Technics SA-GX100 power transformer

[stiebrs]

Hi. I got a 30 year old Technics SA-GX100 amp for repairs. I do have a bit of digital electronics experience (mostly self-taught), but this is essentially first time me dealing with analog/power electronics.

So, the patient was brought in with complaint that the screen won't turn on. Standby light is on, upon switching the power button relay can be heard, but nothing else happens. Skimmed through the service manual I popped the case open and straight off the bat saw regulator transistor (Q752 - 2SC3940AQSTA) surrounded by soot. Replaced that with 2SC1384(R) together with zener, just in case replaced most of the electrolytes in PSU. Tried powering up and found out that the main transformer is dead (open on primary, secondaries are fine). Didn't think of measuring that before powering it up, maybe I killed it.

The problem is that I can't find direct replacement for RTP1M5C001AV transformer (center-tapped 54.4 (27.2+27.2) + 3.9V from schematics, no datasheets available). Generally not much is available for specified secondary voltages. So the question is - are those transformer output voltages in schematics no-load voltages (implying that it's actually 24V) or full-load voltages (30V transformer)? Can I replace it with a dual 30V toroid (60V center-tapped) in parallel with some simple 4V source? Or do I really have to rework PSU from scratch to provide necessary +-27.2 as designed by the Germans?

Any help much appreciated

[shakalnokturn]

Usually voltages are given unloaded.
Expect any AC or unregulated DC voltages to vary quite a bit from SM indications they are directly subjected to transformer primary voltage setting (220/240V) and local line variations.

Before searching for a replacement transformer try checking the existing one for a burried thermal fuse on the primary some times there is one that can be extracted by cutting away a small amount of plastic.
Note the primary pinout with intermediate taps and resistance between each. Is there a end connection that reads open while linked to mains input alone?

[stiebrs]

Ok, thanks for the info. I'll try to disassemble the transformer. Although amp manufacturing date is 1990, probably designed in at least a couple of years older transformer, not sure when they started to use thermal fuses on transformers. If this doesn't work out, I'll whack a toroid in, just have to find appropriate secondary voltages and power rating.

For that I'll try to power the amp up directly from 2x24V series wired bench supply to estimate quiescent supply and supply with a dummy load. Then my guesstimate would be to take 2x40W RMS output power which would be slightly below 2A on 24V rail, multiply that by 1.4, add 40% margin + quiescent. Totaling about 3.5A per channel 200VA one should be enough. It doesn't pass the sanity check of having 125V 2.5A fuse on primary though.

[stiebrs]

I did destructive disassembly of the old transformer and found thermal fuse buried under the primary windings (which were internal, so I had to unwind everything). Since the whole transformer assembly was welded and filled with epoxy, I decided that it would be easier/cheaper and quicker to just replace it.

Got 2x24V 200VA toroid, unloaded output voltage was about 27V. Wound extra 10 turn secondary with .2mm transformer wire and got 3.85V on it. Connected it up to the amp and it works just fine.
When mounting toroid I used mounting kit provided (2 rubber pads, steel washer, steel bolt and nut) and mounted directly to the chassis. Luckily only thing that blew was fuse - center tap of secondaries is also wired to chassis. So I guess I'll just have to use zipties for mounting toroid, because in my village no sane merchant is holding nylon bolts in stock.
Anyhow - thanks for the info on transformers, this case is closed. I'll probably write it up with pictures in my blog, but don't want to do no plugs here

[PKTKS]

windings trafos like this  is not an easy job..

I always give the task for people that do that for living..

Here I have a small list of choices...

Worth the money.. it is really not a trivial thing

Paul

[shakalnokturn]

When mounting toroid I used mounting kit provided (2 rubber pads, steel washer, steel bolt and nut) and mounted directly to the chassis. Luckily only thing that blew was fuse - center tap of secondaries is also wired to chassis.

Steel bolts shouldn't be a problem with or without secondary centre tap linked to chassis.
The rubber pads are there for padding, the large steel washers (if the toroid is not epoxy filled) to hold the transformer firmly.
Having either end of the bolt / nut in electrical contact with the chassis is no problem either, where it does become a problem is if you have the other end of the bolt or large washer touching the chassis (top cover?) elsewhere because that closes the loop and is more commonly known as a (quite effective) single turn short circuit.