Power Transformer Replacment

Amplifiers are not 100% efficient, so there will be *more* power coming out of the power transformer into the amplifier than coming out of the amplifier into the speakers.

Assuming you know the output voltage, I would suggest matching the overall size and weight of the transformer. What is the output voltage (and how many output windings are there)?

your best bet is to figure out the design of the amp. \

is it a chip amp, or discrete amp? if discrete, try +/- 35V.

if its chip amp, look for datasheet of chip to figure specs.

What's the make & model of the amp?

Eh? If W = I x V then W I = --- V

so the current is 0.26 amps.

However, when you say continuity test, what do you actually mean? The primary will not be a dead short to DC, and some continuity testers are designed to give a fail if the resistance is more than a few ohms. So if you can safely power it up, look to see what the voltage is or isn't across the output(s).

It's a discrete amp, and in looking at the amp, I neglected to include that it is rated at 10 watts (RMS). There are two secondary leads (I haven't dissected the transformer to see if there are any unused leads, etc).

Louis

I'll make a quick addendum to this: there are only two diodes on the circuit board, so the transformer is probably (?) centre tapped.

overall

To calc what tf is needed would require at least: amp output power speaker impedance some idea of the output stage topology

Its possible to come up with likely figure without those, but they'd be fairly hit & miss.

NT

If the primary is really open circuit, there's a better than even chance that it's because a thermal fuse buried in the windings, has failed. Quite often, the wires from these fuse elements are brought out to tags on the transformer outside, so it would be worth looking to see if there is an extra tag on the primary, with a silver rather than copper coloured wire going to it. This will usually be one of the line power input tags. If you do manage to identify such, it is easy enough to just put a ( diagnostic ) short across it, then repower, preferably via a variac, and measure the output voltages. I say via a variac, because if the the thermal fuse has failed, it may be for a genuine reason. These items do fail quite regularly for no apparent reason, but they will also fail if the tranny is subjected to a serious overload for any length of time.

There are ways of making valid guesstimates of the secondary voltages, but last time we got a thread going on that subject a couple of months back, it descended into such a pissing contest, with so many opinions and cross-rubbishing of posts, that I really don't think that it would be a good idea to go there again ... :- /

I take it that you can't obtain a genuine replacement tranny from the manufacturers? I've never been very happy about substitute power transformers, but that's another bone of contention that many disagree over ...

Arfa

messagenews: snipped-for-privacy@davenoise.co.uk...

Arfa, Thanks for your input on this - the stock replacement isn't available from the manufacturers, so I'll probably go with something the same size as the old one.

messagenews: snipped-for-privacy@davenoise.co.uk...

If the manufacturere is known, they might have a schematic even if they don't have the transformer.!!!

H. R. Hofmann

I would get in touch with the manufacturer before you end up spending more on blown up components than the amp is worth. You could get a clue from the PSU electrolytics as most amp designers run the amp fairly close to the electrolytics capacity.

Is the secondary voltage from the transformer usually just below/ around the value of the caps?

You don't want to go here ...

This is exactly how the last argument on the subject started ... :-\

Arfa

Yikes! Point well taken. Well, I did, at long last, find a number. It says "Wuxi Radio Transformer Factory" with part number DBE-180 (E139334). Also, I found a schematic that lists the secondaries as 12VAC. As for the amperage, would 500mA-1A be sufficient?

No. A 12-volt 500-mA transformer only delivers 6 watts (or less if the voltage and current are out of phase; I don't want to start that argument again). Your amplifier must have more electricity going in than coming out; it's not a perpetual-motion machine.

Match the size and weight of the transformer and you'll get it close.

There is a good bit of loss in heat in the output transistors.

Yes, that's my point. The power transformer will be rated for considerably more watts (or volts * amps) than the output of the amplifier. Matching the size and weight of the original transformer is likely to result in a good match to the power rating of the original.

With 10w rms output youre looking at roughly a 12v 1.5A transformer.

NT

I'll give that a try - thanks everyone for your help in this. It's much appreciated.

Sincerely, Louis