Multiple Problems Solved In This Marantz Integrated Amplifier Model PM-75
This set was brought to me by a regular customer reporting that it suddenly stopped working and no control in the front panel worked. After my usual practice of opening and cleaning inside thoroughly, I brought it to my service table, struggling a bit because of its heavy weight. I feared that my low back pain would return! Little did I realize that the set was so heavy! But luckily nothing happened!
The customer had provided me the circuit diagram and the trouble shooting therefore was easy to some extent. I checked for the voltages present and did not find anything wrong. My focus turned on to the Protection and Power On IC, TA7317.
In order for us to check the bottom side of the PCB, we need to take it out, as there was no detachable bottom cover to view it from behind. As mentioned by me in my previous articles on service of Marantz, I dislike its crude design and inter connections making it extremely difficult for service. I released the wire connections from its cable ties and somehow managed to keep the board inverted.
I noticed that the set was serviced before and some crude modifications done perhaps to match with the original design (pun intended!). The relay was not its original. A 24V relay was fixed with a power dropping transistor circuit to reduce the voltage from 60V to 24V. Anyhow, the person who did it might be a real expert as he knew what to do. I noticed that he has input the 60V to a transistor (TIP122) through a resistor (it was covered with a heat shrink sleeve) and used another resistor and a 24V Zener to enable the drop and handle the current flow when the relay starts its operation. Let us have a look at these:
I checked the voltages at the pins of TA7317 as given in the circuit diagram, which was slightly different. In the circuit diagram, pin three is not shown as grounded. But in the set pins 3 & 4 were grounded. Let us have a look at the circuit diagram:
I checked the voltages and found it to be different, indicating that there indeed was a problem somewhere in the amplifier side. I tracked the pin 1 & 2, which were getting the voltage feedback. I detected one leaky transistor; 2SA970 in the amplifier pre-stage. I replaced it.
Now the voltages at pin and 2 were normal (-0.65 & -0.66). But even then the relay did not get on. I checked the voltages at the speaker outputs to make doubly sure and found these to be quite normal. No DC voltages were present. Let me give you an idea of what the reading was at this stage:
As you will notice, the pin 8 showed zero voltage, whereas it should have been 1.26. Pin 8 gets a voltage through a 220K resistor from pin 9, which was the VCC. So, if the IC works fined with all other input sensing ok, there should have been a voltage of 1.26. I tried replacing the resistors and capacitors around the IC, which could cause problems. But nothing worked. The conclusion was either the IC had gone bad or some other components down the line were defective. I combed the entire board, sometimes removing one leg of it to make sure that there was nothing else that could trigger a fault. Ultimately, I decided to replace the IC, which I had two numbers in stock. I replaced it and found that the voltages at pins were odd! It never matched with the original IC and even the voltages at pin 1 & 2 were missing! I knew immediately that this was yet another case of fake ICs. Even though another one that I had was also of the same batch, I tried that also with the same results. Luckily, these fake ICs did not cause blow up of working components. I communicated this to the customer, who was good enough to get in touch with his friends and he located the original IC in Mumbai. The price was almost 20 times more than the market price for this, but he had no other go except to get that as the supplier was known to be very genuine. He got the parcel a few days later and brought it to me. As I was busy with other works in the meantime, I could take up the Amp for trying out the replacement IC only later. Luckily, this time, the relay got on and the Amp started working. I took the readings for keeping a record and was amazed at the differences from that of the circuit diagram of the company!
I just could not understand why it was so! When I tried the amp giving audio input and connecting speakers, I noticed that the output was very, very low. It had no power at all! Moreover, the weak output was present even if other inputs like Phono, Aux1, Aux2, Tuner or Tape were pressed. The bass, treble and balance control also did not work, though the master volume control was ok. As I had already checked the voltages in the set, I did not pay any attention to that and tried to trace the signal at the input level. The signals were very weak, upon which I checked the voltages at the input board and found that +17.5V was missing! Then returned my attention to the original supply point and noticed that there was an Op-Amp, 2068DD which perhaps serves as a current controller. (I expect our expert readers would throw some light on its use here) Let us have a look at the circuit:
I checked all the components in the positive side and found that Q801 was getting 24V at its collector, but instead of 17.5V in the emitter, only around 8V was present. Pin 8 of the Op-Amp also was showing zero. Then I checked R817, which was a 0.22 Ohms resistor in the set and found it to be open. I suspected that some load has blown this resistor and wanted to check that all were ok, before replacing the Op-Amp. I removed the Op-Amp and fixed an 8 pin socket in its place. Then removed Q801, Q803, Q805 & Q807 and checked these in my transistor checker and found all of them were good. Checked all the components around and did not find any defects. So, put the transistors back.
Then replaced the Op-Amp with 4558 (I did not want to put the 2068DD suspecting it to be defective) as both were identical, and before powering on, I removed the jumper wire of positive rail that goes to Pre-Amp stage and the connectors to Front Panel and Input Boards. When I checked the voltage, it was showing 17.5V and I put the connectors back one by one, watching for any voltage drop and finally connected the jumper wire also. The voltage was steady. This time, the Amp worked very well and output was very powerful. The selector switches, bass & treble as well as balance controls all worked perfectly well. I am giving below the pictures of the defective TA7317 and that of the Op-Amps.
I fed audio signal and checked the output wave forms in both the channel and found these to be even:
I tried the Amp for several hours before fixing the board back and putting the top cover. It was giving an awesome sound output and I enjoyed the songs, as a part of fringe benefits for a technician. Thus a marathon service work came to an end with positive results giving happiness both to the customer and the technician. These types of service work are very thrilling and I enjoyed every bit of the challenges thrown. Mission accomplished very well, rather a war was won, with intense satisfaction swallowed by the collection bag without waiting for any approval!
This article was prepared for you by Parasuraman Subramanian from India. He is 74 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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