L2 Audio, PA1000 slave amp

N

N_Cook

Guest
No schematic seemingly available, anyone know if circuit similar to
anything else?
Owner no longer remembers what the problem was.
Protect LED goes from red to green and blue power LED comes on.
Cold testing the top layer of bipolar output devices seems ok.
Says "professional" on the front but 1/3W resistors thrown in "6
transistor" radio fashion. One base resistor lead is broken, presumably
from vibrational resonance fracture of the thin 1/3W wire, but that
would not be the problem
 
2009 date but no PbF statement anywhere seen.
Strange protect cct, +140V on the red supply and +65V on the black,
continuity to +ve of the main bridge rect, but not for the -ve tab.
Time to take the amp apart to get to the othe rside of the ps board, no
obvious failings there topside. For the moment the smokey staining above
one of the 4 TOP66 I'll take to mean it was taking more than its fair
share of the remaining 3 after the base resistor failed on one of the 4.
 
A fused piece of 1.5mm pcb trace, warmed up laquer and then a clean
break, no smoke staining. I make that about 4 amps carrying capacity for
5 deg C rise along 25mm assuming 2oz board.
Anyway this is the main trace between earth bond point and common of the
secondaries and the speaker lines, all of which is 20 amp wiring.
Another 20A line from this bond point to the input pcb and no heavy
wires out from that pcb.
So 1/4 inch spade connections to the wiring off the ps board, but on the
board just this thin trace to the earth bond wire.
Just about goes with the "6 transistor" radio component placement and 2
fudged, topside only, rework replacement components, ie using the cut
stubs of the original leads to solder replacements to
 
Uses 2x uPC1237 sil protect ICs one at mains level for full mains rather
than bleed for startup and the other for amp problem.
Perhaps Green protect LED means pa failure, red for mains switch over
failure and orange for both.
from user manual
"PROTECT indicates at the point of switching the amplifier on for a few
seconds. Once they
have turned off the amplifier is ready for normal operation. If at any
stage, they stay
illuminated, a fault has occurred. See section under PROTECT LEDs for
more information
....
If a fault has occurred on one or both channels the relevant PROTECT
LED(s) will stay
illuminated and there will be no output "

The output audio relay had one pair of contacts welded closed so perhaps
earth failure, due to some sustained imbalance of power out, was picked
up by pa protect , and relay cut out on high current passage
 
Replaced earth trace with an earth wire, tiny DPDT 8A relay replaced
with 2 SPST 15A relays stacked on top of each other. So far so good ,
stable -80.0,80V rails , mV output, red ->green-> no protect LED
sequence at power up . Cannot sensibly check audio until that rat's nest
is complete. Just as well took pics, as had not realised black wires
were for Dc gnd, plum for -ve and red for +ve
 
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