This is my 300W 2SC5200 + 2SA1943 amp circuit. Wiring is completed, +48/0/-48vdc, in&out, speaker in&out... but there is no sound at all, not even a slight pop from speakers. I suspect from speaker protection relay. I supplied it +18vdc and GND, however green light is not lighting up. It states AC&DC 12-18V, what could I do wrong? Conductuvity is there.. When I turn power on power transistors are getting overheated... Any leads?
btw, there is a 1k variable resistor, is that master volume or bias? Maybe I need to tweak that...
I made the bias around 40 ohm, transistors are still getting way too hot, and relay still not working.
Did you short the output somehow? That could overheat half the output transistors. How are the transistors mounted on the heatsink?
Why aren't the output transistors attached to heatsinks and why don't they have Mica/Silicone insulators? Something is wrong, it seems that you have too much DC offset at the output and the protection will not turn on the relay. That's why it's called DC protection.
There is no short, I haven't mount them on the heatsink, I wanted to hear if they make sound.
@NIXIE62 I just recived the amp circuit, I wanted to have a little sound test to make sure it's functioning.
Update:
I managed to light green LED, but still no sound.
Symptoms:
What I tried:
I managed to light green LED, but still no sound.
Symptoms:
- No sound at all from the output.
- Output transistors were getting very hot at idle, even with no input signal.
- Speaker protection relay LED doesn't turns on, but there's no mechanical click — relay doesn't actually switch.
- Bias trimpot (1k) is present and tested, but adjusting it had no effect.
- DC offset at output seemed high (relay protection possibly active).
- All power rails are correct.
What I tried:
- I suspected the bias/Vbe multiplier transistor (a H669A in my case) was either faulty or wired incorrectly.
- As a test, I shorted the collector and emitter of the Vbe multiplier transistor, effectively disabling the bias circuit (forcing near-zero bias).
- After this:
- The output transistors stayed cool — no more idle overheating.
- The relay LED turned on, but still no click — so relay is not engaging.
- No sound at the speaker.
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This is never done, not even with the AB class, they can easily overheat. What is the DC offset at the output?
Meanwhile I will de-solder and check each 2sc5200 & 2sa1943 pair, they might be short. These transistors do look like second-hand.