Signal tracing and noise reduction for old drum machine - Page 1
Forum section: electronics / beginners

author: cod65
author_rank: Contributor
timestamp: January 29, 2025, 06:10:39 am
content: So I have this friend who gave me this drum machine. He wanted me to add a midi kit mod. I successfully did this and it works, then he asked me if I could make it less noisy. I obliged by switching out the largest caps in the power supply board, but that didn't seem to do much. I'm wondering if there's anything else I can do. I don't see a whole lot of electrolytic caps besides the power supply. I would like to learn how to signal trace the noise and use my new scope + isolation transformer to get a picture of where the noise is most prevalent. Maybe it's just the old design that is hopelessly noisy? There's a heck of a lot of stuff in here. I should just move on but I would like to learn how to trace noise and find out what components are making the noise. I want to be safe with my scope. I'm aware that I don't want to blow it up so I got an isolation transformer. Now I'm looking to move forward and maybe use a battery powered audio signal tracer as well. The whole philosophy of signal tracing was never really taught in my electronics courses- the most Hands-On useful thing I would like to know. So here I am.

author: special_K
author_rank: Regular Contributor
timestamp: January 29, 2025, 06:29:37 am
content: Noisy like what?Like 50/60Hz hum from the wall? Or hiss like a cassette tape?

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 29, 2025, 07:35:28 am
content: To be able to trace the signals, you have to know where they are. For this a schematic is the tool to use. When you can't find it try to draw it yourself. This is a lot of work, but can be a good learning experience.Don't rely on an isolation transformer to not blow up your scope. You have to know how to use both (the scope itself and the combination with the transformer) in order to protect your scope, and if you know what you are doing the risk of blowing up your scope without is not that high.Before scoping the circuit with your scope check the voltages with a multi meter to make sure they are within range of what your scope can handle. Stay away from the primary side of the power supply. This drum machine uses an old fashioned transformer and a linear power supply, so it seems to be safe enough to probe.The design of the drum machine is old and there are a lot of wires in there connecting lots of parts together. These are like antennas and can pick up noise when located in or near an audio path.The cables you added to midify the machine can also add to the noise, even when they are just switching at very low frequencies, they can carry higher frequencies from the MCU.Trying to measure noise with a scope is also not that easy, because even your probe works as an antenna when not used properly.I'm not trying to discourage you, but aim to make you aware of what is needed to know for jobs like this.

author: CaptDon
author_rank: Super Contributor
timestamp: January 29, 2025, 01:51:32 pm
content: If it is power mains frequency hum and the power supply rails look good then there are a myriad of possible causes, poor power supply design / implementation, insufficient heavy grounding wires / straps throughout the units design, capacitors to chassis ground on the power mains input causing ground loops, etc. If the noise in question is hiss like steam escaping, well that is par for the course in these old designs. Some hiss can be reduced by looking at your gain structure. Try running the output of the drum machine near maximum but reduce the gain of the mixer channel. I have found that sometimes the hiss of a given device remains sort of at a constant level but when you turn up the output control the 'signal to noise ratio' improves drastically. I have one case in particular with a piece of gear used in our band. I crank its output to nearly max and then run a 30dB pad inline with the signal output. That places the noise so far down below the signal level that the noise is no longer objectionable. The keyboard player loves this particular keyboard and won't part with it or replace it so I had to do a 'work around'. We had a Kurzweil with so much ground loop hum it had to be run with an isolation transformer on the mains input.

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 29, 2025, 04:01:01 pm
content: In these old designs the "snares" of the snare drum are made with the use of a noise generator. Mostly a reverse biased transistor pn junction and amplified with an opamp. The same principal can be used for the cymbals and high hat.These noise generators are running constantly and might leak through due to the envelope generators not fully dampening them anymore. This would be more of the hiss sound CaptDon and special_k are referring to.Can also be caused by component deterioration. Looking at the photo makes me think it is a machine from the seventies maybe early eighties, so quite old.I posted some schematics of an old design here:https://www.eevblog.com/forum/microcontrollers/stm32f103-dds-pwm-dac/msg5707399/#msg5707399

author: floobydust
author_rank: Super Contributor
timestamp: January 29, 2025, 04:53:29 pm
content: Roland Rhythm TR-77 was released in 1972. This is old hardware! But simple circuit not too bad to go through.The blue Matsushita electrolytics I find dry out, I would outright replace all of them. Some will leak a little. They are end of life.The transistors can go noisy and I would suspect that as well. 2SC372, 2SC1000 etc.More info, partial schematichttps://mezzoauto.blogspot.com/2017/05/roland-rhythm-tr-77-how-it-works.html

author: pcprogrammer
author_rank: Super Contributor
timestamp: January 29, 2025, 07:53:30 pm
content: Quote from: floobydust on January 29, 2025, 04:53:29 pmWow good eye in reading or recognizing the make and model.I'm just not elderly enough to have the image in my mind. I'm more familiar with the hardware from the eighties and ninetieths.Edit: link to the service manual:http://manuals.fdiskc.com/tree/Roland/Roland%20TR-77%20Service%20Manual.pdf

author: cod65
author_rank: Contributor
timestamp: February 05, 2025, 02:45:52 am
content: Quote from: special_K on January 29, 2025, 06:29:37 amHey Special_K thks for the reply ,sorry for delay, busy work week.The noise is definitely mostly a tape hiss kind of thing to my ear. I also hear the metronome clock leaking through the noise floor. The particular midi hardware hack (https://tubbutec.de/blog/tr-77-midi-with-unipulse/) requires to have the metronome running without any pattern selected for it to work, so I probably need to reduce the noise floor of the metronome/clock circuits in order for this implementation to work with any usefulness.attached is a recording of the noise floor with the internal clock started but no pattern selected. this is the default setup to get the unipulse board to trigger sounds via MIDI.kind regards for your time and knowledge.

author: cod65
author_rank: Contributor
timestamp: February 05, 2025, 03:03:58 am
content: Quote from: pcprogrammer on January 29, 2025, 07:35:28 amthanks pcprogrammer for your time .I did indeed find a service manual and Ive been taking a look. It seems fairly detailed with schematics and other alignment info. I have been studying the noise floor and posted a recording in reply to member special_K. I think the hiss suggests its not power supply , correct? And the clicking in regular intervals is tied to the speed of the metronome/clock in the original circuitry, so I want to see if I can make that as clean as possible, as I will need to keep it running in order to trigger MIDI with the add-on UniPulse MCU board. SO I think my intuition tells me I want to look for schematic sections related to internal clocking and sequencing .I intend on disconnecting the V+ to the Unipulse board and comparing the noise floor to get a sense of how much it might be contributing to any of this noise floor. I think that would be a good test?Quote

author: cod65
author_rank: Contributor
timestamp: February 05, 2025, 05:24:06 am
content: ironically (or not!) this is why I have 'the fear of the electricity god' baked into my psyche:Quote from: pcprogrammer on January 29, 2025, 07:35:28 am

author: cod65
author_rank: Contributor
timestamp: February 05, 2025, 05:38:57 am
content: Quote from: CaptDon on January 29, 2025, 01:51:32 pmThanks CaptDon!I would request a more detailed description of " how to check if the power supply rails are good", if you would be so kind.I noticed in a recent video by a one Mr Carlson that I subscribe to - he was judging the quality of various single board mains-to-dc power supplies( for a bluetoothing project for classic small tube radios). He would judge the quality of the output with a voltmeter, to some degree- the fluctuation in the hundredths or thousands of milivolts seemed to give him enough information to conclude the quality of the output of these small boards, in terms of stability, etc. I recently got a decent meter used (a Fluke 115) which seems to read out to thousandths of volts. It's an RMS reading- so what can I deduce from reading the voltage fluctuations with it (or my newish scope) in terms of quality of my power supply output ? How much fluctuation of power supply voltage is acceptable in this case , and does a noticeable variation around the thousandth or hundredth digit represent harnessable noise ?Or isit insignificant after capacitor filtering downstream ?Should I do this with isolation transformer ? Where do I attach scope ground?( or meter ground for that matter ?)regards, Chris O

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 05, 2025, 07:06:13 am
content: Quote from: cod65 on February 05, 2025, 05:24:06 amAnd sure it should be, because it will keep you on your toes and think about what you are doing.In your case with the device under test already using a old fashioned mains transformer there is no need for an isolation transformer. Just make sure to stay on the secondary side of the transformer, and when using more than one probe check where you connect the ground leads, like warned about in the video.Quote from: cod65 on February 05, 2025, 03:03:58 amThis will tell you whether your added circuit contributes to the noise or not, so yes a good test.Quote from: cod65 on February 05, 2025, 05:38:57 amTo check your power rails, I think your scope is the best tool to do so. Since you stated that the noise is more hiss than hum, the ripple on the supply will be low, but still worth checking. To measure this ripple with your scope set the channel to AC coupling to get rid of the DC component and turn up the sensitivity to zoom in on the ripple signal.This will also show how much high frequency noise is on the signal, but this can be misleading since the probe also works as an antenna for this. The ripple will either be in the range of 50Hz to 120Hz, depending on the mains frequency and the rectifying method. Single sided leads to lower frequency hum than double sided rectification.You can at the same time probe the drum machines output and check if the power supply noise bleeds through.

author: cod65
author_rank: Contributor
timestamp: February 07, 2025, 05:00:31 am
content: Quote from: floobydust on January 29, 2025, 04:53:29 pmthanks for that info! that guy has the machine pretty well sussed out - i was looking for a point to use the unipulse clock -even the creator of the unipulse wasn't sure where it was in hs directions - this is very helpful info.

author: cod65
author_rank: Contributor
timestamp: February 12, 2025, 10:00:48 pm
content: So I'm making some progress with the noise by basically replacing all the electrolytics. I have to make an order of 100 microfarad, some of them are 6.3 volts. Some of them are as much as 25. Is there any reason not to just use all 25s? Is that 6.3 volts going to have a certain 'bias' on the audio signal because it's possible using a different type of electrolytic material? Or does it not matter. You may notice in this screen grab that there's a hundred on either end of this diagram, one is 6.3 volts, one is 16. What is the implications of just using 25 volt?

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 13, 2025, 06:41:19 am
content: I'm no expert on this, but I think it it is not a problem when you just use 25V rated capacitors for all of them. In mass production it saves money to use lower voltage ratings on these capacitors when possible.The capacitance is what matters, the voltage rating just has to be high enough to not make it blow up.

author: cod65
author_rank: Contributor
timestamp: February 14, 2025, 02:13:57 am
content: Quote from: pcprogrammer on February 13, 2025, 06:41:19 amthanks, yes I had heard that "voltage just needs to be high enough", but I had also read a possible other points of view- that in order for a cap to maintain being 'formed' it is designed to run at a certain voltage- s in my case perhaps a 6.3 volt cap relaced with a 25-50v cap wont be running at its designed voltage(does this mean temperature ? could it be running cooler ?), and may not stay properly 'formed' .It could be just nerds on audiokarma opining about nothing though

author: cod65
author_rank: Contributor
timestamp: February 14, 2025, 02:49:29 am
content: so I'm at the point where i need to order a handful of electrolytic , mostly 100mfd, from 6.3v to 25v. I'm inclined to just get a bunch of 25v caps,Ive been warned that amazon has a lot of crap and counterfeit caps, and I think since this is an audio circuit and im trying to minimize noise I will go the extra distance and order from Mouser or digikey some nichion or panasonic or at least something known. Its expensive though ? IM trying to build up a supply of likely caps for my work with audio devices. It seems that 100mfd, 47mfd, 4,7 , 10 are the most common values, maybe all around 25v. I was looking at kits at jameco and digikey , but man the jump in quality is surely a jump in price! I don't want to go too cheap though because IM working on some cool gear that deserves quality caps. I just hate the shipping cost though- and there's always one more thing Ive wished Ive bought that isn't in the order!looking for a good quality assortment of electrolytics for audio and hifi use.

author: cod65
author_rank: Contributor
timestamp: February 14, 2025, 03:16:29 am
content: jameco has a good price on nichicon 100mfd(39c each), but they are 63v max. Is there any reason I wouldn't put these all over my project, even if they are replacing a cap rated at 6.3v ?

author: Simmed
author_rank: Frequent Contributor
timestamp: February 14, 2025, 04:57:37 am
content: i screenie the audiothe timebase is so long like over 2sso i wonder if you change the bpm of the MIDIdoes the timebase change ?then maybe with this you could find a way to locate/probe which part on the pcb is this timebaseand maybe this only needs correction on that particular sectionthen the cap bypass in that section may not be 100u? it could be like 220u? 470u? as in largermaybe even use a polymer which has low ESR.then beef up the GND return to supply? with a flywire maybe 14AWG ?just my thoughts, but i cant say for sure this would hit the nail 100%also the wires are old, maybe there could be unseen corrosion just on the inside edge of the wire insulation. where the solder transition to the copper wire, during soldering, the flux will clear the oxidation, the insulation normally covers this, but this is also where some corrosion might start to occur. might be a good idea to check (normally the wires will just fall off the board )i think the spec of the cap could be more impt than whether it is "audio", like as in lifespan ? ESR ?when caps fail it presents high ESR which is mainly due to lifespan issue and not if it was "audio"

author: cod65
author_rank: Contributor
timestamp: February 14, 2025, 08:22:58 am
content: Quote from: Simmed on February 14, 2025, 04:57:37 ami disconnected the midi module V+ so I know its not coming from there. its coming from the built in metronome/pulse which drives the whole drum pattern logic. Even If I switch off all the patterns there is still a slight distant click(subdivision) and a bigger thud(metronome pulse) - that is what you were listening too- its like the residual sound of the internal clock and metronome leaking thru, this may just be how the original design is , but i need to improve it because I will be triggering sounds with the midi brain and don't want to hear that internal clock leaking in- but the caveat is it has to be running in order for the midi brain to trigger the sounds. Its OK now, it is useable , but the ghosted metronome is just unfortunate extra noise in the noise floor.QuoteI have been using my ESR meter to check the caps I removed. Im new to that meter and process, so I vaguely understand equivalent series resistance means a cap shouldn't show high resistance. The specs of the cap are confusing to me , moslty in terms of voltage. I dont understand if theres any reason to not use all 100mfd 60v nichions if they are quality , rather than 100/25, 100/16v, 100, 6v, etc ..... as in schematic- i have a handful of caps i still need to change and hopefully this will further clean up the noise floor, but ...what if all my 100mfds are 60v? does it matter ?

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 14, 2025, 09:18:16 am
content: A quick search on the net shows that the common census is that it does not matter as long as the voltage rating is higher or equal to what the manufacturer specified. Also used as a rule of thumb is to double the value. If the circuit runs on 5V use 10V capacitors.Like I wrote before, it is the capacitance that counts. For stabilization of power rails a higher value means a lower ripple, but also means a higher inrush current.Don't overthink it is my advice.

author: cod65
author_rank: Contributor
timestamp: February 20, 2025, 05:40:10 am
content: well I'm really kind of heartbroken right now. i had a perfectly fine working drum machine this morning, slightly noisy, but i couldn't leave well enough alone. i was trying to learn something about using the oscilloscope. i touched a transistor with the probe and now everything is messed up. I thought i could just touch any leg on a simple old transistor like this, with a high impedance lead and not have a problem. Maybe i somehow shorted 2 legs together ? Wouldn't be the first time- ruined a guitar amp doing that. I feel like a total loser , really crest-fallen . Now looking at hours, maybe weeks of work, costs. .. dunno what i burnt out.....I don't even know how to begin to figure it out(test transistors in situ? Is it even possible ?). i barely understand this circuit.the midi in board doesn't light up anymore- maybe some sort of power surge went through the whole thing because of my clumsiness....I need to just walk away for now . god I hate this moment.

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 20, 2025, 05:48:10 am
content: Yeah that sucks. Most likely is that you shorted something with the probe. Can you pinpoint the transistor in the schematics and post it here?

author: cod65
author_rank: Contributor
timestamp: February 20, 2025, 07:46:44 am
content: OK! SO, I walked away, was resigned to go to bed, then had a flash of an idea- what if the midi board is shorting out ? It's not lit up, so there's an issue there.... I unplugged it's connectors and the drum machine came back to life ! hopefully a replacement board isn't needed or too expensive- but I am back at it!

author: cod65
author_rank: Contributor
timestamp: February 23, 2025, 01:28:55 am
content: Quote from: floobydust on January 29, 2025, 04:53:29 pmSo I replaced most of the the electrolytic and there's not a huge difference. the ticking I hear is directly tied to the metronome tempo from the master clock, so i suspect you are right about the noisy transistors. I am thinking q1 and q2 in schematic because they seem to be the master clock( or first subdivision of this clock).2sc828r is the transistor types for q1 q2. used as a flipflop, I guess? Im having a lot of trouble touching the collector with my oscilloscope probe, since its the middle lead.. how do people do this ?I also note 2 tantalum caps tied to collectors. I read bad things about them somewhere. I'm researching . Can I effectively replace them with modern electrolytics ?Why did the designers choose tantalum in first place ? Are they prone to wearing out? failing ?they are little orange drop shaped

author: cod65
author_rank: Contributor
timestamp: February 23, 2025, 04:02:18 am
content: C1 and C2 are the tantalums, and in the manual there's this bit of gibberish that is trying to explain something to me about adjusting the Master clock by changing these capacitors

author: cod65
author_rank: Contributor
timestamp: February 23, 2025, 04:07:49 am
content: Here's screen grabs of the various clocks off the collectors that deal with quarter notes and half notes I believe. Am I seeing a slight amount of DC offset? Is that significant?

author: floobydust
author_rank: Super Contributor
timestamp: February 24, 2025, 05:19:39 pm
content: What does the signal at the "FC" terminal look like? It is the control voltage for the VCA IC MCF6040. Higher voltage is more attenuation; 3.5V is 0dB, 6.0V is -90dB.To see if the envelope generation is OK verses it's just on all the time letting the voice generators keep sounding. We need to narrow down where the noise is coming from. I keep looking for a muting circuit but did not find one, or where the "FC" terminal ends up lol.It would be hard to single out which voice gen might be noisy, if that is the case. There are 13 of them.Have to use an audio signal tracer (to listen to each one) or maybe the scope could show the noise if you know what the signal looks like. That would look like scope check on each one, is it a noisy snare drum, maracas, guiro section etc. Another method is to greatly load down the voice output (to make it quiet) with a 10k resistor. Say the snare drum is not properly silencing at the end and it is the noise source, then shutting it up might reveal that.The main audio mixing point seems to be everything coming together at Q12 base, you see all the signals get summed up there.Does the Fade-out function work? As another check if the VCA is working OK.------------The clock signals all sit on a 1V offset at C25 100uF cap and look OK. As long as the +24VDC "B" and +15.5V "BB" rails are reasonable in voltage for that board.When doing repairs you start with the power supply section, check all voltages before digging in.Tantalum capacitors are more stable and accurate in their capacitance value, so C1, C2 0.33uF are for the Master Clock which you want solid and not drifting around.If the clock works I would leave them alone for now. Replacements would better to use a film cap here.Sorry I was busy this weekend to help out by drilling down into this. Can you attach another mp3 etc. of the noise to hear it?

author: Langdon
author_rank: Frequent Contributor
timestamp: February 24, 2025, 05:34:09 pm
content: Whoa!Such beautiful old curvy PCB traces.Sound better than modern 45 degree charp curves.Looks like a beauty.Make sure all devices are connected to the same ground.You don't necessarily need like 3rd-prong ground, that can introduce mains ground hum into your audio.If anything is connected to a turntable-esque ground screw, make sure the screw , bolt, and spade connnector are brass, copper, bronze, or something else that is hightly conductive.Make sure you have flawless ground continuity between all connected devices.Beware of ground loops!

author: Langdon
author_rank: Frequent Contributor
timestamp: February 24, 2025, 06:15:38 pm
content: I have a c. 1970s Bang & Olufsen Beomaster 4000.It sounds beautiful with a turntable.However, without the ground screw, it crackles above 30% volume.With the ground spade connection attached, it crackles above 70%.

author: cod65
author_rank: Contributor
timestamp: March 24, 2025, 03:39:38 am
content: Hey , thanks , sorry for the delay. life called me away. im re-digesting all your suggestions and starting to revisit this thing .In the meantime heres a couple scope readings from a couple of collectors on the trnsistors in the timer/sequencer circuit section . hopefuly its informative with the audio from my camera phone.https://drive.google.com/file/d/1Urs_VC8LWKzq7aI8KQzfyEqWWMVz6CpM/view?usp=sharinghttps://drive.google.com/file/d/1UrmcuTSrO_8MAc2SwaivRmrhFoo5W3tz/view?usp=sharing

author: cod65
author_rank: Contributor
timestamp: March 24, 2025, 03:42:01 am
content: thanks floobydust, going to dig into your suggestions this week.Sorry for delay, life calls .... I will check FC and post something soon.Quote from: floobydust on February 24, 2025, 05:19:39 pm

author: floobydust
author_rank: Super Contributor
timestamp: March 24, 2025, 03:57:59 am
content: It takes a lot of patience, working on vintage gear.

author: cod65
author_rank: Contributor
timestamp: March 24, 2025, 04:51:24 am
content: FC is a clean looking 8.8 vDCB is 24.4 vDCI'm not sure I know what you mean by BB terminalJust to be clear the thing works. It's just if I'm going to use this addedmidi brain it needs to run in a state where no pattern is being triggered , but the clock is running. Only then can the midi brain trigger any voice by sending a pulse. The problem is there's this clock leaking through all the time- it's not huge leak , but it's not great. Somethingmaybe my client will have to live with? But it's distracting for the purpose of this midi brain mod. Those movies I sent today were basically all voices off, all patterns off, the clock running. It should be as close to silent as possible on the outputThe way I'm seeing it there isn't a problem with the voice board. It's the clock board is just a little more noisy that I'm hoping. The way I understand it, it feeds a complex clock sequence into the matrix switcher, so the clock is really probably those aforementioned transistors. Transistors. I'm wondering if I should just start replacing those or do more reads off of the collectors. I'm trying to interpret what my movies show. There seems to be these negative spikes that go away below the waveform, and then these little spikes here and there. There I'm wondering if those are unusual or to be expected. I guess I'm leaning towards what you said about the transistors being leaky and noisy. I just don't know how to interpret the scope waveforms yet

author: Langdon
author_rank: Frequent Contributor
timestamp: March 24, 2025, 01:34:55 pm
content: Make sure your grounding is good.Beware of ground loops.See if the amp has a low-pass filter.(My B&O has a HI and LO filter, which can be switched on mecahnically)

author: cod65
author_rank: Contributor
timestamp: March 24, 2025, 04:31:14 pm
content: Thks.this unit doesnt have an earth ground though.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 24, 2025, 04:41:27 pm
content: My amplifier has no earth ground.But the turntable has a ground connector to connect the turntable's chassis to the amplifier's chassis.Neither is actually "grounded" to earth, but both chassises are electrically bonded to each other.That's what matters really; a common ground.

author: cod65
author_rank: Contributor
timestamp: March 24, 2025, 04:53:53 pm
content: I see. The noise I'm trying to mitigate is a rhythmic click that corresponds to the master clock tempo, how would that be related to grounding? How would i test for quality if common internal ground?

author: Langdon
author_rank: Frequent Contributor
timestamp: March 24, 2025, 05:01:37 pm
content: IDK, ohmmeter?The ground screw on the amp was initially loose.I had to tighten it.Also, I think the grounding screws should be brass or bronze or some copper alloy that is electrically conductive.Although your case it sounds like a potential filter cap (or lack thereof) issue.

author: Langdon
author_rank: Frequent Contributor
timestamp: March 24, 2025, 05:23:18 pm
content: I had to make a breadboarded traffic light circuit for an electronics competition.It had a 555 timer and a 4017B counter and some LEDs.The LEDs flickered due to the a lot due to the 555 timer.I had to add filter caps.TTL chips draw lots of current, which can mess with stuff.Maybe try soldering a cap across the GND and VCC pins of ICs you can or across the VCC bus to GND.Make sure you have filter caps everywhere, not just the input power supply.Also consider: is the sound coming out of the speakers or out of the machine iteslf?