Revisit Of MARANTZ PM75 Amplifier With Different Problems
I request you all to see this link, in which I had reported about the first visit of this complicated, crudely designed amplifier with a hell of a lot of interconnecting wires.
https://jestineyong.com/multiple-problems-solved-in-this-marantz-integrated-amplifier-model-pm-75/
I am including only the images relevant to the problem handled. Though I had told the customer that without dismantling the entire set, cleaning all boards, replacing all electrolytic capacitors and retouching every point in the boards, this set would not work without any trouble for a prolonged time, budget as well as time from my side and my aversion in handling such crude sets prevented to carry out such mass repair works. Result was that it returned with the complaint that only power on indicator was working and there was no other response to keys pressed. The first visit was in July, 2024 and the second visit was in December, 2024. I did a primary check of all the voltages and found these to be present. Next step was to check whether the µController was getting the voltages. I took print out of all the relevant pages required for trouble shooting. In order for me to access the µController which was fixed under the front panel, I had to, not only detach the front portion, but also disconnect dozens of wires, most of which were wire-wound connections!
Without detaching the wires, the control board could not be removed from the panel as these were fixed with screws from behind. To get the panel, we also need to remove other boards such as headphone socket, power indicator, speaker selector switch, bass/tone control board, record selector switch and volume control board, all of which were inter connected with short wires. How these people assembled the set itself was a mystery! I really struggled to take it out without causing any man made damages and if I had cursed the designers, I cannot be blamed for that! Let us look at some step-by-step images:
After removing the boards, I was shocked to see the crude modifications done by a previous technician who had worked on it, as reported by me last time and I promptly shared the images with the customer. Two ICs were missing in the volume control board and a relay was fixed at the bottom, function of which is known only to the guy who did it. The lever of the power on/off switch had broken and luckily it was found inside the front panel. The rest of the lever came off while I was taking the panel. I left that to deal with it later. Let us have a look at these modifications:
The volume control board itself had 9 connectors, which I removed after giving different markings for identification and also took snaps to avoid making mistakes. Then covered all the adjacent boards in bubble sheets and put rubber bands so as to prevent its accidental touch anywhere else. Then applied power to check the µController based on the circuit diagram.
Noticed that the power on voltage to the IC was missing. Found two resistors near that rusted. Replaced the 1k and 330Ohms resistors, upon which the power came and the µController started working. The default LEDs were also found lit as mentioned in the service manual:
I decided to do all preventive maintenance works on all the boards as access to these boards was made very complicated and difficult by the designor. I separated the µController board, replaced all electrolytic capacitors, pressto-on micro switches and did dry solder patch up.
Did dry solder patch up on all other boards as well. Then took up the case of on/off switch lever fixing. Fist I fixed the broken leg and inserted component cut leads using soldering iron for ensuring proper hold in place. Then the two legs which go into the grooves under the front plate of the panel were steady and rugged. Then removed the front plate, removed button switch holding assembly one by one. Finally located the place where this lever had to be inserted and pushed it in into its groove. It was found held in place properly. Now Let us have look at the connected pictures, which might not be of much help as we need all these in front of us to do anything. But overall we might get an idea and that is all what we can expect:
Then fixed all boards back, tied the wires in a few places to hold together, and made it look as much better as possible.
Then played the set for a few hours before fixing the top cover and moving it out for delivery to the customer. Mission accomplished with aplomb which resulted in satisfaction getting added to the prestigious collections!
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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Albert van Bemmelen
September 7, 2025 at 5:59 pm
Although Marantz is a well known manufacturer this example clearly showed why your aversion in handling such crude sets prevented you from carrying out such mass repair works at first.
Whoever designed this amplifier probably wasn't aware of the bad lifespan of capacitors and why the need for a friendly easy serviceable set is mandatory.
And that it was designed in a time when they still used the wire-wound connection method.
In december 1980 Marantz came in the hands of Philips. Only Marantz America and Canada stayed at Superscope. This Amplifier was made around 1989-1995. Maybe this design was made after Philips had taken Marantz over?
https://www.radiomuseum.org/r/marantz_digital_integrated_amplifier_pm_75.html
Parasuraman S
September 7, 2025 at 8:12 pm
Thanks for your valuable inputs and a dig out history of Marantz!
Waleed Rishmawi
September 7, 2025 at 9:39 pm
Good job on the repair and that is an ugly modification that the previous technician did on this device, two different c’s missing? What a mess.
Parasuraman S
September 7, 2025 at 11:04 pm
Yes, we need to put up with such tamperings! What to do!
Mark J
September 8, 2025 at 12:16 am
Parasuraman great job once again. Thank you for sharing.
Parasuraman S
September 8, 2025 at 12:47 pm
Many thanks, dear Mark!
Yogesh Panchal
September 10, 2025 at 4:32 pm
Good Job! Sir,
Every technician has certain limitations in terms of knowledge, availability of materials and workshop set up and these limitations are reflected in the quality of work done by him.Now technicians are not wasting their time on the equipment where they don't get enough remuneration.
anyway you handled the case very professional way.
Parasuraman S
September 10, 2025 at 10:44 pm
Very true, Yogesh Bhai! Your analysis is correct! Many thanks!