Kenton Midi->CV inside Moog Prodigy high pitched whine

kynsi · Joined: Sep 10, 2008 Posts: 14 Location: cookietown, NL

Hi,

I'm trying to fix a problem with a Kenton midi->cv kit mounted inside of a Moog Prodigy . The kit consists of two boards: one digital and one analog. The analog board gets its power from the prodigy (+12V, 0V and -12V), and the digital board gets its power from a separate transformer and onboard power circuit (for 5V).

Both boards are connected with a 16-pin cable, connecting both grounds.

The problem is that there is a very annoying high pitched whine bleeding into the Prodigy-circuit. If I disconnect the 5V transformer for the digital board it disappears, even though the analog board still gets its power from the Prodigy.

The noise is before the VCF because lowering the cutoff frequency makes the noise inaudible.

I've already replaced the capacitors on the digital board and replaced the 5V regulator but that didn't help. Kenton told me that there might be a ground loop but I can't find any..

Any suggestions? Almost all of the ic's are soldered to the board so that makes troubleshooting pretty hard (also Kenton not giving a schematic doesn't help)..

Thanks!
KS

Grumble · Posted: Wed Oct 26, 2016 11:23 pm Post subject:

Is there a possibility to swap the 5 volt power for another one?
maybe the 5 volt power supply is at fault...

blue hell · Posted: Thu Oct 27, 2016 2:11 am Post subject:

The ground loop might be caused on the MIDI side. Did you try some other MIDI cables?

When the MIDI side is connected to a laptop you could try to run the laptop on batteries to see if that helps - not as a solution but as a test to find out where the ground loop is.

kynsi · Joined: Sep 10, 2008 Posts: 14 Location: cookietown, NL

The weird thing is that it also occurs without the midicables connected. The midisockets are connected to the Kenton board with a header which I disconnected and the whine is stil there. I also tried it with a lab powersupply and again, still there..
But as soon as I disconnect the 5V supply the whine is gone.
Could it maybe be a faulty 74-cmos chip somewhere?

Thanks!

blue hell · Posted: Thu Oct 27, 2016 7:03 am Post subject:

It would surprise me when any part would turn out to be defective - it really sounds like a ground loop.

Did it always have this problem, or did it start after you changed something?

kynsi · Joined: Sep 10, 2008 Posts: 14 Location: cookietown, NL

It's not mine, I have it in the shop for servicing. But it looks like I've found the problem. The owner (or previous tech) had installed a different s-trig socket and had connected the ground wire coming from the prodigy to it's ground, which was already making ground-contact directly to the chassis through its body. Together with the ground from the audio-output this was causing a ground-loop.
Also the grounds of the other sockets wired connected together, although they were already connected through the chassis. Another small loop.

The whine is still there but it's much less audible now. I also added two extra 100nF caps to the power-lines going to the analog-board (from the prodigy board), which I connected to the central ground-point directly and that also took away some of the whine.

PHOBoS · Posted: Thu Oct 27, 2016 10:57 am Post subject:

kynsi · Joined: Sep 10, 2008 Posts: 14 Location: cookietown, NL

blue hell · Posted: Sat Oct 29, 2016 8:36 am Post subject:

They used a z80 for their rack mount stuff too, at least for the oolde pro 4 I have here.

Ideally you should not let current flow trough the chassis ... so connectors exposing ground to the chassis should be insulated from it, and then run the ground wire to the PCB ground star point (which possibly is ill defined with a couple of PCBs - best point may be the power supply ground .. but then again you seem to have a couple of those too).

The chassis should be grounded at one point only - ideally close to the most sensitive input, also going to the ground star point.