Okey, I guess I'm ready to place an order at Cricklewood Electronics.
Great company to deal with.
Very fast delivery and very good pricing.
i.e. irfp240 RS £5 Cricklewood £1-50.
O
jaycee: About the quality of Cambridge products. The following is printed in large letters on the input control board:
"Designed with care and attention by Cambridge Audio - Research and engineering Enland"
The design is not the problem. They are quite well designed, but then manufactured in chinese sweat shops using the cheapest parts they can get. It might be different now but the inside of several A500's ive seen all paint the same picture. This is one instance where I belive re-capping with decent capacitors such as Panasonic or Ruby/Nichicon would bring a big improvement in performance and reliability - and i'm normally very much against that stuff!
I'm still waiting for my parts from Cricklewood. I will post some pictures after I have replaced the faulty parts.
It's quite hard to find the micas needed for the transistors so I'll probably use the old ones, if they don't brake that is! They seem to be quite brittle, does anyone have some tips in removing and cleaning them?
Will a thermal grease of 4W/mK be sufficient?
Thanks!
It's quite hard to find the micas needed for the transistors so I'll probably use the old ones, if they don't brake that is! They seem to be quite brittle, does anyone have some tips in removing and cleaning them?
Will a thermal grease of 4W/mK be sufficient?
Thanks!
Yes, because +/-45V supply voltage without parallel output devices at 4Ω load is too much (check out your speaker impedance in their datasheet).The SAP15's are favourites plus collateral damage
One is an NPN and one a PNP.
Additional I see one thing in the schematic, that I don't understand: the connection of short protection sensor resistor R239 (pos rail) and R260 (neg rail).
Why the connection to the integrated emitter resistor from the first of the integrated SAP15 darlington stage instead to that one of the second, as normally usual?
An other shortcoming I discover at first look: variable resistor RV202; what happens at short interruption as a consequence of a contact problem?
I would say the highest possible idle current flow and thus destroy of the SAP's after milliseconds.
As a precaution, both trimmers should be substituted by new. After find the correct idle current through the output, desolder both trimmers and substituted it by solid resistors - exactly measured out - to the adjusted resistor values from each trimmer.
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and also by Profusion in the meantime:
Power Darlington
my proposal is to modify the output stage (discrete darlington and external Vbe multiplier). A good choice it could be the topology of formerly integrated amp from Cambr. Audio. Check out therefore this thread:
http://www.diyaudio.com/forums/solid-state/162751-help-cambridge-a3i.html
You must choice a TO-3P instead a MT-200 outline.
But if you absolutely want to maintain the original condition, ask for remaining stocks by diyaudio member "jannemann" - go to
http://www.diyaudio.com/forums/swap...ap15n-sap15p-std03n-std03p-if-compatible.html
about Sanken's website successors are listed: STD03N/P - go to
SANKEN ELECTRIC : Power Transistors : For Power Amplifier
(scroll down), but I don't know about the exact compatibility.
Obviously the solution with integrated VBE multiplier diodes (1 pc at SAP15N and 4 pieces at the SAP15P) wasn't as elegant as it has promised by advertising (more stable idle current and better sound through thermal direct coupled Vbe multiplier diodes). Consequently, Sanken have been discontinued and released better version.
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Guys, I don't quite follow. You want me to redesign the entire output stage? Is 4Ohm too much even for the SAP16s? Whats the lowest impedance recommended for this amplifier?
The parts will probably soon arrive, I will replace the parts and hold my thumbs.
Still looking for answers to my previous questions though!
Is 4Ohm too much even for the SAP16s? Whats the lowest impedance recommended for this amplifier?The parts will probably soon arrive, I will replace the parts and hold my thumbs.
Still looking for answers to my previous questions though!
Hello NiklasF - mica is a silicate mineral that is brittle - i.e you can easily snap it, however it is perfect for insulation as it transmits heat well and it's structure allows it to be easily split into thin wafers. It's tougher than it looks and careful cleaning is OK with a solvent that dissolves the heat compound - alcohol / white spirit, etc - the mica won't melt!
PS I concur with the views above - however for your sanity's point of view, I'd replace all four SAP's with the replacement versions, plus change the driver transistors and any obviously burnt out components - check the resistors, as described. If you get the sucker working OK, then go back and change the caps. On the heatsink issue, comments above are spot on - you may wish to source a larger replacement heat sink and mount the new output pairs on this. Thermal problems with these are the most common causes of failure. The heatsink canna take it, Captain!
If any caps are suspect, replace them. If you want to do the job thoroughly, replace them with good quality equivalents - such as the Rubycon audio grade. As I've learned, the cap quality is directly proportional to the depth of your pocket. I've got two Cambridges and they are reasonably well designed, indifferently built, with components sourced by a tone deaf Scottish accountant. I'd do the minimum to get it working, then think about upgrades - this is my view - many others will differ!
I finally got the time to finish repairing the amp, it’s alive!
All went well. I followed the adjustment procedure in the A500 manual and set the SAP16 S-E voltage to 13mV. Hopefully I won’t break down again anytime soon. I actually noticed that all the caps were of 50V type even though the schematic says 63V, that’s a bit odd I think. Anyway I ordered the 63V type of the 100uF, 10 in total. They were a bit bigger in size but I managed to fit them anyway.
Thanks for all the assistance. I'm not sure whether the amp was worth fixing but it’s a nice feeling getting things to work again.
All went well. I followed the adjustment procedure in the A500 manual and set the SAP16 S-E voltage to 13mV. Hopefully I won’t break down again anytime soon. I actually noticed that all the caps were of 50V type even though the schematic says 63V, that’s a bit odd I think. Anyway I ordered the 63V type of the 100uF, 10 in total. They were a bit bigger in size but I managed to fit them anyway.
Thanks for all the assistance. I'm not sure whether the amp was worth fixing but it’s a nice feeling getting things to work again.
Well done for persevering
63V is more of a standard voltage than 50V. Either is acceptable. They probably got 50V rated capacitors slightly cheaper and went with those.
They're good little amps for the price if you bear in mind the limits - don't use 4 ohm speakers, and dont use them for parties.
63V is more of a standard voltage than 50V. Either is acceptable. They probably got 50V rated capacitors slightly cheaper and went with those.
They're good little amps for the price if you bear in mind the limits - don't use 4 ohm speakers, and dont use them for parties.
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