Drip STA-Level - help needed

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morls

Well-known member
Joined
Jul 15, 2013
Messages
203
Location
Australia
Hi,
I feel like I'm posting a lot about this project, sorry for the different threads.

I've got both channels up, the voltages are great in each, but there is an issue with the second channel I'm troubleshooting. While doing this I noticed there was audio signal coming from the chassis - I have a probe for tracing through the audio path, which grounds to chassis. I'm getting audio from channel 2 through this ground connection, which changes volume as I increase the input level, and disappears when the input is turned to zero. I'm now checking the grounding, starting at the power transformer.

First thing, the power transformer is buzzing during operation. I'm not sure whether this indicates a problem or not, but it's going to be a pain long-term in the studio.

What's got me worried is that I've found continuity between ground and the high voltage (600VAC) secondary connections to PSU board. I've attached the data sheet, the wires are red, red and red/yellow. For this measuring the PT wires are connected to the PSU board, which is isolated from the rest of the circuit, so it's just PSU. There is continuity between each red and centre-tap (red/yellow), but not between the two reds. There is continuity between each red and ground, and between CT and ground.

The HT leaves the PSU after filtering and a choke. There is no connection between ground and this HT line exiting the PSU board.

I double and triple-checked everything while assembling the PSU. It's been a difficult build because the documentation is all over the place, and I've had to refer to different builds and errata. I'm very happy with some of the voltages on the main board - test points just after the HT regulator on each channel call for 300V, and I'm at 299V for both channels. Other voltages are close, but not as close.

At this point I'm hoping someone can advise me whether it's safe to continue working with this PSU? I don't know what to make of this continuity between chassis and high-voltage secondary input to PSU.

Cheers
Stephen
 

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morls

Well-known member
Joined
Jul 15, 2013
Messages
203
Location
Australia
OK, so I've worked out that the CT is grounded, so have moved on from the PSU. I'm still getting audio through the chassis on channel 2, so am working my way through the schematic, matching up with the PCB. There are a few wrong labels on the board, but the values match the schematic so I'm working it out.
Any thoughts on how the audio might be leaking to chassis?
 
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