I am trying to fix an amplifier that has a micro-controller in it that is not functioning properly, it is a M50747-XXXSP,all of the voltages to it according to the service manual for the amplifier are correct except for pin#2(standby)supposed to be 4.0volts and pin#49 (turns power relay on) supposed to be 4.7volts,It seems to me that these 2 voltages are supposed to come FROM the micro-controller? But then again I'm a complete ignoramus when it comes to micro-controllers I have read the M50747XXXSP Datasheet but it has just made me more confused. If I can get some suggestions and insights into this problem it will be greatly appreciated.

 

The datasheet on this micro-controller says it has 4 modes,single chip mode,evaluation chip mode,micro-processor mode,memory expanding mode.Seeing as it is in an amplifier what mode should it be in so I can set it to that?

 

Can you provide a link to the datasheet. I've never heard of this part.

 

M50747 should be at least 20 years old by now. It's a mcu with 65c02 core. Was your amplifier made by Yamaha? I have never used this chip before. The ones I used were M50734 without ROM and RAM built-in in HS-8 Yamaha Electone.

Pin #2 and pin #49 are Port6/B7 and Port0/B7. They can be of any function depending on the program inside. Without looking at the schematics and program flowchart, it is hard to determine whether the chip is bad or not.

May be you can check with a scope/logic probe to see if pin 31(phase2 clock) is giving off pulses. You may try a reset on pin 28 (refer to the service manual carefully) to see if it can revive the mcu. Or have the crystal changed.

Allen

 

Can you provide a link to the datasheet. I've never heard of this part.

http://www.ic-on-line.cn/search.php?part=m50747-xxxsp&stype=part Here is the link.

 

M50747 should be at least 20 years old by now. It's a mcu with 65c02 core. Was your amplifier made by Yamaha? I have never used this chip before. The ones I used were M50734 without ROM and RAM built-in in HS-8 Yamaha Electone.

Pin #2 and pin #49 are Port6/B7 and Port0/B7. They can be of any function depending on the program inside. Without looking at the schematics and program flowchart, it is hard to determine whether the chip is bad or not.

May be you can check with a scope/logic probe to see if pin 31(phase2 clock) is giving off pulses. You may try a reset on pin 28 (refer to the service manual carefully) to see if it can revive the mcu. Or have the crystal changed.

Allen

Ill try again here, Pin31 is measuring high on the logic probe and 2.73 on the DMM.No pulses ? So it could be the the crystal?

 

Ill try again here, Pin31 is measuring high on the logic probe and 2.73 on the DMM.No pulses ? So it could be the the crystal?

Most probably. Was it using 8MHz crystal? The 2 capacitors can also get faulty sometimes. But why 2.73V, is your logic probe confirmed good?

If you refer to P2 (block diagram) of the datasheet, the xtal, cap and phase2 are all on the same "clock generating circuit" block.

Allen

 

Ill try again here, Pin31 is measuring high on the logic probe and 2.73 on the DMM.No pulses ? So it could be the the crystal?

A DMM and a logic probe? You need a scope to see what is going on.

 

A DMM and a logic probe? You need a scope to see what is going on.

I have a scope but it didn't show anything on pin31(there must be something wrong with it) My DMM reads 2mhz on pin31.

 

Most probably. Was it using 8MHz crystal? The 2 capacitors can also get faulty sometimes. But why 2.73V, is your logic probe confirmed good?

If you refer to P2 (block diagram) of the datasheet, the xtal, cap and phase2 are all on the same "clock generating circuit" block.

Allen

The 3 pin resonator is 8mhz,there are no capacitors for it on the schematic at all its just( in, gnd, out )with a 1m ohm resistor between in and out,schematic shows 2.3v on pin31,so its fairly close at 2.73. Logic probe is OK as far as I know?

 

I have read the datasheet for the micro controller carefully but most of it is beyond my ken,this is the first time I have had to deal with one.what I don't understand is how to reset it to make it turn on? The datasheet says to short pin 1 to pin 28? Thats 5v to 5v and hold it there until the crystal frequency settles down? Well of course I tried that but still no result? Ive attached a copy of the micro-controller schematic showing voltages,so that maybe someone can make sense of it. It has four modes to operate in and I have no idea which one?

 

Yes it is a yamaha I think the crystal may be playing up because Im getting 7.97mhz out on pin30 but 150khz to 280khz on pin 29.

If you're getting around 8Mhz on pin 30, the xtal is probably OK. After reading more on the mcu datasheet, I found out that the clock output pin should be outputting 1/4 of the Xtal Freq ie 2 MHz if some conditions are fulfilled. Look at the diagram attached.

The output of internal clock is governed by the conditions of Reset, WIT instruction and STP instruction. You're not getting clock pulses from pin 31 may be due to that.

I have read the datasheet for the micro controller carefully but most of it is beyond my ken,this is the first time I have had to deal with one.what I don't understand is how to reset it to make it turn on? The datasheet says to short pin 1 to pin 28? Thats 5v to 5v and hold it there until the crystal frequency settles down? Well of course I tried that but still no result? Ive attached a copy of the micro-controller schematic showing voltages,so that maybe someone can make sense of it. It has four modes to operate in and I have no idea which one?

To reset the mcu is simple. Either you send gnd with a 100 ohm resistor to pin 28 or make Q302 C/E conduct momentarily. It was all in the M50747 datasheet. Your problem may not be as simple as it seems. Yesterday I repaired a 20 years old Yamaha Electone. Some of the tones menus were distorted and I though the problem was due to a bad Digital to Analog converter YM3009. But the fault turned out to be a bad 74HC367 buffer chip.

So it's better to check the parts around the MCU and only suspect the MCU as a last resort. I am not saying that the MCU cannot get faulty. But have you try hard enough to trouble-shoot the surroundings of the MCU. Even though you dont know about mcu, do read the datasheet and there's a lot of info that you can understand especially on RESET and clock oscillators that might be usful for trouble-shooting.

Allen

 

Hi Allen,Basically the question Im asking is! Does anyone have any ideas or suggestions for how I can get the MCU to turn on the power relay and the display? Im trying to fix an amplifier and I don't want to spend an inordinate amount of time trying to figure out the ins and outs of an MCU,I mainly rebuild vintage amplifiers that dont have these MCUs.If you want the service manual for the amplifier so you have an idea of what Im talking about it is at Audiokarma in the digital docs section on page ten of the Yamaha section. It is a Yamaha RX550 not RXV550.

 

I'll take a look at the schematics when I'm free and let you know if I find out anything, OK?

Allen

 

I'll take a look at the schematics when I'm free and let you know if I find out anything, OK?

Allen

Hi,Allen,just a few things Ive found,I am getting 2mhz on pin31,the battery backup type capacitor c376 4.700uf,5.5v is down from 4700uf to 3100uf I measured the voltages around it and they came up ok but if it was leaky could that put noise on the reset line and stop the MCU from turning on? Ive read something along those lines where a leaky polyester cap in the power supply can do that. If you have a gmail email account I can send the service manual to you that way,gmail accepts fairly large attachments.

 

You can send the service manual to me at allen#bsf#6502@gmail.com##

remove # before sending.

Allen

 

Hi,Allen,just a few things Ive found,I am getting 2mhz on pin31,the battery backup type capacitor c376 4.700uf,5.5v is down from 4.700uf to 3.100uf

Good, so your crystal is working fine then. The battery backup capacitor is 4700uF or 4.7mF (milli-Farad) not 4.7uF. If you're only getting 3.1uF, the cap is probably open-circuit.

Allen