i am not a qualified electronic engineer , i am not even a hobbyist in the electronics field , i own some hi-fi and it needs some maintenance from time to time , if you undertake these simple but still DANGEROUS repairs , do so at your own risk ,
try and do the work when someone else is in the house just in case you manage to turn youself into a lightbulb :-)
tools, flux core solder , soldering iron of more than 15watts with a small tip , desolder wire braid , pozi driver for all the screws and a small pair of needle nose pliers , a small slotted screw driver , a reliable multimeter ,,good quality coffee or tea and a space to safely do the work .
this blog is to keep a record of the restoration work on a pair of old cambridge audio p500 power amps , i wont be modifying them as such , just replacing all the old capacitors before they let go and cause more damage , both are absolutely stock and have never needed repaired in all the years i have owned them , i bought them new just after cambridge stopped making them ,
they are not great but should be on every aspiring audiophiles starter amp list , plug in a good pre amp and source with well matched speakers and this little amp is great fun , it loves older large multi way speakers as much as small stand mounts , just watch the speaker impedance , usher s520 works superbly with it , old monitor audio ma 5 etc , the venerable p500 can be quite breath taking , feed it with dynamic material and it does its very best to deliver just that to the speakers , choose your pre amp wisely , the best result i have had from mine is using a Nakamichi CA-5e mk1 , (avoid the mk2) , ,
these are the main offenders , if your p500 starts to hum , chances are these bulk reservoir caps are dying the death , leave them too long and the sap15's will give up ,
choosing the caps ,
buy from a reputable vendor of electronic components , so many are now being faked , if you see a great deal on some chances are they are fake cheap items that have been re-wrapped in a quality brand jacket , such as this video showing some sanyo WG ,
in the case of those in the p500, they are cheap but perfectly fine 2200uF 50v radial electrolytic aluminium jobbies , i will be fitting panasonic FC series , they are good reliable caps that can be found easily , they dont bring any nasties to the sound and are perfectly justified in this amplifier , if you want to go posh thats fine , its your money , i would suggest you dont go any cheaper .
i will also be replacing the electrolytic's in the circuit , especially those that are close to the main heat source , heat kills caps , dry's them out to be precise ,
the one with a yellow oval is directly in the input signal path and is a bipolar 1uf 50v item , this is quite important as far as influence on the sound ( as far as they can influence the sound) , i will be spending some time asking around to see what is best in here , i suspect it may be worth using a good polypropylene in this location but may find a pair of good panasonic FM polar caps that add up to 1uf with the caps negative terminals connected and the two positives used to connect into circuit may be an interesting approach ,
i have read ( but have yet to verify ) that this is a very good way to go and can yield nice results at low cost . the cap in the red oval should be replaced as should the trim pot also in the red oval.
heres a close up
This trimmer pot ( 100 ohm ) is a nasty-ish device , its prone to dusting up and only has about 320 degree rotation, Its job in life is to regulate the power feeding the SAP15 and needs to be adjusted quite finitely , the fitted one is a pig to adjust as even a tiny adjustment can go too far when you are trying to get the voltage spot on and even for each channel , if we replace this with a sealed multi turn item adjustment becomes far easier , i will be using elecsound items ,
link to elecsound 3296 cermet trimmer pot
the contact material in these pots must be quite soft , if you look at the spec sheet it is suggested a total rotation life of just 200 turns before +/- 5% deviation beyond specs so, dont go twiddling the knob too much (as the nun said to the choir boy )
they are cheap and fit the space , they should also make adjusting easier later as we need to set the voltage on pins S&E on the transistor side SAP15 to 13mv ( equivalent of around 60mA) with the amp idling and nothing connected , dont worry , the pins are easy to access and i will put up a a pic showing the pins to use and how to adjust .
the board is littered with ceramic disc and tantalum bead type capacitors , i will check around to see if these need replaced , likewise some of the resistors could be replaced but for now i will just be doing those potentially leaky or dry metal can types .
speaker connectors ,
the ones that cambridge selected are horrid , the plastic nuts bind up and are unpleasant in use and also need checked every couple of months as they tend to work loose , they dont properly accept 4mm plugs unless you want the cable sticking up from the rear of the amp , all in all they are simply cheap nasty crap that panders to the EU and its stupid mission to solve a safety issue by replacing it with something that has multiple safety issues . , while i am at it i will be blanking off the two extra sets as i only use a single wire set up , bi-wiring is not worth the effort and expense .
i will use something like these ,
The binding posts are held with nuts, the whole speaker connection terminal block lifts out and is easy to modify to accept most types of post , i will most likely arrange them on the outer 4 holes and blank the 4 in the middle , this will make it safer and easier to use , notice the red and black wires , these only run to the circuit board to distribute the output to the extra speaker outputs , i will be running the wire directly to the output terminal on the single row of speaker posts that will remain thus replacing two solder joints and a screw down terminal with one solder connection , i may even up the wire gauge a bit too .
more soon
