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Old 23rd February 2010, 03:18 PM   #1
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Default Help troubleshooting a Cambridge A500

Iím having problems with my Cambridge A500. The right output is silent but the left one is still OK as far as I can "hear".

I took some pictures, 2 resistors R256 and R260 has gone bad, possibly also R258 but it might only be discoloured by its neighbour. I have looked underneath the board as well but no trace from burnings there! I attach a link to the circuit diagram also. Link!



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Click the image to open in full size.


I canít see any other apparent problems but something must have caused this right?!

I would be really thankful for some guidance! What should I look for, could I try to replace only the bad resistors?!

Regards, Niklas.
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Old 23rd February 2010, 08:28 PM   #2
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You might get away with just replacing the resistors but I suspect you wont.

Probably a semiconductor or more have gone and fried your resistors.
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Old 23rd February 2010, 08:31 PM   #3
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Yes that's my thought also! Any idea what to check and how?
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Old 23rd February 2010, 08:36 PM   #4
gootee is offline gootee  United States
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Since the two channels should be identical circuits, you could try measuring resistances across corresponding semiconductors' lead/terminal pairs (one in good channel and same one in bad channel) and look for differences.

The unit should be disconnected from power, when doing that, of course.
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Old 24th February 2010, 06:25 AM   #5
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Yes that’s a good idea, I'll try that. I was hoping someone could help me narrow it down though, there are a lot of transistors. I'll will give it a go.


Thanks!
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Old 24th February 2010, 05:49 PM   #6
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You need to check U206,u205, q216, q215 and upstream BJTS for possible shortage and replace then apply only 1/3 of rail voltage to see DC offsets and if you can adjust voltage of U205 and U206 Base (B) by turning RV202 before you plug into AC outlet directly to avoid fire work and smokes. Good luck
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Old 24th February 2010, 06:07 PM   #7
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birdyman: I'll get right on it..., tomorrow. I'm not sure that I understand what you mean by "to see DC offsets". Where should I look for these? Thanks for concrete advice!
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Old 24th February 2010, 06:08 PM   #8
Mooly is online now Mooly  United Kingdom
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The SAP15's are favourites plus collateral damage
One is an NPN and one a PNP.

Data sheets here... they are unique Darlingtons with built in thermal sensing diodes,
SAP15N Datasheet pdf - Darlington transistors with built-in temperature compensation diodes for audio amplifier applications - Sanken
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Old 24th February 2010, 06:09 PM   #9
Mooly is online now Mooly  United Kingdom
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Oh, and as Birdyman hints at "be carefull"

Always power up with a 100 watt bulb in series with the mains first.
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Old 24th February 2010, 06:15 PM   #10
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Thanks guys, you are all really helpfull! The bulb will make sure that I wont fry the circuit? Why 100W? Will it be enough?, the current is probably still high enough to fry something right? Isn't a dimmer a better approach?
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