amp output vs. headphone out - upstream or downstream?

[chowbird]

Hi everyone.  Please be patient with the noob slap across the face coming your way.  I am a novice in amp repair and am trying to move forward.  Thanks.  I procured a Marshall MG100hdfx for  small amount of money.  I am trying to determine whether or not it is worth the repair deposit.

Loud noise occurs on power up (loud but not killer, sounds like static or putting your finger on the tip of the instrument cable 1/4 jack.)  Does not matter whether an instrument cable is plugged in or not.  Still makes the noise.  Volume control has no effect on noise...it is constant.  It will not accept a signal from the guitar.  Interesting fact is all knobs and functions (except master volume) operate properly when using headphones.

Quote from reputable repair guy (who I have not informed yet that the headphones work):

Best case: $60 to fix minor stuff
Worst case: $45 dposit gone if amp is toast.
Next worst case: $120 to fix bad power amp and associated goodies that fail along with it.

If the headphones work, can the power amp still be bad?

Does this scenario ring any bells with anyone's past trials and tribulations?  Thx    Tim

[J M Fahey]

1) stop turning that amp on !!!  :grr You will kill the speaker and needlessly add U$150 to your bill !!!
2) You *VERY* probably have a burnt power module. Marshall sells them or they can be repaired, your choice depending on circunstance.
Here in Argentina I repair them (8000 miles away, freight and import taxes triple its cost); in USA getting the replacement usually saves on bench cost.
3) unusual among MI amplifiers, headphones have their own amplifier, that's why they work even with a blown power amp.

Remind your Tech to unplug the speaker because it's receiving DC through the dead Power Amp.
Oooops! is not an option.

Tell us how the saga ends, and good luck.

[chowbird]

Thanks J M.  It's just the head and headphones I tested with. I unplugged the cab.  Any idea how much a power module goes for?  I'm going to go searching on the web and Marshalls site to see what I can find out.  It is nice to have knowledgable help from the forum.  I really appreciate it.

[J M Fahey]

Marshall sells them.
Maybe you can also find an aftermarket one, because they burn so much that there is a cottage industry making them.
That said, that price applies if *you* replace it at home; but if the Tech does, the price is whatever his bench services cost, the same way that 4 ounces ground beef and a bun at the supermarket do not cost the same as a hamburger at Mc D. ... although both are essentially the same.