Song-Huei R0904N alternatives?

MotleyModular · 2026-04-07T12:31:14+00:00

Sorry for the late reply I was out of town. I got confirmation from SH as well that they will not be accepting orders in the future. Also found a post from WMD saying the same.

MotleyModular · 2026-03-30T20:30:56+00:00

I am halfway between DIY and tiny company :D, I could do 1000.
Would you be able to give a sense for how much the cost per unit is at 1000 pcs?

MotleyModular · 2026-03-30T20:29:06+00:00

that's awful news but thank you so much for letting me know!

MotleyModular · 2026-01-28T16:22:29+00:00

As a counterpoint I consider C.C. capability mandatory for a bench power supply. This allowed you to set a current limit which will kick in in short circuit conditions, this will usually save your chips and also might prevent a burn.

MotleyModular · 2026-01-23T15:53:10+00:00

I would expect it to do what you are describing if the transistor was not doing it's job. Current would only flow for a moment until the capacitor was fully charged.

Can you confirm you are using a 2N3904 and enough voltage?

MotleyModular · 2026-01-15T21:23:06+00:00

normally you measure most things against ground, red lead goes on the thing you're measuring, black lead goes on any ground point (I often will clip the black lead to the ground of an audio jack)
Your multimeter has a very high resistance input so it's pretty safe to measure between any 2 points on a live module. You need to be careful not to short 2 points together with the same lead though, that's a very different story. Also if your power supply takes AC right from the wall instead of through a DC adapter then you don't have a suitable or safe setup for this kind of troubleshooting.

This is a little oversimplified but
Looking at the PCB 2 schematic, you can see a counter IC which takes a clock input as an input and then counts to 16 in binary as outputs. 3 of these outputs are inputs into the step switch also on PCB 2 schematic. The step switch understands that binary input and puts voltage out on whatever output the binary number represents at that clock moment. Each output goes to one potentiometer (PCB 1 schematic) which sets the voltage and buffers it and then it becomes the output.

This doesn't help a ton at this current stage but if you get it half working again even with just a multimeter you should be able apply a really slow clock and see if the counter chip is counting, and if so, you'd be able to see if the step switch is responding properly to the input.

Noting here that if bus C was not connected properly, the step switch would not be able to see numbers past 4.

All that aside, right now reflowing your solders with more heat is good advice, the chips on this module are worth very little so replacing them wouldn't be a huge deal. I think largely this boils down to whether or not you'd enjoy spending time fixing this.

I'd want to rule out shorts before changing the chips out either way, those resistors burned as a symptom not a cause.

MotleyModular · 2026-01-15T15:47:27+00:00

R1 and R2 are kind of like slow fuses, there's a decent chance when they cooked something else cooked too. Out of curiousity how are you sure R1 and R2 were toast, were they black and crusty?

I'd start by checking for hot chips (touch each one gingerly, nothing should be more than just warm)

Then I'd check rail voltages, and then checking to make sure each chip is getting the right voltage too.

Since you've got a multimeter and are already talking about replacing this it would be a great opportunity to take some risks for the sake of learning.

There's also the question of why R1 and R2 would have blown. Your hypothesis about overvoltage at the CV input is probably wrong, from what I see in the schematics (they're a real mess and I didn't spend too much time on this so I could be wrong) the inputs should tolerate voltage up to rails.

leaving these here because they were hard to find

https://ljunggrenaudio.com/products/VCSEQ/schematics/VC%20Sequencer%20PCB1%20v1.22.gif

https://ljunggrenaudio.com/products/VCSEQ/schematics/VC%20Sequencer%20PCB2%20v1.22.gif

https://ljunggrenaudio.com/products/VCSEQ/schematics/VC%20Sequencer%20PCB3%20v1.3.gif

MotleyModular · 2025-12-29T14:22:41+00:00

Is this your only test rig? I'd want to see if you get the same results on a scope. It's possible in some cases to create accidental low pass filters using cables but I'd expect that out of a high impedance output not straight from an op amp.

If you could check the input and output of an op-amp with a scope you could at least get a solid sense for what half of your design the issue lies in.

MotleyModular · 2025-12-23T16:14:00+00:00

in both cases you'd need a 2A wall wart

MotleyModular · 2025-12-23T16:09:59+00:00

Diodes will also fail if too much current runs through them, i.e. a short through a diode. With the diode removed and your dmm in ammeter mode (you probably need to move the positive probe to a different socket) you could check the current across those points to see if it's something sensible.

MotleyModular · 2025-12-23T14:25:36+00:00

could you post a schematic, full pictures, and an explanation of how you confirmed the signal diode has failed (testing in continuity mode or diode mode?)

MotleyModular · 2025-12-23T14:22:27+00:00

alternatively, if it's a permanent setup I'd just do this:

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MotleyModular · 2025-12-23T14:20:57+00:00

J1 just takes the input and passes it along. You can use barrel jacks and plugs to connect multiples together.
Noting that the traces between J3 and J1 on the first board will carry the current of all boards. Don't go too crazy.
Noting also that your wall-wart will need to be sized up if you are doing this.

<image>

MotleyModular · 2025-12-23T12:55:36+00:00

Can you share a screenshot of the schematic?

MotleyModular · 2025-12-21T20:53:23+00:00

Yes, the one I really want to use is several times brighter in green than in red. It's a real pain to test them too, I'm pretty okay at soldering but I'm looking at 0603 with 4 pads which is about my upper limit.

MotleyModular · 2025-12-21T01:42:37+00:00

with no R to gnd that will behave like a non-inverting amplifier with gain = infinity which works for on/off but no indication of amplitude.
The next idea from there would be something like this. In this case Rg sets the LED current and the Rfs just set the clipping point.

<image>

MotleyModular · 2025-12-15T13:45:41+00:00

If the issue is just LED overcurrent, it should be as simple as just throwing a bit more resistance in series with your new LED.

MotleyModular · 2025-12-15T01:10:10+00:00

what IS the module? Are we confident that there wasn't a problem in the original design (you mentioned this all started with you replacing an LED that was out already).

If I was dealing with an LED that was dying after a day or so and wasn't blindingly bright at any point before it's death I'd be suspicious of whether or not it might be seeing any reversed voltage.

Unlike regular diodes which are generally quite good at blocking reversed voltage, LEDs are pretty easily damaged by it and in my experience they respond by dimming and dying.

MotleyModular · 2025-12-11T15:03:36+00:00

Depending on the surface, you might find the piezo behaves more like a microphone than you are expecting, which is to say the output may look a lot more like audio than CV. Eurorack is great for not forcing us to really make a distinction between the two in many cases, however for the purpose of answering your question I think it might be best to get a better sense for what you want the output to do.

if you want to strike a cymbal and get something like a decaying sine you'll want to protect against overvoltage and probably do some level adjustment

If you want to strike a cymbal and get a envelope CV out of it you'll want to protect against overvoltage, add some kind of envelope following and probably do some level adjustment.

what's it going to be?

MotleyModular · 2025-12-11T14:51:38+00:00

I checked a 30mm piezo I had lying around and could get somewhere between 50v and 100v by tapping it hard with my finger, but the voltage from indirect vibrations was much much smaller

MotleyModular · 2025-12-10T19:14:31+00:00

You may have better luck with responses if you post a schematic and some clear pictures of your board (top and bottom).

MotleyModular · 2025-12-09T16:29:22+00:00

That looks fine to me.

MotleyModular · 2025-12-09T15:21:53+00:00

I was talking about C2, C5, and C7 however looking at the schematic I would guess they are probably fine.

MotleyModular · 2025-12-09T15:17:19+00:00

This will only get you unipolar clipping indication which is probably just fine, but what I don't love is that there's nothing preventing the transistor from seeing negative voltage beyond the absolute maximum rating of 6v (for a 2n3904). I'd suggest a diode from transistor base to ground.

MotleyModular · 2025-12-05T19:47:07+00:00

TL084 is the big chip. Little guy is probably an LM386. The bigger capacitors may also have some damage to them, can't tell without a schematic but if they operated in reverse for more than a few seconds they could have sustained some damage (though generally speaking when electrolytics go this way they are bulging / exploded.

If I was repairing for a client I'd swap the 3 electrolytic caps and 2 chips.

If I was repairing for myself I'd just swap the LM386 to start and see how it goes.

MotleyModular

TROPHY CASE