Marshall JVM410H Re-Bias and Servicing
Customer Complaint
The customer had replaced the power tubes with a new matched quartet of EL34 power tubes and needed to have the amp re-biased to use the new tubes most effectively. The customer also complained about about noise in the amp and asked that it be checked
Equipment Information
The Marshall JVM410H is an extremely versatile tube amplifier meant to cover lots of tonal ground with one amp. The amp has 4 channels, each with 3 "modes" (green, yellow and red) making for 12 distinct tones. The amp also has foot switchable master volumes and two separate effects loops. Each channel on these amps has separate volume, gain and tone controls and there is a lot of control flexibility in the output section as well. because of this, there are lots of controls that can get noisy over time and the 28 potentiometers on the front panel don't make it particularly easy to service.
Initial Inspection
I performed an initial inspection by plugging the amplifier into a dummy load so that I could read the output on my oscilloscope. This would allow me to read the output waveform to check for noise and irregularities.
The output from the amp was fine, but whenever certain controls were touched, there was a ton of scratching and noise. This points to dirty pots and switches in need of cleaning and lubrication.
Repair
Once I determined that the pots were in need of some maintenance, I got to work pulling the 28 knobs off the front of this beast and removing the nuts and washers from each control. I then carefully removed the control boards from the amp so that I could properly access the pots and switches to clean and lubricate them effectively.
I first sprayed each pot out with an electronic cleaner and turned them to loosen up any dust and oxidation. Then after drying I applied a contact cleaning and lubrication solution and turned the pots a few times. This solved the control noise on the amp so I reassembled the amp and double checked it on the scope.
I then set to re-biasing the amplifier. With the amp still plugged into the dummy load, I located the bias measurement points in the amp and let the amp warm up for a few minutes on the bench before checking the voltage on each pair of Output tubes. there are two measurement points on these amps and two trim pots to adjust the bias on each pair of the four total tubes. This amp is said to sit happily at 70mV from the test point so I biased each pair to 35mV to reach that total.
These somewhat newer Marshall tube amps have pretty handy test points for the bias. The slightly older JCM amplifiers have very handy exterior test points and trim pot access do that you don't need to remove the chassis from the amp to get to them. This amp however is a little more troublesome so the chassis mush be removed to access the bias adjustment and measurement points.
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