Vintage rhythm machine ringing sound

[drumusername]

Hello, I have a vintage drum machine (a National SY-50, similar to Acetone and early Roland organ drum machines), and it has always made a loud ringing sound, like a dial tone. The sound is constant and is proportional to the volume, ie. gets louder or quieter as I turn the volume knob. I was hoping somebody might be able to identify what kind of component or issue could be causing this sound. Here is a sample of it: https://vocaroo.com/i/s0PvaxUC42ud

I opened it up and looked at the circuits and didn't see anything visibly problematic.. This website has some pictures to give you the general idea of the insides https://estecho.com/2016/12/national...-rhythm-sy-50/

Everything else seems to sound and work fine.

I would greatly appreciate any suggestions so I can go about trying to fix this on my own,

[The Dude]

Do you have a scope? If so, the page you linked shows where each voice output is located on the circuit boards. Scope those and see if you can narrow down where the noise is coming from.

[Leo_Gnardo]

Hello, I have a vintage drum machine (a National SY-50, similar to Acetone and early Roland organ drum machines), and it has always made a loud ringing sound, like a dial tone. The sound is constant and is proportional to the volume, ie. gets louder or quieter as I turn the volume knob.

Does the pitch of the ringing sound change as you vary the tempo? If it does, I'm thinking the power supply filter caps need to be replaced, as they often do on any old equipment. The caps are there not only to get rid of buzz & hum from the power supply rectifiers, but to keep noises generated by various stages of the circuit - clock I'm thinking in this case - from showing up in the audio output.

If pitch doesn't change the ring frequency then I'm at a loss. But it might not be a bad idea to refresh those old filter caps anyway.

[drumusername]

Do you have a scope? If so, the page you linked shows where each voice output is located on the circuit boards. Scope those and see if you can narrow down where the noise is coming from.

Unfortunately not, I just have a basic multimeter and performed a visual inspection.

Does the pitch of the ringing sound change as you vary the tempo? If it does, I'm thinking the power supply filter caps need to be replaced, as they often do on any old equipment. The caps are there not only to get rid of buzz & hum from the power supply rectifiers, but to keep noises generated by various stages of the circuit - clock I'm thinking in this case - from showing up in the audio output.

If pitch doesn't change the ring frequency then I'm at a loss. But it might not be a bad idea to refresh those old filter caps anyway.

No change in pitch, and even if the drums buttons are all off (but the volume is still up) the sound remains.

Thanks for the suggestions guys

[Enzo]

Thoughts come to my mind are that they used to make drum sounds by setting a tone, then gating it to represent a drum hit. If the gate is stuck, you just get a steady tone. I wouldn't expect the tempo to change tone, but there might be a pitch control.

As to filter caps, it doesn't sound like he is describing hum.

[Leo_Gnardo]

As to filter caps, it doesn't sound like he is describing hum.

Somehow drumusername's reply, entry #4, didn't show up until after I typed a paragraph which has now proved to be irrelevant & now edited into digital nowhere. So if anybody read it, fuggedaboudit all right? Then read the next tale of repair derring do:

Enzo, I like your theory better now. A couple years back I had an old drum machine in with a similar problem. Inside was a row of trimpots that set the decay times for various instruments. The metal parts had oxidized to rainbow hues, so you know there was some corrosion going on. No schematic of course. I poked at the trimpots with a toothpick while running a rhythm pattern. When I hit the defective one it sure was obvious. Just a dot of D5 on the trimpot track, followed by a couple of swipes back n forth with a mini greenie screwdriver was all that was needed, then fine tune that bongo trimpot so it hung in there neither too long nor short, 'n' bob's yer uncle. So ... to the OP I'd say have a look inside and if you see a batch of trimpots, try that first. Don't blindly flail them back n forth, just touch lightly with a toothpick, a little poke with a chopstick, something like that. One may identify itself then you can concentrate on it.

[Enzo]

One of my earliest recalls was listening to WWV on the short wave, and learning about it. It was the NAtional Bureau of Standards time signal, and it ticked once per second. The tick was actually exactly five cycles of a 1000Hz tone. It is perceived as a tick or tock. But in a drum machine gating a tone like that can easily become a clave hit.

I have seen simple rhythm machines like this that start with a white noise generator. Gate it off and on, and it is a snare drum. A little tone filtering and it can be different drums. Mix that with a tone gate and you get a different drum . White noise can also become cymbals.

[drumusername]

Somehow drumusername's reply, entry #4, didn't show up until after I typed a paragraph which has now proved to be irrelevant & now edited into digital nowhere. So if anybody read it, fuggedaboudit all right? Then read the next tale of repair derring do:

Enzo, I like your theory better now. A couple years back I had an old drum machine in with a similar problem. Inside was a row of trimpots that set the decay times for various instruments. The metal parts had oxidized to rainbow hues, so you know there was some corrosion going on. No schematic of course. I poked at the trimpots with a toothpick while running a rhythm pattern. When I hit the defective one it sure was obvious. Just a dot of D5 on the trimpot track, followed by a couple of swipes back n forth with a mini greenie screwdriver was all that was needed, then fine tune that bongo trimpot so it hung in there neither too long nor short, 'n' bob's yer uncle. So ... to the OP I'd say have a look inside and if you see a batch of trimpots, try that first. Don't blindly flail them back n forth, just touch lightly with a toothpick, a little poke with a chopstick, something like that. One may identify itself then you can concentrate on it.

Good ideas guys, there are mini trim pots inside that can control aspects of the different instruments. I will experiment a little and see if I can alter it. I think I remember reading somewhere that the way these work is that all the drums are triggering at all times regardless of the pattern, and pushing a rhythm preset just engages select ones. I think this would account for why the tone (it does sound like a sine wave) would appear even when no pattern is selected. I'll report back.


My reply showed up late because it needed to be approved first, new user and all

[drumusername]

...and the follow up, you guys had it right on. I adjusted the internal trimpots, which were a little stiff at first due to some corrosion I couldn't quite see at first, but once they were turned the strange ringing resonance disappeared. I'll have to play around with them a bit more and maybe get some contact cleaner in there, but it is working and sounding good. Thanks so much!