Hi!
My Technics SE-A3 was working at 200W into 4 ohms but gave up after 5 minutes or so. The "Protection" light comes on but no fuses are blown.
Does anyone have any idea of what might be the cause of this?
Did I read somewhere that there are a couple of hidden fuses somewhere?
Many thanks
David P
My Technics SE-A3 was working at 200W into 4 ohms but gave up after 5 minutes or so. The "Protection" light comes on but no fuses are blown.
Does anyone have any idea of what might be the cause of this?
Did I read somewhere that there are a couple of hidden fuses somewhere?
Many thanks
David P
Hi, I have just been given one of these amps to look at too. Protection light comes on. All supply voltages are OK. I haven't worked out yet what amount of DC voltage at the output will trigger the protection circuits. I have noticed both channels are putting out around 2 or 3 volts, which is a lot. Tried to adjust the DC offset but runs out of range.
I read back through some of the older posts on this thread and comments like the output devices run standing currents from 7 amps and higher. I doubt that, considering the schematic shows 0.03V across the emitter resistors of 0.47 ohms, ie about 64mA per transistor!. They are probably dissipating only around 5 W each at idle. I haven't had time to look at this thing yet and don't know if it runs fixed class A operation or some kind of sliding bias yet.
Agree that with Class A operation, it would be a lot safer to have output devices that can handle 200V or more. 150V for the original devices seems dangerous.
Someone mentioned using MJ15024 and MJ15025. In theory yes, but try fitting TO3 devices in there.....good luck!
When I find out more I'll let you know,
Cheers
I read back through some of the older posts on this thread and comments like the output devices run standing currents from 7 amps and higher. I doubt that, considering the schematic shows 0.03V across the emitter resistors of 0.47 ohms, ie about 64mA per transistor!. They are probably dissipating only around 5 W each at idle. I haven't had time to look at this thing yet and don't know if it runs fixed class A operation or some kind of sliding bias yet.
Agree that with Class A operation, it would be a lot safer to have output devices that can handle 200V or more. 150V for the original devices seems dangerous.
Someone mentioned using MJ15024 and MJ15025. In theory yes, but try fitting TO3 devices in there.....good luck!
When I find out more I'll let you know,
Cheers
Agree, doesn't appear as class A. They call it "new Class A" on the amps front panel! 
Out of curiosity, I tried sourcing the original complementary pair output devices, OD503A (in one package), but no luck as yet. That's probably no surprise based on previous comments I guess!
David P, if you are still there, I don't know of any hidden fuses at this stage. There are quite a few inside, about 10, accessable when the bottom cover is removed.
Out of curiosity, I tried sourcing the original complementary pair output devices, OD503A (in one package), but no luck as yet. That's probably no surprise based on previous comments I guess!
David P, if you are still there, I don't know of any hidden fuses at this stage. There are quite a few inside, about 10, accessable when the bottom cover is removed.
The Amp is still working on
After a long time i looked at my old thread, and i was surprised that he is not closed.
I finnished my amp 10 years ago and what should i say he is still working without any problems or trouble.
The sound is very clear and powerfull.
In my opinion the Sanken transistors was he right choice.
For all interested people to repair this amp my best wishes and good luck.
greetings from germany
Bocky
After a long time i looked at my old thread, and i was surprised that he is not closed.
I finnished my amp 10 years ago and what should i say he is still working without any problems or trouble.
The sound is very clear and powerfull.
In my opinion the Sanken transistors was he right choice.
For all interested people to repair this amp my best wishes and good luck.
greetings from germany
Bocky
I noticed that the proposed replacement transistors vary a lot in the fe parameter. For example, the 2sc2922 is rated at 50typ / matching pair 2sa1216 is 40typ, while the 2sc5200 it's pair 2sa1943 are rated both at 30mhz. Then there is someone on audiokarma that used 2SC3858 which is rated at 20, while it's pair is also rated at 20.
Finally on second page of this thread someone apparently measured the originals (bad measurement ?) fe at 80mhz ?!!
From what I've read around the amp can start to oscilate if a transistor with higher fe than what it's designed for is used. What are the expected consequences if fe lower than what it's designed for is used ? Distortion ?
I still have some originals that are ok, is there a reliable place that could have them measured ?
Finally on second page of this thread someone apparently measured the originals (bad measurement ?) fe at 80mhz ?!!
From what I've read around the amp can start to oscilate if a transistor with higher fe than what it's designed for is used. What are the expected consequences if fe lower than what it's designed for is used ? Distortion ?
I still have some originals that are ok, is there a reliable place that could have them measured ?
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