Shorted Mosfets and Bad Capacitors In Amplifier
This NAD 360 Amplifier was brought to me with the complaint that the output was weak. As usual, first job is to open the set and clean it thoroughly, which I did.
On observation I found that the speaker connectors were broken, the volume knob was missing and replaced by a crude looking cap of some plastic bottle.
I decided to fix this first using superglue. So, dismantled the PCB from its case and removed the speaker connectors.
After it got fixed properly, turned my attention to the defects in the unit. Observed that the DC fuses were blown, proving that the customer was either taking me for a ride or this would have happened during last use at the time of switch off, which would have gone unnoticed. Unfortunately, there are a few customers who resort to covering up the actual defect or damage that themselves know as there is a general feeling that if they tell us what happened, we will take it as a free license and may charge more! Because, the prevailing tendency is to get the work done to their satisfaction, but not to spend befittingly! Anyhow, I called him up and told him sternly that what he told was not true, upon which he replied that it would have gone unnoticed. Anyhow, I ensured that the message got registered!
On further testing using Analogue Multimeter, noticed that two of the Mosfets were showing bad reading. So, removed these. But removing the portion was a bit tricky, as the heat zinc was held by the mosfets, and the pins were bent up, any strain would tear off the tracks and cause further damages! Replaced the mosfets.
Then looked for reasons for the failure of these mosfets.
Many of the capacitors were way out of range of ESR and values. Following pictures are self-explanatory:
This amplifier has two modules for left channel and right channel, housed in aluminium cases. Just see one cap almost burnt! Also how clogged were the connectors!
Removed the Display PCB from the front panel for check up and dry solder touch up.
After all these marathon efforts, cleaned the PCBs thoroughly. Connected Audio Input and checked up. Noticed that the volume control was having dirt collected in it and it was intermittent and lubrication and cleaning did not work. So, removed it and opened it up, did manual cleaning using cotton dipped with IPA. As you will notice, this has a motor for remote control of the Volume.
Refit it and noticed that it was working well. Tested it for a very long time and for a couple of days and ensured that there are no faults in the Amp. As the customer did not want to change the volume control knob, I left it as it is!
Here are the components replaced in this Ampere, which could be an all-time record!
Another tedious but teaching job completed to my utmost satisfaction. The customer did not seem to be very happy about the cost of service, as it looks it was borrowed for use from someone and this has burnt his fingers. Well, I cannot help it!
Like what I mentioned earlier, there are some who do not want to spend, but expect the best in return, of-course, for their own reasons! For me, if I had borrowed something from someone, and it failed at my place, I will opt to return it in better condition than before and definitely not just a patch to make it work! Well, we have to manage with such multifaceted customers in our profession! What else to do!
This article was prepared for you by Parasuraman Subramanian from India. He is 69 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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