I recently unpacked a Mastersound 845 reference that had been boxed for 10+ years. it's makes loud crackling noises even with the source on mute. the bias voltage measured 25-45V with the trimmers turned to lowest settings. per instruction it should be 15V. I think it's not from the tubes as I had swapped tubes from left to right channels but the measurements and noise do not change positions. the right channel that measures 45V bias voltage is also the noisiest. I've also swapped in a completely different set of tubes. I'm hoping some of the gurus on this board can help me debug this thing. TIA.
If unused for a long time, electrolytic capacitors dry out and cause issues. Replace, with the same voltage and capacitance, the electrolytics. Check other components like anode load resistors and give the controls a good clean with switch cleaner and lubricant. There are many makes out there, I prefer Electrolubes Contact Treatment Grease.
SGB200DR | Electrolube SGB 200 ml Contact Grease Aerosol for Contacts, Thermoplastics | RS Components
SGB200DR | Electrolube SGB 200 ml Contact Grease Aerosol for Contacts, Thermoplastics | RS Components
I think the high bias is the first problem to address. The Mastersounds use a common bias feed for all the valves, so if one valve is showing high, look for the components that could effect one valve: the valve's bias pot, cathode resistor and cathode bypass capacitor. Which exact model is it?
First, the amp you have place the bias test point on a set of cathode resistor. A 45V measurement indicates that the plate current (Ip) is three times what it is suppose to be. Very bad things will happen to the amp if you leave it at this stage for too long. This being a very expensive unit, you might consider letting someone who know what is doing to fix it. In addition, 845 uses extremely high voltages (ie. 1000V+) to archive power. Personally, I wouldn't touch it (unless with a 3 feet pole 
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There are a number of things that could cause this, I would check the negative bias voltage rectifier circuit (the four trim pots and the surrounding parts) and check the four coupling capacitors for leakage. Watch out for cap charge after power down. Stay safe.
.There are a number of things that could cause this, I would check the negative bias voltage rectifier circuit (the four trim pots and the surrounding parts) and check the four coupling capacitors for leakage. Watch out for cap charge after power down. Stay safe.
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