Cambridge Audio A5 repair job

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Hi all,

I have a Cambridge Audio A5 amplifier that has a fried SAP15 transistor. I took it to richer sounds to get repaired but they want £80 to do the job, unfortunately I don't have that money so I'm looking at repairing it myself. I can use a soldering iron fine but I was hoping anyone with experience of this could tell me if any other components are likely to be broken too, and also where I could source the SAP15 transistors from. I checked rapid electronics but they only have SAP16.

An externally hosted image should be here but it was not working when we last tested it.


Theres a pic for you to take a look at if it helps.

Thanks loads!
 
ok, first of all you really need check if there is any evidence of burning around any resistors then replace both positive and negative transistors on the blown channel. also i would recomend replacing the driver transistors mje350, and mje340. having done this check if there is any evidence of burning around any resistors, after you have done this and before doing anything else build a dim bulb tester this site gives you the basic idea http://www.antiqueradio.org/dimbulb.htm obviosly ground goes to ground which this site dosent show!
then with your amp pluged into this power it up, if the bulb lights up and stays on you still have a problem, if not you now need to adjust theidle current which is done by monitoring the voltage between pins s and e of the output transistors (see their datasheet) to achieve a voltage of 13mv.
now check your dc offset this should be anywhere between 0-20mv, now normally would connect a dummy load to the amp to test it fully but seems your a beginner at this i doubt you have one so connect back to your speakers at your own risk as they say!

chris
 
This is for the A500
Part 1

Regards
James
 

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Hi Pulseezar,

I have just been given the same amp with exactly the same problem, have you had any luck with your repair job?

Also, to Jaycee, I have some SAP16's with which to replace the SAP15's (I was told they were a drop in replacement) should I replace all four of these so there is symmetry?

Thanks,

Charlie
 
Each channel will have a trimpot on it, a variable resistor if you like, to adjust the bias. On the schematic, these are RV201 for the left channel, and RV202 for the right.

Using a voltmeter that measures millivolts, connect clips to the S and E pins of the SAP16N (this is the two pins on the right as you look at the transistor from the front, with the screw at the top). Make sure these clips do not short - it might be better to tack solder some wire in place if you do not have suitable clips. The best kind of clips for this are these:

An externally hosted image should be here but it was not working when we last tested it.


Switch on the amp with no speakers or sources connected. Adjust the trimpot for that channel until you get 13mV. Do this carefully in small increments - don't rush the job as you can blow the transistors if you increase it too much. The bias will "drift" as the amp warms up, so adjust it to 13mV cold and then let the amp warm up for about half an hour or so. You could also gently heat the heatsink with a hair dryer if you're impatient but I wouldn't recommend it. Re-adjust to 13mV as the amp warms up.

If you are replacing blown transistors, check that the trimpot and the capacitor across it are intact with an ohm meter before you power on. Sometimes the pot can go open which will blow the transistors pretty much instantaneously.
 
Very similar problem

I took the back of my Cambridge A5 last night and there is a crack right through the middle of one of the SAP15's.....am I safe in assuming that is why one of the channels is very crackly and very quiet?!?! If so all of this info is very useful as I think even an audio novice like me will be able to have a crack ata repair (assuming I can source a pair of SAP15's in Australia that is).

Jaycee,

You say to check the trimpot and the capacitor are intact with an ohm meter before you power on. How do I a) identify them both and b) how do I check they're intact.....apologies for the numpty questions!!

Thanks all - great stuff.
 
Re: Very similar problem

Originally posted by sgrafton
I took the back of my Cambridge A5 last night and there is a crack right through the middle of one of the SAP15's.....am I safe in assuming that is why one of the channels is very crackly and very quiet?!?! If so all of this info is very useful as I think even an audio novice like me will be able to have a crack ata repair (assuming I can source a pair of SAP15's in Australia that is).

You're lucky your speaker isn't damaged but yes, thats a big problem!

Originally posted by sgrafton
You say to check the trimpot and the capacitor are intact with an ohm meter before you power on. How do I a) identify them both and b) how do I check they're intact.....apologies for the numpty questions!!

Thanks all - great stuff.


Measure the trimpot with an ohm meter. If it still reads 100 ohms it's ok. The service manual for the A5/A500 (same power amp circuit) is around on here. RV201/202 are the trimpots in question.

The capacitors are marked as C220/C238. If they look like they've exploded, replace them.

I say it again with these amps - these are NO GOOD for parties or extended cranked volume. The heatsink is simply too flimsy and the SAP's overheat and die. An L angle with some folded sheet is an incredibly poor substitute for a proper solid aluminium extruded heatsink.
 
Hi, I have got an A500 amp that has got a faulty pre-amp.For the last couple of years or so the turn on click has gone from being almost silent to a full turn on thump. At the same time it developed a turn off fart. The transformer blew about 6 months ago by overheating and the resettable thermal fuse failed to reset. This may be irreverent (but shows that the amp hasn't exactly been treated nicely).
About a month ago the pre amp developed a nasty HF whine that could be got rid of by pressing tape monitor / banging the amp / removing the phono module. As a note when the amp was whining, the LED on the phono module wouldn't light. The amp has also developed a noisy volume and balance pot. I don't think it is just dirt as it started effecting both pots at the same time and also its not just noise, it looks like DC is being applied to the power amp, but is filtered out quickly by the power amp input capacitors.
It sounds to me like one of the pre amp rails has developed a fault, maybe the voltage regulator, and the positive rails are now at a different voltage to the negative rails. What do people think? I will try and measure the voltage of the +/- 15v and 45v rails and see where they are at. If they are radically different, what would be the best fault finding method.
Thanks, Nick
 
Hi all,

I have a Cambridge Audio A5 amplifier that has a fried SAP15 transistor. I took it to richer sounds to get repaired but they want £80 to do the job, unfortunately I don't have that money so I'm looking at repairing it myself. I can use a soldering iron fine but I was hoping anyone with experience of this could tell me if any other components are likely to be broken too, and also where I could source the SAP15 transistors from. I checked rapid electronics but they only have SAP16.

An externally hosted image should be here but it was not working when we last tested it.


Theres a pic for you to take a look at if it helps.

Thanks loads!

£80 - I think, that's very cheap. If you don't find and remove the reason for the defect of the SAP's, it can easily be more expensive than £80, even if you do everything yourself. The reason lies in most cases in the components and PCB traces for the quiescent (idle) current setting for the SAP output devices.
 
Fixed it temporally by removing CN6 (output buffer connector). Basically there was a dry joint on C14, the 47uf -15v capacitor (it had probabily cracked when the gold cover was first applied) and the regulator must have finally died after years of oscillation. It is odd as this section deals with the pre-amp output and isn't in the signal path.
Cheers for your advice Jaycee,
Nick
 
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