Hello,
I'm working on a 4140 and have a spare parts unit. Two of the channels on one side work fine and bias up well. The other two need attention. I'm aware of the errors on the schematic (thanks Catrafter). In short, I can't get the bias voltage down to the required 10mA. The lowest I can get on one channel is about 85mA and on the other, around 115mA. I've tried adjusting the balance pots still can't get close.
What I've done so far - I've extended the leads on the bottom of the board so it can be removed enough to access the components for testing voltages. I've removed the four power transistors as well as the next eight transistors, testing for voltage drops and shorts. All test good BUT when I put in a pair of power transistors from the parts unit, that side's voltages got more stable and the VU meter from that side, which was bouncing, quieted down to an almost negligible bounce.
I've replaced the two 220uF capacitors with 470uF 50V caps.
I'm getting good voltage readings on the output transistors although the emitters on H001 and H004 do fluctuate quite a bit, but are generally in the low range as indicated.
I am used to working on tube gear and although my soldering and testing skills are good, I am still fuzzy about the transistor circuits, NPN vs PNP and what is happening in general. In addition to repairing this unit for my brother I 'd like to learn some theory as well. For instance I am not clear on how the bias voltage is developed and which components control it.
Any help appreciated.
I'm working on a 4140 and have a spare parts unit. Two of the channels on one side work fine and bias up well. The other two need attention. I'm aware of the errors on the schematic (thanks Catrafter). In short, I can't get the bias voltage down to the required 10mA. The lowest I can get on one channel is about 85mA and on the other, around 115mA. I've tried adjusting the balance pots still can't get close.
What I've done so far - I've extended the leads on the bottom of the board so it can be removed enough to access the components for testing voltages. I've removed the four power transistors as well as the next eight transistors, testing for voltage drops and shorts. All test good BUT when I put in a pair of power transistors from the parts unit, that side's voltages got more stable and the VU meter from that side, which was bouncing, quieted down to an almost negligible bounce.
I've replaced the two 220uF capacitors with 470uF 50V caps.
I'm getting good voltage readings on the output transistors although the emitters on H001 and H004 do fluctuate quite a bit, but are generally in the low range as indicated.
I am used to working on tube gear and although my soldering and testing skills are good, I am still fuzzy about the transistor circuits, NPN vs PNP and what is happening in general. In addition to repairing this unit for my brother I 'd like to learn some theory as well. For instance I am not clear on how the bias voltage is developed and which components control it.
Any help appreciated.