PCB repair help need to Identify or Choose suitable component

[What_NZ]

Hi there,

I have been given a PCB to repair (photo's attached) with 2 failed components (no brand or value) and no circuit diagram/part list or any help available from the original manufacturer.

The two failed components I believe are resistors, as they appear to be Wirewound on a cement former. Hence I have labelled them as R1 and R2 in the simple circuit diagram I put together.
They feed a Toroidal Transformer rated at 230V AC / 10V AC 1000mA, 10VA (as printed on the transformer)

It appears this design is using R1 and R2 to limit the inrush current.
I was taught that a Toroidal at this low power only requires a Slo-blow fuse so are they gilding the lily here?
The PCB is burnt under R1 and R2 so I feel that reinforces they are Wirewound resistors.

If so what would be a suitable value for R1 and R2   1 Ohm?
I really have no experience with Toroidal transformers and inrush current protection so your experience/help would be greatly appreciated.

Kind regards,
Bruce

[Chris56000]

Hi!

Use two small 10 ohm disc–thermistors (NTC!) about 5 mm in diameter – these will fit the original wore–wound resistor holes, and give you more surge–protection than a wire–wound resistor on it's own would!

A 10VA transformer, allowing for 20% losses, would take about 80mA from a 230V AC supply, so when the thermistors heated up they're probably about 2 ohms!

Chris Williams

[SeanB]

Those were probably 22R 5W vitreous enamel coated resistors, and being mounted right next to each other they got hot, and then the glaze became conductive and then passed mains from one to the other, leading to destruction. Replacing with 10R NTC insush limiting disc thermistors will work well, and as well, replace all 3 capacitors on the power input side, those X2 and Y capacitors are crazing, so they have either failed short circuit ( which would have caused the resistors to overheat rather spectacularly) or will do so soon after you put power on the board.

As well replace that small electrolytic capacitor by the optoisolator, it will be suspect as well, after having had a hot life with high ripple current through it, and as it provides the power fail warning for the Z80 processor allowing it to save the data to the SRAM replacing it will save you lots of odd restarts.

Just wondering what this is, Z80 in a classic minimal configuration, battery backed SRAM ( watch that NiCd battery for leaks, it gets nasty), 6 digits of common anode BCD displays, 8 relay outputs. Looks like from the firmware it is a back gauge, something to do with a sheet metal brake or cutter, or at least that is the only result for the exact number.

[borghese]

Have you tested the diode bridge, the large electrolytic capacitor or if there is some short circuit after the voltage regulator? For me the condenser is oversized; 1000 or 2200 uF are sufficient; the larger the capacitor, the greater the peak current flowing in the diodes, increasing the probability of failure and increasing the power dissipated in the transformer. When I was young as a rule of thumb 1000uF each ampere.

[tkamiya]

I keep wondering why all this "stuff" is necessary on input side.  Looking over the entire board, there doesn't appear to be any special components....  I would just remove all that "stuff" except for a fuse, and add a TDK noise filter kind of thing.

What is this board anyway? 

[What_NZ]

Hi and Thank you for all the help.

@SeanB was right on the money as the PCB is from a DYE Metal Shear machine and I believe this PCB is responsible for setting the back stop. Yes, LED displays and Key matrix on the other side. Looking at this took me back to the days of Z80 designs and brought back fond memories of wire wrapping / breadboarding and minimalistic circuits, erasing EPROMS with the sun on the kitchen windowsill   then getting enough money together to build a UV Eraser.

I connected another transformer and tested the Power Supply on load with the scope and it appears to behaving correctly.

The Toroidal Transformer was open circuit and when I unwound to see if this was the source of the fault, I found the area of the short which was close to the other input so appears to have failed Shorted Turns. It was a bad short as it took out the series Resistors and the Mains Fuse in the process. The Transformer will be replaced along with the X2 and Y2 Capacitors and the small capacitor as @SeanB pointed out.

Thanks once again for the advice on the possible resistor values and also the possibility of replacing with NTC's

Kind regards,
Bruce