Bias Circuit
Posted: Tue Feb 01, 2005 11:42 am
Okay, I finally got around to checking all the components on my '76 Superlead. Firstly, I'm working off Marshall Drawing 78331, I'm just wondering if this drawing is accurate to the model I'm looking at? There are a few tone related capacitors that seem to differ in value, though I'm not overly worried about these.
The anomalies that are bothering me are all in the bias circuit. I have the original problem with the 47k resistor in series with the trim pot (R28 on the marshall drawing) drifted up to 66k or so. What is also puzzling me is that 47K is, in the first place, the "USA Variant" according to the schematic. All of the other components follow the standard value, including "R8", the 27K resistor just in front of the diode in the bias supply. The next problem is the two earthing capacitors C15 and C16. In each case I have a pair of 22uF electrolytic caps instead of the single 10uF. The marking on the PCB seems to indicate the original 10uF. There's messy soldering around these parts, as well as on R8, the diode, and R27, the 15k between the two caps. This leads me to believe that these components were all replaced at some stage.
I'm going to replace R28, C15, and C16 - the bias resistor and the two caps. What I'm not sure of is whether to replace R28 with a 56K as indicated in the drawing, or a 47K as it originally was. With R8 at standard spec I would have assumed that 56k would suit better, but as it sits (at 66k) I can't even get my bias above 10mA. Any thoughts?
The anomalies that are bothering me are all in the bias circuit. I have the original problem with the 47k resistor in series with the trim pot (R28 on the marshall drawing) drifted up to 66k or so. What is also puzzling me is that 47K is, in the first place, the "USA Variant" according to the schematic. All of the other components follow the standard value, including "R8", the 27K resistor just in front of the diode in the bias supply. The next problem is the two earthing capacitors C15 and C16. In each case I have a pair of 22uF electrolytic caps instead of the single 10uF. The marking on the PCB seems to indicate the original 10uF. There's messy soldering around these parts, as well as on R8, the diode, and R27, the 15k between the two caps. This leads me to believe that these components were all replaced at some stage.
I'm going to replace R28, C15, and C16 - the bias resistor and the two caps. What I'm not sure of is whether to replace R28 with a 56K as indicated in the drawing, or a 47K as it originally was. With R8 at standard spec I would have assumed that 56k would suit better, but as it sits (at 66k) I can't even get my bias above 10mA. Any thoughts?
) which was a fairly common way of adjusting the bias from 6550's to EL34's here in the states.