Identify Diode in Casio Midi Guitar

[RanjerSmith]

Hi

I have a Casio Midi Guitar MG-500. The Midi part does not work and I would like to fix it. The first port of call would seem to be the Diode D1 which has disintegrated on the lower board. Thre is a barely discernable figure '3' (or possibly M or W) which appears to be replicated on the diode D2, is it possible to identify and replace this component effectively?

Here are a couple of photos

Casio PCB - Windows Live

Thanks in advance to anyone that can advise me.

Cheers Mark

[Jazz P Bass]

You need a service manual that has the schematic.
Link: Casio MG-500 MG-510 MIDI Guitar Synth Service Manual. | eBay
I would like to know what took that diode (& anything else) out!

[jrfrond]

D2 also shows signs of being cooked, and above them is a 78L05 voltage regulator, so it is quite possible that those diodes are in the +5V supply. The "F3" designation on the diode signifies that it is a 1SS193 ultrafast switching diode.

Without a schematic, it is impossible to tell what is going on there, but a short is indicated by the catastrophic failure. Couldn't tell you where, but if it were in my hands, I'd replace the regulator and two diodes (since I have the parts in stock anyway), plus check the surrounding e-lytic caps, then power it up. Not exactly scientific troubleshooting, but the intuitive approach sometimes works when you have no service docs. You can even tack-in standard package UF diodes across the solder pads if you don't have the correct SMD components. Here's the pinout: http://us.100y.com.tw/product_jpg_original/A025622.jpg

[Ronsonic]

I've worked on a number of these and not seen that sort of fault. My initial guess is a wrong polarity / voltage / AC power supply.

Usually it's faulty caps throughout, like dozens of 10uF/16V smd caps and failed opamps in the input section. Occasional digital and power supply problems, but never saw anything fried like that.

[MarkusBass]

It seems like a typical problem caused by connecting reversed polarity power supply. Find a schematic or draw it yourself (because it's just a simple power supply). Check whether this diode is connected parallel to DC input (of course it has to be connected reversed).

Mark

[RanjerSmith]

Thanks to all who have replied;

Jazz P, I found the schemtic and parts catalogoue through this forum midiguitar : MIDI Guitar user's list

I am not sure where and when this happened, but concur that there was a mismatch of power supply to guitar of some sort at some time.

At this point I will try and get hold of the ISS193 Didoes and a Voltage Reg 78L05. I have looked on CPC Farnell, Mouser and RS; 78L05 comes up ok, ISS193 draws a blank! Is there another designation or alternative?

The research so far has found that there are some mods that can be done to the board or at least replacing the old caps that improve the midi operation!

Cheers Mark

[MarkusBass]

I would be nice if you post the schematic (at least the power supply part). The diode is available from many suppliers e.g. mouser: 1SS193T5LFT Toshiba Rectifiers
Don't start with modifications until it is fixed.

Mark

[RanjerSmith]

Hi there

sorry it has taken a while, not a great image but here is the power supply circuit: MG500 Power Circuit.jpg

Hopefully clean up and order the parts today!

Mark

[jrfrond]

Looks like a simple shorted regulator issue to me, most likely caused by reverse polarity power.