Arcam A65 flashing cd led problem

Hi all,

I picked up an Arcam A65 amplifier recently with a reported protection fault. On power up the mute relay clicked repeatedly and it wouldn't come out of the protection state.

I've replaced some components around the protection IC, resoldered the output transistors and bypassed the microcontroller controlled mute transistor and now when you switch the amplifier on it comes out of protection after a few seconds (which is correct). However, the microcontroller is still pulsing the mute signal and flashing the CD selection led with a 10mS signal repeated every 120mS.

The supply on the microcontroller is ok (5.1-5.4v) , the 4MHz resonator is ok , the /RES line is +5 and there is also data and a clock to the eeprom which contains the amplifier settings after power off. So all seems ok there but the flashing must signify something.

I haven't seen the amp working properly so I don't know if the led normally flashes when you switch the amp on. The mute and premute are both shown as output signals and there doesn't appear to be a discrete input signal to mute it.

I emailed Arcam over a week ago but they haven't replied.

Has anyone else experienced a similar problem ? Does anyone have any suggestions ?

Cheers,

Jon
 
hi Jon
i have always had very good support from arcam, i would phone them.

a couple of sugestions is the rotary encoder (input selector) working ie does it select inputs? if not then i would check it is ok, also i would check for dry joints / broken tracks around that area. sometimes false input to the micro can do odd things.
 
Hi Bob,

Thanks for the reply.

The rotary encoder has two outputs which produce a low pulse when you turn it and the phase difference between the two indicates the direction. I've checked that both of the encoder input pins on the microcontroller have a pulse on them when I turn the shaft (although I haven't looked at both together to check the phase although as this is done mechanically I can't see how it could be wrong).

The microcontroller doesn't respond to the input selector it just flashes the cd led every 120ms.

I've examined the pcb and resoldered the pins on the microcontroller, the ribbon connectors and the rotary encoder but it's made no difference.

I've also checked the protection sense pin and it changes level (5/1.4) when the amp comes out of protection after power on.

I wonder if corruption of the data in the eeprom could cause this effect. All address lines are tied low so it shouldn't be too hard to decode the data stream. I assume it's there to store the last selected input or something like that.

Perhaps the microcontroller is faulty but it's odd that it's producing this 120ms signal on some but not all of the port pins - as I say it's like it's trying to indicate something.

Do you have any additional thoughts ?

Cheers,

Jon
 
hi Jon
i had a similar problem (not on this model however) when one of the input pins was floating due to break in track, the pin floated (or was pulled internaly) high, this caused a very similar fault to yours.
yes the eeprom stores the last settings data.
i have the manual if this helps.
i think a call to Arcam would help you
 
I thought it would be worth closing out this thread for the benefit of anyone else who experiences a similar problem. The fault in the end was the microcontroller (Z900).

Having disconnected the 9V AC winding from the transformer (at the socket on the left hand side of the pcb) and replaced it with a bench supply I found that the microcontroller circuit appeared to work. Back to the AC supply and the led flashed/output relay chattered again. The difference between the two - the level of ripple. The AC is half wave rectified by D906 and filtered through C914, this is followed by paralleled R902/R903 (to share the current and avoid overheating). These resistors supply a zener configured as a simple shunt and finally an RC formed by R904/C915. Although the schematic shows the zener as 5.1V, a 5.7V had actually been fitted to the amp. There is only around 20mA drawn from the +5V supply so if you discount the charging current for C915, R904 (10R) only drops 0.2 volts. The microcontroller is specified for a 2.2-5.5V supply (although nominally 3.3 or 5V). With less current drawn the voltage will be even higher than 5.5V. I am not sure why Arcam decided to change this zener and run the microcontroller at the absolute upper limit of its working range (in fact overvoltage when you factor the supply ripple and varying current demands of the circuit). This just seems like a poor design choice imho. I can only assume this has something to do with the PROT signal from Z101 (TA7317P). The microcontroller datasheet specifies a 'low' input below 0.3 times the supply. The TA7317P datasheet states that the output ON level is typically 1V but can be up to 2V. This output is connected to the microcontroller through a 1N4148 diode (D132) so as the output from the protection IC switches ON the input to the microcontroller will track down 0.7v lower - it is 1.4v on my amp. 5x0.3 = 1.5v which doesn't really give any head room for variation in component tolerances. Perhaps there was another reason but a zener shunt with no fixed load to ensure a maximum voltage = bad idea.

Anyway I'd found that when the microcontroller was fed from a 5.5V supply it worked but if this supply was taken below 5.45v it shutdown. What was happening with the AC supply (with the supply ripple) is that the micro was constantly starting up and shutting down, starting up and shutting down which lead to the flashing led and chattering relay. I can only assume that it must have been damaged in some way by the long term over voltage it had been subjected to.

Having replaced the microcontroller with a new one I have also removed R904 and D908 and fitted a 5V regulator (TO92 style 100mA) wired across R904 with the ground wire going down through the adjacent hole and onto the ground plane under the pcb. R903/902 can remain as there is still enough headroom for the regulator to operate even at the minimum AC input and dropping a bit across these resistors means less across the regulator. I've replaced the caps with the same value Panasonic FC. I did consider fitting another 220uF in-place of D908 to form a CRC filter but decided it was unnecessary. The output from the protection IC is now connected to the base of a surface mount PNP mounted under the pcb at the microcontroller which pulls the PROT input down to 0V so no more worries about the 'low' 0.3x threshold. Whilst I was there I also improved the decoupling on the motor controller Z905, decoupling each half of the motor drive circuit to 0V as recommended by the manufacturer's datasheet rather than to each other as Arcam had decided to do.

Even with the AC source the supply is now a much cleaner 5V and everything works again !

Finally I can investigate the effect of varying the bias level on the distortion of the Sanken transistors which was my reason for purchasing the amp in the first place ...
 
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I thought people trying to following what I've done might find this photo useful. D908 and R904 were removed and the 78L05 regulator (TO-92 package) fitted instead, the wire on the centre pin goes through the pcb to the ground plane.
 

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geni

Member
2011-11-17 9:41 pm
Vienna
Hi Jon,
I have a similar problem with the A65+, in my case the input selector works for a couple of turns, then it freezes. All signals to the uC are just fine, the switch pulses from the selector have the correct phase relationship, but the output MUX signals don't react. Also the zener is 5.7V instead of 5.1V, so supply voltage is close to or above maximum ratings, leading me to the same conclusions as you - the micro must have been damaged by the overvoltage.

I also wanted to replace the micro, but could not figure how to get the appropriate SW and program the device.
I have emailed Arcam for a replacement source for the micro, but received no answer yet. How did you achieve the programming ?

Greetings, Georg