Hello,
PREAMBLE
This is my first post so please forgive any protocol errors.
On Christmas Eve my well-loved (old!) central heating boiler suddenly failed (post electric storm). Examination showed the PCB had a couple of baked switching diodes. None the less, I decided it would be better to get a replacement. That way I could de-solder / test etc at leisure.
However, the replacement, though sporting an identical part number, features several new components and also differs in the number of spade connections. Moreover, the thing is OLD, so I decided attempts at reforming the caps might be in order. All of which led me to try and understand the circuit layout.
The more I looked the more it became clear that I did not understand certain aspects of its operation. Which brings me to my question.
QUESTION
I've attached a photo of the PCB with an overlay of the new circuit components / connections. The grey path from the "Aquastat" seems to be entirely redundant as no sooner does it enter the board than its progress is blocked by a diode (labelled D1 - coloured blue) - at least this is what I believe given that everything to the right of this diode and the rectifier -ve has zero resistance. FYI, D1 is labelled T4148, which AFAIK is simply a switching diode (with a reverse breakdown of 100v - well in excess of the 21v put out by the transformer powering this circuit) with presumably no chance of zener breakdown.
I've uploaded bmps so that members can easily colour parts of the circuit to trace paths easily.
CAVEAT
I should perhaps caution that the layout of paths coming into the "Aquastat" (which features two independent thermostatically controlled bi-state switches) shown in the attachment is how it used to connect to the original PCB. The new PCB has markings on its transformer alluding to connections that just might be different, though exactly what the transformer marking mean is not clear to me ( a lil photo is inset in the 52671 - new.zip attachment).
ONE LAST THING
Because it's an old boiler I've had precious little help from teh manufacturer, so any advice people here can give will be much appreciated. I include a pic of the old PCB with schematic overlay, and a pic of the circuit diagram of that unit (though slight differences exist even there - eg no reed-relay fly-back diode).
Thanks in advance.
PREAMBLE
This is my first post so please forgive any protocol errors.
On Christmas Eve my well-loved (old!) central heating boiler suddenly failed (post electric storm). Examination showed the PCB had a couple of baked switching diodes. None the less, I decided it would be better to get a replacement. That way I could de-solder / test etc at leisure.
However, the replacement, though sporting an identical part number, features several new components and also differs in the number of spade connections. Moreover, the thing is OLD, so I decided attempts at reforming the caps might be in order. All of which led me to try and understand the circuit layout.
The more I looked the more it became clear that I did not understand certain aspects of its operation. Which brings me to my question.
QUESTION
I've attached a photo of the PCB with an overlay of the new circuit components / connections. The grey path from the "Aquastat" seems to be entirely redundant as no sooner does it enter the board than its progress is blocked by a diode (labelled D1 - coloured blue) - at least this is what I believe given that everything to the right of this diode and the rectifier -ve has zero resistance. FYI, D1 is labelled T4148, which AFAIK is simply a switching diode (with a reverse breakdown of 100v - well in excess of the 21v put out by the transformer powering this circuit) with presumably no chance of zener breakdown.
I've uploaded bmps so that members can easily colour parts of the circuit to trace paths easily.
CAVEAT
I should perhaps caution that the layout of paths coming into the "Aquastat" (which features two independent thermostatically controlled bi-state switches) shown in the attachment is how it used to connect to the original PCB. The new PCB has markings on its transformer alluding to connections that just might be different, though exactly what the transformer marking mean is not clear to me ( a lil photo is inset in the 52671 - new.zip attachment).
ONE LAST THING
Because it's an old boiler I've had precious little help from teh manufacturer, so any advice people here can give will be much appreciated. I include a pic of the old PCB with schematic overlay, and a pic of the circuit diagram of that unit (though slight differences exist even there - eg no reed-relay fly-back diode).
Thanks in advance.
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