Having had a chance to unpack some of my shop from storage and settle in to the new space, I’ve got an idea of what projects I’ll be working on myself in between projects with my shop partner.

Bose Active Equalizers

Of course, since I used to be a top service provider for these, I have a whole bunch in stock which I’ll need to get through. I picked these up as parts units whenever I could find one for a steal, which was often due to cosmetics. The earliest 901 Series I cases were made of solid wood and hold up pretty well, but past the first ~10,000 units produced switched to particle board with a contact veneer until the Series III was released. Those get to be in pretty bad shape. I wonder if my partner can reproduce them on the wood shop side of the house.

With four Series I, a Series II, two Series IV, and a pair of 800 Series I equalizers (the same as a 901 Series I except with 1/4″ connetcors and none of the tone/tape controls) this should keep me busy for a little bit.

A 901-I/II has about 90 components to replace for a full makeover and takes about 4 hours from start to finish. The 800 equalizers have about 60 components and take maybe 2 and a half hours. Bose switched to a different design for Series III, and the Series IV only has a handful of capacitors in the power supply and signal chain. Some models used LF353N op-amp chips which were subject to a service bulletin and need to be replaced with TL072s. (Interestingly enough, some of these seem to be fitted with chip sockets from the factory and some have soldered chips, so sometimes it’s a really easy swap.)

This will keep me for maybe 10 sessions in the shop. I don’t make it in every week, and when I do, it’s usually for 3-5 hours.

1934 HMV 444

I’ve also got a His Master’s Voice Model 444 radio which I added to my collection more than 15 years ago. (I have the blog post to prove it!) At the time, I paid $60 (plus driving two hours each way) for the radio which had been restored by a long-retired Master Carpenter who found stuff locally at garage sales and gave it away for the same price he paid after re-doing the wood.

It’s not a big radio – maybe up to my waist – and it was only a 5 tube radio so nothing especially interesting electrically. But this is a 220V UK radio, and the truly striking art deco styling sets it apart. The schematic is drawn very differently from U.S. conventions but knowing the general theory, it’s pretty easy to follow. Should be a fun project, although it’ll take a few sessions of detailed measurement and tedious component replacement to get it across the finish line.

Sony TA-5650 VFET Stereo Receiver

I ended up with this Sony TA-5650 VFET receiver tin 2019 or 2020. I’ve done a good handful of these in the past, including several for multiple Grammy-nominated record producer Kavi Alexander of Water Lily Acoustics. This one is from an original owner, even. They’re really special, and deceptively simple, except for one detail.

These are from an era of heavy experimentation in semiconductor technology, the early-mid 1970s, and ended up using output transistors (FETs) which are a drastically different design and topoogy than anything else in common use before or after. They all need to be perfectly matched within the same rank as each other, and some traditional service best-practices (like using a variac to slowly bring an amp online for the first time after service) are guaranteed to destroy the output finals if used with this particilar piece of kit.

If you end up in that situation…good luck finding those semiconductors anywhere else ever again. There are some mods floating around to let you use a more traditional output device, but it’s a heavy rework job, and you lose some of the character that way. (These also use a small-signal VFET in the preamp chain for some reason.)

Adding to the mystery, when initially inspecting this one on intake, I found that it looks like there’s a factory revision to the final circuit board which wasn’t documented in the service manual. There are a few pieces of service errata – a couple of component swaps, changed bias values – but this one looked to have a few extra components and some different values installed. I didn’t look into it further to know whether it’s an early production or a late prodution model. Should be interesting.

I also need to adjust one of my two Audio Precision System One analyzers (and probably replace a few components in the power supply), so realistically, this set of projects will take me the rest of the year. I’ll post about it here as I go.