Technics SU 600 Repair – Fixes Required – Bodge Done

The unit seems fully functional after a few hiccups – blew the fuse as the transformer windings shorted by the ribbon cable not being in the connector properly – these need replacing badly – they don’t latch, and one is now broken. Don’t know where I’ll get them – can’t find the type even on the web. Don’t want to have to hard solder bodge them to the board…

There is stereo sound from my sig gen via a splitter, and with good connections at all ribbon cables, I get a strong output from all 4 channels for a 1Vpp input at the rear phonos.

The Phono channel is distorted obviously at that input level:

and the rest all show similar output just at pre clipping about 80Vpp:

I can’t do a power test until the 8 ohm dummy load resistors arrive (too loud for the ears and flat mates!).

Specifications

Power output: 30 watts per channel into 8Ω (stereo)

Frequency response: 5Hz to 70kHz

Total harmonic distortion: 0.07%

Damping factor: 50

Input sensitivity: 2.5mV (MM), 150mV (line)

Signal to noise ratio: 71dB (MM), 90dB (line)

Channel separation: 45dB (line)

Output: 150mV (line)

Speaker load impedance: 4Ω to 16Ω

Dimensions: 430 x 97 x 240mm

Weight: 5kg

The power calculations are from Ohms Law:

V=IR; I=V/R; R= V/I

P=IV; V^2/R

At full volume with X10 probes with no speakers, about 85Vpp clipped:

Power output: 30 watts per channel into 8Ω (stereo)

For a 500mV input (1kHz sine) at the tuner input, at volume level 5 (1/2):

This is a gain of 10V/0.5V = 20

At this volume 5:

P=IV = V x V / R = 10^2 / R = 100 / 8 = 12.5W from one channel. Probably about right, as assume linearity for volume 10 gives 25W per channel for a rated 30W per channel amp, at this point and level of accuracy.

I need more parts; fuses, leads, solder station etc. to arrive before I can do more on this amp.

I have heard it through the Minimus 26s at low volumes- they’re really clear and quite bassy for small speakers – like them!

I’ve decided to call this complete as it isn’t worth the hassle to replace the ribbon connectors on an otherwise working amp. These work by having the curved side bent sufficiently up and toward the opposite side so the wires are caught between the the sharp bottom edge of the curved side so when pulled slightly up again, they grip the wires.  The clip then pushes the curved side in toward the other if they aren’t worn or broken.

I managed to get the wires to grip enough to then use a drop of super glue on the ribbons each side to stop them vibrating loose.

The main danger there is the transformer secondary ribbon coming out and shorting the transformer when powered. This is made worse by Technics NOT including fuses on the secondary sides on this model AND the SU X840 – which has the blown secondaries!

After bodging the setup of 100W, 8 ohm resistors for a rough power test – given my crap working environment – not exactly Mt Carlson’s Lab shall we say?

I got a 500mVpp stereo signal from my sig gen into the CD input

This gave a volume 7 clean output up to overheating and breaking down (these power amps don’t like a non reactive load it seems – I’ve had this behaviour on other STK power amp models before)

As power in an AC circuit is the RMS equivalent of DC, then continuous DC power in a load would be equal to both half cycles of AC or the same as each RMS peak (as if unregulated DC rectified). So, the peak to peak value is the same as having 2 rectified DC humps, not one, on one positive side of a 0v ref so the peak to peak value is divided by 2 first to get one peak value for the equal RMS energy expended, then root2 to give the equal RMS value for work done in a DC circuit:

This shows 24Vpp output into 8 ohm. So..24Vpp = 12Vp / sqrt 2 RMS = 12Vp / 1.414 = 8.5V RMS

P = IV = V^2/R = 8.5 x 8.5 / 8 = 72 / 8 = 9W per channel at volume 7

All the power comes from volume 7-10 with this amp.

Frequency Sweep: Pointless with a phone App as the result depends on the phone output circuitry.

24/5/20

Here’s the definitive power output through 8 ohm dummy loads – at full volume with NO clipping for a 500mV input and a gain of 24V/0.5V = 48:

24Vp / 1.414 = 17V RMS

P = V x V/R = 17 x 17 / 8 ohm = 289 /8 = 36 Watts per channel of a rated 30W amp with no distortion!

Lessons Learned

These ribbon connectors are common on Technics amps and need to have the bent side pushed out and up toward the other side to have a chance of gripping the wires again. I can be done carefully with my trusty Homebase sharp, pointy tool. The plastic tops don’t hold if too worn  and my only solution without replacing these (where from??) was to super glue the ribbon in place once connected ok.