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Old 1st Mar 2020, 12:56 am   #1
retailer
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Default Technics SU 3400 first steps in restoration

Back in the mid to late 90's when 70's manufactured Japanese amps were falling out of favour I managed to pick up a collection of them from from a salvage yard that was attached to a local rubbish dump, the electronics gear that was dumped was separated and most ended up in skips to be recycled, I passed the salvage yard 3 or 4 times a week and usually dropped in for a look and over the years managed to score around a dozen 70's manufactured Japanese amps Sansui, Pioneer, Techincs, JVC - from the days when cost did not seem to be a huge consideration in the design and manufacturing of the amps. I had gotten to know the guy that ran the salvage yard and the cost to me was a reasonable (I thought) $5. Since I've been retired I gone through just about all of them and brought each one back to their former glory before being offered to family free of charge - or sold off on Gumtree.

A Technics SU3400 is the last one - left till last as it is the US model designed for 110V mains operation - I knew it was a long shot but I had hoped to pick up a suitable 240V transformer but no suitable transformer has turned up so I decided to have a go at rewinding. My main concern here is that US mains frequency is 60Hz so there is a strong possibility that the core may be a bit on the small size for a rewind at 50Hz and I guessed that the 50Hz version probably had a taller stack of laminations. Looking at the circuit there are 3 secondaries windings - preamp - power amp - and indicator globe, interesting that a separate winding was used just for the lone indicator globe.

I removed the transformer taking a pictures as I went, I planned to used the same flying leads to keep the amp looking as original as possible. Getting the laminations apart was not too hard and I set them aside to soak in MEK so the varnish could easily be removed, I measured no load voltages and counted turns and measured insulation thickness for each secondary, as I went, once it was all unwound I set to to calculate the wire sizes and turns for the new wind. As I suspected it was going to be a tight fit. When winding a transformer I like to have my fill calculations come out at less than 80% - I had 82% fill. After testing the core with a dummy winding I decided that the original turns per volt was very conservative and to decrease the turns per volt slightly, while this runs the core slightly closer to saturation it was still a long way from saturated and well within its ratings.

My home made transformer winder did a great job, I was able to duplicate the position and orientation of the leadout wires - over the years they had taken a set and would be much easier to fit the transformer back. The voltages came out almost spot on with the exception of the indicator globe winding which was almost 1 volt under - not worth stripping the the thing down to remedy. An overnight soak in a tub of varnish finished the winding - the last job before assembly was cleaning up the laminations, MEK makes it an unpleasant task at any time, but it had to be done. All back together the amp powered up on 240v and amazingly still worked but with issues, intermittent switch contacts, noisy volume control and a bent mode switch shaft that I managed to partially break in an attempt to straighten. Still a fair bit of work to do, I'll leave it for another post after it is done.
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Old 1st Mar 2020, 9:25 am   #2
Jac
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Default Re: Technics SU 3400 first steps in restoration

Quote:
Originally Posted by retailer View Post
My home made transformer winder did a great job.
That certainly looks like a very capable winding machine you made!
The winding is very neat. Very well done.

Jac
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