Dr Nomis asked me to try and fix his trouble prone MA100C amp after he had taken it back to the shop for "warranty repairs" ( that didn't fix the problems ) and then tried to fix it himself.
I took it on as a "rainy day" repair project and after I had a few attempts at it, the amp now seems to be running reliably for the first time since it was new. Phew.
As
phatt said in previous posts, most current production EL34s are unreliable when hit with high screen voltages and the first thing I found when testing Dr Nomis' amp was 510-515 VDC B+ and 500 vDC screen voltages, resulting in one EL34 red plating and another EL34 drawing way too much screen current.
Playing with bias currents and fitting fresh EL34 valves didn't solve anything, so I knew there were some problems inside the amp. Here's what I found and what I did...
A] The Mains transformer only has 115 vAC and 230 vAC primary taps. Connected to our local mains supply which averages around 245vAC, the HT secondary measured around 365vAC at idle and heater voltages were 6.7 vAC.
I mounted a 7 way tag strip on the chassis near the rectifier circuitry and inserted 2 x 22 ohm 10 watt resistors (in parallel for 11 ohm total resistance / 20 watt ) between the mains input IEC socket Active and the mains power switch to drop the primary voltage under load.
This dropped the secondary voltages to around 345vAC HT at idle, "sagging" to 325 vAC at full power, or around 445-450vDC. Heater voltages dropped to 6.3 vAC for the EL34s and 6 vDC for the preamp valves.
B] I cleaned carbon off the inner pair of output valve sockets using acetone and non-residual contact cleaner and re-tensioned the pins on all the sockets. One EL34 (V7) had "flashed over" at some stage, which apparently hadn't been spotted by whoever did the previous repairs.
C] I replaced the heater ground reference 100 ohm 1 watt resistors with 2 watt metal oxide resistors. The factory 1 watt heater resistors looked like they had been running very hot and were reading over 120 ohms.
D] I changed the 1k / 5 watt screen resistors to 1.5k / 5 watt . This reduced the screen currents and gave the current production EL34 valves a better chance of surviving,
At this point, the amp (with newly fitted EL34's) was working OK up until around 70% of full power, then the output waveform would go badly asymmetrical with one half of the waveform going into hard clipping with some nasty spikes.
I had a couple of known good Marshall OT's at hand, so I de-soldered the leads from the MA100C OT and "temp-clipped" a SLP 100 reissue OT in. Same asymmetrical clipping, so I changed the driver / PI 12AX7 valve, checked the voltages on the pins and tried again. Still nasty asymmetrical clipping, so I unhooked the SLP OT and "temped" in a JCM 2000 OT, with the same results.
I now knew that the problem was somewhere on the PCB, and since the output at the FX Send was clean and symmetrical at very high levels, I started testing the FX Return circuit and viola! There was the asymmetrical signal between the output of the FX Return gain stage and the PI valve input.
E] Testing voltages and components in the FX loop indicated that several parts were faulty and needed replacing. TR91 (TN2404KL MOSFET) and T92 (J202 FET), D90 (24v zener), D93 & D94 (3v3 zeners) and R101 (2k2 0.6 watt resistor) were all suss or measured out of spec., as was R102 (220k / 0.6 watt resistor), which I replaced with a 220k / 2 watt metal oxide resistor.
I refitted the PCB, connected everything up and was rewarded with a nice symmetrical waveform through the FX loop and into the test load at all levels up to clip point, and textbook symmetrical clipping from about 90 watts RMS output.
After checking power supply voltages and setting output bias currents I ran the amp at full power for 10 hours with no problems. Yay!
Hopefully this will help other bunnies when they're trying to diagnose and fix these MA series amps.
Cheers, BB
