Hello, I'm fairly new to the world of solid state amps, more of a tube guitar amp guy. A buddy brought me a Parasound HCA1000A which needed new filter caps. The amp seems to be working fine now but I would like to verify that the bias is correct. My understanding is that the bias calculation/adjustment procedure is to measure voltage across output transistor emitter resistors then use ohms law to drive current and adjust for 30-40 mA. So I'm measuring voltage on the
0.33 ohm emitter resistors on the big 2SC3519 and 2SA1386 transistors but I measure 0V on all of them. Is there something I'm doing wrong?
J.
0.33 ohm emitter resistors on the big 2SC3519 and 2SA1386 transistors but I measure 0V on all of them. Is there something I'm doing wrong?
J.
It appears that the DC voltage across each emitter resistor should be 10mV.
The DC current per output device would then be 0.010V/0.33R = 30.3mA.
Don't adjust the voltage higher than that, as the amplifier could overheat or fail.
Do you have the DVM set to DC volts, and on its most sensitive range?
Are you measuring directly across the emitter resistor? What kind of DVM
do you have? It should be able to read 10mV accurately, with one zero
after the decimal point when the readout units are in mV.
The DC current per output device would then be 0.010V/0.33R = 30.3mA.
Don't adjust the voltage higher than that, as the amplifier could overheat or fail.
Do you have the DVM set to DC volts, and on its most sensitive range?
Are you measuring directly across the emitter resistor? What kind of DVM
do you have? It should be able to read 10mV accurately, with one zero
after the decimal point when the readout units are in mV.
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OK, thanks for the tips, guys, I was able to adjust the emitter current to 10mA for the 12 output transistors. However, I have an additional problem I was hoping you could help me with, it would save me starting a brand new thread. Since I first turned it on yesterday the "current overload" LED on the front panel is lit up for the right channel. After adjusting the bias it is still on. The channel seems to be functioning normally, no different than the other channel. Any thoughts on this?
Hmmm, this has got me stumped for the moment. Looking at Q003 (right channel) and Q004 (left channel) both have 15V at the base and emitter. Q004 has 0V on the collector and same for R009 and the left channel overload indicator LED. But on Q003 there is 5V at the collector and 1.8V on the LED side of R008, hence the lit LED. I assumed Q003 was the culprit but when I pulled it it tests normally on my DMM diode test and when I put another general purpose PNP transistor in place of it I still have the exact same scenario. I can't find any difference in the components from one channel to the other. Does that information provide any clues?
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