CAMBRIDGE A3i cutting in & out help!

[Eggzy]

I got a CAMBRIDGE A3i from the car booty last week for £10. [I also got 2x 6metres of LINN K20 cable with LINN banana plugs for £3!!!]
…I didn’t really expect the amp to work but it was worth the gamble. As soon as I switched it on with nothing connected, it started clicking. I hooked up some mission 731’s & a cd player and it just kept cutting on & off from the outset, I got about 3 second bursts of music which sounded fine.
I put this down to the relay behind the big pots, I feel the pots may be the culprits, has anybody else had this problem? Any input would be gratefully accepted.

Eric

[Eggzy]

i think i'll bypass the relay

any thoughts?
also i've got a scematic of the a3i if anyone is interested.

.................................................. ...........................................hello?

[clem_o]

It's not a relay problem. The protection circuits are finding a fault. Don't bypass until you've found why the protection circuit's cutting!

Does the relay click in and out with the speakers disconnected? If so, remove the speakers and start measuring voltages with a VOM. First suspect is DC offset. Trace to the amplifier outputs BEFORE the relay and see if there's any DC voltage above 0.5V on any channel.

Cheers!

edit: check R201 and R202 if any are open or off-spec. It would cause the relay to chatter on and off, as the protection circuit would 'think' that the amplifiers are taking in too much power...

[anatech]

Hi Eggzy,
Also check C207 (?), 47uF. It may be failing.

Clem is right though. Never bypass a protection circuit. The relay is being turned on and off in this case.

Do you have the audio section schematic?

-Chris

Edit: Scratch the last part, I got it. Nothing pertaining to your problem.

[Eggzy]

thanks for that guys,
yes it occurs when the speakers are disconnected too.
i read somewhere that replacing all the caps is a good idea anyway.

thats the only schematic i could find online.

now i'll try to do some swatting to understand dc offset

[x-pro]

Originally posted by Eggzy
thanks for that guys,
yes it occurs when the speakers are disconnected too.
i read somewhere that replacing all the caps is a good idea anyway.

thats the only schematic i could find online.

now i'll try to do some swatting to understand dc offset

Can you check if one or more green LED's on the main amp PCB are blinking or going out when the relay clicks?

Cheers

x-pro

[Eggzy]

X-PRO
the 4 green leds on the main pcb & the one on the phono pcb are lit up constantly. i noticed the relay clicking when i pushed the mute switch,

Are they just used as resistors? do you have an A3i yourself?

thanx x-pro
eggzy

[Eggzy]

any chance of a quckie tutorial on how to check dc offset etc. on the A3i? i only have basic elec. knowledge or even a full schematic so my friend can have a go

thanks in anticipation

[x-pro]

Originally posted by Eggzy
do you have an A3i yourself?

I've designed it.

Originally posted by Eggzy
any chance of a quckie tutorial on how to check dc offset etc. on the A3i? i only have basic elec. knowledge or even a full schematic so my friend can have a go

thanks in anticipation

I've sent you an email through the forum - and will send you the schematics tomorrow if you reply on this mail.

Cheers

x-pro

[anatech]

Hi eggzy,
Measure from ground to each emitter resistor. If you get the wrong point you will only be a few mV off. The correct point is the junction of the two R22 resistors.

-Chris