Need electrical advice for my cassette-deck Akai GX-32

radio, tape, stands and accessories
H. callahan
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Joined: 25 Feb 2013 17:59

Re: Need electrical advice for my cassette-deck Akai GX-32

Post by H. callahan » 19 May 2019 03:34

Hello dear listeners, nice that you´ve tuned in again at home at your receiving sets, to again listen to the adventures of our stubborn, erm, i mean fearless hero who is fighting the evil and wicked mysteries of electronics.

As you might remember from the last episode, our hero has been able to make a partial victory against the powerfull evil, being equipped with a new and almighty weapon, a new multimeter for 12.99$. And because the meter was very multi he could wrest uncertainty from the electronic mystery, resulting in the root of evil possibly lying within the ominous and cursed FR701, also known as the "resistor to fusible them all".

So here i am again, having repainted my horse to white again and at least having covered the rusty spots on my armor, i once again went into battle to hopefully free my poor Akai GX-32 from the spell it is possesed by - this time with a replacement for the FR701.
As the original FR701 might have been burnt due to me mismeasuring the reel motor, resulting in the color code rings having changed color, i decided to go with the service manual which seems to state that the FR701 needs to have just 12ohms. Because as the FR701 anyway appears to be rather a fuse than a resistor, the value i had obtained by cracking the color-code, being about 92kohms, seemed way too high.
So i bought some 12ohms fusible resistors, and as i didn´t knew what wattage these needed to have i decided to go for 1Watts.

Actually the FR701 isn´t directly put onto the circuit borad, but they let him "some legs" which means you can attach some cable clamp to the legs of the resistor, without having to unsolder the resistor. So i put some cable clamps onto the probably dammaged FR701 and put an replacement fusible resistor on the other end of the cable clamps, to find out if there is any use in replacing the FR701. And guess what?
VICTORY!!
The reel motor did run again - in play mode, rewind and fast forward!!

So finally i had found the cause of the problem, the FR701 had been burned by me mismeasuring the reel motor.
Also this means that i mannaged to correctly replace the BA6109, which i wasn´t sure of as it has about 10 legs and just desoldering the old BA6109 was very frustrating - and you easily can rip the conducting path off the circuit board if you cannot dissolder a leg completely.
This already can happend when you just have to dissolder two legs, like with a resistor, but with an integrated circuit having like 10 legs its even more likely to happen. And when a conducting path is ripped off the circuit board you´re having big trouble.
So i was worried the last weeks maybe having even more wrecked my Akai while trying to replace the BA6109, but today i got relief.
The FR701 was defective and i managed to replace without issues and now everything works again, which also means that the BA6109 had been replaced without further damage.

Phew.

Also i have checked the service manual to see if there are several fusible resistors, also hoping to find another FR which might have the same value so i could make a measurement to find out what value the likely burnt FR701 did have - but i couldn´t find any. It seems like the FR701 is the only fusible resistor in this tape deck - and it seems like the designer of that circuit did know that, within about 30 years of time, a guy like me would mismeasure the reel motor - and to avoid damage to the electronics the did put a fusible resistor into the circuit.
So the designer must have been a very foresighted person, because he could see me comming - and my incompetence - for which i am very gratefull, as this foresight did save my tape deck from further damage.
...

Now i have ordered some diodes to, as Athermos recommended, swap the diode D706 for a diode only passing 3.3V to slow down the reel motor in play mode. I managed to get a set of diodes having stepped values like 3.9V, 3.6V, 3.3V and so on, so i have several values to try and see which value fits best to slow down the reel motor, without slowing down the reel motor too much.
Again not knowing what wattages these diodes need, service manual doesn´t say anything about this regretfully, i again decided to go for 1Watts. As far as i understand this only should result in the diodes not burning as fast as a 0.5Watts diode would.

I am aware though that it might be a problem if the FR701 is replaced with higher wattage than recommended, as this should result in longer fusing time, which again could result in damage of the electronics. But unfortunately the service manual does not state what wattage the FR701 should have, so i decided for 1Watts - and i did ran the tape deck for 45 minutes without any problems, so it seems to work.
...

So dear listeners we hope you were thrilled by the new adventures and the great victory over the evil mystery of electronics, as our hero finally has won the big battle against the spirits of evil - but there still is a last battle to come which will be the final question:
Will our hero be able to slow down the reel motor successfully without damaging anything else?

Stay tuned as the adventure hasn´t come to and end yet! This episode has been brought to you by "Hero-burgers, food for the stubborn, erm, the fearless!"

H. callahan
senior member
senior member
Posts: 857
Joined: 25 Feb 2013 17:59

Re: Need electrical advice for my cassette-deck Akai GX-32

Post by H. callahan » 27 May 2019 05:39

Ok, this time i won´t try to be funny or iconic, erm i mean ironic (ok, last one).

So the set of diodes i ordered has arrived, containing different values like 3.3V, 3.6V, 3.9V and so on.
I again opened the deck and decided to dissolder D707, because it looked to me like this diode is for play-mode. I then cut off the legs of the broken FR and soldered just the legs where D707 was sitting, so i could attach two cable clamps to the legs and try several diodes without having to solder and dissolder.
To check whether i had dissoldered the correct diode i ran the deck without having anything attached to the cable clamps - just to find out that D707 is for winding and not for play. When pushing the play-button the reel motor did turn as usual, but when pushing fast-forward or rewind the reel motor did run at full speed. This means though D707 was dissoldered and the circuit was open at this point, the reel motor still did run - and even faster than with D707 in place.
As my electrical knowledge is very limited i shrugged my shoulders, dissoldered the two legs and resoldered D707. Luckily it still did work and was not dammaged by soldering. Then i dissoldered D706 and soldered the two legs at its place. Again i tested the deck with the circuit being open at D706 and again the reel motor did run at full speed, this time when being in play mode.
So for what reason so ever the reel motor does get full power when D707 or D706 is dissoldered and the circuit is open at these spots.
Then i attached the cable clamps and tried a 3.6V diode, as a 3.9V diode would have been too close to the original D706 which is 4V.
I used a "test-cassete" to check for torque. This cassette i had found on the street about 15 years ago, when car radios still had a cassette-drive. The previous owner probably had thrown the cassette out his car window, because the tape was tangled up, several feet of tape where hanging out of the cassette. But nobody had run over it and the tape wasn´t badly crinkled, so i carefully cleaned it and spooled it back into the cassette. It only was half full recorded with music i don´t like, so i used it for short test recordings - and due to the tape once being tangled the reels now turn with a little more friction than on a flawless cassette.
So i took this cassette to test the worst case, when the reels of the cassette have more friction than average.
With the 3.6V diode the reel motor was able to wind up the tape flawless, at the beginning and at the end of the cassette. So i tested a 3.3V diode and again the tape was wound flawless. But with a 3V diode the reel did stop turning smoothly and sometimes got stuck, at the end of the cassette when the reel spindle had to turn all the tape.
So i decided to go with a 3.3V diode, dissoldered the two legs and soldered in the 3.3V diode.
Luckily again this worked out without any problems.

Now when i play a cassette the random slips seem to have dissapeared. Also there still are some short test recordings on the "test-cassette" which i had made before i had replaced the capstan-belt and pinch-roller on the Akai. These recordings had audible w+f and w+f still is audible, but i feel like w+f might have decreased a tiny bit.
I also made a new test-recording and when played back w+f is as good as the source (vinyl). So the plan, reducing the torque of the reel motor to gain better w+f, seems to have worked out.
Strangely though winding time still is the same as before. I assumed fast-forward and rewind to be slower now as it seemed to me that the voltage of D706 and D707 is added when winding - and as D706 now provides fewer voltage i assumed winding to be slower now, but it isn´t. The tape still is wound as fast as before which is puzzling - on the other hand this spares me further tinkering. Because i allready had been thinking about changing the D707 for a higher value, like 7.6V for example, to compensate for D706 providing fewer voltage, to regain same speed than before when winding.
But obviously this isn´t necessary as winding still is as fast as before.

So now everything is as desired and the "final battle" is over.

Many thanks to everyone who tried to help me, especially Athermos who pointed me to changing D706 to reduce torque of the reel motor, it does work!