Return Of Panasonic RX-M70M2 Two-In-One With Mains Transformer Failure
This set belonged to one of my regular customers and he brought it with the complaint that his wife saw smoke coming out from the rear and the set became dead. This had visited me around one and half years back in dead condition. That time, I had replaced all the electrolytic capacitors in the board and did thorough dry solder patch up.
Also fixed the FM/USB module on the Cassette Door, as they did not want to use the tape. The reason for failure was dry solder and component failures at that time.
Before opening the set itself, I checked the primary of the Mains Transformer, after checking and ensuring continuity of the power cord and found it to be open. So, I knew very well what needed to be done, before opening the set itself.
After opening and doing a general cleaning, I rechecked the TX from inside and confirmed it to be dead. I had noted down the secondary of the TX during my last repair and it was 0-7.5V, 1A. I had a replacement TX with me and I removed its clamp and fit the original clamp, without which it cannot be fixed inside. Then powered it up keeping the TX outside itself and found all functions working fine. So, fixed the transformer and wired these like before.
Just kept the front cover, which had the speakers fit without fixing the screws and tried Blue Tooth, USB and its own Radio. Everything worked just fine!
In the following picture, look at the battery compartment, where you will find packets of components replaced last time and the parts of tape that I removed to accommodate the USB/FM module.
Here is the picture of the defective TX, by concluding this article and declaring about satisfaction that got added!
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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Albert van Bemmelen
June 18, 2021 at 1:02 pm
Is it maybe possible that the previously added modifications that obviously use more current maybe overheated the original transformer that apparently was unable to handle the extra power needed at the given max 7.5V 1A? Why they also use to say to buy a bigger audio amplifier with a larger transformer even if we only use normally just the half audio output power at max to be sure it never breaks down for those reasons.
Parasuraman S
June 18, 2021 at 10:45 pm
No, dear! Not at all! The load is taken by the 7805 IC and the module draws only very negligible current. These things happen due to very bad power fluctuations! The other day, suddenly the 230V AC shot upto 300V. Many home appliances in many houses got damaged. It was because of wind and rain that uprooted big trees. Though the surge was momentary, before the power got shut down, that was enough to cause damages. Secondly, bigger transformer will never fit in that place! Many thanks for your observation and comments!
Waleed Rishmawi
June 18, 2021 at 5:23 pm
wow, that is a well burnt dark transformer... what caused that? was the original transformer has the same specs of the modified one? any way, good job on the repair and have a blessed day
Parasuraman S
June 18, 2021 at 11:32 pm
Hi, dear! Kindly read my response to Albert van Bemmelen above. Highvoltage surges knocks off these transformers. Anyhow no fireworks like in your welding machines! (LOL) Many thanks for your comments!