signs of dying transistor?

[KWilcox]

Hi there, I'm trying to fix a Danelectro Dan-Echo pedal. At first the pedal worked but made background "ocean" noise (like the random hissing/popping sounds old bad tubes can make). after not very long, the sound began to distort... then the pedal went mute. Well, not quite mute - if I turn my amp up full blast I can barely hear the guitar along with a lot of noise (like the result of putting a guitar through some simple buffer designs without powering the transistor/opamp).

Before I start hauling components off of the circuit board and replacing them, I'm just wondering if what I described is likely a transistor dying, or not necessarily so?

there's two transistors (C1815, http://www.ortodoxism.ro/datasheets/unisonic/UTC2SC1815.pdf ) on the board. one gets 0V at the collector by design (surrounded by capacitors) and confuses me, the other is getting 5V at the collector, 1.6V at the base, and 1.0V at the emitter, and I'm fairly sure that's all at design spec. The power LED goes on fine and the batt is pushing 8.25 volts. The pots are a little scratchy but otherwise functional. I've verified that all the electrolytics measure their indicated capacitance (haven't tested the paper film ones though)

if anyone's dumb enough to try and help me out with this , there's some pics of the circuit board on this page http://www.e-basteln.de/harp/ in case there's any other components you think I might be suspect of...

A really big thanks to any attempts of advice!

[frankclarke]

Apart from the obvious advice, there is a sticky about debugging things. The Audio Probe will let you trace the audio signal through the circuit. It would help if you had the schematic.

[PerroGrande]

Well, without a schematic, it is a bit of a shot in the dark at best.

The transistor voltages of 5, 1.6, 1.0 seem okay on the surface.  From looking at some of the links, one of the transistors operates as an emitter-follower input buffer, so if that's the one your measuring, it looks okay.  0V on the collector is possible, although a bit unlikely.  Then again, I don't know the circuit.

The audio probe is going to be your friend here, no doubt.  If you can find the stage at which the signal dies, you're in good shape.  That said, electrolytic caps are *always* worthy of a close look (especially given the sound you described).  Again, the probe will help you find if a coupling capacitor has gone out, or if you've got an active component bad.

[KWilcox]

Holy cow, how did I not find the audio probe before... that should be a massive help. Thanks guys, I'll give that a shot.

[petemoore]

  Probe.
  Bias Voltages.
  Hfe Meter.
  Diode tester.
  And continuity checker.
  These are about the only ways I can think of.
  One of my faves is socket it in a known working circuit [observing polarities and pinout, maybe bias] and see how it fares.

[MikeH]

The main problem is that the Dan-Echo is a total pos.  My friend had one that made the "ocean" sound you're describing.  I took it and gave it a thourough cleaning on the inside, and the sound went away... for about a month.  So I cleaned it again.  And again.  After 4 months I made him go out and get something else.