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Old 27th June 2012, 08:21 PM   #1
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Default Cambridge A500 help needed

Hi!

My old friend - the Cambridge Audio A500 failed a couple of days ago.

I have figured out that only one channel is working properly (left one works). Following some other threads I have measured the DC voltage on the speaker terminals and it is low and does not look disturbing. The interesting detail is that the RED channel status LED is off for the failed channel. I've got no experience with repairing amps. Were should I start the investigation?

Thanks in advance for any help.
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Old 28th June 2012, 06:05 AM   #2
MarianB is offline MarianB  Romania
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How does the failing channel acts? is it dead ( no sound at all on the speaker ), or does it distort the signal or the power is too small, you need to give more detailes.

Anyway, a good starting point is isolating the power amplifier inside from the other circuits ( preamp, tone control, etc ), that means unpluggin the IN connectors on the main board this is to make sure that the fault is in the final amplifier stage, than make sure the fuses are ok, check the supply, caps and then measure the transistors inside starting with the output darlingtons, see theyr datashhet for more info.
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Old 28th June 2012, 08:42 AM   #3
jaycee is offline jaycee  United Kingdom
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It is not a status led but rather a voltage reference for two current sources. This does pinpoint where the fault is.

Get a copy of the service manual - you should be able to find that by searching here.

Be warned that output transistor failures are common on this amplifier due to the poor heatsink. This is complicated by the fact that the Sanken SAP series are now discontinued.
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Old 29th June 2012, 09:02 AM   #4
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I found the schematics, if service manual is a separate document than I cannot find it neither on the forum or google. Good news is that I've found a company in Poland selling the SAP15's - hopefully they really do have them somewhere in the basement.

Today in the evening I'll try to troubleshoot it a little more & look around the reference LED. Thanks for all the info you have provided. As soon as I have additional details I'll send an update.
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Old 11th July 2012, 07:44 PM   #5
RS232 is offline RS232  United Kingdom
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I also need some help....
My case is a bit different and is driving me mad
I have repaired a few A300/A500 and A5 (they all use the same amplifier board) but this last one is doing my head in.
As usual this one had one channel fryed with the usual victims(Sap/trimpot/cap and a couple of resisitors)
All parts replaced and no problems in setting the idle current (13mV stable after about 1 hour)
Now to the problem...
The DC offset on the output...
I replaced all MPSA43/93, MJE340/350 and I can get it to go under 110mV...
I know about matching the input pair but I tested on the good channel and without matching I have 3.5mV on the output.
Can some one point me in the right direction ?
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Old 11th July 2012, 11:10 PM   #6
jaycee is offline jaycee  United Kingdom
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check for a bad capacitor in the feedback network.
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Old 12th July 2012, 06:12 PM   #7
RS232 is offline RS232  United Kingdom
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Hi and Thanks
I allready changed the 100uF 63V (C228) connected to ground through a 1Kohm resistor (R246) the only cap left is a 12pF (C221) that together with R263 (220 Ohm) bypasses resistor R262 (33K) on the feedback network.
Is it possible that the 12p poliester cap to go bad ?
Can I change it with another value ? I don't have any in my spares but i have smaller and bigger ones (in capacity) my local Maplin is a wast of time.

Ric
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Old 12th July 2012, 06:56 PM   #8
jaycee is offline jaycee  United Kingdom
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I'd doubt it's that - that's there for compensation. If it was faulty the amp would probably oscillate

I can only think there might be pcb damage or something like that going on. AP seem to have used the crappiest material they can find.

edit: if you replace that cap you must use the same value. It's for compensation and changing it will make the circuit unstable. It's very unlikely to be damaged. Consider Rapid Electronics for spares, they're pretty good.

Last edited by jaycee; 12th July 2012 at 07:12 PM.
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Old 12th July 2012, 07:11 PM   #9
jaycee is offline jaycee  United Kingdom
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Ah, have you any way to check its oscillating or not ? what about the Zobel components on the output?
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Old 12th July 2012, 08:33 PM   #10
RS232 is offline RS232  United Kingdom
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No. I haven't checked for oscilations but will do tomorrow, just need to get my scope out.
Will have a look on the zobel and let you know

Thanks
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