208p build thread (papz's PCBs)

[papz]

Cheers !

[xahdrez]

Switch arrived and all is working like a dream again. Still have some calibration to do, soon as I can tear myself away from having fun with it! Thanks again Papz for an excellent project - realised the other day that it the only thing that went wrong for me was hardware (broken switch and reverb) so extra thanks for making such a big project so straightforward!

[papz]

You're welcome, thanks for the feedback.

[Peti]

i thought about using Buchla 200e style knobs with this build. - i would need 16mm d-shaft potentiometer (for the bigger pots on the right) , is that correct?
or may i just use the 9mm d-shaft for all pots, if they are nowhere to find?
Peter

[haternob]

Hi folks,

I've finished putting my 208p together and everything appeared to work when I plugged it in. I've been trying to follow Dave Brown's calibration notes but I've run into a bit of trouble.

I'm using the pA726 boards with the build. I've managed to calibrate them so they show the correct voltage (well, there or there abouts, around 0.613v) once they have warmed up.

The trouble I'm having is with the complex oscillator. The sine wave looks to be ok:



When I turn the waveshape knob fully clockwise the spike and square don't show what I was expecting although the triangle seems to be ok:

Spike:


Square:


Triangle:


I'm using a selected 2N4339 purchased from Samodular.

Does anybody know what might be causing the wave forms to appear this way?

[papz]

Obviously something's wrong on card 8.
Try and swap IC1, IC2, IC4, VT1, double check solder joints, resistors and caps value...

The sine is ok, so the 2N4339 is ok.
The pA726 manages pitch only, it's not related to the waveshape.

[haternob]

Thanks for your reply Papz. I've ordered some more chips so will try them out when they arrive. I think I've got a spare vactrol so I'll switch that out in the meantime and see if that changes things.

[xahdrez]

After months of having lots of 208 fun, yesterday I decided that I'd try and calibrate my modulation oscillator (previously only CO was calibrated properly).

This actually revealed that the modulation oscillator tracks great at 1.2v per octave, but the CO is actually way off (currently an octave is split over 1.5 octaves, rather than 1 octave - I just managed to trim my 218 wildly out to get it to work, hence why the MO wasn't tracking in the first instance!).

I've not had a chance to experiment properly with R14 on Card 7, but am assuming that if I increase the resistance here that should bring it into line? Main reason I ask is I also realised my CA3080 on the pA726 is quite a bit warmer on the CO than MO, and doesn't seem to be able to be brought into line with other one (T0 is around 0.60v when cold, but drops to 0.38v when warmed up, and trimmer doesn't do much to affect that!). Seeing as it'll be a few days before I get to trouble shoot, I thought I'd ask if anyone else has experienced similar and where best to focus attention - presumably the pA726? Any ideas welcome!

[papz]

You should lower R14 value, not increase it. Did you use the values suggested in the build notes ?

The pA726 may be faulty, check for +10V at the 78L10 output, double check resistors values.
The CA3046 (not 3080 !) may be in cause too, I use https://www.befr.ebay.be/itm/5PCS-IC-IN ... 1950329911 they work perfect in the pA726.

[sendepause]

Help!
Weird problem going on and sice this threat is full of experienced 208 builders i rather asked here than the roman 208 v2 thread:
My 208 was working completely fine. Tracks well, bit of a problem with timbre in the last octave range, but that's not the problem: my pulser stopped working correctly and won't go into self cycle mode. I can trigger the pulse from keyboard but if triggered it wont sequence the sequencer nor the random. Manual firing the pulse, same thing.
Did swap all ic's except for the cd4001, they will be delivered tomorrow. Also, if I patch the pulse into the LPG, I get a bleed when opening the attenuated and not firing the pulser.

Any idea?

[xahdrez]

You should lower R14 value, not increase it. Did you use the values suggested in the build notes ?

The pA726 may be faulty, check for +10V at the 78L10 output, double check resistors values.
The CA3046 (not 3080 !) may be in cause too, I use https://www.befr.ebay.be/itm/5PCS-IC-IN ... 1950329911 they work perfect in the pA726.

Thanks papz - yes, can confirm have the right value from build notes (120k + 68k in parallel), and 150k for R44. I've double checked resistor values on pA726 and they seem to be okay, so will check the 78L10. I've swapped the CA3046 (oops can confirm it's a 3046 not 3080, that certainly wouldn't help!) in my other pA726, which works fine - they were ordered from your recommended source so hopefully they're okay! Good to confirm it seems likely I'm looking in the right place, so will dig deeper asap!

[papz]

The 4001 is most probably in cause in the pulser, this is a typical issue.

[sendepause]

The 4001 is most probably in cause in the pulser, this is a typical issue.

I’ll report back when I changed the ic’s. I need this Easel back, only thing that is keeping me saine at the moment...

[sendepause]

Probably found the problem:
I accidently used the wrong powerbrick for my Samodular powersupply, fried some chip I guess because the +15v rail is putting out 11,5v now...

[papz]

If so it's good to check everything when the PSU is fixed (or replaced, which may be easier) there are likely other issues.

[xahdrez]

Well that backfired....swapped out the 78L10, was checking voltages on the pA726 and then shorted two of the pins (think 7 & 8 or 8 & 9) and turned the CO into an LFO!

Ah well, at least it'll give me the opportunity to get more to grips with the schematic!

[lilskullymane]

hey everyone! I hope it’s okay that I post a quick question here about my 208p. I recently moved it into a new case (keen association 8u smartcab), and a few weeks later noticed that the 3 stage switch on the sequencer only moves 2 stages. I read somewhere on here in passing that a chip might have fried somewhere to cause this. I have also noticed that the modulation oscillator is no longer calibrated to the main oscillator. calibrating it is no problem, but are these two things related in any way? I’m fine with the 2 stage sequencer, but don’t want to knock anything further out of whack by calibrating the oscillators. thanks so much!

[papz]

Hi Richard

I'd start by checking the +15V, +12v and -15V rails on the new case's PSU.

If not ok, this may be the cause.

If the power rails are ok, recalibrate the oscillator and before swapping ICs, try and increase R9 value from former 100k to 120k or 150k on card 1 for the sequencer stages and add 100nF caps as shown on viewtopic.php?p=3284065#p3284065

[xahdrez]

Okay we're back on track. Fixed the CO (turns out was a blown CA3046) and the MO (blown CD4016). In the process I've managed to improve my t0 readings (both now sit on a steady 0.64 after calibration so guess some of my chip replacing did some other good too). MO tracks fine again, CO is clooooose but not quite there (octave is C0 - D1, so only just slightly off). So guessing I need to lower R14 slightly from the ~43k value? Or should I explore elsewhere? assuming if T0 is in range then the temp compensation etc. will be fine so should hold okay. Any other tips/ideas to explore?

[papz]

If the octave intervals are too large, I'd rather increase R14.

[xahdrez]

Thanks Papz! Well, the plot thickens! Turns out that R14 isn't making any difference at any value between 10k and 300k so guess the issues must be elsewhere. Tried substituting the pa726 for 2 x 2N3904 as suggested above for another builder, and behaviour remained the same. As far as I can tell, their issue was resolved by swapping the ICs on card 7, so perhaps that's a good place to start looking? Sure I'm missing something obvious! As ever, any ideas very welcomed, I'll get there in the end!

[papz]

IC2, IC3 and Q1.

[lilskullymane]

hey, was finally able to check the rails - 11.91 (edit - between 11.75 and 11.82 between the unused connectors with the 208 powered) on the +12v and -14.97 on the -15v on each of the 8 connector slots. +15v was dead on. is this enough to cause the issues above (2 seq. stages, mod osc uncalibrated)? this is in a keen association smartcab btw.

also, received my 208 toolbox today - thanks so much, build quality is beyond fantastic and I can’t wait to use it when I get these issues sorted. thanks papz!

[papz]

Thanks for the feedback on the ToolBox. Hope you'll can use it soon.

+12V and -15V are ok.
The 208 wouldn't work without +15V, maybe a reading error ?

Do you still have the former case in which the 208 worked without the errors ? If so could you try it again in it ?

[lilskullymane]

ah sorry, I meant it was exactly +15.00v on every connector so no issue there. glad to hear the other rails are fine. do you think I’m safe to calibrate the mo at this point and connect the toolbox/218e? thanks so much.