Technics SU 600 Repair – Mains Transformer Tests

Using this as a major refresher for everything as I’ve not done any electronics since travelling 6 months ago and moving to a new area. It will be diverse on topics recapped and covered to get my head back into things…

Beware figure 8 mains leads in these 2 wire ungrounded chassis – the mains connector AND the transformer connections are LIVE inside the unit if the ON switch is OFF OR NOT and 50/50 which pin is LIVE or Neutral!

Ebay repair – sold as not working – layer of decade dust that won’t hoover out, so used a soft paintbrush with nail polish remover (acetone – all I could get at Poundland! Lol. No switch cleaner – queues too long at Wilkes!).

Looking for possible reasons for a nonstarter, the On button PCB had screws missing so was pushing back so not locking the button On. This would mean the unit may seem non responsive as no connection is made?

A 3mm screw added to the power board seemed to solve this – see later!

The fuse was not blown but had a thin piece of fuse wire wrapped around the contacts in parallel with the fuse – someone stupidly trying “anything” to get this working no doubt..

I thought the filter caps glue was leakage on first visual so dismantled the unit.

Had to use a sharp point tool to get the link ribbons out the connector bodies after the push clamp cover was lifted. Getting them to make a connection again on re-inserting them was a nightmare as they are now worn and don’t hold tight.

Once the main board was out I checked the transformer primary windings with a meter –  15.4 ohms – good.

The secondaries are too low DC resistance to discern a shorted winding so used my ESR meter to find 2 pins (0 and 4) seemingly shorted:

but the others having some resistance so sourced a schematic from elektrotanya.com to check:

technics_su-600 schematic

The end of the 8.8V winding is connected to pin 4 so explains the “short”.

This was also checked by viewing the PCB connections under LED light – the tracks linking these pins (far left = pin 0 = pin4) can be seen connected under a magnifying light:

I wondered if my Hantek digi scope can do XY mode to check the windings also, so got the manual. It can using two probes, but it’s useless – it just shows a small angle line for a short circuit and no ellipse for a good inductor…need an analogue scope for that.

dso5102p_manual

The mains S2V20 Shindengen Rectifier diodes all checked OK each way with a meter in diode mode. 200v, 1.7A rated. Obviously a step down secondary as mains 240V RMS is 1.414 x 240V = 339.5V peak else would overate the diode voltage.

Happy that the mains transformer seems OK, I checked the filter caps and realised it’s glue not leakage, and they test to 4100uF with the ESR meter but should be 4700uF at 50V DC:

The power amplifier – like the Sanyo ST range – seems an original Panasonic SVI 3102 B as no evidence of desoldering is visible.

Can’t find much data on it – maybe 28W per channel into 6 ohm:

SVI3102C 2x28W 6 ohm HY1o (package)

Maybe the C pinout is the same as B version?

The 50V filter caps are on the limit of supplying the +/- 47V DC max this power amp requires from that schematic above.

Not sure if it’s original or chinese replacement as the heat paste seems too sloppy for factory? Need to check the solder joints…

The Phono circuit uses an AN6552 chip for gain.

 

https://industrial.panasonic.com/content/data/SC/ds/ds4/AN4558_E_discon.pdf

See XRaytonyB videos for the explanation of why RIAA EQ circuitry is required for the mechanics of vinyl record usage:

https://youtu.be/G8LS6FDfdZY?t=20

This diagram from an SU-V1X manual describes the process well:

If you follow the red line from Phono In Left and Right to both Tape OUT LR, you can see both op amps in the AN6552 handle each LR channel.

Checking the mains transformer windings at low voltage – 20Vpp sine into the mains connector to check values on schematic:

Pins 0/4-1 is 8.8V AC on schematic. In Phase

Measured = 20Vpp to 0.75V = 27:1 step down. 240V RMS / 27 = 8.8V RMS Correct!

Pins 2-4 is 22.3V AC on schematic. Out of phase

Measured = 20Vpp to 2V = 10:1 step down. 240V RMS / 10 = 24V RMS Correct!

Pins 3-4 is 22.3V AC on schematic. In Phase

Measured = 20Vpp to 2V = 10:1 step down. 240V RMS / 10 = 24V RMS Correct!

Pins 4-5 is 32.1V AC on schematic. Out of Phase

Measured = 20Vpp to 2.5V =  8:1 step down. 240V RMS / 8 = 30V RMS

Correct!

Pins 4-6 is 32.1V AC on schematic. In Phase

Measured = 20Vpp to 2.5V =  8:1 step down. 240V RMS / 8 = 30V RMS

Correct!

So, testing mains transformers with small signal generators can be quite accurate if you’re not sure about measuring with full 240V RMS connected, and risking unknown windings blown states. Doesn’t mean the insulation will hold up at full voltage though..!

Decided to put it back together to see what happens as there are no more obvious faults and the tranformer, filter caps and rectifier diodes all check ok.

Putting missing screws in the chassis help the broken front edge fit better but the power button still sits too far back in the front so isn’t latching properly without being hard pushed all the way, so inconsistent.

I have to get a fig 8 mains lead so can’t power it for now…