The amp was now running well, and I had biased the new matched quad tubes down to a more "within spec" 0.8V, not 1.35+ Volts that Marshall released the amp with. Mounting it with a heatsink and a couple of centimetres off the circuit board for heat and ventilation. Next, for good measure and because it's so cheap, I replaced the stock bridge rectifier with a higher rated one. So I repaired the circuit board and sorted that all out. Where those original resistors had died, the tracks on the other side were burnt and lifting. The amp was now running stable and I was getting sound, albeit, extremely crackly. In fact, considering the age of the tubes, with my friends permission, I replaced the entire set of tubes. At this stage, I looked at the tubes and replaced all 4. The new ones I replaced again worked fine. I replaced the resistors again and removed the tubes. What was suppose to be around 18V DC in standby mode and about 375V DC when switched on, was closer to 475+ I turned everything off, let it cool down and bled the caps. I decided to leave things on and measure the voltages. The resistors I just replaced fizzled, exploded and let all the magic smoke out. Next was to see what happened when I turned things on and flicked the standby switch. I replaced them with the new values (knowing, now that I did some reading, that Marshall requests replacing them with higher value components due to the older DSL401's having been biased incorrectly from the factory. 2 of the 4 resistors were black and some were completely disintegrated. I noticed that some of the resistors near the tube sockets were blown. So I was just gathering info to learn its history. These weren't any special test points, at this stage I had no schematic and I had not worked on this model Marshall before. I measured around the place in various spots to get a rough idea, and noted down the voltages. Before flicking the standby switch, let's see what the voltages are light inside. Next up I got the power light/switch lighting up. This was a very glamorous fix, replace the main fuse. I have a multimeter and a gas soldering iron.įirst thing was to see what life this amp had in it. I don't have a variac, nor do I have an oscilloscope. I got the amp from him with no lights or anything working at all. I also thought I'd post this for anyone interested in reading how I got the amp working from being completely dead. However, I thought I'd write up about my friends JCM2000 DSL401 that I repaired for him after it had been through years of hell with various "amp technicians". I am a subscriber here and I don't often post or comment too much. So, my knowledge and skills come purely from basic understanding and eliminating the causes. However, more than 10 years ago I was involved in a pretty catastrophic vehicle accident that left me unable to walk for two years.ĭuring that time, I learnt to build my own tube amps thanks to the AX84 community, and some local Yahoo groups I joined at the time. TL:DR - Fixed a Marshall JCM2000 DSL401 from completely dead to 100% working better than factory.
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If you decide, whether suggested to you here or not, to open your amp and tinker with it, PLEASE BE CAREFUL, and learn how to do so safely. Tubes amps make use of very high voltages! These voltages can kill you dead. Whether you're a toob noob or Leo Fender reincarnated, you're welcome here. Welcome to r/ToobAmps - A place to discuss that sweet tube tone.