Akai GX-635D full restoration

[hp4766]

the 2SC458's are the ones yes and I had 10 pairs on the preamp board. There are also some 2sc945's that I've been advised to change while I'm in there so I stocked up on KSC945C which puts the collector in the center making them an almost drop in replacement for the 2sc945's. But, OnSemi is discontinuing the KSC945c, I bought 2000 of them and they still had around 18K left so I'm sure there's plenty to go around for anyone who needs them but I would get them quick. I bought a few thousand of several different transistors that are being or have now been discontinued by OnSemi which should last well into the rest of my life provided I don't become the only HiFi repair Mogul overnight hahahahahahahahahaha, never gonna happen! I might have a piece of paper that says I've been trained in electronics but there's just no replacing the countless years of experience found on AudioKarma. There are guys here that really know their stuff and are very humble as well. I have a LOT to learn but I continue learning from these folks on a daily basis! I'm merely trying to pay it forward and document some of my restorations mistakes as well as success's so that hopefully someone else may gain some knowledge from those documented projects when they come to AudioKarma looking for answers. There are so many well Documented procedures and restorations alike on here that it blows my mind when I go through them and just read, it leaves me in awe. I sincerely hope that the information I'm able to provide in this thread helps you in some way with your 635. All the best my friend!

Steve

Hi, I need suitable transistors for my unit. What might be an available sub for the KSC945c transistors? Thanks

 

[bafio]

Dear all, first of all thank you all for you high value contribution on our challenges here. I'm starting the renovation of my gx-635d. I have changed the transistors and now I'm watching at the capacitors. I have the service manual and references of the caps. Regarding the few stiroflex coming straight from the 70' mines looks very very bad (picture joint)
My question: do you know where can I buy those (radial) in Europe or do you have any recommendations for replacement?

 

[radiokom]

Dear all, first of all thank you all for you high value contribution on our challenges here. I'm starting the renovation of my gx-635d. I have changed the transistors and now I'm watching at the capacitors. I have the service manual and references of the caps. Regarding the few stiroflex coming straight from the 70' mines looks very very bad (picture joint)
My question: do you know where can I buy those (radial) in Europe or do you have any recommendations for replacement?
View attachment 3230685

Why you considered they looks bad? Wrong color? If you decided to make full recap, then replacing of capacitors should be first, transistors after that. I am not an expert of Akai decks (had one GX636 30 years ago, and considered it is really not what I need. Good looking but that's all), but if there are no well known troublemakers (like Frako and Rifa miniprint in Studer and Revox), I would leave them alone.

 

[bafio]

Why you considered they looks bad? Wrong color? If you decided to make full recap, then replacing of capacitors should be first, transistors after that. I am not an expert of Akai decks (had one GX636 30 years ago, and considered it is really not what I need. Good looking but that's all), but if there are no well known troublemakers (like Frako and Rifa miniprint in Studer and Revox), I would leave them alone.

Those ones look more as mushrooms

 

[radiokom]

Those ones look more as mushrooms

And it offends your noblest feelings?
What happened between you and Mushrooms? Most of them are forbidden even in Amsterdam.
But do not worry, they are not mushrooms.
Install back plate and do not look on them.
And do not forget to delete all pictures, however it is not guaranteed you will not see them in your nightmares

 

[oldvinyldude]

Those grey ones are polystyrene caps. They rarely fail. I would not replace them. They are quite heat sensitive, so be cautious. As Radiokom stated, experience with Akai decks of this vintage seldom have cap issues.

On the other hand, the 2SC series transistors are known trouble makers-gotta go.

 

[bafio]

Those grey ones are polystyrene caps. They rarely fail. I would not replace them. They are quite heat sensitive, so be cautious. As Radiokom stated, experience with Akai decks of this vintage seldom have cap issues.

On the other hand, the 2SC series transistors are known trouble makers-gotta go.

Ok thanks guys. I was at this point. I just changed the 16 transistors with KSC1845FTA ones. I put all together and power on

 

[bafio]

Ok thanks guys. I was at this point. I just changed the 16 transistors with KSC1845FTA ones. I put all together and power on

Hi Kai's
So for now I left the "magical" capacitor in his forest of capacitors . Power and insert a old tape to test playback and record... Seems

 

[bafio]

New problem with the capstan motor. Now it starts spinning slowly and stops. If I turn manually the motor I turns freely. Other transistors or capacitors maybe need to be replaced?
Additional remark : in low speed the motor is turning... Strange

 

[bafio]

New problem with the capstan motor. Now it starts spinning slowly and stops. If I turn manually the motor I turns freely. Other transistors or capacitors maybe need to be replaced?
Additional remark : in low speed the motor is turning... Strange

Solved, it was a bad contact on the speed switch

 

[bafio]

I come back to you as I get this issue with high speed
Low speed is ok. When I push fast speed. Capstan motor accelerate but not the r2r motors. Could it be the tr3 transistor (c2336) or the big capacitor on the top of the machine?
Sorry I have poor electronic knowledge...

 

[bafio]

I come back to you as I get this issue with high speed
Low speed is ok. When I push fast speed. Capstan motor accelerate but not the r2r motors. Could it be the tr3 transistor (c2336) or the big capacitor on the top of the machine?
Sorry I have poor electronic knowledge...

Changing the tr3 transistor didn't resolved my speed problem dropping down.
More over, now when I power on the motors are turning opposite side
Any ideas?

 

[Grbluen]

Changing the tr3 transistor didn't resolved my speed problem dropping down.
More over, now when I power on the motors are turning opposite side
Any ideas?

Have you tried the Tapeheads forum?
There is usually a lot of active reel to reel guys there!

 

[bafio]

Changing the tr3 transistor didn't resolved my speed problem dropping down.
More over, now when I power on the motors are turning opposite side
Any ideas?

No Now the symptom is that the capstan and motors are turning over speed. I put back the old tr3 and same ... Over speed. Pushing the speed button nothing happens

 

[bafio]

Have you tried the Tapeheads forum?
There is usually a lot of active reel to reel guys there!

Thanks for the idea, I will open a discussion there

 

[bafio]

Have you tried the Tapeheads forum?
There is usually a lot of active reel to reel guys there!

Thanks for the idea, I will open a discussion

 

[bafio]

From Schematic to PCB - Page 1

From Schematic to PCB - Page 1

www.eevblog.com

Here a useful link with components list for a part of the sys board

 

[bafio]

No Now the symptom is that the capstan and motors are turning over speed. I put back the old tr3 and same ... Over speed. Pushing the speed button nothing happens

For info was D9 dead
Replaced by kbp02m

 

[Pcbuilder]

more pics

Thank you for the pics.

 

[sisstem]

AKAI SCM-200 Capstan Motor – Speed Problems Caused by Deteriorated Solder Joints? My Repair Experience and Measurements​

The SCM-200 capstan motor used in the AKAI GX-747, GX-646, GX-635, GX-636, GX-620 and GX-625 tape recorders can develop a rather unusual problem. The motor initially runs at the correct speed but gradually slows down, sometimes stopping completely. In other cases, it starts running too slowly and, after some time, gradually returns to its normal speed.

I suspect that one possible cause of these symptoms is deteriorated solder joints where the motor winding wires are soldered to the lead wires.

At first glance, the solder joints may look perfectly fine. However, if you carefully try to unwind the winding wire from the lead wire, it may come loose without applying any heat from a soldering iron. This suggests that the solder joint may have deteriorated over time, possibly due to prolonged exposure to heat. A poor electrical connection could cause intermittent resistance and lead to unstable motor speed.

I experienced this problem myself. When I saw that replacement SCM-200 motors were being offered on eBay for around €170, I decided to take a risk and investigate whether I could repair mine instead.

Before disassembling the motor, I measured the resistance and inductance of both windings, as well as their dissipation factors, first with the motor cold and then after heating it to approximately 60–70°C with hot air.

The measurements below were taken from my motor after the repair, when it was working properly.

Winding measurements after repair​

Winding 1

Parameter	Cold motor	Heated to approx. 60–70°C
Resistance	100.5 Ω	100.8 Ω
Inductance	99.6 mH	99.8 mH
Dissipation factor (D)	5.69	4.88

Winding 2

Parameter	Cold motor	Heated to approx. 60–70°C
Resistance	165 Ω	169 Ω
Inductance	211.6 mH	214 mH
Dissipation factor (D)	6.88	5.76

Most of the measurements looked reasonably good, but the change in the dissipation factor after heating the motor continued to bother me. This was one of the reasons I decided to investigate the motor further.

I eventually disassembled the motor, resoldered the connections between the winding wires and the lead wires, applied fresh insulating varnish where appropriate, and reassembled everything.

The result has been excellent so far. The motor has been working perfectly for several weeks, and the speed problem has not returned.

Unfortunately, I did not record the same set of measurements before the repair, so I cannot directly compare the electrical parameters before and after resoldering. Therefore, I cannot say for certain that the dissipation-factor changes were caused by the faulty solder joints.

Nevertheless, based on my experience, I believe that these connections are worth checking before spending a lot of money on a replacement SCM-200 motor.

Has anyone else encountered similar problems with the SCM-200, particularly a motor that slows down after running for a while or initially runs too slowly before recovering its normal speed?

I would also be interested to hear whether anyone has measured the winding resistance, inductance and dissipation factor of a healthy SCM-200 motor, both cold and heated.

I hope this experience may help someone repair their original motor instead of replacing it.