SWR SM-900 bass head broken wiring
 

I had this SM-900 rack bass head come in for repair (fan failed and
caused a meltdown) and someone had already been in there mucking around
with it. Since there are no connectors for the pwr amp modules, the
previous "tech" has broken off a couple of wires from the pcb. The
service manual details one side of the output section (its a
stereo/biamp head) but not the other, and they're not identical. There
are 2 20awg brown wires that have been torn loose. One comes from the
AC leg of the main bridge rectifier and connects to the corner of one
output pcb, which is shown in the schemo and has been repaired. The
other brown wire is attached at the opposite pwr amp pcb and connects
(I assume) to the other side amp pcb. Anyone worked on this unit and
know where this wire is going? I assume it would be after the rectifier
diode at the filter cap and they're just jumping voltage to the other
amp pcb but I want to be sure so I don't have a smokeshow after I just
rebuilt the damn thing.

The techs at Fender were of *no* help whatsoever......just tried to
talk me into sending it out to them. No thanks.....I'm not eating the
time I already have into it.

Also....anyone know the idle current for the output amps and where they
check it at?

Thanks
Travis

"boardjunkie" wrote in
message
oups.com...
I had this SM-900 rack bass head come in for
repair (fan failed and
caused a meltdown) and someone had already been
in there mucking around
with it. Since there are no connectors for the
pwr amp modules, the
previous "tech" has broken off a couple of wires
from the pcb. The
service manual details one side of the output
section (its a
stereo/biamp head) but not the other, and
they're not identical. There
are 2 20awg brown wires that have been torn
loose. One comes from the
AC leg of the main bridge rectifier and connects
to the corner of one
output pcb, which is shown in the schemo and has
been repaired. The
other brown wire is attached at the opposite pwr
amp pcb and connects
(I assume) to the other side amp pcb. Anyone
worked on this unit and
know where this wire is going? I assume it would
be after the rectifier
diode at the filter cap and they're just jumping
voltage to the other
amp pcb but I want to be sure so I don't have a
smokeshow after I just
rebuilt the damn thing.

The techs at Fender were of *no* help
whatsoever......just tried to
talk me into sending it out to them. No
thanks.....I'm not eating the
time I already have into it.

Also....anyone know the idle current for the
output amps and where they
check it at?

Thanks
Travis

You're lucky. I have one in the shop already
repaired.
However, I need to get offline soon and get to
work.
Email me about exactly what you need and after
work
I'll reply with the info you need. Sometime today,
I'll
reopen it since the schematic is not all that
informative.

Biasing it requires a scope and I'll have to look
up
the procedure or just take a reading
of the idle current and give you that.

Email me at
(sub "psychic" for "science" and "south" for
"sound")

Ed @ Sonic Surgery

You have mail.

"boardjunkie" wrote in
message
oups.com...
You have mail.

In case anyone else was following this and may
need to know, the following is what I sent to
Travis:

A brown wire goes to one of the AC leads of the
bridge rectifier to the anode of the diode
on the far right bottom corner of the left channel
PCB (the module toward the front of the amp).

Another brown wire goes from that same PCB at a
junction of a 2.7K 1 watt resistor and a
..1mfd 250 volt cap next to that above mentioned
diode on the right bottom corner to the
right channel's PCB 2.7K .1mfd junction similar
location.

On the schematic, the brown wire goes from and is
labeled going from AC bridge terminal
to D7's anode. However, not indicated on the
schematic, but on my notes, the other brown
wire goes to both channels' Q14 collector, C17,R7
and R8's junction - from one channel
to the other.


This is the SWR bias method as follows:
1KHz on signal generator
2 ohm load - one channel at a time
Signal in to Mono Return
Master pot full CW
Effects pot full CW
Bias trimmers full CCW
Variac set to 115VAC RMS
Signal generator output full CCW
Amp on Stereo mode

Raise signal to get 2 volts RMS at output of amp

Set Sweep to 50 µ seconds
Set Volts/Div to .2 volts

Position waveform to find tiny notch in waveform,
turn trimmer CW until notch removed. Same
procedure
for each channel.


Hope this helps,
Good luck,

Ed @ Sonic Surgery