# Power Transformer Replacment

Discussion in 'Electronic Repair' started by ll, Jun 4, 2007.

1. ### llGuest

I am in the process of replacing a power transformer in a musical
instrument amplifier (solid state). As the existing transformer
doesn't currently pass the continuity test across the primary, and the
coil wire seems to be broken somewhere in the coil, I am thinking of
just replacing it. How can I find the power rating of the existing
one? I know that the amp is rated at 30 watts.

If watts=amps x volts, then 30watts = 115v x 3.83amps. Would I need a
transformer that was less than or equal to the 3.83amps with the same
voltage?

Thanks,
Louis

2. ### mcGuest

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)?

3. ### MikeGuest

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.

4. ### DaveMGuest

What's the make & model of the amp?

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

Life is like a roll of toilet paper; the closer to the end, the faster it goes.

5. ### Dave Plowman (News)Guest

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).

6. ### llGuest

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

Louis

7. ### llGuest

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

8. ### Guest

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

9. ### Arfa DailyGuest

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

10. ### llGuest

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.

11. ### hr(bob) Guest

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

H. R. Hofmann

12. ### MarraGuest

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.

13. ### llGuest

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

14. ### Arfa DailyGuest

You don't want to go here ...

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

Arfa

15. ### neon

1,325
0
Oct 21, 2006
power transformers for the 120v ac range usualy the pimary is in the range of ~15 ohms now as far as the power of the amp of 30w usualy that means nothing because is music power peak power RMS power power to the speakers what? you and i both don't know. so my advice is to replace the power transformer with as close as possible to the physical type or more. the voltage that you don't know look at the caps after the rectifier and see what they are rated for 50vdc or 150 vdc. finaly i have never encounter an open primary before burnt transformers yes but that you can smell it hours before and hours after so make sure

16. ### llGuest

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?

17. ### mcGuest

Well, I did, at long last, find a number. It says "Wuxi Radio
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.

18. ### MarraGuest

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

19. ### mcGuest

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.

20. ### Guest

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

NT