Recently recapped my dh-220. on the left channel was able to set the bias current to 275ma and dc offset to 0V with no problems.
However, on the right channel, I cnat get more the 60ma on the bias. I check P2 and it seems fine. Resistance goes up to 1K.
So I started checking the other resistors in that network. R22 and R23 both measure to spec. R38 isn't the 28K resistor called for in the parts list, but is 17.4k. This is the same in both channels, however in the right channel, it measures at 17.1K (on the left it measured to spec).
So, I'm thinking I should replace that resistor. 300 ohms is almost 30% of the range of P2.
But, I thought I'd check in here first to see what the resident Hafler experts thought ;-)
Note all measurements where done with components in the board.
Thanks,
James
However, on the right channel, I cnat get more the 60ma on the bias. I check P2 and it seems fine. Resistance goes up to 1K.
So I started checking the other resistors in that network. R22 and R23 both measure to spec. R38 isn't the 28K resistor called for in the parts list, but is 17.4k. This is the same in both channels, however in the right channel, it measures at 17.1K (on the left it measured to spec).
So, I'm thinking I should replace that resistor. 300 ohms is almost 30% of the range of P2.
But, I thought I'd check in here first to see what the resident Hafler experts thought ;-)
Note all measurements where done with components in the board.
Thanks,
James
How do you set the bias when a regulated power supply is connected to giant capacitors and then connected to the driver board on the DH200. When're do I put the test leads and where do I disconnect to put my meter leads in parallel to get the MA reading? My Halfler is built in a Bryston 4 chassis and there is very little room to do anything! I can however see the bias pot and can easily adjust it.
There is no current sensing resistor in the output stage. The way to adjust bias is to pop out one of the fuse and put an amp meter in serial with the power rail instead. I use alligator clips to secure the connections. In that way, you could see the current. Make sure you select a proper range big enough on the multimeter before you power on, otherwise you may blow the fuse of your multimeter.
Last edited:
Does it make any difference that I have a fully regulated power supply feeding the board. I have two giant capacitors connected in parallel on the output of the supply. The amp has been running flawlessly for 35 years!