fuse flows

[Salva]

HI am new to this forum but I really like it, I have a question and don't know where to start, I have a guitar combo amp dean markley, one day the fuse flowed, i went and got new ones but as soon as I power up the amp the fuse flows right away, I am little familiar with electronics but not sure where to start, can any body give me an idea what components I should check 1st.

[Rob Mercure]

Salva,

Perhaps your first action should be to go to a service shop - hopefully managed by someone who loves their work and likes to talks about it - and ask them. The fuse is a link of metal that sacrifices itself by melting to prevent a manfunctioning component or series of components within an amp from adversely affecting your house's wiring. And secondarily protecting the internal components. If we use the analogy of a river - everything is downstream of the fuse so it is quite difficult to separate out the other factors that are defective without some skilled knowledge. I believe you amp is a solid state machine. If you are willing to purchase several more of the correct fuses (and who knows, you might have had two defective fuses in a row) you could try unplugging the power transformer on the side where the power comes in and see if the new fuse blows when powered on. If it blows then there is some simple short within the few components on this side of the power tranny. If that doesn't make any difference replug the primary of the PT (power transformer) and unplug the secondary - then with a good fuse turn on the switch and see what happens. If the fuse blows then you've got a bad power transformer or any componenets that didn't unplug - if no change then it's something "downstream" of the PT and desribing how to check each individual piece via internet could take weeks. Now is the time, if not before, to approach you service shop and let then know you are interested in learning about electronics. Who knows, you might wind up with a sympathetic technician who has lotsa small jobs that he'll trade for helping you diagnose or fix your unit.

Hope this helps.

Rob

[Gtr_tech]

You have a shorted component either in the pwr supply or in the output section. If its solid state check the diodes and caps in the pwr supply, then the output/driver transistors. DM did make a tube amp at one time, so if you have that one the same rules apply, only you'll be looking at the output tubes rather than transistors.
Which model is it? We'd need to know that to guide any further.

[Salva]

the model is DMC-40 and it does not have tubes, I disconnected the transformer from the circuit and the fuse did not blow, I am checking the power capacitors and they seems to be ok, do I need to take the power caps out of the circuit to test them?, I will be checking the power bridge diode tomorrow, the one I don't know which one is it is the output/driver transistors if you can give a clue, thank you Gtr_tech for trying to help me, I took electronic classes long time ago and I am getting back, just trying to remember every thing is not easy, thanks again.

[Gtr_tech]

You're looking for a dead short, so you'll need to remove components until the short is no longer there. The schematic is on this page: http://www.deanmarkley.com/Info/LegacyAmps/Docs.shtml


I'd start at the output sections (there's 2 since this is a stereo unit, tip121, tip126 and related components) and work back to the power supply (mainly check the diode bridge, c89,c88, zd3,zd4).
The short will appear at the main filter caps either on the pos or negative side, possibly both.....so you will use you meter's continuity function to find the component(s) that have shorted.