Restoration Of A Badly Tampered Technics Cassette Deck Receiver Amp Model SA380S
An unusual friendship developed with a courier delivery person resulted in getting this antique piece on my table for service. He noticed that I was getting a lot of parcels containing electronic components and was a bit curious to know it though did not mention it openly. He was a good guy with pleasing habits and I had no hesitation in exchanging niceties and discussing about weather etc. which are preludes before one understands the other and develops a friendship. One day, I called him in and showed him my service setup and explained what I was doing with all these components. He was very amused and excited as he himself was an electronic technician once! Birds of the same feather flock together; the old adage goes and it became true in this case! Something common within us attracted and bonded us like a thick long-lost-but-now-found friend! He told me during one of his visits that he had this deck which he bought from a faraway place at a fancy price knowingly well what was the condition, because of his craze for a Technics Set. Though he did a bit of work on the board like removing, cleaning and putting back the speaker connectors etc. he could not fix it properly mainly due to lack of time and essential tools.
After opening, dismantling and cleaning the set, I kept it aside as the boards looked very badly tampered and I was in no mood to set my hands on it. It was only after a few months, the bonded friendship and his repeated gentle persistence forced me to take it up. You can have a look at the insides and decide how much of work I had to do!
Look at the condition of the board:
After cleaning the board inside and outside to the extent possible, I retouched the Amplifier Board, which had the STK IC in it and connected it for checking up, after lubricating the connectors. It was dead and the voltages dropped indicating a short in the AMP board. Then I removed it once again and checked up the voltages which were + & – 43V. I was shocked! The STK IC 419 can take a maximum of 35V (+ & -) and generally found to operate from 25 to 30 V. So that’s why I found the burning marks in the connector and burnt tracks on the board. So, I informed the customer and told him that none of the STK series ICs compatible to this connection can ever take such a high voltage.
There was only one way, discard this amp board and connect a new one which will work on +/- 43V. As we had a common friend, who was manufacturing Amplifier Boards, he developed one using transistors, which I asked him to fix it on the same Heat Sink so that fitting will be easy. In the meantime, I dismantled the front portion and took out the PCB, then did my everlasting thrilling work of replacing all the electrolytic capacitors. After that I did a 100% retouch of the board from one end to the other. Look at an IC that I found burst, which was mute handling IC for the tape recorder portion, replacement of which was not available. Also given is the base of the capacitors to know how bad the board was!
Then I took the PCB to the bathroom, thoroughly cleaned with detergent and dish washing liquids and pressure washed it. Then used my Blower and blew the water out thoroughly from all the connectors, coils and other difficult to reach areas. Then dried it in hot sun for a full day, by turning it up after a few hours of exposure so that it is fully dried. Just see how the board looks after all these exercises: (Fixed two 10,000uF/100V Caps as replacement of 6800uF/56V were not available)
The fresh look cleared my dithering mind and it got fully aroused to set right the Amp to the best possible levels! I also removed crude looking jumper wires used by the previous technicians and did patch work, for which purpose I had to remove the selector switch, study the circuit, as a service manual or schematic was not available. After completing this work, and re-fixing the selector switch, I wired the following Amplifier Board, providing appropriate interconnectors:
When I applied audio input in AUX and checked, there was no output. Checked the signal and found that it was coming to one Dual Op Amp IC No.TA75559S, but there was no output. I checked injecting a signal to its output and it was reaching the Amp out very well. So, removed the same and used same IC of Phono-input and fixed it here. This is an IC, which can be connected without bothering about the order of pin 1 to 9. Since the pin one was left blank and 9 was used in the AUX-in-IC, I had to connect a wire from pin 9 to 1 as pin 9 of Phono-in-IC was rusted and cut. Once I did that, the audio reached the output. Now let us have look at the in-build of this IC from the following diagram, and the jumper wire that I connected to the replaced IC: (Replacement of the IC TA75559S was not available)
The last picture above would also show you how neat and clean the board became after the unusual cleaning work done. (I had to do lubrication of all connectors and controls once again after the cleaning.)
As customer did not want the tape to be working, I just rolled the connecting wires and fixed the mechanism in its place. Then fixed the Tuner board on top after cleaning the board to the extent possible using IPA. Then when I tuned to stations in FM and AM, I was amazed to see the pick up, clarity and output! Then I mentally appreciated my friend for buying the set and changed my accusation attitude that set in at the beginning.
I tested the set in AUX and Tuner modes for several hours, before giving delivery to my friend, who was very happy to take it. He brought his son too at the time of taking delivery. Well, my joy was equivalent to an Olympian who accomplishes a very tough feat! Satisfaction just entered into the collection bag and bulged in pride befittingly! (LOL)
One question remains unanswered and perhaps some reader might clear it: Why STK was found in this Amp, which had very high voltages? Did someone change it? If so, what was the original IC used in the set? We had already probed the possibility of increased AC output due to some tampering and did not find any. The AC selector switch was in 230V position only. The TX was not heating up abnormally due to any possible internal short. Let us hope to get an answer!
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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10 Comments
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Yogesh Panchal
April 13, 2024 at 3:56 pm
Good Job! Sir
Parasuraman S
April 14, 2024 at 11:06 am
Many thanks!
Waleed Rishmawi
April 13, 2024 at 4:56 pm
Very interesting story and great repair. Have a blessed day
Parasuraman S
April 14, 2024 at 11:06 am
Many thanks!
Mervin Binda
April 13, 2024 at 6:23 pm
nice piece of work keep it up
Parasuraman S
April 14, 2024 at 11:07 am
Many thanks!
Albert van Bemmelen
April 13, 2024 at 7:05 pm
Great repair and nice story with unusual STK voltage findings! With indeed a lot of invested cleaning and replacing all capacitors time!
It is incredible to know that not only transformers are fixed in your country by rewiring the broken or shorted coils but even these amplifier modules are made by engineers who even take the time to replace original bad ones.
Of which here the amplifier STK IC 419 module was replaced by a common friend who developed one using transistors to easily fix it on the same Heat Sink so replacing was easy.
Parasuraman S
April 14, 2024 at 11:08 am
Many thanks for your expert analytical comments!
Tayo
April 13, 2024 at 11:45 pm
Another satisfying and informative article Sir.
It's a shame the burst IC could not be replaced. Would have loved to see this thing back to 100% operational status.
Parasuraman S
April 14, 2024 at 11:08 am
Yes, true! But there are excepts which we should learn to accept! Many thanks!