Marantz Amplifier Repaired
This Marantz SM500DC Amplifier was brought to me with the complaint that there was no output from the Amp, though the dial lamps were lit. As usual, the first duty is cleaning up the inside of the Amp. After opening the set and finishing that, brought it to my workshop for further study of the problems.
The relay was not getting on. I checked for any DC voltages present in the speaker outputs. Found none. Then suspected the power supply and checked the Tank capacitors.
On checking the ESR of the capacitors, I found these to be way out. In one of them, it was just NIL. So, removed the Tank Capacitors in the Power Supply section and look at what I found:
Replaced the tank capacitors with 10,000 uF/63V, in place of 8200/50V as I was not having the same rating or was it available in the market. I also replaced all the electrolytic capacitors in this Amp, as the ESRs were either way out or in the brim. Applied power and noticed that the relay was not getting on, even though the power supply was now stable and ripple free and there were no faults because of which, the Protection IC, TA7317P should be in protection mode. Downloaded the service manual after paying for it, as free was not available, and studied the voltages that should be present in the IC pins:
The voltages at the pins 3 and 7 were absent. At pin 9, it was low. I removed the caps in the 3 & 7 and checked for any short or leak. Could not find and in my capacitance meter, it showed perfect reading. So replaced this IC and the relay clicked on when tried, after its prescribed delay time.
I checked the outputs through the headphones, injected an audio signal at each inputs. It was perfect and even the VU meters showed it properly balanced. Here, you can have a look at the inside of this lovely Amp:
Here is the toll:
Tested the Amp for several hours without any hitch, tried switching it on/off several times at several stages. It worked very well!
Another job satisfactorily jumped into its collection bag!
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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