Cambridge A500 amplifier

[Cave Miner]

Hi,
I have a Cambridge A500 amplifier with problems on the preamp stages. Both power amps work fine with AF injected into input sockets but when the inputs from the preamps are plugged into the power board the output on the right channel is hardly there at all and the left hand channel lacks any real gain. Also the input selector control fails to select phono but deck works on CD inputs. When the circuit board containing the controls is pushed the right and channel sometimes appears. Could this be an internal connector problem or something more serious? I have a schematic but not a component layout. Any help appreciated.

[AlanBeckett]

Sounds like Switches and Dry Joints in the Front End.
Can you Post the schematic?
What Test Gear have you got?

Alan

[Cave Miner]

Alan ,
Thank you for your speedy reply. I have a RF/RFmod and AF sig gen, twin channel osciliscope, digital multimeter and capacitor test meter. I am more at home with vintage valve equipment but can find my way around most circuits. Will post schematic when on my PC tomorrow. The fault now seems 'hard' frustratingly the schematic I have contains no voltage levels which is not a lot of help. Will get back to you with more info soon
Dave

[AlanBeckett]

You should be OK with that lot and at least this won't bite as hard as a Valve Amp

Hard Faults are a lot easier than Intermittents and at least you've got two Channels to compare.

Alan

[Cave Miner]

Thanks, a bit late for diagnostics tonight but will attack when brain is refreshed in the morning, espresso and common sense might win the day. Will get back to you after some further investigation
Dave

[Murphyv310]

Hi.
I agree with Alan's advice. I'd also look for any cracked tracks on the PCB and carefully prod the components, some resistors especially are known to "fall apart".
The old adage of eyes, ears and nose will repair more faults than scopes and meters!
Good luck.

[Cave Miner]

Thank you everyone, Eyes and ears failed me this time bur a trusted passive ohm meter test revealed an O/C on a connecting pad to the volume control. A bit of de-soldering fond the pad detached from the PCB. Fixed with a direct wire link. Tested and working fine on both channels. Low output was down to me using a low output turntable. Most of the work involved getting at the board. Once again thanks for your advice and help. I think I can close this one
Dave

[AlanBeckett]

Well done young Dave.
Back to the Valves

Alan

[Cave Miner]

Yes thank goodness. A single valve (EL 84 ) Densette record player should not cause many problems, oh I know you should never say that!
Maybe a 1956 USSR Rigondo Syphany may be more of a challenge. I will get back to you.
Thank you for your help