Annoying noise from the speaker when connecting a GOTEK internally

[eto]

I just connected a GOTEK internally to a CPC 6128 and now I can hear a weird buzzing noise from the speaker. When I remove the power from the GOTEK the noise immediately stops.

Everything works fine and I can read data as expected - just the noise is really annoying. 

Normal sound (you can hear a fan from a power supply in the background):
https://www.dropbox.com/scl/fi/68wgyq3gadu0kcg2ke9wd/nonoise.mp4?rlkey=7t44o0902ibjie6zq81e9vlvv&dl=0

Sound with noise:
https://www.dropbox.com/scl/fi/wjfi5x56ohbsng9w9td30/noise.mp4?rlkey=qt8rhuiw4xl0qx597771uh7s8&dl=0

It's much louder in reality than I could catch with the phone.

Any idea what's causing that or if it's possible to eliminate that noise?

[Bryce]

This is called "Conducted back EMI" (Read about it here: https://en.wikipedia.org/wiki/Electromagnetic_compatibility ). Probably coming from a switching regulator on the Gotek. If you have a scope, connect it (AC coupled) to the Gotek power wires and check what frequency the Gotek is feeding back into the CPC (probably somewhere around 140kHz). From this value you can calculate the value of inductor needed to remove the AC component from the signal. This is what L04 on the CPC Plus is there for, but on the plus the EMI is due to the motors in the floppy drive.

Bryce.

[eto]

Should I be able to measure that with a multimeter? My multimeter can detect frequencies in the 4MHz range properly.

I tried but it is stuck at 0Hz. Or do I need a scope to properly measure that?

I also compared it to a second GOTEK that I have here. It's an identical model and the noise is much much less. The only difference I can spot is that the 8MHz crystal is soldered in opposite direction. Can that have an impact?

[pelrun]

I'm not aware of a single gotek version that has a switching regulator on it. They all just have the cheapest AMS1117 clone on them.

[Bryce]

I'm not aware of a single gotek version that has a switching regulator on it. They all just have the cheapest AMS1117 clone on them.

Ok, I wasn't sure, because I've never owned one.

@eto: No, the direction of the crystal shouldn't make a difference. Things that would make a difference:

- Changes in the ground plane of the Gotek.
- Age of the capacitors on the CPC.
- Physical position of the Gotek.
- Dimension of the power leads to the Gotek.

Try wrapping the Gotek in a plastic bag and then wrap that in Tinfoil. Then you'll know whether the interference is conducted or radiated. That will let you know what will help resolve the issue.

Multimeter: I don't know of any DMM's that can read up to 4MHz, what model is it?

Bryce.

[eto]

- Changes in the ground plane of the Gotek.
- Age of the capacitors on the CPC.
- Physical position of the Gotek.
- Dimension of the power leads to the Gotek.

Strange. Same CPC, same GOTEK model, at least from what I can see on the surface. 

Try wrapping the Gotek in a plastic bag and then wrap that in Tinfoil. Then you'll know whether the interference is conducted or radiated. That will let you know what will help resolve the issue.

okay, will try. 

Multimeter: I don't know of any DMM's that can read up to 4MHz, what model is it?

UNI-T UT61E

I used it a lost for checking the signals in the CPC and the values always were what I would have expected - or if not, something was wrong with my circuit(s). 


Thanks a lot for your help

[Bryce]

- Changes in the ground plane of the Gotek.
- Age of the capacitors on the CPC.
- Physical position of the Gotek.
- Dimension of the power leads to the Gotek.

Strange. Same CPC, same GOTEK model, at least from what I can see on the surface.

Try wrapping the Gotek in a plastic bag and then wrap that in Tinfoil. Then you'll know whether the interference is conducted or radiated. That will let you know what will help resolve the issue.

okay, will try.

Multimeter: I don't know of any DMM's that can read up to 4MHz, what model is it?

UNI-T UT61E

I used it a lost for checking the signals in the CPC and the values always were what I would have expected - or if not, something was wrong with my circuit(s).


Thanks a lot for your help

Careful, the frequency measurement on the UT-61E needs a signal of at least 300mV to detect anything. Conducted EMI will be a lot lower than that.

Bryce.