NAD705

[damith74]

NAD705 CPU (IC701) malfunctioning. cant store FM presets.

[jackand08]

If the actual receiver is working and will store presets whilst powered on but loose them once powered off, I'd imagine that instead of the CPU malfunctioning, it could be a faulty memory backup capacitor. These are usually low voltage, high capacitance capacitors which are charged while the unit is turned on and provide a small current to keep the presets maintained when turned off.

[Mooly]

I'd second that

Faults like this are never (99.9% never) "the micro".

[damith74]

It has no memory Cap. Memory is on eeprom. I think eeprom is ok.

[damith74]

NAD705 has no Memory Cap. Memory is on eeprom. I think eeprom is ok. all memory and settings goes off when the unit turned off. and the another thing is if i change the tuning modes to preset mode the frequency of the tuner is going to 197 mhz and speakers turned off.

[Mooly]

You need to consult the service manual and use a scope and DVM to confirm every pin on the micro and eprom has what is expected. That includes supplies/grounds/clock/reset/I2C bus if it uses that etc etc and make sure levels etc are OK.

[stratus46]

Originally Posted by damith74

It has no memory Cap. Memory is on eeprom. I think eeprom is ok.

EEPROMS have a limited number of write cycles. Even the old ones were like 10,000 (new ones are many more) and while it's unlikely you've used that, the chips do fail.

It's also possible that the programming for the micro is going stupid. Mask programmed ROMs don't fail but EPROMS have been known to start slipping bits.

G²

[jaycee]

Is it a seperate eeprom IC, or did they use the micro's built-in eeprom ? If the latter is the case, then you will have to replace the micro.

[stratus46]

Originally Posted by jaycee

Is it a seperate eeprom IC, or did they use the micro's built-in eeprom ? If the latter is the case, then you will have to replace the micro.

If it's a One Time Programmable that is likely true though if you are programming EXACTLY the same thing into it, it may just 'restore' the faulty bits. If the chip is an EEPROM or EPROM (UV window on top) type then just erasing and reprogramming it would work. There are security bits in many micros and if it is set, you're screwed n that you won't have access to the code that way.

FWIW Tektronix used 8751 microprocessors in the 17xx series scope and did NOT set the security bit so it is possible to read out a known good chip and program into a new chip. This isn't just theory, I've done it several times.

G²

[jaycee]

Originally Posted by stratus46

If it's a One Time Programmable that is likely true though if you are programming EXACTLY the same thing into it, it may just 'restore' the faulty bits.

This is not what I meant. Microcontroller program ROM is usually either OTP or Flash these days, however micro's often now contain a portion of serial EEPROM that is supposed to be used for storing settings and suchlike, separate to the main Flash used for the programming.

This is fine for use where the EEPROM is not often written to, but for a domestic device which is saving settings frequently, it can be bad as it wears out, meaning a microcontroller replacement is required. It's much better design to use a 24C08 or suchlike that is external, and can be replaced. Of course, that costs more!