Tour 450 DI output issues

[byacey]

Numerous times I have run into hum issues trying to use the DI output. With the ground lift switch closed, I measure about 8.5 ohms between pin 1 and the sleeve of any of the 1/4" connectors. The schematic doesn't show any resistor. Measuring signal on pins 2 and 3 with the scope, the signals appear to be equal in level, as they should be.

I haven't opened it up to see what's going on yet. Did Peavey at some point add a series resistor between the pin 1 ground lift switch and ground?

[byacey]

Perhaps I'm missing something here. The Shell ground and pin 1 are tied together, and the shell ground has the XLR jack self tapping mounting screw threaded through the amp chassis, into the tab from the shell grounding tab. How can the ground lift switch possibly ever lift pin 1 from chassis ground? After I loosened and tightened the fastening screw, the measured resistance went to 0 ohms between pin 1 and chassis ground. If this screw is indeed meant as a grounding point, they should have put a lockwasher under the screw. Regardless, the pin 1 lift switch still won't function as intended.

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[Enzo]

Look on the power amp page, just left of center. Find R123, seems to be grounded at both ends. You will note the ground symbols differ. This amp has multiple ground circuits. Chassis and circuit commons are not always the same thing. That 4.7 ohm resistor could be involved if you used chassis as a ground for your meter.

[byacey]

Thanks for the reply .

I see that, however all the 1/4" sleeves are grounded to the back panel of the chassis; there isn't any isolation here. Pin 1 is also effectively grounded to the chassis via the shell ground contact connected to the back panel with a self tap screw.

If I remove that screw, with the ground lift switch open, it measures infinite between pin 1 and chassis, 1/4" sleeves, and power supply ground.
With the screw in place, it measures 0 ohms to chassis, 1/4" sleeves and power supply ground, regardless of the ground lift switch position. With the screw in place, the switch does absolutely nothing.

The schematic does not reflect any chassis ground connection on the XLR shell / pin 1 connection. When the ground lift switch is open, pin 1 should be completely isolated.

Either the wrong XLR connector was installed at the factory, or the schematic is wrong.

R123 bridging the two grounds: The only other incidence I see of the chassis ground symbol is the center node of the output transistor emitter resistors. The schematics I have don't show the power transformer primary, AC input power and AC power grounding.

[Enzo]

Ground circuits are separate, but often as not the chassis points are wher they bond together. If this is not your situation, oh well.

"Either the wrong XLR connector was installed at the factory, or the schematic is wrong." Or they made a change not reflected on the schematic. MAny updates to circuits do not have a new schematic to go with them.

You could put in a call to the engineering section at Peavey and ask about it. I'd like to know, myself.

[byacey]

I called Peavey Tech Support this morning, and all I got was auto attendant menus and voicemail. I guess the days of a real person answering the phone are bygone.

I'm satisfied that the ground lift switch is now doing what it's supposed to after cutting the link on the connector between the shell ground and where it grounds to the chassis back panel.

[Roger Crimm]

What time did you call? Business hours are 8am-5pm Central Time, Monday- Friday.

[byacey]

I called 10:30 MST.