Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: spunko on August 24, 2023, 11:45:01 am
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Hello, a friend of mine bought this Marshall DSL 100h kinda cheap, not working, and acting weird. These are the series of events for the initial test:
1.- First power-on was using a bulb limiter that inmediately light at max bright, so I suspected a short somewhere.
2.- Took the chasis out, visually inspect and noticed FS2 (500mA) was lifted from one clip, I pushed it back to its clip, tested again and the bulb limiter didn't light bright this time, that was weird. Kept checking and noticed mains fuze (4A) and FS1 (1A) where blown and soldered a wire between their terminals :sad2:
3.- Replaced the fuzes with the correct ones, using a bulb limiter powered the amp and when engaged the standby switch the amp didn't sound and the pilot light from the power swicth went off (no light), the channel switching didn't work either, but as soon as I swicthed stand by off, the pilot light went on again. Also tested the pre amp using the FX loop with another power amp and no sound. All this time the power limiter bulb didn't light bright.
4.- Took out the 4 power tubes and tested only with preamp tubes, this time the amp did work, I had sound on the preamp sent to another power amp, and also the channel switching was working. So I tested with another quad set of EL34's but the same happened in point 3, no sound, no channel switching, no pilot light.
5.- This time I tested with 2 of the original power tubes and the amp seemed to work fine again, then took those out and put the other pair and again it worked in the middle or in the ends, but when the 4 power tubes are installed the amp behaves like point 3.
6.- One day have passed. The amp is sitting without power tubes, only pre amp tubes. Always with the bulb limiter, I powered on the amp today just for my pleasure and channel swicthing didn't work, and I'm pretty sure I didn't hear the relays acting. Then I powered the amp off and on again and this time the channel did swicth normally. So this time it failed without the power tubes.
Does anyone have experince with this type of fault, or with this amp, that can help me?
Thanks.
(https://i.imgur.com/5SyfLQ7.jpg)
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When was this amp made?
Early DSLs from the late 90s to early 00s have gained a rep of being extremely problematic. Dry joints, rectifier modules that overheat and desolder themselves, conductive pcbs that need replacement.
Main and bias pcbs are available, so it can be resurrected.
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I think is from 2017. It has a sticker that may be a date. 09/17. Is the one with the digital reverb.
(https://i.imgur.com/V3e36XZ.jpg)
(https://i.imgur.com/f9HwuV2.jpg)
(https://i.imgur.com/kcEll3c.jpg)
(https://i.imgur.com/Xfin9Lq.jpg)
(https://i.imgur.com/kx8z2EG.jpg)
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So the problem was the bulb limiter this whole time. Someone in another forum suggested me to test the amp without the limiter and it worked flawlessly. They mentioned that the logic board needs an specific voltage to work and pass signal. I was trying to be safe but I have learned my lesson.
I'm pretty sure what blow the fuze was a poorly biased amp. One side was reading 40mA and the other 120mA, also those 2 power tubes have signs of over heating. Now both sides are at 86mA and 84mA, couldn't get to 90 mA tho.