Fuse Blowing In MARANTZ PM-25 Amplifier Resolved
This amplifier and tuner belonged to my schoolmate. He brought this to me with the complaint that when they tried to use the system after a long gap, they heard a loud noise and on opening the amplifier, found its DC fuse blown. They tried by replacing the fuse, which blew again.
As usual, my first job was to open and clean the AMP thoroughly. After that, I straightaway checked the output transistors and found one PNP 2SB633 transistor dead short. When probed further, found one Zener diode cracked. Removed both and joined the Zener diode using Fevibond to read its value, which was 16V. Then when I probed further, I located another shorted transistor in the preamp section, number of which was C2274.
I replaced all these defective components and the fuse. As B633 was not available, I used MJE15031 in its place and changed the other working transistor also to make the output matching. On powering up (I have a series lamp connected to one plug and hence I had no fear of another fuse blowing) , I noticed that the B+ and B- voltages were not equal and ripples were present in the DC. So, removed the tank capacitors, 6800uF/35V and checked it up. This is what I found:
As you can see, it was not even half of its value. Having got enough reasons for involving in my amusement of replacing all the electrolytic capacitors, I went ahead with that job merrily! I also retouched all the solder points on all boards.
I had kept the three legs of both the output transistors up and disconnected, before applying the voltage. On switching on, the voltages were found to be stable. But DC was present in the Left channel speaker output, indicating that something was wrong in that section. I tried to fetch a schematic but could not find one in the web. Wrote an email to Marantz and got a reply that they could not part away with a schematic, as they had their authorized agent in Mumbai, India.
Though I wrote back to them stating that no agent would ever be ready to part with schematics and would insist on taking the set to them, which was not possible in this pandemic period and further that Mumbai was located thousands of KMs away and Marantz had helped me in the past by providing schematics of some other models etc., they declined and did not want to violate the agreement with the Indian agent. Well, I was groping in dark without a schematic, but managed to look through every component in that section. I changed all the transistors in that section. Also changed the fixed caps and a few resistors. But nothing could get rid of the DC presence in the output. I even made a chart in the computer and compared the voltages present in the other channel.
I subjected the removed transistors to thorough testing both in Peak Atlas and analogue Multimeter in X1 and X10 ranges as advised by Mr.Jestine Yong in his troubleshooting methods. But all of them tested to be good. So, you can imagine my predicament. No schematics. No defective components found. But none of the transistors were conducting due to some unknown reason. Since this use complementary symmetry circuit, using direct coupling methods instead of through capacitors, if anyone transistor does not conduct, none else will ever work! It is like laborers going on strike together! I had tried even feeding an input audio signal. It was present in the Right channel output, but absent at the output of the first transistor in the preamp of left channel. Since I removed and put back transistors in that section several times, three more transistors failed in further switch on testing. As a last resort, I replaced all the transistors once again. Let me remind that the output transistors, MJE15031 were not connected yet. I mentioned the number in the chart only to know the location. After this replacement, the DC was absent in the speaker output. Then I fed a signal at the input and got equal waveforms at the bases of the MJ15031. So, confidently soldered its legs and played an audio input and was happy to listen to the melodious songs!
Checked the voltages that go to Tuner from this Amp and found these to be normal and connected the Tuner and tuned FM/AM stations. Got good output with crystal clear music reaching the ears soothingly!
Work accomplished and satisfaction got collected! Here is the usual picture of defective components. (I have kept aside the components which tested ok for testing it elsewhere in future, because I wanted to find out ‘who’ was the real culprit or black sheep!) I advised the customer to check the speakers before connecting in order not to blow the amp again.
This article was prepared for you by Parasuraman Subramanian from India. He is 72 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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