Restoration And Modification OF Panasonic RX-CS730 Two In One
This set was brought to me by a boy doing his post-graduation in Marketing, with a request that it be restored to working condition and a USB/FM/Bluetooth module fixed in place of cassette player. Evidently this set has been lying in the attic and a sudden interest brought it out for the present generation to revive it and return to the halcyon days and nostalgic childhood, adding latest technology!
Well, I love to do such restoration work and I kept aside all other work and took it up immediately. As usual, I opened and dismantled the set to find tons of dust as expected. One speaker was found replaced by crude looking rectangular one in the past repairs on the set. Following picture of the other speaker, which I opened for cleaning would give you an idea of what I would have found in the rest of the inside:
After thorough cleaning of inside and wiping the inside well to the extent accessible by cloth and brushes, I brought the PCBs for retouching and replacement of all electrolytic capacitors. Found a lot of dry solders, with a few leads of components dislodged from the PCB. After finishing this work, I cleaned the PCBs thoroughly using IPA. Then located the tape head pre-amp IC and disconnected its power supply, inputs and outputs. For connecting the following USB/FM/Bluetooth module, I needed 5V supply, ground, FM antenna in and then wire the Left and Right channel outputs to the inputs of the AMP, i.e., at the outputs of tape preamp IC, which I had isolated. The IC number was BS3313L, pin details of which are given below (Please use cntrl+scroll to zoom out/in):
Pins 5 & 9 were inputs, while 1 and 12 were outputs. The VCC was given to pin 8. I scraped the tracks of all these pins in order to deactivate its function and also to arrest random pick-up from the tape head, as the leaf switch of play/FF & REW was to be used for 5V supply to the module. The mechanism had a connector giving in 12V for the motor; two wires for the leaf switch (in and out) with a ground wire. I used the same connection by isolating it from the motor. The 7805 regulator IC with a further 220/25uF smoothing capacitor at the output of the IC was fit on the mechanism, screwing it well to dissipate possible heat.
Then tapped the Antenna connection from the board through a 0.01 cap, L & R channel inputs and ground. Made a hole on the cassette door for a panel mount of the USB MP3 module. Inserted it and fixed it with two self-thread black screws. Inter connected the power supply and output wires.
Then inserted the connectors in the appropriate places. Rechecked and applied power, keeping the play button pressed. Was very glad to see the USB lighting up.
I noticed that the headphone jack was having something was obstructing and I could not take it out. So I removed it and dismantled it to find a broken pencil tip inside!
Then turned my attention to the FM Radio of the set (not that of the module.) As I am experienced enough to know that the 10.7M crystals would normally be the cause, and from the crystals what we get from the market, only 2 or 3 will be working out of 20, I fixed a three pin socket, cutting it away from a six pin IC socket.
Then checked the crystals one by one. Out of 29 pieces I checked, only 6 were working well, six were weak, and 17 nos got rejected! Even out of the six, only one was excellent!
After ensuring that FM was working very well, I replaced the two speakers and provided two tweeters in place of defective diaphragm type one that it had. Then played the set for a long time, before fixing it back and then rechecked it and found it to be working very well. Then I polished the cabinet and made it shine like new.
The output was so good that the boy was rejoiced to get back his favorite childhood day set in perfect working condition with new features added! He even went to the extent of making a short video of all stages that I had shared with him, with the title ‘nothing is impossible!’
Mission accomplished with a great satisfaction of a job well done, promptly adding it to the collection.
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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You may check on his previous article on Restored A CCTV Power Supply With A Perfect Output