Hello,
I am in the process of repairing my Clarion MA-7800 which suffered from a weak channel. After isolating the problem to the power amplifier and further isolating the source using an audioprobe and a multimeter I found that one of the output transistors had gone bad. The output is build around two toshiba devices, the 2SD428 and the 2SB558, of which one of the 2SD428's is bad. After a quick scan through the datasheet I found that the replacements are readily available; 2SD718/2SB688.
Should I just replace the one bad transistor, should I replace both 2SD428's or should I replace all of the transistors? Also should I gain match the pairs?
Thank you very much!
Greetings from Holland,
Roy Vorster
I am in the process of repairing my Clarion MA-7800 which suffered from a weak channel. After isolating the problem to the power amplifier and further isolating the source using an audioprobe and a multimeter I found that one of the output transistors had gone bad. The output is build around two toshiba devices, the 2SD428 and the 2SB558, of which one of the 2SD428's is bad. After a quick scan through the datasheet I found that the replacements are readily available; 2SD718/2SB688.
Should I just replace the one bad transistor, should I replace both 2SD428's or should I replace all of the transistors? Also should I gain match the pairs?
Thank you very much!
Greetings from Holland,
Roy Vorster
Both, and the parts you mentioned are super rare. Where do you look when you say they are readily available? Beware of fake parts.
You need to be careful using different or more modern devices as they can sometimes have more gain and cause oscillation on the output.
I put new transistors in a Maplin amplifier and it oscillated badly. I had to increase the VAS capacitor a little to stop it.
I put new transistors in a Maplin amplifier and it oscillated badly. I had to increase the VAS capacitor a little to stop it.
Well, at least heren in Holland those are readily available. So that won't be a problem.
Thank you all for your help, I'll see what I can do!
Just a little question; Does any one of you have any experience with the philips PM series oscilloscopes? Particularly the PM 3233?
Thank you!
Sent from my HTC One using Tapatalk
Thank you all for your help, I'll see what I can do!
Just a little question; Does any one of you have any experience with the philips PM series oscilloscopes? Particularly the PM 3233?
Thank you!
Sent from my HTC One using Tapatalk
Only fakes are readily available, and this applies to the whole world. Put them in and watch the fireworks. Those transistors were discontinued in the 70s/early 80s.
It's a moot point, anyway, as the transistors you are looking for have the wrong package. Those originals appear to be to3 metal cans, whereas the b688 is a to3p plastic package. Not compatible.
It's a moot point, anyway, as the transistors you are looking for have the wrong package. Those originals appear to be to3 metal cans, whereas the b688 is a to3p plastic package. Not compatible.
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All the replacements available for this transistor seem to be in a TO-3P package; 2SD1667 and 2SD718. The transistor I am getting is however the real Toshiba transistor and coming from a trusted source.
After looking at some datasheets I didn't fin any other transistors that would be suitable. Is there anyone in Europe that has replacements for me?
After looking at some datasheets I didn't fin any other transistors that would be suitable. Is there anyone in Europe that has replacements for me?
Alright! I just found two modern day replacements which are probably going to work fine, the bandwith frequency is not substantially higer so no modifications have to be made! The 2SD428's are going to be replaced with the MJ15003G and I am going to order the MJ15004G (the complementary transistor) as well to avoid future problems.
Thankyou for your help!
Thankyou for your help!
Buy your replacements through an accredited Onsemi dealer, or direct from Onsemi.
Is the output stage using pairs of devices in parallel?
If yes, then you must select from a small batch, a pair (or more) that have closely matching parameters.
The two most important being Vbe @ your quiescent bias current and Hfe @ your quiescent output current.
You might get two pairs from 5 off, but buying 6 to 10 off would get you more chance of selecting good pairs.
BTW,
Onsemi confirmed with me (a few years ago) that they have no problem with supplying 5 devices using their "samples" distribution.
I don't know their current attitude to "samples" supply to amateurs.
Is the output stage using pairs of devices in parallel?
If yes, then you must select from a small batch, a pair (or more) that have closely matching parameters.
The two most important being Vbe @ your quiescent bias current and Hfe @ your quiescent output current.
You might get two pairs from 5 off, but buying 6 to 10 off would get you more chance of selecting good pairs.
BTW,
Onsemi confirmed with me (a few years ago) that they have no problem with supplying 5 devices using their "samples" distribution.
I don't know their current attitude to "samples" supply to amateurs.
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The devices were bought from a certified trusted dealer. The power amplifier in the amplifier is made up of one pair per output channel. The transistors will get here tomorrow.
Than you!
Than you!
MJ15003/4 are old as F.
MJ15022/3 or MJ15024/5 are better substitutes for the Toshiba TO3's.
(also cost pretty much the same as the 15003/4, e.g. at Conrad)
fT of the Toshibas is 6MHz average.
Gain-bandwidth product of the On-Semi's is 4 MHz minimum, in reality both NPN and PNP could have a higher fT than your original devices.
MJ15022/3 or MJ15024/5 are better substitutes for the Toshiba TO3's.
(also cost pretty much the same as the 15003/4, e.g. at Conrad)
fT of the Toshibas is 6MHz average.
Gain-bandwidth product of the On-Semi's is 4 MHz minimum, in reality both NPN and PNP could have a higher fT than your original devices.
I have bought the MJ15024/25 set. I'll report back when I get the transistors and have installed them! I will finally here my Clarion on both channels😀
Tank you!
Tank you!
Hello RoyAudio. Got a PM3200 and its service manual [detailed schematics] if it can help.
Hello Antoinel,
Thank you for your help! But I already have the schematic, I am working on it right now. What is your experience with the scope?
Greetings
Hello RoyAudio. PM 3200 is a well-made, reliable and a great scope which was a donation from a friend at work. Once in a while, I clean the rotary switches with cotton swabs and acetone. I also spray the inside of the volume controls with an electronic cleaner I had bought from Radio Shack. The high voltage section may have degraded. An AC signal already displayed on the whole screen distorts/bends at its maximum and minimum levels when brightness is increased.
Best regards
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