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Rescue of a Sound Craft Mixer EPM8
This Mixer was brought to me in dead condition by my friend. This was being used by his friend in US, and went dead with a sound. Since getting it serviced is a very costly affair there, it was brought to India and given to my friend to find out whether it can be rescued.
Before dismantling, I noted down the colour of the knobs and its position in each row. Opening this unit was a laborious task, as I had to remove not only 38 screws from top, sides and bottom, but also remove 26 nuts and 90 knobs!
Since the power supply unit is at the bottom, I had to remove 12 screws of the PCB and other screws holding the SMPS (The set has totally around 60 screws). (Unfortunately I forgot to take snaps of the board before fitting it back). The SMPS was using UC3842 IC and an N Channel mosfet. Fuse was blown indicating shorted components. Found one 100V 5 W Zener diode (PKIN5378B) connected between the primary winding of the SMPS transformer shorted! A few other components were also charred.
So, I removed the IC and all the components on the primary side, marking the symbol on the PCB and noting down the values in a sheet of paper, with reference to its number printed on the PCB. Checked the SMPS transformer using Blue Ring Tester bought from Jestine Yong, and ensured that there was no problem in the secondary side. (Thanks to him again, for his wonderful book on SMPS repair). Connected a universal smps switching module to the primary and got rated voltages of 48V and dual 15V. So, the SMPS transformer as well as the secondary circuit were ok. ESR of caps were also ok. Replaced the IC, mosfet, diodes except the 100V Zener, as it was not available anywhere here.
Put back the other components that were found ok in thorough checking. Since 100V zener was not available, which was provided as an over voltage protector, I tried using four numbers 25V 1W zener in series. But that also got shorted. So decided to forgo this! Connected a bulb across the fuse and switched on, got the rated output voltages. So, SMPS was ok! Here are the components replaced on SMPS:
Turned my attention to the board and found one op amp IC 5532 charred, PCB and smd components burnt beyond recognition. Downloaded the service manual from net. Since this has two channels, left and right, I compared the values with the other. Removed the charred ics and components, thoroughly cleaned the PCB. Scraped the black marks. Then replaced the SMD IC along with the 7 SMD components. Since the positive and negative supply tracks going to pin 4 and 8 were burnt on this double sided PCB, I wired it directly from the other IC. See picture below:
Totally 8 components including the IC were replaced, 4 SMD resistors, 3 SMD Capacitors and SMD Op Amp IC 5532
Checked and found the set got back its life! Checked all the inputs feeding songs from a self made USB player, by connecting two RC jacks to stereo jack and used the left and right channel fader and found it to be in perfect working condition. Put back the PCB and all the screws, nuts and knobs.
A job accomplished well!
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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