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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https://jestineyong.com/grundig-tv-repair-tips-china-kit-and-rejuvenation/
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Albert van Bemmelen
August 29, 2016 at 2:35 pm
Again Kudos to you dear Parasuraman Subramanian!
When you wrote "Took back the IC" I first thought you had brought it back to the Seller. But you just meant out of the Board to replace the old one back.
Good to know that gluing Belts is possible. I never tried it but maybe the new Nail Glue I already used successfully on broken DVD Boxes called 'Fingers' is excellent in these cases too? I wonder why you were reluctant to show any part of the circuit or the Device Serial number in your repair?
Parasuraman S
August 29, 2016 at 7:10 pm
(1)I have not used nail glue and do not know about its availability here. In other surfaces, when you want things to be bonded permanently, we can apply Fevibond first on both sides, wait for it to dry and then apply Superglue and stick it together. It holds better. But that is not possible on belts (2)The service manual, which contains the circuit diagram is owned by the customer, as he has paid for it. I have no right to reproduce it here. (3) Erasing serial number: To protect the privacy of the customer.
Albert van Bemmelen
August 30, 2016 at 12:08 am
Thanks for your fast reply Parasuraman S. I still do wonder how the serial number is going to harm the privacy of the customer. Since it is an old device and he (or she) is the legitimate owner, why bother. And sorry for asking but you also wrote in the article: "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!". And this got me confused since you now also wrote : (2) That is not possible on belts. So if only Fevibond is ideal for sticking leather, rubber, rexine, canvas, cork and really does work on belts, and not just superglue, I must order some Fevibond on-line, for these special repair cases. Thanks for the much appreciated info!
Albert van Bemmelen
August 30, 2016 at 12:21 am
I re-read your Post and take it that only superglue really does work on belts. So there is no need at all for me to buy Fevibond. I did once the same as what you did with fevibond. I used first Bison tix to kind of plasticise the material to be glued together and mixed it with superglue afterwards. It indeed glues thing much better that way.
Parasuraman S
August 30, 2016 at 2:26 pm
(1) Serial number will identify the set and in the event of the next buyer looking at this article, by chance, this will cause drop in the value, reasons for which are obvious! (2) What I said is combination of Fevibond and Superglue cannot be applied on belt, because of its narrow surface. I have suggested combination of both only on surfaces such as plastic or plastic+metal broken joints, like a cassette door, front panel etc. The tips of the belts will melt and join together tightly when you apply Superglue. Fevibond cannot hold belts, as the technology is different. It sticks itself to the surface, and sticks both sides through the glue. But Superflue melts the surface and bonds it, aparts from its own glue. That is the difference. Fevibond does not melt the surface. It can be easily removed also! Hope I am clearer now!
Albert van Bemmelen
August 31, 2016 at 12:52 am
Yes thank you dear Parasuraman S. Although I am not familiar with Favibond nor with the Superflue glue you mentioned I will give it a go once I am able to obtain it. And try it according the way you say to best use it. Thanks for the Info!
Albert van Bemmelen
August 31, 2016 at 1:07 am
PS: I once build an ELV (German Kit Manufacturer) GLP 7000 Gleich-Prüf-Gerät (a Tapedeck Synchron Test Meter) which I am able to check and adjust the Tape Speed, Wow and Flutter and also Drift of any Cassette Machine. It has two Test tone settings : CCIR 3000 Hz, and a second being DIN which is a tone of 3150 Hz. Plus a Calibration, Drift and Wow & Flutter Setting. Cassette Decks sometimes may need to be recalibrated on Speed and tested on Drift and Wow & Flutter. And Most Capstan Motors have an integrated electronic speed adjusting controller that can be tuned with a small screwdriver for these circumstances. I may also have a schematic of this Device. And maybe you are able to download it from the internet? Cheers.
Yogesh Panchal
August 29, 2016 at 2:56 pm
Good Job! sir,
At last you caught the culprit.
S Ramachandran
August 29, 2016 at 3:41 pm
Mr parasuram thanks for sharing this . I have done this type of repairs by adjusting the motor fixing screws by drilling holes in the chassis and also using the self wound cotton belts by using nylon thread as per spècification.. thanks a ton for your effort.
suranga bandara, Suranga Electronics
August 29, 2016 at 6:06 pm
Hi, Mr- Audio Expert parasuram,
Very Good and Clever Repair.
Humberto
August 30, 2016 at 12:24 am
Good repair Parasuraman, you have saved another device from the dump.
Debirt Maynard
August 30, 2016 at 7:08 am
Great effort. Love your approach.
beh
August 30, 2016 at 11:01 am
actually these kind of machine are not in use in my country
since cassette recorders contain many problematic mechanical parts
manufacturer are not producing any more . Parasuraman
thanks for article you remind me my young hood when i repair such machines .
Andre Gopee
August 30, 2016 at 11:27 pm
Great Repair article. I myself has done repair in such manner and works great. Thanks for sharing.
Benjamin E.
August 31, 2016 at 8:21 pm
I hate when manufacturers do that. It's quite annoying - if you're going to make the same part as other manufacturers, make it the same!
And heheh, I've heard of rare instances where transistors fail without shorting or going open circuit. Usually there is a reduction in the gain (hfe), however sometimes, they can simply become noisy. This is an issue that pops up now and then in audio amplifiers, since the noise manifests itself in the speaker/listener's ear ... 😛
Robert Calk
September 2, 2016 at 6:25 pm
Nice work, Parasuraman. Thanks for sharing.
George
September 9, 2016 at 2:48 am
But how this is possible that an ordinary transistor to become a mosfet ? I don't get it.Can someone explain this ?
Ezekwem Lawrence
September 17, 2016 at 2:51 pm
i learned a lot from this article, please i have a problem with a DVD player which when turned on it comes on but after some initial display on it's LCD screen it then displays "protect" and goes into standby power mode and doesn't work. please how do i fix this, i cant see any damaged part in its circuit board but can i say it is a bad lens Sir? please i am in urgent need of a reply. thank you
Parasuraman S
November 3, 2016 at 2:32 pm
Sorry for the delay. I saw the comments only today. If you know the model and make, look for a free service manual for download and take help from there. Check whether any child lock or any other lock system has been activated from the remote. The protect mode normally comes on whenever there are power supply problems, one or more of the dependent functions such as loading motor, sliding motor and spindle motor does not turn, or the sensor switches are faulty. It can come even when the lens is dirty. You may have to do a thorough check up.