Category Archives: Hi-Fi

Everything to Know about Bose 901 Series V Active Equalizer and Speaker Repair

Posted on October 12, 2026 by retrovoltage

A detailed look at the Bose 901 Series V speaker system and Active Equalizer, with a brief history, plus repair tips and tricks, schematics, equalizer curves, and a parts list with direct links to buy what you need. Continue reading →

Posted in Audio, Bose, DIY, Electronics, Hi-Fi, Stereo, Vintage | Tagged , , , , , , , , , , , , | 2 Comments

KLH Transistor Substitution Guide (“Solid State Components Cross Reference Guide”)

Posted on October 6, 2026 by retrovoltage

This post contains the transcribed text of the Februrary 1970 KLH Solid State Components Cross Reference Guide (aka, transistor substitution guide), obtained from an original copy of the document. It’s in this format because it’s easier to search and find quickly. Continue reading →

Posted in Audio, Electronics, Hi-Fi, Resources, Vintage | Leave a comment

The Speaker Spotter — October 2, 2026

Posted on October 2, 2026 by retrovoltage

A recurring feature curating interesting speakers for sale on Craigslist from around the Pacific Northwest. Links to Craigslist postings might go down at any time if they’re sold or pulled from sale. 1977 Klipsch La Scala $3,500 · Portland Someone’s … Continue reading →

Posted in Audio, Collections, Electronics, Hi-Fi, Vintage | Tagged , | Leave a comment

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

Posted on August 10, 2026 by retrovoltage

A detailed look at the Bose 901 Series IV speaker system and Active Equalizer, with a brief history and explanation, plus repair tips and tricks, schematics, a discussion of the 5 different Active Equalizer versions for this generation, and a parts list with direct links to buy. Continue reading →

Posted in Audio, Bose, DIY, Electronics, Hi-Fi, Stereo, Vintage | Tagged , , , , , , , , , , , | 5 Comments

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 →

Posted in Bose, Electronics, Hi-Fi, Stereo, Vintage | Tagged , , , , , , , , , , , , , | 5 Comments

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 →

Posted in Audio, Hi-Fi, Projects, Speakers, Stereo, Vintage | Tagged , , , , , , , , | Leave a comment

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