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War with a Teac A500 Stereo Cassette Deck
I got this Teac A500 Stereo Cassette Deck with the complaint that tape does not run. As usual I opened the set and did a thorough cleaning and dismantled the mechanism assembly, as I saw a cut belt lying inside.
The capstan belt was found loose. So, I removed the motor assembly to remove it.
I tried my best to look through entire stock and to fetch this special belt, which was very broad. But unfortunately, I could not get an equivalent one. So, I tried to increase the tension by adding a roller next to the motor, fixing it on its screws. See the first picture, which shows the size of the belt and its path. The belt from the motor turns the capstan and on its return, turns the wheel that rotates the play wheel. As you can see, both sides of the belt are in contact.
Even though this should have served the purpose, it did not take the cassette load, as the belt was unable to turn the play wheel and motor stopped turning. Though I adjusted the position of the roller closer to the pulley of the motor, it did not work. I made several attempts, each of which required removing the motor assembly and fixing it back. Finally I gave up and removed the roller. Another idea, which had worked in one of the Decks I repaired many years back, came to fore of my mind. This is to cut a larger broad belt, and join it with superglue. We should take measurement of the inner size of the original belt and cut same length of belt from a larger belt. Then, we have to keep both ends together, and cut it straight with a sharp blade. Then keep both ends just touching exactly matching with each other, take a pin drop of superglue and apply it on the joint, which should not have any gap. Within seconds, the two ends will bond together! Wait for a few minutes allowing it to bond firmly. I did the same thing for this case also and fit the belt in the mechanism. (Unfortunately I could not take snaps of these stages because I had to use both of my hands, that too with quick actions!). The mechanism started working properly.
It is now that a twist took place in this interesting service experience! The tape will run for a few seconds or minutes and will go off in all modes; play, fast forward or rewind for no apparent reason! Sometimes the touch button command does not respond at all! All of a sudden, it will respond and work normally! Then again will go off! So, this intermittent on/off/seizers continued. First I thought it was due to some mechanical problems and checked everything including the leaf switches for cassette load, recording sense, counter, which has a hall sensor. I had replaced the counter belt and also the belt that was found lying cut outside, along with the capstan belt earlier.
Tape Deck Key Input Processor IC No. M54410P (16 pin through hole) was used in this. Though I could get the datasheet from the web, (http://pdf1.alldatasheet.com/datasheet-pdf/view/128949/MITSUBISHI/M54410P.html) , I could not study further in the absence of voltages that should be found on each pin for operation of this set. As there was no free service manual available, with the consent of the customer, I bought it from http://www.teac.owner-manuals.com/A500-service-manual-TEAC.html , paying US$ 4.99. Studied the service manual and circui diagram thoroughly. Noticed that the power supply for functioning of the tape deck is controlled by high-low situations on the input of the IC. I checked the voltages at the IC input (Pin 1 to 7) when the set worked. Also checked it when the set stopped. After several attempts, because of the intermittent nature of the defect, I tried isolating the circuit one by one that has a control on the voltage output from the power supply board. (Since the service manual belongs to the customer, I am unable to share the circuit diagram or its parts here. My apologies!). On repeated check-up, I more or less concluded that the output from the IC was not coming, even though the relevant requirements at the inputs are met. So, ordered for a new IC and replaced it.
But the situation was same, i.e., intermittent on/off! On one of these attempts, suddenly the set stopped and did not start on its own for a longer period. Then I checked the input voltages and noticed that at pin No. 2, which controls the stop function, the voltage should be high (4.9V) whereas it was zero. Then I checked and analyzed the various circuits such as (1) cassette loaded sensor (2) Counter rotation sensor (hall sensor) (3) Timer setting sensor (4) Power Mute circuit etc and did not find anything wrong in hot and cold tests. I even removed and tested some of the transistors. But in vain. By this time, several hours and days of sitting on this set, started getting into my nerves! Finally, yesterday night after hitting the bed, one idea stuck me and went back to the workshop and did some tests once again and mentally decided to attack a suspected area today early morning.
Got up at 3.30 AM and continued my check by removing the transistors in the suspected area one by one. Tests by Multimeter showed no defect. Then I used my Peak Atlas DCA and see what I found when I checked one transistor, 2SC945 (NPN). This is an ordinary NPN transistor without any diode or resistors built in. But Peak atlas was showing it as some N-Channel Mosfet! So, I checked the transistors in my stock, reading of which are on the second and third picture. Please note the difference in Base, Collector, Emitter positions in the same number of transistors!
As you can see, the C945 transistors showing a Y in the second line were having the center pin as Base, whereas the other two were showing Base at pin 1, on the side as Base. This is a very important aspect we have to check whenever we replace any transistor, as cautioned by Jestine Yong in one of his wonderful books on repair work. Replaced this transistor and hooray, the set started working perfectly well, after several hours of trial, with all functions of the tape! Took back the new IC and replaced its old IC and checked and found it was working fine. (Though the customer was ready for paying for the new IC, I refused and added it to my stock for any possible future use. I rather took this self-punishment for my inability to trouble shoot the complaint properly!) Fit the mechanism back and kept it running for several hours thereafter. Found potter wasp had blocked one of the Line Input sockets, so cleaned it up.
After completion of lubrication of rest of connectors and touching on the PCB on all suspected dry solder joints, fit the set back to its original state. Once again run it for several hours and checked all its functions, including recording. The set was giving superb performance! Here are the pictures of the bottom side of PCB, set in finished form, and the back plate (I have erased the serial number on the picture, to protect the customer’s interest, though shark eyed Sherlock Holmes can still identify the set, if at all they purchase it!)
Another job completed to my relief and satisfaction!
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company.
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