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Monday, 28 August 2017

Sinclair Stereo Sixty Amplifier.

Colin popped by, with an unusual amplifier under his arm.

Chap's bought this off me, can you take a look?

Yeah, why not...

It's a Sinclair (Sir Clive... think of the ZX 80, 81 and Spectrum computers, and the C5 electric tricycle, yeah, that bloke) "Stereo Sixty" Amplifier.



Now back in the 60's and 70's, before Sir Clive hit the big time with home computers, Sinclair sold electronic kits, and the Stereo Sixty was the preamplifier part of "Project Sixty", which consisted of a Stereo Sixty Power pre-amp, a Z8 power supply and a Z30 or Z50 Stereo amplifier module.

There was also a "Filter module"... and a quick web search show's it looks nothing like the module fitted to this module, but does show a similar one in "Project 80", although not available as a separate unit....








Lid off, and it's definitely home-built.... Coax used to get the audio from the din plugs, what looks like coax screening stripped off and used to screen the mains cable to and from the switch (?!) ...









and some rather too short wiring between some badly fitting phono sockets, to an attenuator hovering above the board ....









Gently powering the thing up produces some audio, and quite a bit of hum ...

That big red capacitor is open circuit...

Note the "Nomark" pass transistor, and the bridge rectifier shaped like a nut!








Dated Nov '71 ... it's almost as old as me, no wonder it's knackered....









There's a few other caps that are a touch low... these are replaced.

There are two output capacitors, each 1000uF, 63v, which read perfectly.... Sadly they can't be trusted. If they fail short, it's good-bye output stage, so they're replaced...








The dodgy mains-switch wiring is replaced... Someone was obviously paranoid about hum pick up... screened indeed!









The pre-amp is removed, and checked through.











The pots are horribly noisy, but clean up well. The co-ax is removed, along with the wiring that's too short ....













... and that floating attenuator replaced with something much nicer...










and lashed up for testing ....












All cleaned up and back together. I quite like it's blue and grey case ....










So... how's it sound?

I expected to be typing "of it's time" , but I think it deserves better than that , I was pleasantly surprised  ... It's quiet, and I enjoyed a couple of Matt Berry albums in the workshop whilst it was soak testing.

Of course, being a home built kit, this would vary dramatically on how it was put together in the first place. Whoever built this example was obviously paranoid about hum pickup. It's got a reasonable star-earthed design.

The bass control's centre frequency is far too high for my liking, and the PU input (for turntable pick up) seems very "flat", I suspect it was designed for a ceramic cartridge. Other than that, a talking point for Sinclair collectors everywhere, and very usable.





















Thursday, 24 August 2017

Oddball prototype Linn LP12 power supply. The Linn Wakonda.

I'm just posting this because I'll probably never see another. Apparently there are only a slack handful in existence ...

It's a prototype Linn LP12 power supply. Apparently called a Wakonda, it's a bit like a bastard son of a lingo and an axis (of evil), with a hole in it!


It's a pukka job. Obviously some sort of prototype...


It's obviously had some use as the board has discoloured from heat underneath the droppers, and, as usual, a re-cap restored operation ..... except 45 RPM....

... which was traced to two missing pull down resistors, not catered for on the board, and never fitted... perhaps the previous owner didn't have any 45's ??!

One interesting feature is the output transistors are bonded to the heatsink with some sort of cement. The digital phase generator is almost identical to a lingo, and allows LP12 users to switch between speeds without all that tedious mucking about with the belt.

Here's some photo's of it's final resting place