A Marshall JCM900 100 Watt Combo arrived in the workshop. This version is the 4102 It was very poorly.
The first issues seen were :
- 10 amp fuse in the plug!!
- Low power output and VERY loud mains hum
- Pots noisy
- Amp plays a dying gasp when amp switched off
- Pops / crackles and very low volume. The valves were tested on an AVO MkII and CT160 tester. The preamp valves were down to a lot less than 50% of the Gm = very low gain. The output valves were all over the map. Definitely not matched at all! And the Gm on one indicating totally worn.
The usual checks were made on the amp and the following things tacked :
- New valves were fitted. Tung Sol 5881’s and Sovtek 12AX7WA’s. The Soviet 12AX7WA is a medium gain valve ad takes the hrashness out of the overdrive which is done with a diode bridge ( at east I liked the sound better !)
- The bias current was set to 60%( 35mA) for a balanced sound and to maximise the output valve life.
- Some hum was due to the mis-matched and worn output valves
- Most of the hum was due to a leaky 33uF capacitor drawing too much current and this cooked the 10K resistor (1 watt). These components supply the HT volts to the pre-amp valves. They were being starved of volts and with weak pre-amp valves and output valves compounding the problem, the result was a 100 Watt amp producing about 25watts ! The amp was stripped down and the 33uF capacitor replaced. A 3 watt 10K resistor was fitted for better volts and power handling and less heat in the resistor. The amp now has minimal hum and within the normal limits. The amp is now extremely quiet from a background noise perspective.
- The amp was re-assembled and PAT tested.
This Marshall JCM900 100 Watt Combo is LOUD !!! the mains hum and weird noises have all gone and the amp is sounding just like a JCM900 should.