Cambridge Audio A500 & P500

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hi all...
I have both an A500 and the P500...
in my A500 the nasty RM caps (2200uf) decided to let go their combined bladders a while back and shorted my pcb...
since replacing the caps for panasonics and generally cleaning up I can only get the left channel working..
several components in the right channel fried completely.. though not the saps funnily enough... several small resistors and the right channel trimpot grew a nice hole...

regarding the trimpot.. I have found it near impossible to source an original piher 100r trimpot to fit...

I have now pulled the lid off the P500 to see about canibalising some parts just to see if I can get it workiing again....
I had judged from the A500 pdf manual that the P500 circuit is a close copy, including the use of 100uf 63v capacitors in the power amp...

yet... when I looked under the bonnet of the P500 the left and right channels have 100uf 50v caps??
eh?
what's it all aboot?

sorry for the preamble....

simply put... can I use this entire board with the 50v caps and scavenge the necessary parts from the A500 board?
or should I go the other way?

thanks for any advice....
 
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I think I would try and fix the duff amp rather thanrisk ending up with two (possible) problems.

Without knowing the details of each its impossible to say whether boards can be swapped etc. As to voltage rating of the caps, the manual should give a clue as to the expected working voltages. Maybe the transformer voltages are different.
 
The A500 has 45V rails. 50V capacitors is a bit close margin. They probably specified 50V caps initially, and then when they found this causes a problem in the field, increased to 63V capacitors.

Are you absolutely sure the SAP's are not dead ? If the trimpot has burnt, that means a lot of current flowed through it. On this amp the trimpot is connected directly between the SAP's internal bias diodes, so the only way for a high current to flow would be if they shorted internally.
 
thanks to both for your replies...
something going on with forum today lost my replies twice now before posting them as I was logged off after 10 minutes???
hey ho....

to summarize...

ordering more sap 16's to swap out both A500 and P500..

ordering new caps for P500 and A500...
to wit...
Read a recent thread on here about modding an Azur 640 and replacing 2200uf caps for possibly 3300's...
good / bad idea? or stick with 2200uf but better quality?

several resistors need replacing.. however there is no indication on circuit diag of wattage required / needed for the smaller film resistors...
specifically R256,257,258,259,260.. ranging from 47r to 22k..
I can source .25w to .6w does it matter?

lastly.. I want to replace a darkend (suspect) pewer resistor..R245
it's listed as 4k7 3w but I can only find a 2w available...
can I use this instead?

thanks for any advice you can give on this...
 
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The darkened 4K7 3W resistor is probably OK. Its not unusual for metal oxide types (assuming it is something like that) to be like this.

As to the other resistors, again, without circuit details it impossible to say.

When powering a repaired amp up for the first time (and for faultfinding) always always use a bulb tester to prevent failure of new output devices.

The difference between 2200 and 3300uf electros won't make much real difference and bigger isn't always better anyway. If used in a PSU as reservoir caps going to large can cause other problems such as increased "resistive" and "copper losses" meaning the transformer runs hotter under high loading and the rectifiers see higher peak currents over a "shorter" conduction angle. But 2200 vs 3300 is no real issue.
 
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Thanks folks...


sorry.. still trying to work out how to post a scan of the diagram on this thread....

couldnt give me a pointer mooly?

For future reference :)

"To add a photo.

First click "go advanced" in the box below the "quick reply" message box. Doesn't matter if you decide half way through a message to do that, it carries it foward.

Then click "Manage attachements". Maximise the new Window so that you can see all the text.

Click browse in the first box at the top and find your picture. Repeat for any more pictures.
Click upload... a message appears "uploading"
When complete the files will show as being attached. Now click the small text that says "close this window"

The pictures should now be attached and when you submit your post they will appear.

Make sure your pics aren't too big, a couple of 100k is plenty, and many members object when they are massive and it alters the margins
It tells you in the attachments window what max sizes are allowed.


If you want to attach a file that has a non standard format for example excel, circuit simulation etc then try putting the files in a zipped folder and attaching that.
"

I'll have a look at the circuits later or tomorrow
 
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Easy ones first, the resistors apart from R245 can all be 0.25 watt if needed. Reasoning for that is based on the VAS current sink which puts a limit of around 17 ma current "flying around" and available. That said I would still go for the more standard (these days) 0.6 watt metal film. Do Maplin still do those ? Even at their prices its an option and saves buying 10 or more of each from wholesalers.

R245 sees around "one rails worth :)" voltage at all times so worst case on 50 volts is still only 0.5 watt dissipation. Why a 3 watt part is specified I have no idea.

Moving on, if all those small resistors are zapped (and you must check them out of circuit to get a true reading) then you are looking at replacing the "driver" transistors and other related parts... still an easy fix.

Check the parts out of circuit first, work neatly, and I would advise just desoldering one end of a part with braid to do an "invisible" test.

When you power up use the bulb tester as mentioned and start with the preset at minimu resistance. In other words the pot should be shorting the base of the output transistors together for minimum current flow.
 
Ordinarily driver transistors would be TO-220 package devices that drive the main output transistors. In this circuit, the SAPs are power darlingtons, so basically they encompass driver and output in one package.

When the SAPs fail it is common to find preceding stages damaged. In this case if you look at the right channel schematic (Page 4 in the A500 service manual), Q214, Q218 and Q219 and associated parts often get damaged. Q215 and Q216 also often get damaged trying to limit the fault current from the failed output stage.

The good thing about this amp is that the transistors (other than the SAPs) are very cheap and generic. The MPSA43/93 and MJE340/350 are very cheap and easy to get.
 
Heh....
I bought this set-up end of 98 and ran it with
MS 906 floorstanders bi-wired...
Might account why it lasted 12 years without
going up in a puff of smoke....
That being said....
I used to crank it right up for 3+ hours at a time with no
noticable problems...
Infact it seems it's the RM 2200uf caps solely to blame
for the break down....
"Electrolyte leaked and shorted the pcb" ��
 
The Cambridge amps are a pretty solid circuit, let down by poor quality parts. The capacitors are crap, and the heatsink is poor for long term use at higher volumes. Frequently seen these killed by people who had parties, turned them up too loud, and they went pop due to overheating
 
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