Q25 sensitive, picking up hum?

Discuss x0x construction and related issues

Moderators: altitude, adafruit_support_bill, adafruit, phono, hamburgers

Please be positive and constructive with your questions and comments.
Locked
User avatar
PM_Orc
 
Posts: 4
Joined: Mon Nov 30, 2015 7:32 pm

Q25 sensitive, picking up hum?

Post by PM_Orc »

Hi,

I'm servicing a x0xb0x (likely not built by the owner) and I've never looked at one before (or an actual 303).
(The reported problem seems to be related to the actual 9VAC supply and some buttons appear to be worn out.
There were also some cold solder joints on the mainboard that I touched up, LEDs for instance).
The PCB is blue and V1.01.

However, while messing around with it, I noticed it sometimes had a very fast modulation (50Hz?) on the pitch, or at least it sounds like it.
I also noticed that if I grab the tempo pot axis and touch the faceplate it gets better and the pitch is steady. (It's a x0xsh0p.de aluminium case). I occasionally get a few static noises when I touch the waveform select toggle switch (metal lever).
Are there usually grounding problems with the faceplate, or is it even supposed to be grounded?

Furthermore, holding my finger about 1cm from Q25 (2SC536 in VCO) makes the sound really buzzy (when triggered).
If I touch Q25 the played notes go insane and remind mostly of mains hum (which it very well may be).

I thought it's because it's a high impedance circuit going to Q28 (2SK30) gate, but touching Q28 itself (i.e. my finger closer to the gate) doesn't seem to have any bad effect at all, which doesn't makes sense then.
The box seems to be working ok, as far as I can tell, but it's just this weird modulation that keeps bugging me.

Is it normal that Q25 is super sensitive and picks up mains hum or should I start looking for more bad solder joints? :P
Anyone with an open x0xb0x care to point your finger at Q25 and see what happens? :P

User avatar
PM_Orc
 
Posts: 4
Joined: Mon Nov 30, 2015 7:32 pm

Re: Q25 sensitive, picking up hum?

Post by PM_Orc »

I might have a ground loop problem in my test setup...
If I run the x0xb0x only using an AC-adapter and headphones, I don't hear it.
I do however hear it in the speakers going through my small mixer :P
Maybe the x0xb0x is unusually sensitive to ground loops because I haven't really had this kind of problem with stuff connected to my setup before.
Ah well...

User avatar
antto
 
Posts: 1636
Joined: Thu Apr 15, 2010 3:21 pm

Re: Q25 sensitive, picking up hum?

Post by antto »

"fast modulation of the pitch" .. it'd be nice if i can hear this, but it really sounds like you have the "filter warble" issue, where the instability in the filter grows enough to significantly move the 5.33V line up and down, which affects everything which uses 5.33V as reference (including the filter cutoff and the vco pitch)
that oscillation is normally about 8Hz but it may easily change both in amplitude and frequency..

iirc C33 is making the actual sawtooth

User avatar
PM_Orc
 
Posts: 4
Joined: Mon Nov 30, 2015 7:32 pm

Re: Q25 sensitive, picking up hum?

Post by PM_Orc »

I don't know what the filter warble everyone is talking about sounds like, but this is definitely something being picked up by Q25 (as I can influence it by placing my finger near it).
I took the PCB out of the case as I was working on it and if I keep it standing upright, it sounds stable.
However when I lay it down on the table I get really fast (though weak) vibrato-like modulation.
I am wondering if it's got something to do with my grounded ESD table mat.
Likewise, when I put the PCB back in the case I get quite bad modulation as well.
But again ONLY in the speakers, not in the headphones.
I really should check if there's something weird going on with my crappy monitors.

User avatar
antto
 
Posts: 1636
Joined: Thu Apr 15, 2010 3:21 pm

Re: Q25 sensitive, picking up hum?

Post by antto »

only in the speakers?
it would really help if you record an audio clip demonstrating the issue

User avatar
PM_Orc
 
Posts: 4
Joined: Mon Nov 30, 2015 7:32 pm

Re: Q25 sensitive, picking up hum?

Post by PM_Orc »

I seriously think there's a ground problem.
If I connect only the PSU to the x0xb0x and nothing else, then check AC voltage between x0xb0x chassis and 1/4" audio cable sleeve (cable coming from mixer, but not plugged in to x0xb0x) I get 60V...
I hooked up my scope as well, connecting the ground clip to one of the CV/Gate jacks and touched the probe to the audio cable shield (still not connected to x0xb0x of course) I got 180V peak-peak...

Either that PSU is doing something really nasty or something in my mixer/monitor setup is causing it.
Likely not a x0xb0x problem in itself...

Locked
Please be positive and constructive with your questions and comments.

Return to “Making x0x”