Hi guys,
I have got a Positive Grid Spark practice amp. Its powered from a typical 19V power brick. Its humming so bad. There are some known issues with these units and its has got something to do with grounding issues. There are couple of suggestions that people have made and have worked for them:
1. Connect a usb cable to the unit and the other end to a properly grounded laptop/computer.
2. Attach a separate cable from the chassis of the unit to the ground pin of the mains.
3. Change the power supply to a ‘three pronged’ power brick.
I didn’t have any issues with the amp out of the box and with in a weeks time the humming began. I used option 1 and connected the USB port to a well grounded computer. It did seem to help at first but then the hum came back. Right now the usb cable remedy is not working. I tried option 2 as well by alligator clipping the chassis to the mains earth and there is no improvement/change on the hum whats so ever. My mains out are properly grounded and I have confirmed this.
I am starting to think that maybe my unit has got a failing component somewhere as the problem kept progressively deteriorating to the point where it is now.
So I was hoping to get suggestions on what to look for.
I measured the resistances between the AGND, PGND and DGND (marked on the pcb). All seems to be well connected. However I did notice that the chassis of the USB port to PGND showed resistance value jumping around before it settled to 0ohms, I am assuming that its error on my side with my probes.
Also I have got a question about the 3rd option (‘three pronged’ power brick) - how does grounding help with noise etc with an SMPS type power brick. I see with SMPS schematics that the ground is not directly connected to anything on the SMPS but sometimes through a Y type cap to com. Is that connection doing the magic?
Anyway I was hoping for more suggestions on what to look for. I have got the unit opened.
Thanks in advance.
I have got a Positive Grid Spark practice amp. Its powered from a typical 19V power brick. Its humming so bad. There are some known issues with these units and its has got something to do with grounding issues. There are couple of suggestions that people have made and have worked for them:
1. Connect a usb cable to the unit and the other end to a properly grounded laptop/computer.
2. Attach a separate cable from the chassis of the unit to the ground pin of the mains.
3. Change the power supply to a ‘three pronged’ power brick.
I didn’t have any issues with the amp out of the box and with in a weeks time the humming began. I used option 1 and connected the USB port to a well grounded computer. It did seem to help at first but then the hum came back. Right now the usb cable remedy is not working. I tried option 2 as well by alligator clipping the chassis to the mains earth and there is no improvement/change on the hum whats so ever. My mains out are properly grounded and I have confirmed this.
I am starting to think that maybe my unit has got a failing component somewhere as the problem kept progressively deteriorating to the point where it is now.
So I was hoping to get suggestions on what to look for.
I measured the resistances between the AGND, PGND and DGND (marked on the pcb). All seems to be well connected. However I did notice that the chassis of the USB port to PGND showed resistance value jumping around before it settled to 0ohms, I am assuming that its error on my side with my probes.
Also I have got a question about the 3rd option (‘three pronged’ power brick) - how does grounding help with noise etc with an SMPS type power brick. I see with SMPS schematics that the ground is not directly connected to anything on the SMPS but sometimes through a Y type cap to com. Is that connection doing the magic?
Anyway I was hoping for more suggestions on what to look for. I have got the unit opened.
Thanks in advance.
Attachments
........could be a bad electrolytic cap - bulged elcap (no more flat on the top)
or built in reverted elcap
or built in reverted elcap
Right guys, so I have discovered something while having look at the pre amp section. For the preamp TP2582 opamp is used. I am measuring 12V at pin 1 which is output A - no guitar plug inserted and no input signal. +VS is 12V!. Differential voltage between the input pins are around 0V. At first I thought that the ‘A’ section of the opamp was bypassed but it looks like the input signal does in fact go through here (would need to double check though). Also I’ll remark that when I touch this pin 1 with the DMM positive probe I can hear a buzzing sound from the main speakers.
Another thing I sort of discovered was that when I checked the point marked with pink with a poor man’s spectrum analyser that I made (basically a pocket radio with probes connected to the amplifier section an idea I borrowed from one of a tech here) I get the same ‘noise profile’ heard at the main speakers.
Could I be onto something? Any more ideas what to check for.
Another thing I sort of discovered was that when I checked the point marked with pink with a poor man’s spectrum analyser that I made (basically a pocket radio with probes connected to the amplifier section an idea I borrowed from one of a tech here) I get the same ‘noise profile’ heard at the main speakers.
Could I be onto something? Any more ideas what to check for.
Attachments
well with no guitar plugged into the input, the "tip" of the input jack should be seeing ground, is it?
and do any of the controls change it, the hum level that is?
i have come across a reference to supply troubles and that the new replacement supply bricks are beefier and better built.
and do any of the controls change it, the hum level that is?
i have come across a reference to supply troubles and that the new replacement supply bricks are beefier and better built.
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with respect to your inquiry about the noise at the point marked in pink, as it's a multilayered board and likely the same trace.
The hum level changes when the gain or volume is brought down. Hum remains when changing the amp modeling knob as well, sounds different but remains.
it may well be that the ground for the input jack is bad i'd inspect it via multimeter and visual inspection of the solder joints with a magnifying glass.
Unfortunately not 🙁 . Its shelved at the moment. I tried contacting the manufacturer regarding on getting a schematic-no luck.
One more thing I would remark is that even when I had the processor board removed the amp still had the same noise on its output.
such is life, so i take that the input jack and all associated solders and lead in's are 100%?
the processor is usually on a parallel path and if your "noise producing fault" is ahead of the point where the signal is taken for the effects input, whether the processor board is there or not you will still have the fault.
good thing is we know that it's not the fault of the effects or associated circuitry!!
the processor is usually on a parallel path and if your "noise producing fault" is ahead of the point where the signal is taken for the effects input, whether the processor board is there or not you will still have the fault.
good thing is we know that it's not the fault of the effects or associated circuitry!!
Thanks @turk, yeah the input jack and the solder lead are as good as it can be as far as I can see. Just pausing the project till I get my hands on a scope.
as i've learned don't trust your eyes... without a magnifying glass things may look ok but under magnification the ugly truth is revealed!
