Help troubleshooting a Cambridge A500

NiklasF · 23rd February 2010, 03:18 PM

I’m 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!

I can’t 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.

nigelwright7557 · 23rd February 2010, 08:28 PM

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.

NiklasF · 23rd February 2010, 08:31 PM

Yes that's my thought also! Any idea what to check and how?

gootee · 23rd February 2010, 08:36 PM

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.

NiklasF · 24th February 2010, 06:25 AM

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!

birdyman · 24th February 2010, 05:49 PM

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

NiklasF · 24th February 2010, 06:07 PM

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!

Mooly · 24th February 2010, 06:08 PM

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

Mooly · 24th February 2010, 06:09 PM

Oh, and as Birdyman hints at "be carefull"

Always power up with a 100 watt bulb in series with the mains first.

NiklasF · 24th February 2010, 06:15 PM

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?

23rd February 2010, 03:18 PM	#1
NiklasF diyAudio Member Join Date: Feb 2010	Help troubleshooting a Cambridge A500 I’m 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! I can’t 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.

23rd February 2010, 08:28 PM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	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. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

24th February 2010, 06:08 PM	#8
Mooly diyAudio Member Join Date: Sep 2007	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 __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

24th February 2010, 06:09 PM	#9
Mooly diyAudio Member Join Date: Sep 2007	Oh, and as Birdyman hints at "be carefull" Always power up with a 100 watt bulb in series with the mains first. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

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23rd February 2010, 08:31 PM	#3
NiklasF diyAudio Member Join Date: Feb 2010	Yes that's my thought also! Any idea what to check and how?

23rd February 2010, 08:36 PM	#4
gootee diyAudio Member Join Date: Nov 2006 Location: Indiana Blog Entries: 1	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.

24th February 2010, 06:25 AM	#5
NiklasF diyAudio Member Join Date: Feb 2010	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!

24th February 2010, 05:49 PM	#6
birdyman diyAudio Member Join Date: Jan 2010	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

24th February 2010, 06:07 PM	#7
NiklasF diyAudio Member Join Date: Feb 2010	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!

24th February 2010, 06:15 PM	#10
NiklasF diyAudio Member Join Date: Feb 2010	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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