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Post by terjewinther » Thu Nov 12, 2015 10:20 am

I while ago I got some PCBs and stuff which was mostly digisound modular stuff, but between everything there was one Tau-1005 VCO PCB, completely stuffed and working. As these are quite rare, I thought I should write something here about it.

It came attached to a large green connector that attaches to the edge card fingers. There was also a veroboard connected to it, which seemed to have been used to bring digisound power to the module. It needed some minor fixing, but apart from that, it worked perfectly.

Refering to the schematics found on Jim Patchell´s web site:

- It was set up as a LFO with a huge polystyrene cap in position C. I swapped this for a 300pF polystyrene cap as described in the schematics, and it then became a nice VCO. I can still switch in the huge original cap, so I have a twin VCO/LFO module now.
- The LM207 sel. (designated A1) is a LM307M
- The HA-2500 (A3) is painted red/orange, indicating it was selected too
- The Q3 transistor is printed yellow, as in the schematics, and it seems to be a 2N5457 (or it could be somehing like a 2N8456 - hard to read)
- The Q4 transistor is a 2N3417
- Transistors Q5 and Q6 are selected, printed in red-white-red stripes
- The two caps C3 and C4 were not present, so I put in 68pF ceramics
- The trimmers are large Piher 20k trimmers, marked 63 8576 7323
- The CV inputs had various 5% resistors (100k - 150k), so I swapped them for 1% resistors, except the fine tune resistor, which was already a 1,5M 1% large blue resistor
- Between Q2 and the CA3080 (pin 5) there is a 18k/1% resistor that is not in the scematics nor in the PCB layout
- The 142k resistor at the CV inputs is a 147k resistor in the real
- The symmertry resistor is not 390k but 1M/1%
- The two 1k resistors connected to pin 2 and 3 of the CA3080 are 390 ohm / 5% in the real
- One leg (pin 5) of A3 (HA2500) is cut off to make it fit the PCB
- The 100k resistor at the bottom of the schematics (between Q5 and CA3080) is a 39k/1% resistor in the module
- The 1volt/octave input: the part order is not correct on the schematics, but it is corect on the PCB drawing

It was surprisingly easy to trim the module to 1V/oct, even though it only has a single turn trimmer and not a multiturn trimmer. It tracks quite well too. Before trimming, it had a whooping range of 8Hz - 102kHz. After trimming it has a range of 4Hz - 91kHz, using both the course and fine tune pots. I thought it would be difficult to tune the VCO using the front panel controls with that large range, but it is very easy.

Switching in the huge cap for LFO work got me this range:
75 seconds (that is 0.013Hz) to 376Hz

Something to note: I originally set up the coarse and fine tune pots as voltage dividers between -15 and +15V. However: the Tau VCO module doesn´t like negative CV on the inputs (confirmed with external CVs from others sources as well), so I rewired the coarse and fine tune pots between ground and +15 volt, which works great.

Another detail: the triangle output is 10V peak-to-peak, and the square output is really a whooping 26 volt peak-to-peak, just as described in the schematics. That is way too much for my modular, so I used a dual op-amp (on the extra veroboard) to buffer and change the outputs so they are 10 volt P-P on both waveforms.

Here are some photos, as well as a SoundCloud test file:




Last edited by terjewinther on Sun Jul 17, 2016 5:10 am, edited 1 time in total.

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Post by oberkorn » Thu Nov 12, 2015 3:28 pm

wow thanks, never seen these in the flesh before.
very nice sounding as well!
you have to pay for the music but the noise is free

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