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Servicing Unisound UB-50 Public Address System

By on November 27, 2018
public address system repair










pa system repair

This Amp was brought to me for service stating that it was lying idle for a prolonged time and whether it could be brought to working condition, as it had some ‘sentiments’ attached to it for the customer!

Well except for the look of the front panel, rest of the body was rusted. As usual, I opened the set and did a thorough cleaning of insides.

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Switched on and found it had life, though the output was very poor and distorted.

Replaced the Electrolytic capacitors on the Power Supply section, as they were in bad shape.

bad e-cap

heatsink transistor

This amp was using two 2N3055 in its output, driven by separate preamps for each mike input. As there were no markings on the board, I had to take a few extra snaps of the preamp board from different angles for fear that should any of the wires break, in my efforts to replace the capacitors, the images should come to my rescue. Moreover, I need to know the polarity of the caps, though I had put (-) marks on the board. For the sake of those who might find it useful in India, I am reproducing all these snaps here:

After I was confident that I was ready for all ‘emergencies’, I removed the holding screws and tilted it for replacing all the electrolytic capacitors, which were having a ‘sick’ look! Replaced these one by one. This was an intricate job, as I had to apply only the bare minimum pressure for removing the caps. As you can see, the preamps were all using a couple of BC149B transistors for each input. Slowly the work progressed in snail’s pace! I breathed a sigh of relief, once it was over!

I could not do a thorough cleaning of this board, for reasons already explained.

This customer wanted to use the amp with a USB input, which was assembled by me like before. Also replaced the aux input socket with RC sockets on the side.

Tested the amp and found it to be working in superb condition, both in Aux inputs as well as mike inputs.

The new USB player had an additional facility of Bluetooth, because of which, I could play songs from my mobile too. I played a couple of songs sung by me and patted on my own back as admiration! (LOL).

Thus the curtain on reviving this amp was pulled down, adding a little more bulge to my satisfaction list!

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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You may check on his previous repair article below:





  1. albert van bemmelen

    November 27, 2018 at 10:23 pm

    Parasuraman the restauration Man! You deserve a break after another satisfactory but intensive repair done!

  2. Robert Calk Jr.

    November 27, 2018 at 10:31 pm

    Good job, Parasuraman. Man, that amp has sure seen better days! lol!

  3. doraiswamy SR

    November 28, 2018 at 1:33 pm

    Dear Sir
    keep up the good work in reviving old outdated equipment and contribution to the environment by not throwing away them and creating e--junk.No one repairs such old gadgets nowadays and dare to touch such cumbersome wirings... you did very great job. keep it up sir. I admire your patient and passion...

  4. Yogesh Panchal

    November 29, 2018 at 4:23 pm


  5. Sachin Joshi

    March 9, 2020 at 8:26 pm


    I have Unisound UB75 amplifier which I want to restore, looks similar to your UB50. Can you please help with circuit diagram. I am planning to restore this machine.



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