Category Archives: Hi-Fi

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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Music Minder Automatic Power Controller for Hi-Fi Systems

Posted on August 24, 2016 by retrovoltage

Mixed in with some other gear I’m servicing, I found this old Music Minder power box. It’s not all that obvious what it’s for at first glance! Some digging around found an explanation in the June 1958 issue of Popular … Continue reading →

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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

“Won’t that ruin the vintage sound?”

Posted on March 16, 2016 by retrovoltage

Audio as a hobby sits in an interesting place in the world. There are hard numbers and objective measurements made with lab equipment on one end, and on the other end you have the subjective and variable linguistic descriptions assigned … Continue reading →

Posted in Audio, Commentary, Electronics, Hi-Fi, Vintage | Tagged , , , , , , , | 3 Comments

Harman-Kardon Festival D-1000 Tube Receiver Repair

Posted on November 17, 2015 by retrovoltage

All vintage gear has a story to tell, but something really rare and interesting recently came across the bench: the Harman-Kardon Festival D-1000. While it looks somewhat unassuming, a fairly straightforward 15-tube receiver with push-pull 6L6GC outputs and an AM … Continue reading →

Posted in Audio, Electronics, Hi-Fi, Vintage | Tagged , , , | 4 Comments

The Speaker Spotter – November 6th, 2015

Posted on November 6, 2015 by retrovoltage

Today’s Speaker Spotter showing the most interesting speakers for sale on the local Craigslist has a number of rare, unusual, and high-end speakers you’re not likely to find anywhere else. As always, I’m not affiliated with any of these sellers, … Continue reading →

Posted in Audio, Collections, Commentary, Hi-Fi, Speakers | Tagged , , , , , , , , | 1 Comment

The Speaker Spotter – September 30th, 2015

Posted on September 30, 2015 by retrovoltage

It’s back! Another installment in this occasional series, “The Speaker Spotter”, picking out the most interesting speakers available on my local Craigslist. Today’s Speaker Spotter focuses on bookshelf speakers for the best sound possible in a small package. As always, … Continue reading →

Posted in Audio, Collections, Commentary, Hi-Fi, Speakers | Tagged , , , , | 1 Comment

Sony ST-J75 Recap and Alignment

Posted on July 31, 2015 by retrovoltage

The Sony ST-J75 is a pretty well regarded, great sounding stereo FM tuner from the early ’80s. It’s one of the first forrays into digital tuning, with a frequency read-out, programmable memory, and seek/scan functionality and variable muting driven by … Continue reading →

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1950s Motorola Modded Standalone Tube Amplifier Measurements

Posted on July 6, 2015 by retrovoltage

I had an unusual project through the shop recently. It’s a Magnavox tube amp from the late 1950s. The owner brought it in reporting he’d purchased the amplifier, pulled from a damaged console stereo and modified to be a standalone … Continue reading →

Posted in Audio, DIY, Electronics, Hi-Fi, Projects, Radios and Tubes, Vintage | Tagged , , | 3 Comments