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On Jun 4, 8:21 pm, "James Sweet" wrote:
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I need your help repairing the voltage conversion circuit of the Bosch
WFL2061BY washing machine. I have the following questions:

1) No operating lights etc. are on. Visual inspection shows a damaged
resistor (first two bands brown and black, next band difficult to tell
- possibly brown, gold or even yellow). As far as I can tell no fuse
is fitted - is this normal? A fuse seems to be essential to protect
against possible overcurrent conditions.


there's likely a thermal fuse in the transformer windings. If the motor
pulls anywhere close to 15A they figure your circuit breaker will do the
trick. BTW fuses don't protect semiconductors, they're too slow.

2) The damaged resistor is directly connected to the mains in series
with a transformer (which I suspect is used to derive the low voltage
for driving the relays). I have experimented with
a 100R resistor (1/4 Watt) connected to 220V, but it is immediately
burned out.
Four (1/4 Watt) resistors of 10k in parallel also got hot in a few
seconds. My initial guess would be that the damaged resistor provides
current limiting?


Something's shorted and drawing too much current. A metal film resistor
tends to be a pretty good fuse in many cases.

3) If the damaged resistor provides current limiting, why is it placed
on the high voltage (220V) side?

Less current, smaller resistor, cheaper. Also, it ensures that the
transformer is not hot in the event of a fault.

4) A number of diodes (likely 1N4001) also seems to be connected to
the high voltage side, before the transformer. If they form a half-
wave or full-wave rectifier, why do they connect the diodes before the
transformer (on the high voltage side)?

The AC cannot be rectified prior to your transformer. Can't can't can't.
I'm confused as to what you've got here... does the main power all run
through the transformer, or is the transformer just to step down the voltage
for powering the electronics with the AC going to power the motor relays? I
don't think you've got any motor relays if you've got Triacs (see below).

5) It is possible that some other components also got damaged. From
visual inspection I cannot confirm damage to other components. In
circuit measurements of the diodes show about 4.5k in one direction
and infinity in the other. An 8 pin IC TP209P is found close to the
(small) transformer. The voltage regulator is a BT916 (or BTB16 -
difficult to tell from visual inspection). Any suggestions on how to
proceed?

The BTB16 is a 16A Triac, not a regulator. It's likely used to switch on
the motor or pump and possibly control the speed depending on the complexity
of the control circuit.

First, can you confirm that if you physically disconnect all wiring from the
pump, that the problem persists? That would effectively rule out the pump.
I might try powering the pump directly by attaching a line cord and confirm
that it works. The triac which controls (I am assuming here, the pump MAY
be controlled by a relay as it only needs to operate at one speed) has
virtually no resistance when biased by the gate voltage, the only thing
limiting the current draw is the load which is in this case is the pump. So
let's rule it out.

If in fact the pump is fine, and if removing it from the circuit does not
eliminate the problem of the resistor smoking, I'd try removing the triac
which controls the pump. When triacs fail they either stop working or fail
shorted. Neither of these matches your scenario. Check the pump wiring and
make sure there are no shorts to ground or shorts from hot to neutral lead.

If it's a relay and not a triac which controls the pump I'd take a good look
at it... are the contacts burnt? What else is around it in the circuit?

Can you confirm the writing on the 8-pin DIP? If there are various motor
speeds it could be a pulse width modulator driving the triac, but I come up
empty with "TP209P"... TP209 yields a DC-to-DC upconverter, 5VDC in and
15VDC out, 50mA rating...

Dave

Best regards,
Theo van der Merwe

There's certainly a shorted component which burned up the resistor.
Resistors don't just burn up on their own.

Thanks for the email message and feedback. The problem occurred when
testing the pumping out of water (which wasn't working at all -
everything else seems OK). However, I have disconnected all the
external connections to the motor and pump from the circuit board. Any
suggestions on how to proceed with fault finding (assuming some
component is shorted on the circuit board)? At present I don't
understand the power supply design on the circuit board.

Best regards,
Theo van der Merwe