Frequency Central Microbus Pulsating +12V

Hi Guys, (it’s my first post so please tell me if I did something wrong)

so I started building my own Kosmo with some of Sams modules. First, off I ordered a Microbus from Frequency Central. I soldered everything down and connected it to an old Halogen Lamp AC/AC converter that I had laying around (SET150-2 12V 12,5AMax 50-150W) All LED’s turn on and I can correctly measure -12V on the corresponding pins and also the 5V.

Now comes the weird part:

When I watch the +12V LED for a while I can notice, that the LED goes slightly dimmer over time until it Pulsates back to full brightness. And it keeps cycling like that. (the +12V LED is the only led showing this behavior). When I measure the Voltage on the +12V Pin I also get the Pulsating Mesures cycling between 8V up to 11.8V.

I’m a little bit confused about whether my AC/AC Power supply is broken or not suitable? (It should have enough juice since it says 12.5A max) Or is my Microbus broken and I did something wrong soldering it? (I’ve checked every component 3 times now)…

I’d highly appreciate it if someone has an idea what might be the problem here…

il you’re interested, I can also upload a video of the behaviour.

Cheers guys and have a great day!
Love from Hamburg Germany <3

That should be a steady 12 V so something’s not quite right with that rail. I’d start by checking the capacitor solder joints, and measuring the input voltage to the 7812 (the voltage between pins 1 and 2).

(how do you measure the voltage, btw?)

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It might be worthwhile to remove the 7812 and measure indeed just the input voltage, as fredrik suggested. If that is stable, then the regulator IC may be broken, especially considering that the -12V rail works fine. The 5V is probably alright because the regulator compensates for the fluctuations (that’s its job after all).

The first time I built an FQ power supply, I also had trouble with the 12V rail. The second I assembled step by step and testing everything before putting in the next component. It turned out that one 7812 was defective. It can help to also trace this with an oscilloscope, but a voltmeter should be sufficient. Also, if you have a multimeter with diode testing option then you can use that to make simple tests on your 7812.

Thanks for your tips… I’ll check that out tomorrow.

One thing I wondered is wether it could be that my 12v transformer is the problem… because if I measure the ac poles when the transformer is connected, I get also a pulsating outcome with my multimeter… 0.4 up to 1.2 V which does not really make sense to me…
(The multimeter is not the best actually a pretty cheap one)

Hi fredrik so I measure with a simple multimeter I think from Voltcraft or something…

Are you using the AC voltage setting on your multimeter?

Yup I do at least it says in my manual that the setting with the wave sign next to the V is for AC measures


Everything past the diodes is DC.

Looks like the transformer module outputs 11.6 V AC which is a bit low, but 11.6×1.4−0.7−2.0=13.5 so you should be ok there (1.4 is the AC RMS to DC factor, 0.7 the drop over the rectifier diode, and 2.0 the margin needed by the 7812 regulators).


That’d be it. But the reading makes no sense — you can’t get 12 V DC out of a transformer that’s putting out 1.2 VAC (not with a Microbus, anyway).


If the transformer really outputs 1.2 VAC, how did the negative rail reach −12 V?

OP, when you say pulsating, what period are we talking about?


Okay, so I tried to follow your ideas, checked all solder joints again, and actually built a second Microbus that I ordered a while ago.

with the second Microbus, the Pulsating behavior was gone for the first time turning it on. After unplugging it and checking the other MB, they both showed the pulsating behavior. I also noticed that both 12V rails fluctuate.

I made some videos to show you how and where I measure hopefully this might give you more insights:
Measure 01 ->Here you can see the Pulsating LED and the corresponding Measurements…


After both MB have the same problem and the Mulitmeter gives me a very weird reading for the AC input i am guessing, that my ac transformer is broken or just not delivering the 11,6V that it should…

That transformer has two pairs of 12 VAC output for feeding two circuits

To me it looks like you have one wire connected to one side of one pair and one connected to one side of the other pair. But it’s hard to be sure from the video.



Oh, good catch! Leaving the secondaries half-floating would explain what we’re seeing (and the dual secondaries are clearly marked on the transformer, so a bit annoyed that I didn’t spot that earlier :clown_face:).

See the “Parallel Connected Secondary” section here for how to wire things up to use both secondaries:

but before you do that, make sure you read the entire page, and then unplug everything and do some continuity tests to verify that the secondaries are indeed independent before continuing.

If this sounds scary, you can start by wiring up just one of the secondaries. They can both provide a lot more amps than a single FC can handle, so no need to use both if you only use one FC supply.


Oh wow guys :DDD hahahaha I can’t believe how stupid and blind you can be sometimes….

Of course you were totally right! I somehow connected the 12v output of the transformer completely wrong… after connecting them to the same coil it all works fine now!

Thanks so much for your help!

Just tested the 1222 VCOs and they sound awesome even though I have to figure the tuning out a bit more :rofl:


There are 528 posts that say “believe it, we all can be…”

Good to hear it’s working!