I recently purchased a nice Luxman MQ-70 from Japan. I’m getting to know the amp as I await a quality step down transformer. Unfortunately it did not come with a service manual, so I don’t know how to adjust current / bias and voltage (if applicable). I did see people have posted their versions of a schematic on this forum, thanks for doing that.
Does anyone have a service manual for this amp, or can anyone provide guidance on how to best adjust it? Seems like the innards have a number of variable resistors, and while I partially know my way around electronics, I only know enough to be dangerous. 🙂
Thanks...
Does anyone have a service manual for this amp, or can anyone provide guidance on how to best adjust it? Seems like the innards have a number of variable resistors, and while I partially know my way around electronics, I only know enough to be dangerous. 🙂
Thanks...
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Unfortunately the schematic you provided is for a different amp - likely the MQ-60, where I own the MQ-70. Completely different circuits. Thanks for the effort, though...
Thanks, I did see that schematic. I also got these values from the same or similar forum:
Output stage B+ = 502V
Output tube bias = 40V
First stage plate voltages = 146V
Driver stage plate voltages - 350-360V
I guess that my question is for each of these locations, am I measuring right at the variable resistors, or are there other particular places I should be putting the multimeter? If you could point out where on the schematic that would be incredibly helpful.
Thanks!
Output stage B+ = 502V
Output tube bias = 40V
First stage plate voltages = 146V
Driver stage plate voltages - 350-360V
I guess that my question is for each of these locations, am I measuring right at the variable resistors, or are there other particular places I should be putting the multimeter? If you could point out where on the schematic that would be incredibly helpful.
Thanks!
Adjust the bias and balance pots for the right plate currents, as measured directly across
each output tube's cathode 10R resistor. Likely to be 0.50VDC, which is 50mA x 10R.
However, the factory spec could be lower or higher than 50mA, so look for a service manual.
Adjust first a couple of minutes after turn on, and then again after about an hour.
You do not need to measure any other voltages. Use an accurate DVM, not a cheapo.
Do not adjust by the DC grid voltage. That will vary with each individual output tube.
each output tube's cathode 10R resistor. Likely to be 0.50VDC, which is 50mA x 10R.
However, the factory spec could be lower or higher than 50mA, so look for a service manual.
Adjust first a couple of minutes after turn on, and then again after about an hour.
You do not need to measure any other voltages. Use an accurate DVM, not a cheapo.
Do not adjust by the DC grid voltage. That will vary with each individual output tube.
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Thank you, Rayma! Very helpful. Will do that once a step down transformer ships in as it’s a 100V unit.
I do have a good multimeter, and I’ve biased amps before but usually there are more obvious points, but what you provided makes perfect sense. Thanks!
I do have a good multimeter, and I’ve biased amps before but usually there are more obvious points, but what you provided makes perfect sense. Thanks!
I just wanted to thank those who provided guidance. I was able to bias the amp, and since have installed a new IEC connector, replacing the thin lamp cord that was in place for the AC and added new binding posts. The amp sounds great and is definitely one of a kind in this country. Tube rolling has also been a delight.