Sunday, 6 October 2019

Strathclyde STD305D power supply.

I'd popped down to Brighton for a few days, to see my good friend Edouard.

He's got a very nice looking Strathclyde STD305D ..

"Got it on eBay, doesn't work, and appears to have bits missing.. Fancy a look?"

Yeah, why not...

A super turntable, which uses a proper DC brushed motor, a tacho and proper servo control...

Those speed adjusting sliders you see there, are actually tiny thumbwheels, which move up and down the track, allowing precision adjustment of the speed. Nice!

There's a digital display (presumably) for speed, and soft touch controls.

Here's the control PCB...

And here's the motor, and what remains of the power supply....

Some bugger's had the transformer!

It looks like they had a hum pickup problem (or galloping paranoia) and moved it outboard. Edouard didn't have it...

Thankfully someone had carefully marked all the voltages on the board, the original transformer had a good few windings on it...

It was (allegedly) wound 18.5-9-1.5-0-1.5.-9-18.5 

This is not going to be so easy , and is likely going to need multiple transformers, as I did to get the Tascam Portastudio 488 going.

I quickly sketch out the supply schematic, and realise something odd's going on with the 1.5 volt side of things... it appears to supply AC off to the control board... The other rails are simply rectified, smoothed and regulated.

Remembering the Tascam again, I trace the AC from the 1.5-01.5 across the control board, and it supplies the filament for the Vacuum florescent display! Ah!

A couple of suitable transformers are ordered for the 18 and 9V supplies, and cobbled up in the usual lethal manner to prove the point. To supply the 1.5-0-1.5 , a 20-0-20 transformer has it's primary supplied by the 9V transformer.

The point isn't proved.. it's pulling the best part of 100W from the mains! 18V transformer has got damn warm.

Tracing the circuit for the 18V through shows whomever measured it in the first place got it very wrong. It's not 18-0-18 .. it's just 18 - 0 ... connected across the two 18V connections. It's not 0 at all.. it's a bridge rectifier circuit. Checking the 9V shows that's a bridge too, not a centre-tapped full-wave circuit as the note left on the board suggests. The other very odd thing is the four fuseholders on the board. They fuse each half of the bridge ... why?? The 1.5V is, however centre tapped.

Once this is sorted, consumption falls to a sensible 14W. Nothing gets warm!

Here's the diagram.

T1 and T2 supply the 9 & 18V supplies.
T3 is a 6VA 0-20 0-20 transformer, the primary of which is wired for 120V operation, but supplied from the 18V supply. It's output provides the 1.5-0.1.5 output for the filaments.

The output is taken to a 9-way D.

Excellent. Well, nearly. The touch selector isn't working. No matter what speed is selected. It's always 33 RPM. Disassembly of the front panel finds a 7400 which is rather warm, this is replaced and all is well :)

 Touch sensors work perfectly, and the speed is stable

Now to tidy up the rat's nest supply!

And done... :)

To sum up, a simple job which took far too long due to me trusting the incorrect work of two people... 1. The person who wrote out the label... 2. The person who laid out the power supply PCB incorrectly.


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