208p build thread (papz's PCBs)

[papz]

never mind

[papz]

100nF caps added like this to the back of IC1 and IC9 (and others if needed) may help.

[tele_player]

That's why I included a bunch of 100nF caps when I ordered the BOM.

[papz]

I have a limited stock of NOS Excelitas VTL5C3/2 dual vactrols for the LPGs. Those are available to customers who order or ordered a 208p PCB set ONLY. Please contact me if you want to buy a pair.

[fg]

it was a long time ago now, but it seems to me that my sequencer had this problem also and the decoupling worked
especially in control mode "both" in mode "front panel" it worked better

i tried differents ics ... new, vintage nos, same problem

but decoupling solve this problem

personnal archive about buchla 208 here ----> http://pjmnc.free.fr/files/_buchla_1v-oct.zip

[tele_player]

Please contact me if you want to buy a pair.

Email sent.

[tele_player]

it was a long time ago now, but it seems to me that my sequencer had this problem also and the decoupling worked
especially in control mode "both" in mode "front panel" it worked better

i tried differents ics ... new, vintage nos, same problem

but decoupling solve this problem

personnal archive about buchla 208 here ----> http://pjmnc.free.fr/files/_buchla_1v-oct.zip

Thanks, I’ll add some caps today.

[AlanP]

Are you using CD4009, or a replacement like CD4049?

[fg]

4009

[tele_player]

100nF caps added like this to the back of IC1 and IC9 (and others if needed) may help.

IMG_1332.resized.JPG

Thanks, that did it.

[papz]

[tele_player]

Failed again, after working fine for a few hours.
Same chip that failed earlier (Card 2, IC2, CD4066), this time it smoked, chip is obviously destroyed, socket is damaged.

When it failed, I had been switching the sequencer stages and Control (board, panel, both) switch. Is that switch safe to operate while powered? Maybe I've got a short on the transfer card?

Fortunately, damage appears to be only on card 2, and I'll be able to repair it, though replacing a socket is a bit tedious. And with card 2 removed, other things seem to be working OK.

[papz]

All switches should be safe to operate while powered.

[tele_player]

That seemed a reasonable assumption.

I'll replace the socket and chip tomorrow.

[papz]

IC2 will most likely fry again, it seems to be the victim rather than the culprit.
You have to find out what makes it fry.

[tele_player]

I think I found a clue. With card 2 removed, all white bananas are 0v, except the one under CO Pitch slider, which shows a negative voltage near -15. This voltage varies with certain switch and slider settings, but I haven’t found a pattern.

First thing, I’ll trace the path from that banana back to card 2. Looking for an unwanted connection.

Various ugly hack solutions come to mind, but first I’ll try to locate the short.

[papz]

The trace is on the MB bottom side between the socket eyelet and a via located under TR5 trimmer and on the top side between the via and contact 21 of the card 2 pin header.
You can see it on the MB pics with parts layout.

[tele_player]

@papz: correct. Right now, it looks like I'll need to unsolder all the bananas, I don't see a problem on the 'easy' side. But then it should be easy to find - better than throwing it in the trash.

[fg]

hello,
I have a config a little particular because I put mini jack in input to ensure the connection between the buchla and a eurorack, and the simple fact of connecting a jack in the input trig was enough to burn the ic 2, I concluded that the entry of ic2 hates any connection on 0v.
I resolved the problem by going through an opamp.
but in your case I think you may have a short circuit that touches the ground ?

[tele_player]

My situation is different. When card 2 is not installed, a negative voltage appears on ONE of the white bananas. This trace should connect only two points: the banana and pin 21 on card 2. When card 2 is not installed, no voltage should be present on this trace.

But I still haven't found any shorts to this trace. I'll keep trying, but if it becomes necessary, I will cut the trace and jumper around the short. This is not my preferred solution, obviously.

[tele_player]

I followed the trace from banana jack to card 2 pin 21 on both sides of the board, using a meter, found no shorts to adjacent traces, pads, vias. Also checked using higher resistance setting on multimeter, didn't find anything.

Powered up board, with cards 3 and 12, and found that the metal case for slider EG Decay has the same voltage I measure on the banana plug. None of the other sliders have voltage on the case. Powered down, I measured resistance between this slider and the banana jack - it varied, but was a high resistance, hundreds of K to Megs.

Could the slider be making weak contact with one of the traces underneath? Was I supposed to put Kapton tape under the sliders? Is it a defective slider? This is my first build using sliders.

[papz]

I don't put tape under the sliders neither did I come across a slider caused error so far, but this doesn't mean it's not a possible cause.

[tele_player]

Wow. I found it. The Alps sliders have small metal tabs, which touch the traces for -15 and the trace to pin 21 of card 2.

With the slider removed, -15v is NOT present on the banana jack or pin 21 .

I’m surprised nobody has seen this problem before.

[papz]

Glad you found it and thanks for the info, this may help another builder.

[tele_player]

Note: there are other positions where these tabs can touch traces. As you can imagine, this could cause troubles which are very difficult to locate.
I would recommend using Kapton tape under the sliders , not precisely as insulation , but to establish a small amount of space between these tabs and the traces.

On a related note, the Alps sliders are rather difficult to remove cleanly. The Alps datasheet specifically cautions against mounting on two sided board with pads on both sides and through-hole plating. Following Alps' recommendation would make these parts MUCH easier to remove safely.