Technics SA 5L Repair – Working – Sound Check and Lessons Learned

So the carbon crap on the board was the problem with this nice little, great sounding unit.

The front face was a pig to get back on – you have to remove the analogue meter to get it to fit properly!?

The digital dial light is a 6.2V AC circuit like on valve amps – why? Just for a wheat grain bulb type? Seems a waste of a 6.3V winding on a transformer, unless it is used elsewhere and rectified, but without a circuit diagram, I can’t say..I don’t see why this dial section needs a light either, with the digital display?

It has an alignment/quartz lock issue on FM as it can’t scan and lock, and I don’t have the kit to align it.

The LW and AM bands scan and lock ok, but there’s no stations worth having anyway.

The dual PNP and NPN power amps are well heat sunk and really kick out the bass with the Bass Boost on for a small unit.

Power Tests (0.5V input):

20Vpp = 10Vp = 10/root2 RMS = 10/1.414 RMS =  7.07V RMS

P= IV = V^2/R = 7.1 x 7.1 / 8 ohm = 50 / 8 = 6.25 Watts per channel

This is at volume 7.2 for 0.5V 1kHz input.

I didn’t get the power transistor types as they were covered by the heat sinks and I can’t find any info for this unit at all!

Lessons Learned

It is now not necessary to remove the main board to check the caps (underside) with the Peak ESR meter now I have a dual rail supply, as I can connect to the rectifier with +/- 30V DC and ramp up slowly watching the amps flow for any shorts OR NO current flow for an open circuit somewhere (probably a constant voltage transistor soon after the rectifier). Shorts on other boards can be proved by disconnecting relevant ribbon/push connectors if fed from the main board.

Low 4 ohm values on secondaries on the Peak ESR can be deceptive! It does NOT necessarily mean the windings are short, as I have since learned on the SUX840 as these values increased once disconnected from the main board (that amp needs more investigation – no voltage on the voltage regulators though they tested OK when removed).

When testing power output with the cheap 100W chinese wire wound resistors – be careful! They get REALLY hot if left at full volume for too long (seconds) and they don’t cool well AT ALL once disconnected and are a burn risk.