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.
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!
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
The unit should be disconnected from power, when doing that, of course.
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
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
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
You measure across speaker terminals without input ( Red & Blk) see any DC voltage readings. If you see less than few hundred milivolt that MAY be OK but if you see higher voltage reading there is something wrong with your amp still. If you apply full voltage by plug right into AC outlet, you will see smoke and fire to burn up all new parts you just put in. SO taste DC offset with 1/3 of full votage by using step down transformers or variacs or big resistance( like 15w light bulb) serial to AC plug.
I wouldn't use a dimmer... that could give really unpredictable results driving a reactive load like a transformer.
100 watts is usually OK, actually you can go less such as a 60 for more safety.
It's really to protect the outputs... if they conduct heavily the rails will be low, and the current limited.
Remember if the bulb is lit (100 watt bulb) then there is correspondingly less power available for the amp to use.
Edit... A500 circuit here a few posts down,
http://www.diyaudio.com/forums/solid-state/124289-cambridge-audio-a5-repair-job.html
Transistors,
http://www.diyaudio.com/forums/solid-state/124289-cambridge-audio-a5-repair-job.html
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Okey! I've done a lot of measuring and compairing components between the channels. I followed both Moolys and birdymans advices.
I'm pretty sure that U206 (SAP15P) and the visibly broken resistors are the only faulty parts. But while I'm at it I might aswell replace all of the SAPs and the drive transistors (MJE340 and MJE350) just to make sure, and perhaps because of symmetry reasons (SAP16 is apperently an improved SAP15?!).
Inspecting the capacitors I found that one of the 100uF cans (top picture, to the left of the upper blue filter resistor) could have been "swelling", the lid is slightly round, not entirely flat anyway. Could this be/become a problem?
Could someone please identify the resistor type, I have no idea what to buy and at what wattage?! cricklewoodelectronics.com seems to have all parts needed (although the SAPs are quite expensive 6 pounds each). Is there a cheaper alternative somewhere?
Any input is appreciated! Thanks guys, or girls but I doubt it!
I'm pretty sure that U206 (SAP15P) and the visibly broken resistors are the only faulty parts. But while I'm at it I might aswell replace all of the SAPs and the drive transistors (MJE340 and MJE350) just to make sure, and perhaps because of symmetry reasons (SAP16 is apperently an improved SAP15?!).
Inspecting the capacitors I found that one of the 100uF cans (top picture, to the left of the upper blue filter resistor) could have been "swelling", the lid is slightly round, not entirely flat anyway. Could this be/become a problem?
Could someone please identify the resistor type, I have no idea what to buy and at what wattage?! cricklewoodelectronics.com seems to have all parts needed (although the SAPs are quite expensive 6 pounds each). Is there a cheaper alternative somewhere?
Any input is appreciated! Thanks guys, or girls but I doubt it!
The 100uF capacitor looks like one of the rail filter capacitors. This capacitor going bad could be what caused the damage. You should replace it.
It looks like the VI limiter turned on hard! I guess U206 failed and the VI limiter tried to shut it off, in vain.
There is no alternative to the SAP16 transistors - power transistors like this are expensive by nature. The resistors are standard 1/4 Watt metal film resistors. You may see 0.6W types, they are also suitable.
These amps are fine for regular home listening, but whenever I see them blown up they have usually been used "cranked up" for a party or so. This they cannot take - the heatsink is not up to the job, and the SAP transistors overheat and die. Cambridge's other thorn in their side is poor build quality - they are usually made in China with eg cheap capacitors. The board soldering is usually lousy too - I'm pretty sure they have people soldering them by hand! The last A500 i saw was a mess, there was flux and solder spatters all over the PCB. The owner had had it from new and had never modified it.
It looks like the VI limiter turned on hard! I guess U206 failed and the VI limiter tried to shut it off, in vain.
There is no alternative to the SAP16 transistors - power transistors like this are expensive by nature. The resistors are standard 1/4 Watt metal film resistors. You may see 0.6W types, they are also suitable.
These amps are fine for regular home listening, but whenever I see them blown up they have usually been used "cranked up" for a party or so. This they cannot take - the heatsink is not up to the job, and the SAP transistors overheat and die. Cambridge's other thorn in their side is poor build quality - they are usually made in China with eg cheap capacitors. The board soldering is usually lousy too - I'm pretty sure they have people soldering them by hand! The last A500 i saw was a mess, there was flux and solder spatters all over the PCB. The owner had had it from new and had never modified it.
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jacee: You seem to understand or to have a feeling of what went wrong. Is it a feasible explanation, I didn't quite follow. Is there something else I should check in regard of this VI limiter?
This is perhaps a detail but there are som plastic sheets inbetween the heat sink and the transistors. Should I try to get hold of something similar or is heat conducting silicone enough?
This is perhaps a detail but there are som plastic sheets inbetween the heat sink and the transistors. Should I try to get hold of something similar or is heat conducting silicone enough?
Insulators
Hello - the thin sheets between the transistors and the heat sink blocks are mica insulators, I believe. I'm currently repairing a Cambridge amp of similar design and the SAP15's all have these. I'd keep them if you have them, although you can source them from most electronic suppliers.
Hello - the thin sheets between the transistors and the heat sink blocks are mica insulators, I believe. I'm currently repairing a Cambridge amp of similar design and the SAP15's all have these. I'd keep them if you have them, although you can source them from most electronic suppliers.
They are indeed mica washers and can be reused if not cracked. You should apply new heat sink paste though. The stuff you get for computer CPU's is fine.
The VI limiter is there to cut off the drive signal to the output transistors if excessive current is flowing... this usually means the amp is overloaded or the output is being shorted. However to cause this kind of damage it is likely the output transistor blew short to begin with. Thermal failure is the usual reason in these amps, the heatsinking is poor and they cannot take being run at high volume for long.
The VI limiter is there to cut off the drive signal to the output transistors if excessive current is flowing... this usually means the amp is overloaded or the output is being shorted. However to cause this kind of damage it is likely the output transistor blew short to begin with. Thermal failure is the usual reason in these amps, the heatsinking is poor and they cannot take being run at high volume for long.
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