Arcam A22 / Alpha 10

[biker29]

My A22 goes into fault protection 15 to 30 seconds after 'on'. Per manual this suggest voltage issues as output transistor over amp should immediately go to "red' state. Arcam provided an internal switch to disconnect preamp and allow external system into power section. With preamp out fault protection clears and power amp section works fine with an external signal. The preamp out shows 0.05 to 0.30 vdc with no input. I believe this is the cause of the fault protection. The LM1972 volume control is showing about 0.15 VDC on both inputs and slightly less at outputs. As the upstream OPamp has 10uf cap on both inputs DC should not be from upstream? Just checking for any experience with this before operating on the LM1972 chip. Thanks

 

[PRR]

> preamp out shows 0.05 to 0.30 vdc with no input. I believe this is the cause of the fault

The later A28 (not a LM1972) seems to put a deliberate DC bias on the volume device recovery op-amp.

_I_ would suspect leaky electrolytic caps before any chip fault.

 

[biker29]

Re: Caps

thank you for the response. I will look at the caps on the preamp board. There are only four.

 

[biker29]

On further investigation, found the one of the six Max4051 multiplexers shorted +/- power supplies. Working now after replacing this IC.

 

[dieselophile]

I wonder if my problem is the same one:

My FMJ A22 was fine until I used it as a power amp (via the fiddly little internal switch), there was a hum I was trouble shooting and I used a three prong to two prong adapter, to isolate any ground loop, the amp didn't like it, I guess I didn't pay attention to the amp instructions, its needs to be earthed at all times. Since then the amp has been temperamental - going into prolonged standby mode, and more recently not switching off and remaining in red light on mode even when the power on button is off. I can't get it to function any more - just goes into red light on mode whether power switch is on or off.
Any help/advice appreciated.