I am restoring an Akai GX 255 reel to reel. It's my first attempt at restoration so I am a nubi. The left channel did not work when I started and the first thing I replace where all the C 458 transistors with the suggested replacements. When I tested it out I had the left channel sounding good but in a few seconds I could hear pops and other noises and then the it went dead. Can anyone suggest where I might start looking for the problem?
You could also see if a voltage check of the first three transistors in the head amp have correct voltages on them.
Look for a mechanical rec/play switch. If there is one, clean it with some proper cleaning fluid and a lot of "massage".
Look for the infamous rotten Hitachi 2SC458.... If you find any... replace them (since you´re using the word "restoring") 😉
Same voltage spec, same pin out, low noise multi purpose TO-92 will do fine.
Look for the infamous rotten Hitachi 2SC458.... If you find any... replace them (since you´re using the word "restoring") 😉
Same voltage spec, same pin out, low noise multi purpose TO-92 will do fine.
Indeed, changing components at random without any real reason is never a good idea.
If that comment was for me, just remember that I didn´t suggest that.
Everyone (at least grown ups) by now should be familiar of the "Hitachi rot", especially for the 2SC458. If not,
just do a Google search.
Thread starter wrote "restoration". That equals replacing all of them (if any) in that particular deck 😉
I've changed many a 458 in Akai and Sony decks. Odd thing is they often measure OK with the diode tester but won't pass audio. You can spot them by their jet black legs. @winggo not to comment on your work but I would check your solder joints. Maybe one has not flowed too well.
No it wasn't, not sure why you thought that. It was a general comment from a seasoned successful design engineer giving the benefit of experience.
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1: Maybe because your post came right after I recommended to replace all the 2SC458?? 🤣
2: JFYI..... You´re not the only one in here 😉
All good.
The solder joints may not be the best. I did this without taking the boards out of the unit so they were rather hard to get at. I am thinking about changing out all the electrolytic caps so I may remove the boards anyway. I have used Deoxit on all the switches and pots but again they were hard to get at. Will clean them again when I take the boards out. I don't have a scope yet and don't know how to use one if I had one. Can you trace the signal with a multi meter? The replacement trans were KSC1845FTA. I will be back here to ask more questions. Thanks for the help so far.
Simple answer to that is no I'm afraid but you should check the DC voltages on those transistors when the fault occurs.
I checked the voltage on all the transformers I replaced last night. The right channel ones seemed to be stable but the left channel ones were all over the place. One leg on one had 0 volts. They seem to get worse as the unit warms up. They start out sounding ok and then they start to "crackle" and fade away. Any suggestions on where I might look to narrow down the problem?
You are going to have to break the circuit down into blocks to start with. For example these sections are AC coupled and so DC values in each should be stable.
For example the collector volts of TR9 should be stable and so should the collector of TR11.
The same applies to the unmarked parts for the right channel.
I am assuming these are the transistors you have replaced.
For example the collector volts of TR9 should be stable and so should the collector of TR11.
The same applies to the unmarked parts for the right channel.
I am assuming these are the transistors you have replaced.
Thanks, I have got a GX600 on the shelves but not yet done anything with it. Will keep a lookout for these.
Dave beat me to it........ Otherwise it´s easy to find 😆
https://www.tapeheads.net/threads/infamous-2sc458-transistors.55286/
https://audiokarma.org/forums/index...d-akai-gx-635d-transistor-issue.693019/page-2
https://forums.phxaudiotape.com/threads/re-the-evil-hitachi-2sc458-transistor.7564/
and many more, if you bother to look for them 😉
In order to look for something you need to know what you are looking for in the first place (and no, google search did not return useful results a first attempt) ... 😉
Problem with these: Look for black, corroded legs. The corrosion goes up the legs and into the housing.
When it reaches the "chip" itself, all kinds of hells break lose.
Noisy/sputtering in amp-sections.
When used in servo´s, motors start running irradicly. DD turntables with CCW spinning, or full RPM take off´s instead
of 33 or 45 RPM. The list is as long as ...................
This is something I regularly came across in general service back in the day (TV's) and you could often spot these a mile off by the discolouration. Typically in that line of work it was T0126 style packages used as voltage regulators, line driver stages or video output stages that gave the trouble. Measuring the base/emitter voltage often showed it would jump to some unrealistically high value and then start working again. Very common issue.