2600 finished

At last I’ve completed my build. All work fine. I’ve made a ghetto case from some scrap shelves, not the best but I’ll make a better case later or buy a metal case from zthee. Not decided yet. Just needed to hold the TTSH in something to being able to calibrate and play it.


Fixing a Behringer MX8000 desk (3)

Some more advancement on  the mixing desk front…

Replaced the NJM4580 I thought was the culprit and no changes. gah

I also made a new cable that connects the main board and the input board but it’s not that either.

Next plan is to change the two 2SC2878 responsible for the muting of the Mix-B channel. I read on a HiFi forum that when they fail they put DC in the audio path. Sounds like my problem….

So I ordered some and waiting again for delivery.

Next things to fix / restore are:

– a dead channel on one of the voice boards.

– a deep clean up of all the sliders. They are filled with dust and gunk. yak

– Find a way to restore the color of the knobs. they all gone yellow and the grey ones are in the same state. Peroxide + UV treatment ahead…

– Get the whole thing back together. hm, hope i will remember what screw goes where… The damn thing has been sitting on my dining table for more than a month now…

And it will be done i think


Fixing a Behringer MX8000 desk

Got my mixing desk back from a long term loan and unfortunately  it doesn’t work anymore :/

On power up, there’s plenty of hum coming out of it, flashing input and solo leds, scratchy pots and faders, silent channels, etc….

I guess it’s a power issue, either in the PSU or in the desk itself. Looking at the components, it’s packed with cheap chinese electrolytics capacitors, but apart from those the rest is standard thru-hole resistors, ceramic or mylar caps with NJM 4580 SIP package dual op-amps (still available at mouser).

So my best bet is that some electro caps have rendered their last breath and shorted the rails…

The adventure begins….


ARP Odyssey VCO Prototype

Sounds good so far. Need another prototype as i goofed up a bit but quite happy it worked first time!

Edit 1 :

Final version will have Thru-hole transistors for the expo pair as I can’t get any good tracking with smd trannies. I’m tracking 3 octaves with MMBT3904/MMBT3906, 4 octaves with BC850/BC860. Quite ridiculous.

With a pair of 2N3904/2N3906, i’m getting 8 octaves of tracking (go figure…)

I’ll also change the Pulse Width trimpot to 1 turn as having a 25 turn there is making adjusting more difficult .

Still need to test how it behaves under +/-12V and to check temperature stability.




Fixing a SCI Sixtracks

Yesterday I fixed a Sixtracks which had a silent Voice (#5). Swapping the CEM3394 from voice 6 to 5, revealed that the CEM chip was ok. The capacitors in the voice section looked a bit funky so i changed the one of voice #5 but that didn’t fix the issue. photo

A bit of scope probing later, i found that the 4051 was busted so i changed it too. After reassembly Voice #5 was working again but the pitch was super low. Checking in the service manual relieved me a bit as they state that after 30 sec after boot up the Microprocessor checks each voices and tune them. Indeed, after 30 secs the pitch of voice #5 was good again. Job’s done.

photo copie

Polivoks VCF Prototype



Finished the prototype for the Polivoks VCF in MOTM format. It includes the mods done by Dave Brown. As i’m unable to find NTE888 here in europe, i’ve used LM4250 in replacement. I had to up the 100R resistor to 110R to work with the 4250 otherwise the filter was stalling when the limit switch was on.

Great little filter and low parts count.

Boards, Bom and schematics in the Project section.