Power transformer for stereo - shorted - why?
 

Hello,

I'm troubleshooting a dead "Brother BR-600XL HiFi Music Center"
(cd-tape-phono-tuner integrated unit). If anyone has info on this
please post - the Internet yields nothing.

The unit is powered by a big (about 2"x3"x4") transformer with two
apparent secondary windings - red-black-red wires both, with red at
ends and black is probably the tap on each.

One of the windings has a capacitor between the 'red' contacts, with
one of the leads also soldered to the black contact, which seems to
have an effect of shorting a part of the winding.

In addition there are two resistors near the transformer, but when
measured they appear shorted, however there is no visible damage to
them.

What could this mean?

Yours, MaxVT

One day MaxVT got dressed and committed to text

Hello,

I'm troubleshooting a dead "Brother BR-600XL HiFi Music Center"
(cd-tape-phono-tuner integrated unit). If anyone has info on this
please post - the Internet yields nothing.

The unit is powered by a big (about 2"x3"x4") transformer with two
apparent secondary windings - red-black-red wires both, with red at
ends and black is probably the tap on each.

One of the windings has a capacitor between the 'red' contacts, with
one of the leads also soldered to the black contact, which seems to
have an effect of shorting a part of the winding.

In addition there are two resistors near the transformer, but when
measured they appear shorted, however there is no visible damage to
them.

What could this mean?

Yours, MaxVT

I'm thinkin you might be in "over your head" on this one ??

--
Regards ..... Rheilly Phoull

MaxVT wrote:
Hello,

I'm troubleshooting a dead "Brother BR-600XL HiFi Music Center"
(cd-tape-phono-tuner integrated unit). If anyone has info on this
please post - the Internet yields nothing.

The unit is powered by a big (about 2"x3"x4") transformer with two
apparent secondary windings - red-black-red wires both, with red at
ends and black is probably the tap on each.

One of the windings has a capacitor between the 'red' contacts, with
one of the leads also soldered to the black contact, which seems to
have an effect of shorting a part of the winding.

In addition there are two resistors near the transformer, but when
measured they appear shorted, however there is no visible damage to
them.

What could this mean?

Yours, MaxVT

You can disconnect the capacitor to see if it is shorted. If it is, it might
have damaged / ruined the power transformer. The primary can be checked with
an ohmmeter, and should read a near-short. If it's an open-circuit, the tran
is bad. If these tips don't help, you may indeed be in over your head. The
AC input / rectifier stage of a linear power supply is a simple circuit, but
with no training or test equipment, your chances of fixing anything more
than a bad fuse would be slim.

Mark Z.

One of the windings has a capacitor between
the 'red' contacts, with one of the leads also
soldered to the black contact which seems to
have an effect of shorting a part of the winding.

You may be misreading which wires are parts of which windings. It's
not too unusual for two independent windings to be connected to each
other in funky ways to choose 120VAC vs 240VAC inputs, for example.

Big leap time: plug it in, see if you get AC out of the transformer and
DC volts across the filter caps.

Tim.

MaxVT:
snipped:
"MaxVT" wrote in message

One of the windings has a capacitor between the 'red' contacts, with
one of the leads also soldered to the black contact, which seems to
have an effect of shorting a part of the winding.

Multiple windings on a transformer are many times jumpered (or shorted) to
produce desired voltages.
Is the "capacitor" really a MOV? .... or if it really is a capacitor or
not, you can unsolder one of the leads from the circuit and measure it to
see if it is indeed shorted...... brilliant idea..... huh?

In addition there are two resistors near the transformer, but when
measured they appear shorted, however there is no visible damage to
them.

If the resistors are very low ohm (did you "READ" the value on the
resistor?) they can appear on some meters to be just about a short.... or if
they are inductors or PICO fuses they can read almost a short. Again,
unsolder one of the leads so you won't be confused with the low resistance
of the transformer winding that it is connected to.

What could this mean?

I am th;inking here that you may want to call in one of your tech friends
with a little more electronics knowledge and repair experience to assist you
with this troubleshooting.

--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Thanks for all input, everyone! Now, the saga continues...

I've checked the transformer: one winding produces +19.2v, -19.2v and
(I assume) ground, the other produces 10v and ground.

Continuing to main board, there is a diode bridge and from there 3
regulators: 5v standby, and +15v / -15v controlled with a relay.

A relatively big resistor immediately before the input to 5v regulator
is cracked and the solder has evaporated from its connection. Also,
there is a leaked cap in the -15v circuit.

I can replace the resistor and the cap, however how can I check that
the regulators (7805C, 7815C, 7915C) are OK and the load is not
excessive, so as not to create any additional leaks and explosions?

BTW, I do have electronics knowledge (B.Sc in EE), however zero repair
experience... One has to start somewhere :)

Thanks,

MaxVT

I can replace the resistor and the cap, however how can I check that
the regulators (7805C, 7815C, 7915C) are OK and the load is not
excessive, so as not to create any additional leaks and explosions?


Check with a multimeter that they're not shorted, otherwise they have
internal overload protection so you can apply power and measure the output
voltages.