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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

Reply to
boardjunkie
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repair (fan failed and

in there mucking around

pwr amp modules, the

from the pcb. The

section (its a

they're not identical. There

loose. One comes from the

to the corner of one

been repaired. The

amp pcb and connects

worked on this unit and

be after the rectifier

voltage to the other

smokeshow after I just

whatsoever......just tried to

thanks.....I'm not eating the

output amps and where they

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 snipped-for-privacy@bellsound.net (sub "psychic" for "science" and "south" for "sound")

Ed @ Sonic Surgery

Reply to
Elvis Kabong

You have mail.

Reply to
boardjunkie

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

Reply to
Elvis Kabong

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