Hi All,
New member here and am looking for any assistance.
Well, I purchased a used GFA-555 and the unit was working until I touched it.
I had opened it up to measure the bias. I was testing on the right channel resistor on the voltage amplifier board (not end board) when I accidentally shorted something.
The odd thing is the fuse on the output board blew. I borrowed the fuse from the left side and powered it up and the orange clipping light stays on and the speaker outputs on the right channel have 82 volts Dc on it.
I'm not a technician but can solder and measure using a DMM and have some very basic understanding of electronics.
I know this is a very old amp but was wondering if anyone can help me troubleshoot?
Now I was reading through the board last week and I came across a thread where someone did the exact same thing but I cant find it now.
I guess my other question is did I blow the Toshiba transistors as looking around the net, they don't make them anymore.
Any help would be greatly appreciated and any parts recommendations especially for the transistors would have me indebted to you if identified as faulty.
I really like the amp and the design looks very simple which is why I want to keep it. I guess this is my incentive to rebuild it from top to bottom.
Thanks everyone,
Rob
New member here and am looking for any assistance.
Well, I purchased a used GFA-555 and the unit was working until I touched it.
I had opened it up to measure the bias. I was testing on the right channel resistor on the voltage amplifier board (not end board) when I accidentally shorted something.
The odd thing is the fuse on the output board blew. I borrowed the fuse from the left side and powered it up and the orange clipping light stays on and the speaker outputs on the right channel have 82 volts Dc on it.
I'm not a technician but can solder and measure using a DMM and have some very basic understanding of electronics.
I know this is a very old amp but was wondering if anyone can help me troubleshoot?
Now I was reading through the board last week and I came across a thread where someone did the exact same thing but I cant find it now.
I guess my other question is did I blow the Toshiba transistors as looking around the net, they don't make them anymore.
Any help would be greatly appreciated and any parts recommendations especially for the transistors would have me indebted to you if identified as faulty.
I really like the amp and the design looks very simple which is why I want to keep it. I guess this is my incentive to rebuild it from top to bottom.
Thanks everyone,
Rob
Hi Rob,
For completeness in case this turns up in a google search ..
You don't need to use the original transistors in this amplifier. What is important is that the entire set is the same and roughly close in beta to each other. I would tend to use either the MJ15022 and MJ15023, or the MJ21193 and MJ21194 for outputs. Be aware that with 80 VDC rails, the outputs and drivers will have to withstand the full 160 VDC if you get into clipping. That's some serious power. You will receive a shock if you touch both supplies, and it will not be pleasant!
If you find one output transistor blown, you need to replace all of them (and the drivers) on that channel. What you can get away with in a low powered amplifier will not fly with higher powered amplifiers.
-Chris
For completeness in case this turns up in a google search ..
You don't need to use the original transistors in this amplifier. What is important is that the entire set is the same and roughly close in beta to each other. I would tend to use either the MJ15022 and MJ15023, or the MJ21193 and MJ21194 for outputs. Be aware that with 80 VDC rails, the outputs and drivers will have to withstand the full 160 VDC if you get into clipping. That's some serious power. You will receive a shock if you touch both supplies, and it will not be pleasant!
If you find one output transistor blown, you need to replace all of them (and the drivers) on that channel. What you can get away with in a low powered amplifier will not fly with higher powered amplifiers.
-Chris
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