I have an NAD C245BEE 4 channel amplifier. I cannot find any schematic, repair manual or any other useful information about it. It it difficult to work on because the protection circuit shorts the outputs . The protection short is “always on” till pulled off. Some of the protection routes through a microprocessor. NAD does seem to offer any help on this discontinued product. If anyone has any information on the circuits, or advice on how to deal with amps that are hard to diagnose do to protection circuits it would be appreciated.
@sds2000 I once (some years back) have searched for it myself with “”no avail”” unfortunately. Well, in my case three were some High ESR electrolytics on the power/protection board. It’s been quite some time ago and cannot remember exactly what I’ve done but I’ve downloaded the service manual for the model C325BEE (which I have found) and kind of… analyzed it and used it as a form of “inspiration” for how things worked or rather supposed to work. I also followed a thread on the repair of C325BEE on EEV blog site and kind of took some conclusions from there too. It is indeed hard to troubleshoot with no schematic but especially when the manufacturers are also employing microcontrollers in it.
The prime suspect was (at first) the 2 power amp board but fortunately wasn’t the case. All I remember was that some voltages weren’t quite consistent and ended up changing all the electrolytic caps on that board along side 2 resistors that were way, way out of tolerance and that was it. It worked. And, also thoroughly checked all other electrolytics all throughout the amp to make sure the amp does not leave without a proper “sanity check”.
Now, in your case is hard to say, it could be a whole array of things but a good starting point would be checking some voltages, check to see if all the buttons are making proper contact, cold/cracked soldering joints, devalued resistors, blown tracks next or around H. Power components such as H. Power resistors or transistors, loose connectors of any sort and in any place, bad /inconsistent electric contact on relays etc. Look for any signs of black spots on components, decolored resistors, blown IC tops, fuse holders with loose contact, bad fuses etc. etc.
Just keep in mind that, for any equipment of any age the most likely component to fail (as a rule of thumb) is always the electrolytic capacitors. !!! and then the rest.
if after changing some caps etc. still doesn’t work it might pay to actually try to reverse engineer the protection circuit in conjunction with the microcontroller to have a better understanding of how things are designed. Who knows but it could lead you to the final answer. You never know.
Also the E-Bay or some other paid sites to see if you can find a service man. to buy. I’ve done that too but… (back then) I could not find anyone that sells such manual…
Good luck and…don’t hesitate to keep ask other questions too. You never know where the help comes from. Perhaps someone might have the manual or had repaired in the past the same amp etc… 👍
PS: another good point to start with when checking the voltages is to verify the 5V DC presence at the microcontrollers +Vcc pin/pins whichever they might be. If you don’t know what pins they are look at the IC,s part number and then download the Datasheet for it and you should be able to easily work that one out. An inconsistent +5V at the Microcontroller’s pins is a good sign that either a local decoupling cap is bad or another around/near the 5V supply and in conjunction with the 5V regulator IC or similar if there is such thing employed. It could be an LM7805 or an ASM1117 or any other type of perhaps (LDO) regulator IC etc.
Thank you to all who hav given me a good start. I guess I should have mentioned the unit failed as the result of a brown out/black out. In the past I have found this form of failure often “breaks ” bits of the circuitry that usually not the usual suspects.
@sds2000 Have you sourced a Service Manual yet for the C275BEE C/AH? I’ve a pdf of it i just downloaded so if you still don’t have anything i’m willing to onsend it to you as a file. Having glanced over it i think it best you use a copy as there certainly is a lot of specific info you will need….