MTX Thunder 2300 board photographs needed

[JonFennell]

They both read zero ohms from pin 2 to the pad.

 

[Perry Babin]

reread

 

[JonFennell]

Rect 1 reads 1.8 ohms from pins 1 and 3 to B+
Rect 2 reads 5 ohms from pins 1 and 3.

 

[Perry Babin]

how about from the center pad to b+?

 

[JonFennell]

I can't get any kind of sensible reading from the center pin back to B+, Perry. This meter is not the best.

 

[Perry Babin]

Ok. lets try this.

For the output transistors on either side, half (low side) will have their center leg connected to the 3rd leg of the other half (high side).

For the high side, the center leg will be connected to the center leg of the positive rectifier.

The 3rd leg of the low side will be connected to the center leg of the negative rectifier.

 

[JonFennell]

OK, broke out a 1970's KEW -66 analogue meter. My Dad's.
Rect 1 is around 300 Ohms
rect 2 is around 1000 Ohms
From center pin pad to B+

 

[JonFennell]

aha!
Rect 1 is positive by that reasoning. Does that match your expected resistances to B+

 

[Perry Babin]

I thought that I would be able to tell from the B+ to the rectifiers but the readings were not what I expected. The readings to the outputs is definitive.

 

[JonFennell]

I'm still not confident enough about rectifier circuits to know which way round these two diodes fit Perry! But I think, from your amp guide and some common sense that the
FEP17CT diode (as it will allow positive current flow through)
Am I right?

 

[JonFennell]

sorry, that's the positive diode I am suggesting

 

[Perry Babin]

What's the number on the other rectifier?

Do they have diode symbols? Diodes facing inward is positive.

 

[JonFennell]

FEP16CT has arrows pointing inwards (I'm guessing it means the P means positive)
FEN16CT outwards facing.
So I'm fitting the positive as you described above. Here goes!

 

[Perry Babin]

The p is positive when there is an N version but that's the old markings. The new ones are all p and the polarity is indicated in the suffix.

 

[JonFennell]

It didn't go well. Smoke happened from a resistor by the main inductor.
Not sure what to do next. Any ideas welcomed.
Many thanks for all your help so far.

 

[Perry Babin]

Did it open?

Is the white cap shorted?

 

[JonFennell]

It connects between CLR on the main inductor and pin three of the negative rectifier diode.

 

[Perry Babin]

The RC network is connected across the transformer (not inductor) windings.

Did you check the capacitor and resistor?

They aren't absolutely critical. The amp can function without them but they need to be replaced before the amp is put back into service.

 

[JonFennell]

the resistor measures around 54 ohms, there was smoke and red hot light from it, the white cap looks ok and is open circuit.
It's sister cap did blow originally and I replaced it as we discussed.

 

[Perry Babin]

The resistor is out of tolerance so let it burn.