Yes it can, I forget the actual mV required - but if you look for bias setting queries for the AU-9900 or AU-11000 the power amp is almost exactly the same, thus the same bias setting procedure can be used. 33mV seems quite a lot, I am in a bit of a hurry right now, but I will check on this and post again with some more complete guidance later - if you haven't found an answer in the meantime.
Thanks Hyperion! I’ll start to check out other threads to improve my understanding, when you get a chance can you confirm which emitters I need to measure against there’s 4 per sideYes it can, I forget the actual mV required - but if you look for bias setting queries for the AU-9900 or AU-11000 the power amp is almost exactly the same, thus the same bias setting procedure can be used. 33mV seems quite a lot, I am in a bit of a hurry right now, but I will check on this and post again with some more complete guidance later - if you haven't found an answer in the meantime.
).
Thanks Hyperion for the detailed answer here much appreciated! I’l aim for 30mv and follow your directions, will report back once doneThe bias current setting method is the same as for the AU-9900/AU-11000 (50mA bias current), but the actual bias current is higher for the BA-2000 at 100 mA.
The method asks you to disconnect the supply to the Output Transistors and the drivers and measure and adjust using the current meter in-line method. This is OK, but because there is a current surge at power on, this could in some cases blow the current range fuse in the meter causing confusion for some. A much better method in my opinion is to measure the voltage across one (or two) emitter resistors associated with one OP transistor pair. In the AU-9900/AU-11000 there are two pairs of OP transistors per channel, this is the same for the BA-2000.
The total bias current for one channel of the BA-2000 comprises bias current for two pairs of OP transistors and also the preceding driver stages. You could set one channel using the in-line current meter, and then measure the voltage resulting from the bias current across one pair of emitter resistors - and from that arrive at the amount of bias current that the manufacturer intended.
As an approximation, this will be 100mA total - subtract 10mA for the drivers, leaving 90mA - divide that by 2 to get the current per transistor pair = 45mA just as a rough estimate.
So V = I x R V= the voltage across 2 emitter resistors, I = the bias current of 45mA or 0.045 A, and R = 2 x 0.33Ω or 0.66Ω
Substituting the values V = 0.045 x 0.66 = 0.0297 V or 29.7 mV - (so 33mV wasn't so far out after all
So, connecting your meter set to mV, to the emitters of one pair of OP transistors (helpfully labeled TR01 & TR01 or TR02 & TR02 ! ) to measure across 2 emitter resistors), you need to adjust for ~30mV.
While every effort should be made to minimise bias current drift and set it accordingly, precise bias current setting is not critical, so plus or minus a few millivolts isn't going to matter one way or the other. Acquaint yourself with the correct trimmer location to adjust the channel you are monitoring. Connect your meter using minigrabbers with the amp powered down, when you are happy with the placement and security of the meter leads - power up - and adjust. (you might need to warm up the amplifier to allow the bias current to settle).
I hope this helps - please ask questions if anything is unclear.
I had a look this morning and the diagram, if I understand correctly, these are the two points of the R03 to measure per channel when checking a pair of TR01?Thanks Hyperion for the detailed answer here much appreciated! I’l aim for 30mv and follow your directions, will report back once done
Thanks! Will give it a go and report backI think you have identified the measurement points correctly, thankfully the measurement points seem to be easy to access.
It doesn't matter which OP transistor pair you make the measurement for, TR01 or TR02. Rather weirdly, the emitter resistors to measure across are on different boards, as you correctly indicate.
Is that these two plugs on the meter board?First, try the obvious - swap the inputs L -> R and vice versa, then do the same with the speakers - see if the problem moves or stays put.
Possibly, you could look at those - remove and test perhaps? (check - especially Hfe) - also a check of R01 & R02 (8.2Ω x 4) and R59 & R61 (68Ω x 2) and R55 & R57 (1.2KΩ x2) - values here as given on schematic, but could be different.
If you didn’t replace all those awful uniwatt transistors, I’m not surprised there’s issues.
I’ll check and report back
Do the pairs need to be gain matched?
