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January 28, 2012

Fixing Old Speakers – The Common Woofer Foam Rot Problem

Audio Research freebie speakers

Audio Research freebie speakers

Got an old pair of speakers that don’t bark as well at the low end as they did in their younger days? You might be suffering from the dreaded woofer foam rot. Recently, I had been hunting for a good, cheap pair of speakers to partner with my newly repaired Hitachi receiver. I luckily found some free on Craigslist; someone just 2 blocks from me was giving away a great pair of 4-way Audio Research speakers with massive 12″ woofers, probably 1980′s vintage. They were “great”, that is, except that the suspension foam surrounding the woofers had almost completely decayed away.

Foam Rot

Woofer Foam Rot

This is the well-known “foam rot” problem seen in woofers and other high-compliance drivers of similar age. A few decades ago, speaker manufacturers started using a glued-on ring of shaped foam in place of the concentric corrugated cardboard surround on hifi speaker woofers, presumably to reduce distortion, especially at high excursion amplitudes. Unfortunately, the use of this foam has a recomplicating effect; it tends to decay away in 20 or so years, rendering the woofers useless. (The foam is apparently far from stable, which is good reason also to wear gloves when doing this repair; I don’t know the composition or the decay products.) Fortunately, there is an easy solution; there are many vendors of new, precise-fitting woofer foam, and the repair can be done quickly. (more…)

January 26, 2012

Most Repairs are Easy – Fixing an Old Hitachi Receiver

Filed under: repair,Uncategorized — Tags: , , , , — admin @ 1:34 pm

As you can see throughout this blog, I do a lot of repairs of durable goods ranging from cars to electronics. I’m writing a book on closely related topics now (stay tuned here for publication news). One of my contentions is that most repairs are quite easily accomplished with only a small set of tools and rudimentary expertise. In addition, often, the source of the problem can be found by simple visual inspection. Here is one such example.

The Failed Switch

The Failed Switch

I had in my electronic “junk pile” an old Hitachi SR-5150 stereo receiver that a neighbour had discarded in the fertile back lanes of Vancouver, BC. The SR-5150 is roughly early 1970′s vintage; it is all solid state and has all discrete transistors (the power transistors are even Hitachi’s own), and an analog FM tuner driven mechanically with pulleys and a flywheel. Nothing high end, maybe 20-30W per channel judging from the power supply. The receiver was dead to the world; it would not show any signs of life when turned on. I was motivated to fix the thing because I was looking for a decent amp to use as part of a new sound system centred on a home computer. (more…)

November 19, 2011

How to rejuvenate rechargeable NiCd batteries

NiCd battery pack, assembled

NiCd battery pack

This post is about a method to restore (rejuvenate) NiCd (NiCad) battery packs that are failing to charge. I have 2 sets of cordless tools (drill, circular saw, reciprocating saw and more) that use rechargeable NiCd battery packs. Three of the battery packs were failing to charge. Having had some previous success with rejuvenating apparently expired NiCd cells, I decided to do some tinkering, and I was able to restore the packs such that they will now charge to a usable level. I’ve previously used the same procedure, at lower current, to rejuvenate NiCd cells for cordless phones.

These tool sets and battery packs are very common. Mine were MasterCraft brand purchased from Canadian Tire, but there are many similar ones on the market with other branding. I’d guess millions of similar cordless sets have been sold, so maybe someone else can benefit from this post and save a few $ on new batteries. Perhaps more significantly, maybe this will save some battery packs and even the tools themselves from adding to our garbage output; the Cd in the cells is very toxic, so the less of it we put into use and the less that goes into the waste stream, the better. (more…)

July 12, 2011

Reducing audio frequency electrical noise in a PC sound system

Kinter Amplifier in PC Case

Kinter Amplifier in PC Case

I recently built a new PC that I use for multimedia and other purposes. It’s Intel “Sandybridge” 1155 -based, with a Gigabyte P67 series motherboard and Realtek onboard sound. Unfortunately, like so many other recent PC audio systems, it doesn’t have sufficient output power to drive unamplified speakers directly. I have some unpowerd bookshelf speakers that I wanted to keep using. So, I added a cheap audio power amplifier to the Realtek output. The resultant system had slightly annoying audio frequency noise, not 60Hz noise but various other noise from misc. stuff in the PC.  Here are some details re the amp and what I did to fix the noise problem.

The audio amplifier is one I found on eBay from a Chinese manufacturer, with labeling “Kinter 500W”.  Price was about $10 including shipping, cheaper than I could build one for myself. I knew what was in the amp from some googling: an LM-series audio amp IC and typical supporting parts, providing a few watts per channel. Not the claimed 500W :-) , but adequate for what I wanted. I run the amp from the PC 12V supply, which is far enough beyond spec for the other stuff in the PC that it’s fine to power the amp with it too. I started with a generic 1/8″ stereo to RCA patch cord about 3ft. long to connect the amp to the PC audio output. (more…)

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