is nothing short of remarkable—effortless grip, commanding low-end authority, and that signature sweet, liquid midrange Stasis designs are celebrated for.
Rescuing this chassis from the brink was an immensely satisfying project from start to finish. With unit H8109641 singing again, the second amplifier won't be far behind—I plan to get it up on the bench as soon as I wrap up the work on Jamar's Bolero.
Michael
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