In article , Phil
Allison scribeth thus
Trevor Wilson wrote:

--------------------

Phil Allison wrote:

Bose are for Bozos.

**Emphatically agreed.

I, too, have worked on some 1801 amps. Burned PCBs, smoked outputs, etc.
Horrible design.



** Did you see failures of the soft start resistor ??

If the amp is ever switched on with strong audio signal present at the input,
the combination of a huge transformer & filter capacitor inrush plus audio power
to the speakers at that moment smoked the 8 ohm, 10W resistor.

Happened to *every* Bose 1800 owned by the hire business I did repairs for.

Main cause was the LACK of input mating at switch on.

As the amps built up more hours, the hot running 3k/2W carbon comp resistors on
the power boards fell in value to under 1kohms casing their associated 15V zener
to fail short. This meant the op-amp at the input lost one rail and sent the
output stage to full DC !!!!

The VI limiting circuit for the output transistors was super fast and
overcooked. With even a mildly reactive 8 ohm load, +/- full rail spikes
appeared on the output at less than rated power.

Oh, and if the AC supply went even a little high, the amp hummed.



My schem is marked with simple fixes for all the above.



Who designed it originally, anyone we might have heard of?...



..... Phil

--
Tony Sayer