I'm a newbie on this.
Did a cleaning(pots/relay/switches) job on this F411R amp, had a lot of weak/bad solder joints. Also cleaned the bias trim pots with contact cleaner after desoldering from PCB, offcourse have measured resistance before cleaning/turning.
Once all in place, wanted to check/adjust bias current. Service manual says:
"This adjustment should be performed under thermal stable condition. Before starting the adjustment, wait for more an hour after switch off for cooling, and perform the adjustment within one minute after switching on."
Is that a thermal stable condition? After switching on it begins to heat. Service manual recommends 2mV (+ - 0.3) over the test points, which connects to the emitter resistors. Two 0.22Ohm in parallel. So recommended bias current should be app. 18-19mA Thing is, when I measure it while switching on amp (cold), within 1 min the voltage raises from 1.7mV to 5mV. After warming up/playing for 10 min, I measure 12mV. I've used two different multimeters, same reading. During measuring no source and no speakers connected or switched to B (speakers connected to A).
Exactly how/when should I measure? With 12mV after warming up, the heatsink doesn't get too warm to touch. I think 45-50 degrees C after playing 10min with 8Ohm stable speakers. Room temp is quite low though, around 15 C Power amp has a pair of Sanken 2sc3854/2sa1490 for each channel.
Did a cleaning(pots/relay/switches) job on this F411R amp, had a lot of weak/bad solder joints. Also cleaned the bias trim pots with contact cleaner after desoldering from PCB, offcourse have measured resistance before cleaning/turning.
Once all in place, wanted to check/adjust bias current. Service manual says:
"This adjustment should be performed under thermal stable condition. Before starting the adjustment, wait for more an hour after switch off for cooling, and perform the adjustment within one minute after switching on."
Is that a thermal stable condition? After switching on it begins to heat. Service manual recommends 2mV (+ - 0.3) over the test points, which connects to the emitter resistors. Two 0.22Ohm in parallel. So recommended bias current should be app. 18-19mA Thing is, when I measure it while switching on amp (cold), within 1 min the voltage raises from 1.7mV to 5mV. After warming up/playing for 10 min, I measure 12mV. I've used two different multimeters, same reading. During measuring no source and no speakers connected or switched to B (speakers connected to A).
Exactly how/when should I measure? With 12mV after warming up, the heatsink doesn't get too warm to touch. I think 45-50 degrees C after playing 10min with 8Ohm stable speakers. Room temp is quite low though, around 15 C Power amp has a pair of Sanken 2sc3854/2sa1490 for each channel.
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You should follow the manual, and no, it is not a thermally stable condition.
The fact the bias current increases with temperature is normal in a lot of amplifiers and simply means that the design is less than optimal in that regard. There is nothing you can do about it. Also, the optimum bias current may well be something different again and it may not be advisable to try and adjust for that due to thermal contraints i.e. the amp may run to hot.
Follow the manual and enjoy your amp
The fact the bias current increases with temperature is normal in a lot of amplifiers and simply means that the design is less than optimal in that regard. There is nothing you can do about it. Also, the optimum bias current may well be something different again and it may not be advisable to try and adjust for that due to thermal contraints i.e. the amp may run to hot.
Follow the manual and enjoy your amp
Still can't understand how to follow the manual
After a cold start, within the first minute voltage raises from ~1.7mV to ~5.5mV.
I shouldn't have played with those trims in the first place. I did that because the first measurement after 4 min warming up showed 12-15mV (can't remember exacly) I worried a bit and lowered it immediately. Now I've leaved it as it was approximately. Sounds very nice and heatsink doesn't get too warm to hold after an hour of loud playing.
After a cold start, within the first minute voltage raises from ~1.7mV to ~5.5mV.
I shouldn't have played with those trims in the first place. I did that because the first measurement after 4 min warming up showed 12-15mV (can't remember exacly) I worried a bit and lowered it immediately. Now I've leaved it as it was approximately. Sounds very nice and heatsink doesn't get too warm to hold after an hour of loud playing.
I think there are two factors contributing to the relative instability here:
1. The bias transistor is a TO-92 connected to the heatsink via a bracket. Not bad per se, a bit slow though.
2. Most of all, however, the drivers are in the µPC1298V, which is not located on the same heatsink and has a small and thin cooling bracket only.
It does seem like they mean 2 mV cold, which would increase to a more sensible value when warmed up. The heatsink looks decent-sized and supplies are not that high at +/-36 V nominal, so 12 mV when warmed up (a good 50 mA and change) sounds quite manageable and would be near the theoretical ideal.
1. The bias transistor is a TO-92 connected to the heatsink via a bracket. Not bad per se, a bit slow though.
2. Most of all, however, the drivers are in the µPC1298V, which is not located on the same heatsink and has a small and thin cooling bracket only.
It does seem like they mean 2 mV cold, which would increase to a more sensible value when warmed up. The heatsink looks decent-sized and supplies are not that high at +/-36 V nominal, so 12 mV when warmed up (a good 50 mA and change) sounds quite manageable and would be near the theoretical ideal.
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