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Akai GX-255 Good News & Expected Bad News

DougMac

DougMac

Super Member
A friend at work is getting rid of all his '80's gear, including his Akai GX-255. He brought it in to work today.

The good news:
It's in great cosmetic shape. The pinch roller rubber is fine. It seems to have had very little use. It powers up and all functions work properly, except the pause stutters, which I think is a switch or solenoid that needs cleaning. Even the pots are fairly quiet.

The bad news:
Like almost all GX-255's, the transistors are shot. There's no left channel output and the right channel is very noisy and distorted. I demonstrated this to my friend.

Since it's got problems, he said if I was interested that I could have it for free. I've read up and it seems that the transistor replacement is pretty straightforward. I've already found a source for them. I'm pretty handy with a soldering gun, I've built several Heathkits.

I'd hate to see it end up in the landfill, so I think I'm going to take it home. Worst case is I'm out a few bucks on parts and shipping and it still isn't right. If the transistor replacement fixes the problem, then I've got a nice looking deck.

If anyone has some other helpful hints on this deck, I'd appreciate it.
 
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Yes I would take it in a heart beat. I looked it up and looks like a nice deck. With a little work like you said it could be a keeper. :thmbsp:
 
Update:
I brought the unit home and took a closer look. My first impressions were confirmed, it has had practically no use.

I've ordered the 12 transistors to replace the bad ones. The layout of the unit will make it fairly easy to service. I've also obtained a service manual.

This morning my friend stopped by at my desk. He had a box that contains the original dustcover, which is in pristine condition. He's also bringing me the original box.

Once the transistors are replaced I'm confident I'll have a nice unit that looks great.
 
Golly, what a find. I missed 747 for $85 on the local craigslist yesterday.
one day i will get lucky.....
Should be pretty easy to repair, i presume it has the 2sc458? Going with 2sc2240?
Is that one of the decks with the relays for switching?
 
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Replacement Transistors

studeb said:
Golly, what a find. I missed 747 for $85 on the local craigslist yesterday.
one day i will get lucky.....
Should be pretty easy to repair, i presume it has the 2sc845? Going with 2sc2240?
Is that one of the decks with the relays for switching?
Click to expand...
Good question. I read on some forums to go with the 2SC1815Y. I have a service manual and looked up the transistors and did a cross reference and it cam up with the NTE85. I've got both on order. The NTE85 is more expensive and I plan to use them. Any thoughts?

Yes, this unit has relay switching. I understand it can be a problem also, but this unit seems to be fine, except for the pause. If cleaning doesn't fix it, I don't care, I don't plan to use pause anyway.

It's nice to be handy. My friend realizes he could sell it "as is" and get practically nothing for it. He also realizes that finding someone to repair it would be hard and even if he found someone, service would be expensive. If I was a repair guy, I would want to run all diagnostics as well as fix the problem. That's a good 1 1 1/2 hours on the bench, $100 minimum (I wouldn't come cheap). He'd rather it go to a good home. It's not worth him fooling with it.
 
Check the grade on the schematic to match up the hfe for the 2sc458s. I don't have the schematic for the 255, but the ones in the GX-4000d i am doing are high hfe
 
hfe

studeb said:
Check the grade on the schematic to match up the hfe for the 2sc458s. I don't have the schematic for the 255, but the ones in the GX-4000d i am doing are high hfe
Click to expand...
2240 hfe (max) 700
NTE85 300
2SC1815Y 700

Interesting. The parts list calls for three transistors, 2sc458, 2sc 945 and 2sc2002. Their hfe's are 300, 600 and 400, respectively. It seems that in posts I've read, they just replace all 12 with one type transistor. Looks like the 2SC1815Y would be the best choice. What do you think?
 
if that is what others have done, then it should be good. its funny, when i looked i found the 2240 as the replacement of choice for 458

what is the letter grade of the transistors? They are graded at the factroy ofr hfe or Idss depending on type.
This is the schematic for the first stage of the GX-77 playback, the grades are E or F for the 2sc1844, which are 300-600, and 400-800, so fairly high hfe for this transistor.
i don't know about replacing them all with the same type. You need to worry about noise gain and bandwidth, and maybe matching output for the eq sections.

and while you are there, it would probably be good to replace the coupling and PS caps
 

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Success!

Yes! I got in the 12 transistors and replaced them. It's pretty straightforward, it would have been easier if I had pulled the board, but I just took the bottom off the machine and worked from there.

After soldering in the last transistor, I set it upright, plugged it in and crossed my fingers. I threaded up a tape and hit play. Oh no! No audio at all! Then I looked down and the toggles had been flipped to source instead of tape.

I flipped the toggles and my earphones filled with clean, great sound. No noise, no distortion!

I've still got some cleaning of pots to do, plus reassembly. I also haven't tested recording, but I only plan to use it for playback, although having it "complete" would be nice. Not bad for free deck, an hour or so of time and less than $15.00 worth of parts!

I want to shout out to the folks here for their help, especially Jeff. He sent me some great information, including photos with all the transistors that need to be replaced marked. Thanks Jeff!
 
Congrats!!
I re-did the playback in a GX-4000D, and the improvement is tremendous. Mine was also free, but is not quite at the level of your GX 255
You should also replace the PS and coupling caps, that should improve the bottom end.
Can you share which transistors you replaced? Did you replace the output Fet?
 
Transistors

studeb said:
Congrats!!
I re-did the playback in a GX-4000D, and the improvement is tremendous. Mine was also free, but is not quite at the level of your GX 255
You should also replace the PS and coupling caps, that should improve the bottom end.
Can you share which transistors you replaced? Did you replace the output Fet?
Click to expand...
I replaced transistors 1,2,3, 8,9,10 and 1b, 2b, 3b, 8b, 9b and 10b.

Replacing the caps is a great idea, will do.

I did not replace the output Fet.
 
Photos and More Testing

After cleaning, I did some more tests. The pause problem was dirty contacts. Fixed.

I hooked it up to my system and it sounds great. I tested recording and does a nice job. Here are some photos:

IMG_0175.jpg


Interior with new transistors:
IMG_0185.jpg


Detail:

IMG_0188.jpg


Heads and pinch roller:

IMG_0189.jpg


Another detail:

IMG_0192.jpg
 
Matched Transistors

vfr800fiman said:
Matched pairs of transistors are recommended for lowest distortion.
Read this: http://slbender.exactpages.com/2sc458.html
I bought a big bag of transistors for my GX-625 and matched them myself, I had 12 to replace in my deck.

JMTC
Glenn
Click to expand...

Thanks Glenn for the info. I think I remember reading about matched transistors elsewhere. I like your idea of buying a bunch, then DIY matching.

I did not match, but I did use the more expensive transistor. I wasn't sure what I had and if the replacement of the transistors would solve my problems, therefore I didn't want to put a large amount of money in the unit.

I spent a few hours playing different tapes, paying bills and waiting for my wife to come home. It is a nice sounding unit. I need to do some more listening, but I think my Teac X-1000R might beat it out sonically. They are pretty close, though. I did listen to some pre-recorded tapes, but I mostly listened to tapes I made using my Viking 87 Pro (tube) I bought second hand in 1966. I even listened to a tape I made in 1965 using a Voice of Music, my very first RTR I got when i was 12. Both the Teac and Akai bring out detail I'd never heard from these tapes, especially bass.
 
Yeah, I'm not sure you could actually hear the difference using un-matched transistors. I think it's just what the factory did to make the unit the best it could be.
You'd probably need some sort of distortion analyzer to actually see how it's performing, again it's a factory setup procedure.
I'm really not sure how well I even matched my set that I installed. We work with what we have ;)

On a different subject.
I'm actually looking forward to trying out my DBX 224 that I just got to see how well it removes tape hiss. I've never used one before on an open reel deck.

Take care.
Glenn
 
Working on Karma (Where's my list?)

Well, my friend came by to tell me how he made out with the other stereo items he was selling. He got $750 for his Sansui G-9000 and $200 for his Sansui SR737 turntable.

While we were talking, he asked me if I was able to get the transistors for the Akai and if I'd been able to fix it. I told him yes, and the unit was now working. Since it's in such nice shape, in a fit of weakness I said "I think I owe you some money." He said, "Well, OK. How about $50?". I said "Sure".

I think that's about right, given what I've seen them go for minus the repair job, which I think would have run $100 easy in a repair shop.

I hope that helps get karma back in alignment. I don't want to miss other deals because of bad karma.

BTW, I intend to keep the unit. It would have been extra bad karma if I had gotten it for free and then flipped it.
 
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