So, the fuse keeps blowing, I'm guessing I need a new power supply! What to do?
Award-Session say they stopped production of the Sessionette:75s in October 1988 and the company stopped making guitar amplifiers of any kind in 2007. No amps left, no parts left, no technicians who know their way around this piece of kit in Dublin. The original Sessionette:75 PSU was known to fail. A replacement PSU (the PM120 power module) used to be sold but is now no longer available.
The PSU in my kit has PM120 printed on it. I wonder if has my box had a replacement power module fitted before? Everything on the inside is tidily soldered and attached, it looks like a production version.
Symptom:
Turning on the power, indicator light (red) glows briefly (1s) then slowly fades. Power fuse is blown. Repeat, same. Switch power on/off quickly, same result. No obvious heat smell.
Conclude: Something is shorting.
Step 1
Inspect the circuit boards visually front and back to spot anything obvious.
Don't forget to use the nose to smell a burn.
Any discolouration on components? Nope, except perhaps a tarnish on one of the balanced power op amps (mj11015 pnp and mj11016 npn op amp). Note: can source the mj11015 and mj11016 from www.awatronic.fr at reasonable prices if needed.
(mj11015: Transistor PNP Darlington 120V 30A 200W TO-3 MOTOROLA (Equivalent: BDX68C) - Diode de protection intégrée)
(mj11016: Transistor NPN Darlington 120V 30A 200W TO-3 MOTOROLA (Equivalent: BDX69C) - Diode de protection intégrée
Conclusion? Nothing obviously wrong.
Step 2
Poke around with a multi-meter (MM) looking for shorts/continuity in unexpected places. No luck with this on either the power amp circuit or the pre amp circuits.
An observation, with everything soldered in place, I can't assess circuit behaviour without power. Why? Because (for example) discharged capacitors are shorts (for a while) until they charge up (sucking charge from the MM).
Continuity tests aren't that useful unless there's a smoking gun (burn out somewhere). Continuity is more for finding breaks (i.e. no continuity) as opposed to shorts (too much continuity).
Question? WHAT is the metal box on the inside of the cabinet connected to the pre amp by two signal cables (co-ax plugs)?
Answer? It's a three spring reverb effect from Accutronics (www.accutronicsreverb.com/). See www.amplifiedparts.com/ for an Excellent explanation of spring reverb effect box design and theory written by Kurt Prange.
Conclusion? Nothing obviously wrong.
Step 3
Hmm, isolate sections and inspect for expected/unexpected behaviour.
This is going to be difficult without a bench supply, an oscilliscope, and other stuff.
