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

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H. callahan
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Need electrical advice for my cassette-deck Akai GX-32

Post by H. callahan » 23 Mar 2019 17:40

Well, this is a longer story and i´ll try to get it as short as possible, but you may need to know the complete story to help:

I have an AKAI GX-32, which should be a mid-level cassette-deck from the late 80s (twin-field-head super GX, w+f 0.09% DIN, SNR with metal tape -59DB@ 3%THD level, Dolby B and C, THD with metal tape less than 0.9%, frequ. resp. with metal tape 25-20000Hz +-3DB). I bought it used and it had some problems like audible w+f. I cleaned the heads, capstan and pinch, i oiled the capstan with special synthetic oil meant for capstans especially, but that didn´t help. So i bought a new belt and pinch roller and installed both. Then w+f was decreased a lot, before when i did record rather fast pop-music and played back w+f was clearly audible, now i can record slow piano-music without having audible w+f - unless sometimes.
And it´s only for a short moment, like 1/4 a second. To find out whether the audible w+f was recorded onto the tape or was originating during playback, i listened to the recording with my finger on the stop-button and hit it as soon as i could hear w+f. Rewound the tape for about a second and played again - and there was no w+f audible at the exact same position.
I also took out the cassette after hitting the stop-button and looked at the tape, slowly rewinding it with my finger to see whether the tape had dammage which might lead to w+f, but there wasn´t any damage visible.
So i assumed the audible w+f to be created during playback, as it does appear random and if i rewind the tape and listen again it is gone, respectively at another position.
So i guessed the capstan motor to need an overhaul, looked at some videos on youtube to see whether there´s something to watch out for (and there is) and managed to open the capstan motor without destroying it. I cleaned the sintered bearings and the contact springs, re-oiled the sintered bearings with special, synthetic oil and reassembled the motor - and it still works \:D/ .

But the occasional and random w+f still was there. So i did the same overhaul with the reel-motor, hoping this to help but it didn´t.
Also when overhauling the reel-motor i assumed there to be some sort of friction clutch to compensate for different diameter of the tape-reel, because at the beginning of a cassette the wind-up-reel has no tape on it so its diameter is smallest and therefore it has to turn faster to wind up the tape than when the cassette is comming to its end - but there isn´t any friction clutch built into the gears driving the reel-spindle. The power of the reel-motor is transmitted without any loss onto the reel-spindle and i found out that the spindle not only turns faster than it needed to wind up the tape at the beginning of a cassette, no, it also does turn with a lot of torque.
You can run the GX-32 without a cassette in and with the cassette-holder open and then touch the reel-spindle. It takes quite some effort to slow it down and some force to stop it from turning - and because of that i now think i know why there sometimes is random w+f:
The tape is wound that strong that it will slip through the capstan and pinch which are meant to keep it from slipping; actually it seems to me that the capstan and pinch don´t have to actually transport the tape, it seems to me capstan and pinch rather have to keep the tape from moving too fast!

So my theory now is that the random w+f is produced by the reel-spindle pulling too hard, making the tape slip sometimes.
Maybe it was not a good idea to overhaul the reel-motor and oil it, but otherwise now the motor should be in its original condition, so probably it is due to construction design that the motor is pulling a little too strong. But i also get why they constructed the deck like that, they wanted to make sure that the tape always is wound onto the reel, also when the reels of the cassette are a little stuck and have friction, so they made the reel-motor turn a little faster and stronger than needed to wind up the tape.
But therefore i also assume that the motor could be slowed down a little without preventing it to wind up tape sufficient with a flawless cassette and by that w+f should become better, respectively the randwom slips should decrease - and that´s what i want.

Now my electrical knowledge is very limited, i know how to solder, i know what resistors, capacitors, transistors and potentiometers look like, but that´s basically it. My plan is to solder a resistor onto the reel-motor, right where the cable is soldered onto the motor, to slow it down like 20% let´s say. But i don´t know what kind of resistor is needed for that, if a 10ohms resistor would be enough or whether i need like a 100ohms resistor - also i don´t know if i should take a 0.25W 10ohms resistor or a 0.6W 10ohms resistor.
So i had a look at the service manual, but that was kind of confusing, because it looks like the reel-motor does get like 5V when in play-mode and additional 7V for fast-forward, or it does get 7V in play-mode and additional 5V for fast-forwad - and how much ampere it does get isn´t mentioned.

So i decided to take a multimeter and measure the reel-motor for volts and amperes when in play-mode. I measured about 2.8V in play-mode, which again was confusing as due to the service-manual it should be 5V or 7V, but i then also measured a 9V-battery with the same settings on the multimeter to make sure i hadn´t mis-set it - and it read 8.8V so i assume the multimeter was set correctly.
Then i set it for measuring ampere and as soon as i touched the motor, the motor stopped turning completely. Now the problem is that there is an IR-sensor which does detect whether the motor is turning or not, because when the cassette is at its end all the motors, capstan- and reel-motor, still do turn and try to transport the tape. If there wasn´t any sensor, the capstan- and reel-motor probably would rip the tape, so as soon as the reel-motor no longer does turn for about a second the IR-sensor will stop the deck from operating, assuming that the cassette is at its end.
Now when i tried to measure the motor for amperes, it stopped turning completely as soon as i touched it with the contact-pins of the multimeter, making the IR-sensor assuming the cassette being at its end and shutting down operation after about a second.
So first time i tried to measure for amperes i could not read the display of the multimeter, because the reading only was shown for about a second, then the tape-deck shut down operation and the reel-motor no longer was supplied with electricity.
Second time i tried to measure i think i saw something like 500mA on the display, but only seeing it for a very short time i tried to measure for the third time and this time i think i saw something like 560mA. To confirm i tried to measure for the fourth time, again restarting the deck into play-mode, but this time the reel-motor did n´t turn at all - without the contact-pins of the multimeter attached to the reel-motor.

In fact the reel-motor now doesn´t turn at all at any setting, whether its play-mode, fast-forward or rewind, it doesn´t run any more, the deck itself still does react when buttons are pushed.
So i measured the reel-motor for resistance, as this should indicate if the coils are fryed as far as i know and when measuring the reel-motor for resitance it says 0.0ohms. So the motor still should be ok, but something else isn´t.
Again having a look at the service manual with my very limited knowledge, it seems like there is a transistor (DTC114YS) which does operate the reel-motor, switching between play and fast-forward - i always assumed transistors only to amplify, but apparently they can be used as some sort of electrical relay, basically a switch without any mechanical parts. Now there also is something i think is an IC (integrated circuit) and this still seems to work, because when i push the play-button for example, the heads still are lifted up towards the cassette, the amplifying circuit still is switched on, i can hear the white noise and the play-symbol still is shown on the display of the cassette-deck. Same with every other operation (except for record), so it seems like the "brain" of the deck, the IC, still does work, but the transistor being placed between the IC and the reel-motor no longer seems to switch on the reel-motor.

And here, finally, there are my questions:

1. Is it possible to fry a transistor, which does operate the reel-motor, by measuring the reel-motor for amperes during operation?

2. If so, how the heck does one measure an electric motor for amperes without frying its operating electronics?

3. Is it likely that i have fryed the transistor or do i have to look somewhere else?

Because i have found that there is no setting on my multimeter for measuring transistors. I have found a source for DTC114YS transistors, but including shipment it would be about 8$ for just one and it seems like i basically had to disassemble the cassette-deck completely to reach the undersite of the circuit board to dissolder and exchange the transistor.

Oh, and:

4. Is my plan, slowing down the reel-motor by adding a resistor, feasible at all?

Thank you for reading.

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

Post by H. callahan » 23 Mar 2019 18:05

I forgot to mention that the reel-motor doesn´t have any additional electronics at its back or in its housing. The capstan-motor does have a little circuit board inside, but the reel-motor doesn´t.

I am also aware that slowing down the reel-motor by a resistor also will slow down fast-forward and rewind, but that wouldn´t be a problem for me.

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

Post by H. callahan » 24 Mar 2019 03:45

In case it helps, here are some parts of the service manual:
Block diagram.JPG
(92.95 KiB) Downloaded 113 times
schematic diagram.JPG
(117.71 KiB) Downloaded 111 times
Looking at it now i see that the "line of command" is the "brain" (LM6413E-542), then the transistor (TR711 respectively DTC114YS), then another IC (BA6109) and then the reel-motor. Maybe i have fryed the BA6109 by trying to measure the reel-motor for amperes and not the transistor (DTC114YS), as the BA6109 is directly behind the reel-motor and not the DTC114YS.
Could be said what of these i most likely have fryed?

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

Post by Athermos » 26 Mar 2019 14:47

How did you attempt to measure the current drawn by the motor ? You were supposed to put the multimeter terminals in series with the motor (and not across the motor terminals). This means basically de-soldering a wire from the motor, did you do that ? Putting the multimeter in the current sensing mode and then across the motor terminals would place a high load on the motor driver and possibly destroy it if not for some short circuit arrangement. Again 560 mA seems to be the short circuit current rather than the current drawn by the motor (continuously) which should be far less. And yes, a resistor in series with the motor would not only decrease the speed but affect the torque offered by the motor and hence is not an option.

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

Post by H. callahan » 27 Mar 2019 08:25

Thank you for your answer, i measured here:
schematic diagram crop.JPG
(25.85 KiB) Downloaded 99 times
without desoldering anything.
I have made a little research about ICs and found out that these basically are an assmeblage of transistors. As transistors also do age i now assume that i have fryed the IC (BA6109).

Basically the reel-motor does have too much torque (and turns faster than needed). Could you please explain why a resistor is not an optinon?

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

Post by Athermos » 27 Mar 2019 15:26

You have basically connected the multimeter across the motor attempting to read the current, as the multimeter was in the current range it has overloaded the driving circuit and had fried it. The multimeter should have been in series with the motor. The BA6109 was not damaged by aging but by overloading.

To control the torque of the motor basically there is a control circuit comprising of 2 Zener diodes with one of them being switched on by a CMOS switch. Now if you need to reduce the torque during the play mode, D706 could be changed to 3.3V from 4V. That should reduce the torque slightly.

A resistor would not control the motor torque so precisely and would waste power in addition.

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

Post by H. callahan » 28 Mar 2019 03:31

Thank you again, i´ll try to explain what i meant about the BA6109 "aging":

I am aware that i have fried it by measuring the motor wrong. I meant to say that maybe the BA6109 fried so easily because it allready had aged; respectively is it possible that the BA6109 did give a little too much electricity to the motor due to malfunction? I have read that a malfunctioning transistor musn´t fail completely, but still can work, in an amp for example a malfunctioning transistor still can amplify but does produce distortion.
Maybe the reel-motor did turn too fast because the BA6109 did malfunction (due to age) befor i fried it?

Is it also possible that i not only have fried the BA6109, but also dammaged the "brain" LM6413E-542, or was the "frying" only spread onto the BA6109?

As my knowledge of electrics is very limited i considered the two diodes to just prevent electricity from "comming back", as diodes only let pass electrictiy in one direction.
When i understand correct those diodes also can limit voltage, so exchanging D706 for another diode will reduce electricity given to the motor - this should be the solution i have been looking for, well, if i can make the reel-motor run again.

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

Post by Athermos » 29 Mar 2019 15:31

I am aware that i have fried it by measuring the motor wrong. I meant to say that maybe the BA6109 fried so easily because it allready had aged; respectively is it possible that the BA6109 did give a little too much electricity to the motor due to malfunction? I have read that a malfunctioning transistor musn´t fail completely, but still can work, in an amp for example a malfunctioning transistor still can amplify but does produce distortion.
Maybe the reel-motor did turn too fast because the BA6109 did malfunction (due to age) befor i fried it?
No, from your initial post you fried it on the third attempt. Semiconductors do not age that way.
Is it also possible that i not only have fried the BA6109, but also dammaged the "brain" LM6413E-542, or was the "frying" only spread onto the BA6109?
The microcontroller does not supply power to the motor and I do not think a bad BA6109 would short the terminals of the microcontroller and damage it. Further the deck still responds when being operated.
To check the logic states of pins 15 -17 of the microcontroller, you may operate the deck and check the voltages of those pins as state high or low using a very high impedance multimeter set in the voltage range. The logic states should be there in the service manual. If you make a mistake here while measuring the logic states particularly when the logic state is high by loading a gate of the microcontroller, it would be damaged. In that case, it would be practically impossible to get a new one. LM6413E-542, LM6413E is the generic controller but the 542 is the customization for the gates for its current mode of operation, making it practically impossible to source a new one unless something just comes up on eBay.

There is a small chance that BA6109 is alright only the microcontroller is not giving command to it and would need to be reset, that should also be specified in the service manual.

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

Post by H. callahan » 30 Mar 2019 13:51

Ok, i´ll then leave my hands off the LM6413E-542 and try to replace the BA6109.

Just for completeness i have checked the service manual and found this:
reset circuit - Block diagram.JPG
(41.39 KiB) Downloaded 74 times
but there isn´t any procedure mentioned, also i cannot find transistor 702-704 on the Block diagram.

Comming back to the BA6109, i also was asking about aging because the magnetic field of reel-motor did intersperse obivous into the heads during replay. I had to put quite some shielding into the cassette-drive to shield the magnetic field from the reel-motor.
After i had fried the circuit the idea came to my mind that the reel-motor might have had gotten too much electricity due to an aged BA6109, resulting in the reel-motor turning too fast and interspersing during replay.

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

Post by H. callahan » 05 Apr 2019 02:14

So i bought a replacement BA6109 and managed to exchange the original IC by the new one. It was very frustrating because some pins of the old IC were bent and i couldn´t remove every tiny bit of soldering tin from the about tens pins so it was stuck, it took me about one and a half hours just to remove the old IC and to solder in the new one.

Then i tried to run the deck again, but the reel-motor still does not run. To find out whether the new IC also is faulty or whether i missoldered it, i decided to again make a voltage-measurement on the reel-motor - and guess what - when i push the play-button there are 3 volts on the reel-motor.
So the new BA6109 seems to work - and maybe the old BA6109 wasn´t defective at all - but the reel-motor still does not run.
So i decided to again measure the reel-motor for ohms and this time it measured 1.8k ohms. Now i also have another GX-32, exactly the same cassette deck on which the reel-motor does work, so i also measured the working motor on the other deck for ohms - and it also measured 1.8k ohms, just like the not working reel-motor.

I also did turn the reel-motor by hand while measuring for ohms and the reading did change. It changed so quick i couldn´t read the display, but when i stopped turning the motor by hand the reading went back to 1.8k ohms.

Now to me this seems like the motor is not shorted, the not working motor measures the same as the working motor from the other cassette-deck, and the not working motor still is supplied with electricity, so i really don´t know what´s going on here.
Can a defective motor measure the same as a working motor?

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

Post by H. callahan » 12 Apr 2019 05:28

Today i had another try at my GX-32, this time i wanted to find out if the reel-motor is broken. I tried some measurements, all indicating the motor being ok, so i connected a battery to the reel-motor and it did turn.
So the reel-motor is not the problem it must be something else.
For not having any other idea i again measured the motor for volts, when the deck is in play mode. And i think i must have misread my multimeter last time, because the reel-motor does get 0.003V when the deck is in play mode - and not 3V. This should explain why the motor doesn´t run but isn´t defective. This also should mean that the IC BA6109 is working, otherwise the reel-motor shouldn´t get any electricity, but its way to few electricity.

So i also tested fast forward and rewind and the motor also does get 0.003V in those two modes. Now this is strange because in those modes the motor should get additional electricity so it can run faster than in play mode. But it gets just the same 0.003V as in play mode, so i think the problem is somewhere at the electricity-supply for the reel-motor, like the diode 706 and 707 respectively the transistor 711. Though the TR 711 was not working when i mismeasured the reel-motor which lead to this defect.
But as D706 and D707 are diodes, which fry when they get too much electricity, i assume the failure to be with those two and maybe also the TR 711.

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

Post by AudioFeline » 23 Apr 2019 06:23

I can't contribute to your electronics discussion, as your "limited electronics knowledge" is beyond my knowledge!
However, I can make a comment on cassette decks and torque. A comprehensive cassette deck technician may have a cassette torque meter - a mechanical meter build into a cassette housing which displays the amount of torque the dirve is applying, and I have assumed that there would be a pot in the circuit to adjust it to spec.
The source for this knowledge is from buying a cassette torque meter cheap out of curiosity in the 1980's; it didn't come with any documentation.

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

Post by Jim Leach » 23 Apr 2019 22:26

Visit www.tapeheads.net

Better yet, join up!

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

Post by H. callahan » 24 Apr 2019 13:45

@AudioFeline:

Thank you for your suggestion, unfortunately there isn´t any pot in the circuit of my GX-32 to set torque of the reel-motor. The GX-32 isn´t a top model from Akai, so i guess that´s why torque of the reel-motor cannot be adjusted. This also is why i assume they set the torque of the reel-motor a little too high, just to make sure the deck will play any cassette, even if the cassette is not in perfect condition.

@Jim Leach:

Thank you for the link, i havn´t yet had the time to further tinker with my GX-32. I wanted to measure the diodes to see if that is the cause for my problem. But if that doesn´t work out i will have a look at tapeheads.

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

Post by H. callahan » 04 May 2019 05:04

Just a pathetic update:

Aaaaaaaaaand once again, as the fearless hero i am (but you know, there only is a thin line between fearless and stubborn), i once more put on my shiny armor (which isn´t rusty at any spot, really!) and saddled my white horse (which i actually painted white to even more look as the shiny hero) and once again went to war against the evil and perfidious mystery - electronics - to save the defenseless but fair virgin (my Akai GX 32) from malfunction, this time equipped with a new, magical and powerfull weapon being a new multimeter which also is able to measure diodes.
Stubborn, erm i mean fearless as i always have been i once again opened my cursed deck to fight the evil spirits it is possessed by... and actually could make a little progress.

I measured diode D706 and D707 and the multimeter did show the "forward voltage drop". This should mean that both diodes still are intact, as diodes don´t pass electricity when they are broken, as far as i know.
As there is a capacitor, C706, being in contact with the reel-motor i also tried to measure it, but using the capacitance-setting on the multimeter didn´t produce any reading. So i measured the capacitor for resistance, as due to the wisdom of the internet i know that a broken capacitor (usually) does not have any resitance but a short.
And then the multimeter did show a reading, no short, which should mean that C706 also is intact.
schematic diagram.JPG
(121.55 KiB) Downloaded 24 times
Then my attention was drawn to the "FR701", which as it turned out is a "fusible resistor". Instead of putting in a fuse, they put in a fusible resistor, which basically does the same as a fuse, but doesn´t need as much space as a fuse and does not burn when overloaded.
Now this is starting to make sense, because these fusible resistors are built into circuits to prevent the circuit from damage in case of overload - and maybe short circuit, as i have provided by measuring the reel-motor wrong for amperes.
So i tried to measure the FR701 for resistance, starting at lowest setting on my multimeter to avoid overload - but got no measurement exept for "1", which means no conductivity. I went a few settings up, but still got "no conductivity", which could mean the FR is broken.
But for not overloading the FR701 i decided to crack the color code on the FR701 - and thats when the rusty spots on my armor became visible, the white paint on my horse started to wear off and my allmighty weapon (my new multimeter) started to loose its allmightyness.
According to some frustrating websearch the FR701 should have 92.5k ohms at +-5%. Now the service manual says different, it looks like it says "12 (G)" or maybe "1.2 (G)", but not "92". Maybe it actually does say "92", but may be badly scanned at this spot, it´s hard to tell.
Now the color code on the FR701 goes:

white red green-or-blue red gold.

It only makes sense in this order and gives me 92.5k ohms at +-5% tollerance (respectively 92.6k ohms if the third band is blue and not green, but this shouldn´t make a difference in this case).
So i searched for 92.5k fusible resistors, but i couldn´t find any. Not on ebay, not with google, there only are other values like 88.6k or 95k or like that.
I hoped to find a picture of an offered FR where i can compare the color code to mine, in case i have mis-cracked the color code, but sellers just use the same picture of some FR for all the values they sell. So there is no visual check possible.

Heck.

At least it is plausible that this FR might be broken and there even might be a little burnt spot visible on the side of the FR, i looked with a loupe, but it also just might be a mechanically damaged spot.

Stay tuned for more adventures in mystery electronics... when i got enough nerves to move on.

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