Odd Tasks Done In This Antique National 2-In-1 Model RX-C51F
This was brought by a close friend of mine with the complaint that it was lying unused for a very long time and he wanted me to check whether it can be brought back to working condition. I opened the set and cleaned it thoroughly.
Let us have a look at inside of the main unit:
After cleaning the board, replaced each and every E-caps as the values were found to be completely out of range. Then retouched the board thoroughly. Connected the board to PS and checked whether the radio was functioning. Except the medium and short waves, FM band of FM-Stereo & FM-Mono did not work. Even the hissing noise that we usually hear was absent. So, after checking all the components of the FM Front End IC AN7205, I replaced it upon which the hissing noise was present. But no stations were picked up. I observed an FM Pass Filter FF76108 in the input and suspected it to be defective:
I removed it and connected a fixed cap of the value of 100pf, upon which the FM stations came through. Replacement of this was not available and in fact, none of the component suppliers even knew about this! The purpose of this filter is to let in only frequencies from 76 to 108 Mhz. I have seen this failing in radio sets and that is why, I could get to the cause quickly. Then took up the tape mechanism for restoration. After replacing the belts and overhauling the mechanism, during which I observed that the auto-mechanism lever was broken, and cleaning the head & pinch roller, I played a tape and found it to be working very well. Next step was to provide a dial cord as it was missing in the set and even broken pieces were not available with the customer.
My hunt for a replacement fetched a few cords of different sizes, but these were found not suitable. Then I took a long cable tie and affixed a flat belt on its knurled side and inserted it into the tuning knob upon which it was found sliding to and fro smoothly. Took a small copper wire, bent it to insert it into the moving slot and fixed it at the end. Placed a holder at the far end so that the cord does not slip off from its path. Let us have a look at the connected pictures to follow these easily:
I inserted a white sleeve on the needle side for easy visibility through the dial window. The needle movement was smooth after forcing the movement left and right a few times to create groove for the bent end to travel on its slot. The bent end was also filed by me using needle file to make it blunt and smooth. The set was subjected to thorough testing in all its functions including Line in and was found to be working superbly well.
Mission accomplished with vigor and enthusiasm to take up the next work and the derived satisfaction dived into the bag to join others!
This article was prepared for you by Parasuraman Subramanian from India. He is 76 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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You may check on his previous article on The Story Of ‘ORGAN’ Transplantation To Save A Lovely Antique NATIONAL PANASONIC RQ-565D 2-IN-1
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Yogesh Panchal
May 29, 2026 at 3:59 pm
Good Job! sir,
Repairing antique is really exited job.how did you determine to put 100pf?
Parasuraman S
May 29, 2026 at 5:31 pm
I think I read it through somewhere on the website or someone suggested in my what sup group. But I do not remember, Yogesh Bhai! Many thanks!
Albert van Bemmelen
May 29, 2026 at 6:12 pm
Supercapman did it again lol!
Parasuraman S
May 29, 2026 at 9:04 pm
Ha! Ha! Many thanks, dear Albert! How I wish I could meet you personally once, spend some good time learning a lot and lot from you!
Mark J
May 30, 2026 at 2:53 am
Great repair and restore. Like I said before you inspire me to work on and restore these old sets. Thank you for sharing Parasuraman.
Parasuraman S
May 30, 2026 at 8:52 am
Many thanks, dear Mark!
Henrique Jorge G. Ulbrich
May 30, 2026 at 9:16 am
Very good job, Parasuraman. Thanks for sharing. You replaced an FM bandpass filter simply by a sole 100 pF capacitor. Of course, something of selectivity was missed, giving chance to interferences. This happened?
Parasuraman S
May 30, 2026 at 7:54 pm
Nothing, such filtering is actually done to avoid other frequencies entering into the radio signal input. That is all. Since it is a tuned circuit, we get only what we select. Many thanks for your comments!
Philip
May 31, 2026 at 4:48 pm
In the part of the world I come from, when there are no audible radio stations whatsoever, we say the radio is 'not catching stations'. This to me oftenly turns out to be a challenging repair encounter. And so not being really conversant with such like presented issues, I resort to hit or miss methods that on an unlucky day take me nowhere. That was a good hint you gave me, though mostly I have to face the difficulty of dealing with smd components in small radios in my guesswork to make them 'catch stations'.
Parasuraman S
May 31, 2026 at 10:20 pm
Magic works only very little in our field of work. We need to put in efforts to learn, learn and continue to update ourselves with latest advancement in technology and train ourselves to face. Many thanks for your wonderful comments and sharing experiences. It is valuable, indeed!
Philip
May 31, 2026 at 5:59 pm
Its hard to believe you have never come face to face with the man who for years promptly comments to your articles without failure. His commitment adds appeal to this forum beyond explanation. If you happen to come one on one with him pass my regards to him.
Parasuraman S
May 31, 2026 at 10:25 pm
We are almost 7000 KMs away from each other! He is in Netherlands and I am in India! Anyhow, let us hope we will meet each other if luck favors! Many thanks for your involved comments, dear Philip!