PORTABELLABZ

208p FAQ


Buying the parts : which, where, equivalents, rare parts, price, BOM, Mouser cart... ?
Corrections : fixes, improvements, differences with other 208... ?
Building : difficulty, help, calibration... ?
Expanding and interfacing : cards, expanders, negative CV... ?
Keyboard : 218 ? tracking, external controllers... ?





Buying the parts

Do you or a third party offer full kits with all the parts needed ?
Yes
SAmodular offers parts, kits and bundles.
 

Should I spend undue money on expensive obsolete rare parts ?
No
Contrary to what some may believe or claim, there's no magic contained in old electronic parts and the expensive obsolete rare ones can be replaced without impact on the sound or behaviour, details below.
What makes a nice sounding and good working synth is neat build, careful selection on test of some components, precise calibration and basic understanding of how things work. No snake oil or esoteric belief will ever make up for the lack of any of these.
This is valid for all 208 revisions and for all synths in general.

The tips, observations and opinions shared here come from my years of experience building and repairing synths and audio gear.
Of course, feel free to use the parts you want, to experiment and compare by yourself and come to your own conclusions, it's part of the DIY fun.

Can I replace the A726 / UA726 ?
Yes
T
his part is insanely and unduly expensive
, therefore t
he PCB set includes a pair of
pA726, a very good working drop-in replacement. This doesn't affect the oscillator's sound or timbre in any way and the tracking and stability are better than
with A726 / UA726.
T
he cards 6 and 7 have the original layout
, so if you have money to waste and do care more of vintage hype than performance, you can use
A726 / UA726 instead of the pA726.
Beware of the cheap Chinese ones, these are most likely fakes that
at best won't work but might also damage other parts in the circuit.

Can I replace the 2N4339, 2N4340, 2N4341
?
Yes
The cheap J201 available on eBay works fine instead of those and doesn't impact the sound or behaviour. Other JFET's selected on test should work as well.

I've seen some J201's in Q1 on card 7 (2N4341) cause the MO to slightly modulate the CO, if you notice this unwanted behaviour, select another J201 on test to fix it.
Only in Q1 on card 8 (2N4339) I couldn't get a proper CO sine with a J201, a selected SMP4339 works fine thanks to this adapter.

Can I replace the 2N3565 ?
Yes
The PN3565 works fine
and doesn't impact the sound or behaviour. It should be mounted this way.

Can I replace the MC846P ?
Yes
Dave Brown explains how to replace it with a 74LS03This doesn't impact the sound or behaviour.

Can I replace the LS352 / 2N3806 / MAT03 ?
Yes
The 2N3807, 2N3810, ECG82...
work fine and don't impact the sound or behaviour. Other matched PNP transistors pairs with similar specs should work as well.

Can I replace the 2N1711 ?
Yes
The 2N3019 is a drop-in replacement and various other NPN transistors should work, the 2N1711's are used as a LED driver.
This doesn't impact the sound or behaviour.

Can I replace the DG200A ?
Yes
The 208p PCB set comes with a
DG200A-dapter PCB for the modern DIP or SOIC Maxim DG200A. This doesn't impact the sound or behaviour.

Can I replace the big 390F Sprague capacitor ?
Yes
A standard easy to source 330F electrolytic capacitor works fine.
This doesn't impact the sound or behaviour.

Can I replace the big API Delevan
2.2uH inductors ?
Yes
Other 2.2uH inductors
such as Tayda's work fine and don't impact the sound or behaviour.

Can I replace the C&K switches ?
Yes
Taiway switches available from Small Bear are very nice and cheaper.

Any toggle switch can be used for the SPST on-off and on-off-on ones, but I don't recommend to use cheap ones, replacing a faulty switch ain't no fun.
For the DPDT on-on-on switches use Small Bear's Taiway or the BOM's C&K ones only, other ones might have a different pinout and not work on this PCB.
The PCB fits both
PCB mount (with thin lugs) and panel mount (with larger lugs) switches.


Which vactrols should I use ? Excelitas, Vactec, Xvive, Coolaudio... ?
A
good selection on test of a vactrol is more important than its age or the manufacturer name printed on it. These parts  have a large tolerance even in a same batch of the same manufacturer.
However the dual Xvive VTL5C3/2 upset proper LPG behaviour. The cause is their decay tail is too short and no difference can be heard between the combination and LPF modes. The Excelitas
VTL5C3/2 work better for this purpose, although selection on test is recommended as well.
More details on the Muff Wiggler forums
Dave Brown compared accurately different vactrols that he has in stock.
 

Do 1% / 5% tolerance, carbon / metal film resistors sound and behave different ? Which ones should I use ?
5% means a 100k resistor's value will be between 95k and 105k.
1% means it will be between 99k and 101k.
So in a 5%
resistors lot, some will most probably be actually 1%.
In some cases, it's important to have a precise value resistor, in most ones 5% are enough.
In the 70s, 1% were much more expensive than 5%. Today the price is the same and 1% metal film resistors can be used everywhere. No, this won't impact the sound.

"
Some people believe that using parts with a wider tolerance range is required to invoke whatever magic they think is contained in old electronics. I built my 208 with all 1% parts, low drift C0G ceramics, and so on. I am very happy. I use it almost exclusively for pitched material so stability and accuracy are more important to me than witchcraft." (nice tarandfeathers quote on Modwiggler).
Some people also believe that carbon resistors sound warmer and metal film ones sound colder, or that components with golden leads sound richer or that old parts sound more vintage and more musical, or that THT components sound bigger than SMD, or that the expo converter impacts an oscillator's sound and timbre, or that tropical fish caps sound more colored, or that the dust in an old tube amp is an essential part of its "mojo"...

Such beliefs border more on the esoterics and "idiophilia" than on actual facts... but some sellers take advantage of this ignorance and credulity.

 

How much I'd end up spending on parts to stuff these ?
Say in the 1000 range as a rough estimate, with a quite large margin depending on the parts used, on how and where you buy the parts (100 resistors cost 1.20 but 1 only costs 0.10 or more, online Asian sellers are cheaper than the shop around the corner), on the shipping costs (all parts are not available from a single supplier), on your skills (risk of damage or wrong mounting while soldering) etc.

SAmodular's full kits are good value for money and will save you a lot of time.


Do you have the BOM as a spreadsheet or a link in order to load into Mouser  ?
Wiggler sanemalkavian did one.
Wiggler thombet did a new one.
I didn't check
their content and assume no liability for their accuracy.

Which slider pots the motherboard will take ? 
Silder emplacements match both old and new slider pots.
I recommend Farnell's Alps ones (part # 1191730) however, the plastic old ones are really crappy.

Will the other 208 panels fit these PCBs ?
Yes
The PCBs are 100% compatible with all the 208r panels available from other suppliers.
The PCBs are not compatible with the 208e and 208c panels, some holes placement is different.

  

Corrections
 
How close to the original 70s 208 or 208r
are the PCBs ?
The PCBs are similar to an original 70s 208 with some corrections to ease the build and improve performance.
Compared to the 208r :
- No extra wirings or traces modifications are needed and all components can be mounted as shown on the parts layout drawings
.
Only a small trace is missing on board 2
between pins 5 and 6 of IC1 of the bare tracks revision, a 2.5mm solder bridge is the only fix needed on the 208p and is mentioned in the  build notes.
- The noisy headphones out is fixed by the addition of 4 resistors on the motherboard.
- The reverb output is via a shielded cable only to avoid the MO bleed common issue, it won't work otherwise, the related trace on board 12 was removed.

- The enlarged holes for switches allow the use of both PCB mount (with thin lugs) and panel mount (with larger lugs) switches.
- The rectified pots holes emplacements avoid bending their legs and causing stress to the resistance element.
- The extra holes for the trimpots wiper on daughterboards allow the use of various types.
- The gold plated transfer card (between the panel and PCB edge connectors) fits 15mm standoffs, to make the switches nuts easier to screw than with 17mm ones.
-
A +5V connection to the card slot was added to work with the iProgram card or other card needing +5V.  The new silkscreened revisions also features a noisy ground connection.
The iProgram card also needs pins 2 and 4 of both rows on the edge connector  to be connected to quiet and noisy grounds.  These  connections are not implemented on the PCB but details are in the build notes.
- The new revision has all parts value and reference silkscreened and a soldermask.

Should I do the fixes as mentioned on
Dave Brown's page ?
No
The traces fixes on the motherboard and board 3 are for the 208r PCBs errors that are not present on the 208p.
The only error that needs a fix on the 208p bare unsilkscreened boards is the small missing trace on board 2
between pins 5 and 6 of IC1 as mentioned in the build notes.
No fix is required on the silkscreened boards.
Yes
Some of Dave's modifications such as the board 5 ones may be very helpful.

Should I use 1N5236 or 1N5228 on board 5 ?
1N5236 (or other 7.5V Zener diodes).

Do Dunnington Audio's envelope infinite sustain fix and other mods work ?
Yes

What do you think of the new card 12 sold by boops ?
A properly built standard card 12 works and sounds well and I see no reason to change it.
The only new card 12 I've seen so far was in a seriously bugged 208r rev2
built and sold by boops that I salvaged for a customer.
Some of the problems were caused by this new card 12
: noise, clipping, faint sound, inverted waveforms, weird reverb mix... and the required Q-N grounds connection was missing.
I simply replaced it with a standard card 12 and
had no interest in pursuing the investigation.
Some builders seem happy with this card however, so these issues may be more of build errors than design errors.

To improve the reverb mix pot course, use a 10k log pot.
If a lin pot is already installed, add a resistor of 1/10 of the pot's value (if the pot is 10k, 1k resistor, if 50k, 4k7 resistor etc) between the pot's wiper and CCW terminal.


Building

Is this a difficult build ?
Yes
This is one of the most difficult
synth DIY projects and it should only be done by those well experienced in SDIY. Having built a pair of pedal kits or module kits is not enough to make you an experienced SDIYer.
The 208p is a complex module. The build itself could be ok if done thoroughly and slowly but in case of problem troubleshooting might be difficult, very difficult.

Basic electronics knowledge and ability to understand schematics are highly recommended.
Read the 208p and 208r rev2 build threads to make an idea of the difficulties you might have to face.
Be realistic and honest with yourself about your actual skills and experience.
It'
s an expensive and time consuming project and you don't want to waste your money and time, do you ?

Which tools do I need ?
Good regulated soldering station with thin tip, good desoldering tool, good oscilloscope, good digital multimeter, good frequency counter or guitar tuner, small pliers, cuting pliers, wire strippers, lead bender, screw drivers, socket set or nut drivers.

I have a problem, can you help me by email ?
Given the complexity of this build, I do not offer individual support, related e-mails or private messages will be ignored but I'll reply if I can help on the build thread on Muffwiggler, thanks for your understanding.

I have a big problem, can I send you my 208p to complete or fix my build ?
Yes,
depending on the issues, the quality of your work and the solder used.
This service is chargeable.

Please send me a detailed description of the issues, good pics showing the build details under all angles and the reference of the solder used.

Is there a detailed calibration procedure ?
Yes
In the build notes and on
Dave Brown's page.

Should I do the wiring as shown in blue on boards 2, 3 and 6 ?
Yes
.
On the new boards, the connection is silkscreened.

A hole is missing for IC3 (DG200A) on board 9.
This is normal, snip the IC's lead 7.

The CO frequency is limited to about 2.4 KHz.
This is normal and by design.


Expanding and interfacing

Is it safe to use the 208p with negative or bipolar CV ?
Yes it's 100% safe.
The restriction for negative and bipolar voltages doesn't apply to the 208p.

Does
the 208p work with the various expansion cards and expanders offered by Portabellabz and other manufacturers ?
Yes
The 208p is compatible with any cards and expanders working with the 208e, 208c and 208r, but the controls related to the few own features
of the 208c and 208e don't work obviously.


Keyboard

Do you sell or will you do a 218 keyboard ?
No
SAmodular offers one.

Does another keyboard work ? And a sequencer ? And a midi to CV/gate converter ?
Yes
Any
CV/gate keyboard or sequencer, any midi to CV/gate converter will work, the 208p can be calibrated to match any scaling.
A pulse generator is recommended to get the typical pulse required  for proper sustained/transient envelope performance.

Which is the oscillators tracking ?
The tracking via the keyboard voltage input can be 1V/oct, 1.2V/oct, 2V/oct, or any other scaling, it's a matter of calibration and  CV scaling resistors value selection, so this depends on the final user's own preference.
Properly calibrated, the tracking is good over 5 octaves.