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Akai GX77 FF/FR winding speed is poor - try to fix it
Posted by
OE2WHP
on 09 Sep, 2016 08:24
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Hi,
i found my old Akai GX77 reel to reel tape recorder that I used back in the 80's in the basement. I still have a lot of tapes and memories connected to this brick so I decided to fire it up again but as you can imagine it didn't work anymore.
The next 4 weeks I was trying to get new belts which was not exactly easy but in the end I found a US ebay seller who offered belt and capstan sets for the GX77 for a reasonable price.
Although it was a bit tricky to get the belts replaced I finally managed that and now the GX77 is back to life.
Except one thing. Under fast forward or fast reverse, when the spooling reel gets loaded the motors seem to have not enough power. It gets slower and slower and at a certain point it doesn't even start to spool unless I give it a helping hand.
I looked up the internet for information on that issue and found, that this is the most common failure mode of the GX77. The only fix I found is a mod. with relay boards that engage the motor of the unspooling reel in the opposite direction to support the spooling. Thats a nice workaround. However, this does not fix the root cause, which is what I would like to do.
I see two possible reasons for the issue.
1. Dirt & gunk in the mechanics
2. The motor has gotten weak over time.
Is this possible? Can DC motors become weak, loose power or torque over time?
Thanks for your support!
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#1 Reply
Posted by
Zucca
on 09 Sep, 2016 08:47
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1. Dirt & gunk in the mechanics
+1, WD40?
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#2 Reply
Posted by
Towger
on 09 Sep, 2016 09:31
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WD40 is good for washing out old dried lubricant and dirt. After that you need to replace it. Depending of the bearing type 3 in One or sewing machine oil would do as a bodge. On quality motors a better job would be the replace the bearings, assuming they are a standard type.
Try not to use spray on oil, a good old fashioned can is best.
I am trying to remember the type of motors used in reel to reels, I have two in 'storage'. Another possibility are the magnets demagnitised over time. If the motor used permanent magnets.
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#3 Reply
Posted by
CJay
on 09 Sep, 2016 11:15
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Old grease has a tendency to become sticky and slow things down as the volatiles evaporate off and it absorbs dust, material from the bearings etc. etc.
it'd be worth removing the belts again and making sure everything moves freely.
Do not use spray oil, if you have to lubricate things find a grease that's compatible with the plastics and bearing surfaces.
only apply it with a precision applicator if at all possible or you could cause more problems than you fix.
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#4 Reply
Posted by
OE2WHP
on 09 Sep, 2016 20:34
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Big thanks for all your valuable tips!
I will disassemble the mechanism and clean and lube (no I won't use spray!) all the shafts and bearings. After that I will adjust it according to the SM. Lets see if that fixes the issue.
@Wilfred
No there is an extra wheel with two notches and two photo diodes for the counter that determines the revs and direction (by checking the phase between the sensor signals).
@Towger
If you have more info on that magnet degradation, would be very interesting! Keep in mind that this old bugger is 36+ years.
I'll keep you posted on the progress.
73
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#5 Reply
Posted by
CJay
on 09 Sep, 2016 21:01
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Wow, that's a service manual.
It uses DC motors, they can become weak over time and the brushes are rarely of great quality but I would still follow through all the checks mentioned before.
The mechanism is actually reminiscent of a direct drive video reocrder deck with clutches and brakes on the capstans.
It's worth checking the motors are running and the clutches aren't slipping or the brakes aren't partially engaged.
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#6 Reply
Posted by
BMack
on 10 Sep, 2016 06:15
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Clean and reapply grease, if you still have an issue check the brakes. Try to FF/Rev with the brake level pushed so it doesn't engage, does that help? If it doesn't change anything then you probably have some caps that need changing.
My guess is that it's just old grease.
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#7 Reply
Posted by
jh15
on 10 Sep, 2016 06:30
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I'm an old eggspurt on magnetic recording and was just glossing over this when seeing WD-40.
In my days (60s) 3-in-one oil messed up most diy audio equipment. I called WD-40 the equivalent from reel-reel, minidisk, 8-track, pinball machines to DJs chair wheels as the AIDS of lubrication. It is not a lube, it turns to epoxy over time. Use it to dry your flooded distributor cap.
I'll help, as I am familiar with those machines, but first:
Is the tape ok? much tape has failed from becoming hydroscopic and sticking to everything.
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#8 Reply
Posted by
tautech
on 10 Sep, 2016 07:50
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#9 Reply
Posted by
jh15
on 10 Sep, 2016 15:53
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Very common on TEAC machines of that design to have the pinch roller arm be seized up by old lube. Especially after long storage. But now that these machines are so old, probably all the lubed parts are sticky.
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#10 Reply
Posted by
OE2WHP
on 10 Sep, 2016 17:20
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Hi,
Short update:
I cleaned all involved shafts and bearings and then lubed them with sewing oil from the sewing store. No milfs there, so I managed to drive home without forgetting to buy the oil....
I applied it with syringe and needle. It helped a little but not much. I checked the supply voltage of the motor. Should be 12V. I measured 11.1V with 100mV ripple. Going to check the caps but I don't think that they are bad.
Here is the schematic of the winding voltage supply. Could anyone explain what this 4 transistor circuit is supposed to do?
M is switched to the motor when winding.
Edit: Caps are good. I also checked the motor current. It's 1.26A. I'm starting to suspect the motors.
Thanks
73
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#11 Reply
Posted by
siggi
on 10 Sep, 2016 19:47
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Here is the schematic of the winding supply voltage. Could anyone explain what this 4 transistor circuit is supposed to do?
Looks like they do current limiting. If the voltage drop over R2||R3 exceeds 2Vbe (~1.4V), TR2 & TR3 will start conducting, which will rob base current from TR1 (and by extension from TR7).
EDIT: Also note that the voltages on the circuit diagram imply a current draw. By my calculation it's (17.8V-17.6V)/(0.65Ohm)~=300mA, though I don't know under what conditions this is expected.
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#12 Reply
Posted by
OE2WHP
on 10 Sep, 2016 20:46
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Siggi, thanks for the explanation. Makes sense. Sort of an overload protection?
Now, the mechanics are cleaned and lubed. Everything moves free and easy. The brakes disengage properly when switching into FF or FR mode. I think its not a mechanical issue.
I did another test. I hooked the motor to my lab powersource instead of the internal 12V of the GX77. The problem stays the same. Current is also 1.26A so there is no current limiter that causes the problem. :-(
What is left? The motor? How can I determine if it got weak over time?
Thanks
73
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What is left? The motor? How can I determine if it got weak over time?
Isn't there the same type of motor for the other reel - assuming it works - could you compare the current consumption between the motors?
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#14 Reply
Posted by
SeanB
on 10 Sep, 2016 21:24
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More than likely the motor bushes and brushes are dirty or worn. Depending on the motor type you can dismantle them to clean the bushes and oil them, along with cleaning the brushes and commutator faces. Spare brushes are often available by using the nearest scrapped inkjet printer, as they typically use a DC brushed motor to run the print head across the paper.
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#15 Reply
Posted by
OE2WHP
on 10 Sep, 2016 21:52
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Yes, the same motor is on the other side too, but it's the same issue on both sides.
I'll try to open one of the motors with a dremel but first I have to find a 0.7mm allen wrench to get it out of the chassis. My smallest is 1.5mm.
EDIT:
Here is something I found on the web:
http://www.microchip.com/stellent/groups/SiteComm_sg/documents/DeviceDoc/en543041.pdf The torque from a permanent magnet brushed DC motor is limited by its stator field, which gives it good low end ( low speed ) torque and a limited high end ( high speed ) torque. The permanent magnet brushed DC motor responds very quickly to changes in voltage. This is due to its constant stator field, thus giving it good speed control capabilities. But, they do have a drawback. Permanent magnet brushed DC motors will loose their magnetic properties over time, thus degrading its stator field and causing the motors performance to decline. 
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#16 Reply
Posted by
BMack
on 11 Sep, 2016 04:28
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Spin the motor without any load, how well does it spin? Is there drag, does it feel smooth or rough? There will probably be holes that you can drop some oil in without splitting open the motor's case.
Is the large cap going to the motor AC or DC? If it's AC, it's brushless.
What's the play speed like? Is there flutter? If it plays normally and doesn't slow down occasionally, you likely have a logic problem...even if the caps check good you may need to replace some of them.
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#17 Reply
Posted by
OE2WHP
on 11 Sep, 2016 14:26
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Is the tape ok? much tape has failed from becoming hydroscopic and sticking to everything.
Yes the tape is good. Doesn't stick at all. I also tried different tapes.
@BMack
Both motors spin freely. Doesn't feel as if there is any roughness, as well. No holes or other access to the bearings.
These are 12V DC motors. I measured the two caps (C1 & C18) good. 250µF ESR0,33Ohm and 2400µF ESR 0,04Ohm. Why do you think they could still be bad?
Besides that, I tried it also with an external supply. Same result.
No problem with playback. The play speed is perfect and no flutter but I think that is done by the capstan motor. The reel motors just spool the tape.
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#18 Reply
Posted by
BMack
on 12 Sep, 2016 05:42
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Is the tape ok? much tape has failed from becoming hydroscopic and sticking to everything.
Yes the tape is good. Doesn't stick at all. I also tried different tapes.
@BMack
Both motors spin freely. Doesn't feel as if there is any roughness, as well. No holes or other access to the bearings. These are 12V DC motors. I measured the two caps (C1 & C18) good. 250µF ESR0,33Ohm and 2400µF ESR 0,04Ohm. Why do you think they could still be bad?
Besides that, I tried it also with an external supply. Same result.
No problem with playback. The play speed is perfect and no flutter but I think that is done by the capstan motor. The reel motors just spool the tape.
Play is done by the take up reel and the capstan, if you hold the take up reel you'll see the tape drop after the capstan but it should be able to take up the excess tape after you release it(obviously, don't hold it for several seconds, just a second will give you a lot of tape slack). If the feed reel was causing excessive drag it would slow.
I missed that you said you hooked it up to an external power supply, it's sounds like you're right. It's rare that the motors are bad on reel to reels, they just don't get that much abuse that the performance starts to decline, normally caked-on dust and lubrication is the issue with the motors.
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#19 Reply
Posted by
jh15
on 12 Sep, 2016 05:59
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I mentioned the problems 90% of the time in my experience of machines setting unused for years then put into service. I did miss the fact only while FF or RW.
Why not put up with it? Helping with a finger gently giving torque won't hurt anything.
Even my Studer/Revox and Tascam machines, I aid a reel during rewind when new. Tension on tape noted.
Unless restoring it, butchering the motors is ok for you, I'd copy all tapes to digital, and save it for impressing your friends with a full reel, hiss, dullness from flaking ferrite heads, etc.
Good thing in your case is your tapes will be playing back on original azimuth heads. (Hopefully PB is aligned to rec head).
Not trying to be a smartass, there is little I don't know about recorders, but I am glad they are gone, just like my chemical color darkroom.
I'd transfer to digital, and learn new stuff like I want to here, like fpla.
Still something romantic playing a 10.5" reel deck 15ips with dbx on good system.
I was an audiophile back then, grew out of it after first CDs.
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#20 Reply
Posted by
OE2WHP
on 19 Sep, 2016 10:12
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Quick update,
I'm pretty sure now, that the motor magnets degraded over time. The shaft turns without any drag. The bearings run smooth. Unfortunately there is no easy way to open it without the risk of damaging it completely, so this is the end of this road.
I couldn't find any original replacement or any datasheet. I'll try to find a new motor that might fit but I have no idea what rpm (without load) would be ok. The range goes from 2.000rpm to 30.000rpm.
I think, even with degraded magnets, the rpm without load should be fairly the same as it was when new. What dou you think?
Here is the data I found so far:
Max. supply voltage: 12V
Current without load: 145mA
Current at stall: 1,26A (I think this originally was at ~2 maybe 2.5 amps as the transistor in the power circuit is rated for up to 3 amps) (I could check at which current the limiter circuit kicks in..)
Housing diameter: 39mm
Housing lenght: 40mm
Shaft lenght: 11mm
Shaft diameter: 3mm
Mounting holes: 4x M3 (circular at 90°, dia. 29mm)
some pics:
Part number is: 432201075112
Datecode is: 230 (1982, week 30)
Don't know what the NM01 stands for. Maybe "NIHON Micro"??
I found a comment in a vietnamese forum that this is a Philips 15V/2A motor but no evidence for that.
Any ideas or sugestions for a replacement?
[EDIT]
The replacement motor shouldn't exceed 45mm in diameter but it could be twice as long as the original. There is plenty of space behind the motor.
Thanks
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#21 Reply
Posted by
BMack
on 19 Sep, 2016 15:48
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Quick update,
I'm pretty sure now, that the motor magnets degraded over time. The shaft turns without any drag. The bearings run smooth. Unfortunately there is no easy way to open it without the risk of damaging it completely, so this is the end of this road.
I couldn't find any original replacement or any datasheet. I'll try to find a new motor that might fit but I have no idea what rpm (without load) would be ok. The range goes from 2.000rpm to 30.000rpm.
I think, even with degraded magnets, the rpm without load should be fairly the same as it was when new. What dou you think?
Here is the data I found so far:
Max. supply voltage: 12V
Current without load: 145mA
Current at stall: 1,26A (I think this originally was at ~2 maybe 2.5 amps as the transistor in the power circuit is rated for up to 3 amps) (I could check at which current the limiter circuit kicks in..)
Housing diameter: 39mm
Housing lenght: 40mm
Shaft lenght: 11mm
Shaft diameter: 3mm
Mounting holes: 4x M3 (circular at 90°, dia. 29mm)
some pics:
Part number is: 432201075112
Datecode is: 230 (1982, week 30)
Don't know what the NM01 stands for. Maybe "NIHON Micro"??
I found a comment in a vietnamese forum that this is a Philips 15V/2A motor but no evidence for that.
Any ideas or sugestions for a replacement?
[EDIT]
The replacement motor shouldn't exceed 45mm in diameter but it could be twice as long as the original. There is plenty of space behind the motor.
Thanks
Just keep looking for an original. I have a Pioneer RT-707 sitting in my shop, everything repaired(I believe) but the reel hold down was broken by the customer. It's been sitting here for a couple months, just last week a guy parted one out and started listing parts on eBay. It may take a month, it may take 10 months but you'll find one(set up an alert and look occasionally). The only thing is figuring out how much you're willing to spend, shipping won't be cheap if it's international.
Until then, leave it as it is if playback is fine. Maybe buy a cheaper unit to use and occasionally use this one. Not ideal at all but that's the story of old electronics, sometimes parts are hard to come by.
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#22 Reply
Posted by
oldway
on 19 Sep, 2016 16:14
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#23 Reply
Posted by
CJay
on 26 Sep, 2016 09:30
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Part number is: 432201075112
That very definitely looks like a Philips part number, try contacting a Philips dealer who may be able to order one for you or track down a Philips model that used the motor (widens your options for sourcing second hand).
Don't worry too much about the age, when I repaired Phillips equipment it was quite common for us to be able to source 25 year old parts direct from them (and service manuals too but obviously only for Philips equipment)
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#24 Reply
Posted by
raducris
on 08 Nov, 2022 15:50
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Many great findings in all your troubleshooting, thank you for sharing. I'm wondering if you found the actual cure of the problem. My only observation so far is that mine works great with good quality tapes like Maxell and it doesn't work with visibly older Realistic tapes. I'm used to '80s East European R2Rs and, by comparison, I can tell that the DC motors in the Akai are tiny little toys, so yes, undersized for the job one can say for sure. As for me, if this is the solution, I'm glad to pay a bit more for a good tape, yet I think this might be just temporary, till these tapes age too, most likely before me
