One of my friends brought this set to me mentioning that the performance of the set has become bad and the Radio is very weak. I switched it on and noticed that the complaint was genuine. (Please notice what he has done on the backside! He had stuck transparent tape, with a piece of paper, placed exactly on top of the screw holes. This is to prevent potter wasp blocking the holes, which is quite a common problem in Kerala, India. Even if you clean such a hole, the screw head will be corroded and will not hold the tip of the screw driver. Sometimes, the worm inside will make it slippery when the liquid spills from it in our attempts to clean the hole! So, we may have to use a soldering iron, heat the screw head and push it in, rendering that portion permanently unusable! Then alternative arrangement has to be made for fixing the cabinet!)
On opening the set, cleaning and dismantling, I could observe that there were several electrolytic capacitors that had burnt marks on the negative rail. The tank capacitor 2200/16V had leaked on to the board and the burning had made a dent on the PCB. A clear case of high voltage problem, unnoticed by the user and left for sometime in on condition. See picture below:
The ESR of many of these capacitors and those in the signal path, which had no visible symptoms showed way out of the permitted values. So, I decided to do my favourite work of replacing all the electrolytic capacitors in the Main Board, Radio Board and Control Board. Replaced the capacitors in the Radio board.
Did a thorough dry solder patch up on it. Replaced all electrolytic capacitors in the main board. This is a laborious job! We need to de-solder the caps, clean the holes, scrape the leads of the new caps, insert it, solder it and then cut it.
I touched on all the joints on all the boards including the power supply unit. After finishing the dry solder patch up, cleaned the board thoroughly. See how it shines line new!
Totally 56 electrolytic capacitors were replaced:
Tested the set connecting the Radio and tape deck, keeping it on the table. Radio worked just fine! Cleaned the tape mechanism (Forgot to take snap of this!). Observed that the left and right channel were not even. Cleaned the head. But there was no improvement. So, fed signal to the tape path. Noticed that when you feed signal to one channel, it was coming to the other also! Traced it to pre-amp IC No.AN6210. Signal from its out was ok. But when give it to inputs of tape, it was showing internal short (Signal leaking to other channel). Removed the IC and noticed that two pins had left burn marks on the board. Luckily I had this in my stock! Replaced. Set worked just fine in all respects. Reassembled it. The funny thing in this set is that we cannot remove the tape mechanism without removing the radio board, as the manual dial unit prevents its extraction. Since the rotating wheel fit on the tuning capacitor is on the other side of the PCB, the dial unit cannot be dismantled or lifted to extract the tape mechanism. Look at this ‘guy’ happily playing tape:
Another job completed satisfactorily!
This article was prepared for you by Parasuraman Subramanian from India. He is 65 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company and presently working as Consultant Manager, Purchase & IT, in Irinjalakuda Cooperative Hospital.
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