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Transistor replacement on GX-625

vfr800fiman

vfr800fiman

AK Subscriber
Subscriber
I've read a lot about replacing the transistors on this series of decks, and I believe most people do not match their transistors. Is this really required in the Akai circuit? I'm about to replace all 12 of the 2SC458's in my 625 and I'd like to do this job once :) I'm using 2SC2240's for the replacements.

According to Mr. Bender's site this is required, but as I said most people don't seem to do this. http://slbender.exactpages.com/2sc458.html
Any opinions from someone who has done this job?

Thanks in advance.
Glenn
 
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have done the procedure a couple of times - think the matching pair issue is a reminder from early days, where the production line was more unstable ...

if the transistors are from the same batch - i don't match them
 
Personally i would avoid them - that transistor (2sc458) has a weird behaviour - apparently the natural corrosion on the leads have a habbit of entering the chip inside the housing - so a transistor where the leads have turned black is suspicous.
 
jan_stevns said:
Personally i would avoid them - that transistor (2sc458) has a weird behaviour - apparently the natural corrosion on the leads have a habbit of entering the chip inside the housing - so a transistor where the leads have turned black is suspicous.
Click to expand...

I use new ones. These are better than the original old stock 2SC458's.
One says that it's better for the sound to use the same types.
I bought a couple of hundred, New, but it seems they are out of stock these days. ;)
 
Well, all 12 transistors swapped out and it works GREAT! Even got rid of the hiss on the line input right channel.

I also fixed the intermittent power connection on the digital display. Come to find out one of the clips on the fuse holder was spread out so far that it wasn't making contact all the time.

Thanks for the help.
Glenn

2SC458.JPG
 
I did this on a GX365D. There were about 36 of them total. Previous to this there were all kinds of anomalies, extraneous noise, distortion, etc. Replaced all with 2sc2240's and EVERY problem described above disappeared. Performance and sound clarity improved, distortion went down to almost nothing, and the little weird things(like the meters jumping randomly) stopped. I also replaced all the 458's on a SANSUI 2000A with 2240's and the noise floor became a black hole. No hiss, no hum, NO NOISE at all. My GX370D and 2 4000D's are next.
 
Just so others are aware, after researching gain on these transistors, the (D) originals had an Hfe of 250-500, the 2SC2240's that I used (GR) have a gain of 200-400, so I guess I could be a little low overall. I suppose I could have sorted the new transistors for gain, I doubt Akai did this though.

I measured the rest of the 2240's that I didn't install, and they were all right around 305-320. I suppose you could match them all if your the type of person that is bothered by things like this :)

I only had two bad transistors with Gains of around 30.

Glenn
 
Hi,

I have a GX-215D with a few of these transistors. I found its service manual on this great free site. The good news is that Akai already gave cross-references (see page 10) for the 458: 2SC963U (or F, P), 2SC1000 (BL, GR), and 2SC1312 (F or G).

Now, there are a number of sellers who offer these references by 10, 20 or even 100. They're either in China, Taiwan or HK. Are these fakes ? One shows a picture of a Toshiba box but for another reference in the 2SA. Should I use the 2240 or would the 2SC1000 be more than enough ? I have a source for 2SC1000 for about $0.25 each + shipping.
 
vfr800fiman said:
I suppose I could have sorted the new transistors for gain, I doubt Akai did this though.
Click to expand...

I replaced the 2SC458's in my GX-267D and came to the conclusion that since I was going thru all that work, why not match them. Since you're replacing transistors in the signal path, matching them as closely as possible only makes sense to me.
 
I picked up a non-working GX-625 this morning off CL primarily for the NAB adapters and metal take up reels.

The deck powers up and will fast forward and reverse but that's it. The capstan turns but when you hit play but the pinch roller won't drop. The stop, pause, and record buttons also appear to be not working.

I have a Revox A77 which I'm happy with but can't help myself from trying to get this going. I haven't opened it up yet but wondering if the transistors could be my problem. Any thoughts?
 
Nice cruddy leads on those transistors.

So is the 2SC458 the problem causer in all of the Akai's? Are there any other transistors that need to be replaced as well?
 
I got a service manual for the GX-625 and think my problem isn't related to the 458s. There are some other transistors that connect to the controls. Probably a lot of bad transistors in this deck.

Can a transistor be tested in circuit?
 
My 625 was a little hesitant to work when I pressed any of the buttons. I took the button assembly apart, and it's one of those controls that is a printed circuit board with a piece of conductive material on the buttons that shorts the copper traces when you press down. Mine worked much better after I cleaned the traces.
Do you have the schematic?

Glenn
 
Sounds like the logical place to start. Thanks for the tip. I downloaded the service manual from a DIYAudio link.
 
I Didtn't Match

I recently obtained a GX-255 (see thread). I replaced the 12 transistors, which were purchased at the same time, but I didn't match them. It went from a non-functional deck with no left channel and a highly distorted right channel to a sweet playing and sounding deck. The deck had very low hours, the heads look new.
 
vfr800fiman said:
My 625 was a little hesitant to work when I pressed any of the buttons. I took the button assembly apart, and it's one of those controls that is a printed circuit board with a piece of conductive material on the buttons that shorts the copper traces when you press down. Mine worked much better after I cleaned the traces.
Do you have the schematic?

Glenn
Click to expand...

Thanks Glenn. Took the controls apart and cleaned with contact cleaner. Contacts didn't look very dirty but the deck works now.

I'll clean the pots tomorrow night and give it a better audition.
 
Excellent! Glad it was something simple like that.
I remember those types of switches being notorious for getting oxidized and not working, especially on calculators of the time.
The 625 is a good sounding deck, I recently added an outboard DBX encoder/decoder to mine, zero tape hiss now.

Glenn
 
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