Category Archives: Stereo

Everything to Know about the Bose 901 Series III Speakers & Active Equalizer Repair

Posted on July 31, 2026 by retrovoltage

A look at the Bose 901 Series III speaker system and Active Equalizer, with a brief history and explanation, plus repair tips and tricks, schematics, and parts list with direct links to buy. Continue reading →

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1961 Audax CA-70 “Sonoteer” Speaker Refurbish

Posted on July 22, 2026 by retrovoltage

This set of 1961 Rek-o-Kut Audax CA-70 Sonoteer speakers got a full overhaul inside and out. Come learn about these interesting mid-century open baffle hi-fi speakers from a little known manufacturer and see how they turned out with detailed interior and exterior photos, and the original advertisement from September 1961. Continue reading →

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HH Scott HHS-20 “Scotsman” Receiver Overhaul

Posted on January 27, 2017 by retrovoltage

A local repeat customer recently brought in his old HH Scott HHS-20 receiver for an overhaul. It worked a few years ago when he put it away, although not without a few issues of its own, and when he dug … Continue reading →

Posted in Audio, Electronics, Projects, Stereo, Vintage | 3 Comments

Reviewing the Gemtune APPJ PA1501A 6AD10 Stereo Tube Amplifier

Posted on August 17, 2016 by retrovoltage

I recently picked up an interesting little tube amp via a group buy on Massdrop, the Gemtune APPJ PA1501A stereo tube amplifier. Try saying that three times fast! It’s a cute little single-ended stereo tube amp using a pair of 6AD10 … Continue reading →

Posted in Audio, Commentary, Electronics, Hi-Fi, Stereo, Toys | Tagged , , , , , , | 15 Comments

Bose® Active Equalizer Test Platform

Posted on April 13, 2016 by retrovoltage

This project is going to take a while since I’m slammed in the shop, but I’m in the process of converting a Bose® 901 Series I Active Equalizer to  a test platform. With sockets for the transistors, and clip connections … Continue reading →

Posted in Audio, Bose, DIY, Electronics, Stereo, Test Equipment | Tagged , , , , , | 3 Comments

Neat Little 60FX5 Stereo Single-Ended Tube Amplifier

Posted on March 6, 2016 by retrovoltage

While digging around, I came across a really simple circuit in the RCA Receiving Tube Manual describing a 1WPC stereo amplifier using only two tubes, the 60FX5 power pentodes, and a single silicon diode. The 60FX5s have high enough gain … Continue reading →

Posted in Audio, DIY, Projects, Stereo | Tagged , , , , , | 5 Comments

The Speaker Spotter – March 3rd, 2016

Posted on March 3, 2016 by retrovoltage

Some interesting stuff on Craigslist in this week’s The Speaker Spotter, a curated selection of Seattle’s local speakers for sale online. As always, Retrovoltage is not affiliated with any of these sellers and you should click through the link to the … Continue reading →

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HH Scott 233 Stereo Integrated Amplifier Overhaul

Posted on December 31, 2015 by retrovoltage

I was lucky enough to get to work on a very nice Scott 233 stereo integrated amplifier. This example was wonderfully well preserved, coming out of a storage unit looking like it had just left the showroom floor. Unfortunately, though, … Continue reading →

Posted in Audio, Electronics, Radios and Tubes, Stereo, Vintage | Tagged , , , , , , | 7 Comments

Harman-Kardon Citation 11 Pre-Amp Overhaul

Posted on November 11, 2015 by retrovoltage

A neighbor brought me his brother’s Citation 11 for a full overhaul. After a few decades of faithful stereo reproduction, then a number of years in storage, it was time for an overhaul to get it working good as new again! Of … Continue reading →

Posted in Audio, Electronics, Projects, Stereo, Vintage | Tagged , , , , | 16 Comments

1963 Fisher FM-100-B Stereo FM Tuner Overhaul

Posted on August 9, 2015 by retrovoltage

Something a bit unusual came through the shop recently, an old tube-type Fisher FM-100-B Stereo FM tuner. Anyone who knows classic tube audio knows Fisher, and the FM-100-B sure lives up to the name. It’s a 12-tube FM/MPX Only tuner with … Continue reading →

Posted in Audio, Electronics, Radios and Tubes, Stereo, Vintage | Tagged , , , , , | 3 Comments