Technics SU-X840 mains transformer

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The primary on the mains transformer of my Technics SU-X840 amp is open-circuit. The transformer has a part number of SLT5M505, but appears now to be discontinued. So I am looking to replace it with one or more alternatives. The service manual tells me the transformer has three secordaries: 0-4.3V, 12.6-0-12.6V, and 0-53.4V with an earthed centre tap. Unfortunately it does not give the power ratings, but the transformer weights around 2-3Kg. The manual says with no signal input the primary should draw 85-140Ma, and the whole unit is fused at 1A. The DIN power rating of the amp is 2x50W.

Any advice on obtaining suitable alternative transformers sought. And advice on what power ratings I should go for in each of the secondaries if I choose to replace it with separate transformers. Fortunately,there is quite a lot of space in the unit.
Cause Of Failure

Hi Trevor99,

I was a service tech for Panasonic for 15+ years and I worked on just about every Technics product that came in for repair. Your transformer has an open primary but all is not lost.

THE CAUSE: Panasonic started to put a thermal fuse on the primary side. If the core got too hot (~105°C if I remember correctly) the fuse would open and the system would shutdown. I think they did this in order to get UL approval. There were customers that would just put in a bigger fuse on the main and the amp could catch fire.

THE FIX: Replace the transformer and maintain the intended safety systems or repair it. Remove the cover plates (I don't remember exactly what your transformer looks like but most had shields) and start looking for the cutout fuse. It was usually crimped to the magnet wire on one end and crimped to an insulated wire on the other end. The fuse and crimps were usually inside an insulated tube. Now you can replace the fuse or eliminate it. If you replace it use the same temperature (printed on the side) or slightly higher (5 to 10 °C more should do it). If you eliminate it and over fuse the main fuse you could destroy the unit as well as burn down your house so use some common sense.

I remember having to replace a lot of transformers back in the early to mid 80's. I did keep a few of the bad ones and I was able to repair them and use them in my personal projects. They worked quite well and the price was right. Again, just make sure you properly fuse the circuit if you eliminate the thermal fuse.

Good Luck,

Phat Daddy
Thanks for yor advice, Phat Daddy, but I think I'm out of luck. Maybe the transformers for the UK market are constructed differently to the US ones due to the higher voltages. Anyway, I removed the covers, which are really just a box to permit the tranformer to be bolted to the chassis, but inside that is another very tightly fitting metal casing which appears to be epoxy-bonded to the epoxy-encapsulated windings inside. I don't think I can remove this without destroying it, and if I could remove it I'd then have to go digging into the encapsulation in search of the thermal fuse. Do you think this is worth doing, or would I end up with a destroyed or unsafe transformer?

So I think I'm back to asking for advice on replacement transformers, particularly what power ratings might be needed for the three secondary circuits. I could guess these, but any advice on guessing would be great! TIA.

Hi Trevor99,

I posted a reply on Saturday but I guess it didn't make it. I will try again. I like to oversize whenever possible. I know it will work and usually the device will run cooler. Generally the cooler something runs, the more stable the system will be (not as much thermal drift).

First the worst case is 230 volts (your AC power) x 1 amp (the main fuse) = 230 watts. Lets make this 250 watts to make the numbers nicer. As I said, I like to round up.

Now, 50 watts per channel x 2 channels = 100 watts. But nothing is perfect so derate by ~70%. This factor will change based on the type of amplifier so adjust as needed. So 100 watts / 0.7 = ~143 watts for the main amp. My guess is that the 53.4 volts powers the main amp so 143 watts / 53.4 volts = 2.7 amps. Round this up to 3 or 3.5 amps when you look for your new part.

For the next winding use the 230 watts - 143 (from the amp) = 87 watts. If we just look at all of this for the 12.6 - 0 - 12.6 winding we get 87 watts / 25.2 volts = 3.5 amps. I would go with this but it would be safe to reduce this to 3 amps since there is still one more winding to look at.

I will guess that the 4.3 volt winding will use no more than 1/4 of the 87 watts. So 22 watts / 4.3 volts = 5.1 amps. For this I would use 5 amps.

These are just estimates. You can get more accurate results by looking at the schematic and cutting the old transformer open and looking at the wire gauges that were used.

I know this is a little late now but I tried to send you this link:

I found this at auction number 130199931522. It ended up selling for £11.27 .

This is how I would do it. I am sure that there are a lot of people on this forum that know a lot more about this than I do. If anyone has any comments I am always looking to learn something. Good luck with your search.

Phat Daddy
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