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Having finished the two BGW 750E Power Amps, I moved the BGW 350A Power Amp up to the bench, as the same owner indicated this amp didn't power up either. This amp was created after the next generation 750F/G amps were designed, prototyped and refined, then put into production. The 350A was a smaller version of the 750G (both having the LED Meter Display. Having done the circuit revisions to the power amp modules of the 750D/E modules for the fully complimentary GTA module, it having seven pairs of output xstrs, and did the PCB layout for that single-PCB layout, I used the basic front-end format in the packaging of the amp module for this amp. Four output pairs of xstrs in the layout, as this module was used in the SPA-3 signal processing power amp, also the TRIAMP Midrange module, and in this 350/350A, and 7500T, it came together quickly. By this time, we had already created the next generation Active input module for the 750F/G, so that was used in this amp as standard.
With the power amp modules removed, I connected the chassis to the power analyzer/variac, and brought it up slowly, watching the analyzer for any signs of fault current. The power supply was still loaded with the rear panel input module (powered off of the +/- 80V supplies, along with the front panel display module, powered from the 12VAC winding. Chassis was drawing 0.32A/22W @ 120VAC, which seemed a little high. I unplugged the display and the rear panel module, as well as the bridge rectifier/filter caps, leaving just the power xfmr connected. Now I could measure the excitation current, which was 0.28A/28.3W @ 120VAC. Still a bit on the high side, but, that transformer wasn't exactly an ideal form factor either. But, no issues found thus far.
I checked the two amp modules for any shorts on the Sanken 2SC3858/2S1494 200W power xstrs. All were fine, as were the second stage current mirror stage and pre-drivers (all TO-126 devices along the middle top edge of the module). All checked ok there too, and both Triacs on the output were nominal, not shorted. I plugged the modules into the chassis, as I had designed the harness so there was enough length to lay the modules down outside of the chassis.
Bias was nominal, around 400mV across the driver stage emitter resistors, DC Output level about 1mV, both channels pass signal just fine, and drive a load just fine. Shown above using Burst Pink Noise to drive both channels in Dual Mono, load connected (8 Ohm), output level around 10V RMS in these images. I discharged the supplies, disconnected the modules and re-mounted them into the ends of the chassis, mounted with #10-32 socket cap screws in a similar fashion that had been done since the original 250 amp built in the mid-70's. Amp powered back up fully assembled just fine.
I haven't done any critical measurements on the amp yet, but the basics all look nominal. Not sure what Jeff had found when he went to power it up. But, it did let me have a look at one of the amps I was part of the design team on.
With the power amp modules removed, I connected the chassis to the power analyzer/variac, and brought it up slowly, watching the analyzer for any signs of fault current. The power supply was still loaded with the rear panel input module (powered off of the +/- 80V supplies, along with the front panel display module, powered from the 12VAC winding. Chassis was drawing 0.32A/22W @ 120VAC, which seemed a little high. I unplugged the display and the rear panel module, as well as the bridge rectifier/filter caps, leaving just the power xfmr connected. Now I could measure the excitation current, which was 0.28A/28.3W @ 120VAC. Still a bit on the high side, but, that transformer wasn't exactly an ideal form factor either. But, no issues found thus far.
I checked the two amp modules for any shorts on the Sanken 2SC3858/2S1494 200W power xstrs. All were fine, as were the second stage current mirror stage and pre-drivers (all TO-126 devices along the middle top edge of the module). All checked ok there too, and both Triacs on the output were nominal, not shorted. I plugged the modules into the chassis, as I had designed the harness so there was enough length to lay the modules down outside of the chassis.
Bias was nominal, around 400mV across the driver stage emitter resistors, DC Output level about 1mV, both channels pass signal just fine, and drive a load just fine. Shown above using Burst Pink Noise to drive both channels in Dual Mono, load connected (8 Ohm), output level around 10V RMS in these images. I discharged the supplies, disconnected the modules and re-mounted them into the ends of the chassis, mounted with #10-32 socket cap screws in a similar fashion that had been done since the original 250 amp built in the mid-70's. Amp powered back up fully assembled just fine.
I haven't done any critical measurements on the amp yet, but the basics all look nominal. Not sure what Jeff had found when he went to power it up. But, it did let me have a look at one of the amps I was part of the design team on.
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