Component C29 is backwards on all TB-303's... (more inside)
Moderators: Altitude, adafruit_support_bill, adafruit, phono, mome rath

Component C29 is backwards on all TB-303's... (more inside)

by bcbox on Tue Nov 30, 2010 5:19 am

I found something very interesting... ok interesting to me at least! I recently stripped down an entire TB-303 mainboard I was doing some work on. When I got around to re-stuffing the PCB I found something curious. The legend for component C29 is backwards on the PCB, and in fact the component is installed backwards versus the schematic. I have verified this with pics from the dozens of 303's I have worked on, a few of which I show in some pics below...
The x0xb0x PCB is laid out the correct way, which would be like the schematic, so C29 is backwards versus an actual 303. This component is part of the circuit that makes up the recovery stage at the output of the filter responsible for converting the two outputs from the ladder to the single ended output at C14.
I also recorded some samples with C29 in both orientations. In each sample the first 2 bars is C29 as per actual TB-303, and the last 2 bars is C29 as per schematic.

File 1 - Setting 1 saw
File 2 - Setting 2 saw
File 3 - Setting 3 saw
File 4 - Setting 4 saw
File 5 - Setting 1 sqr
File 6 - Setting 2 sqr
File 7 - Setting 3 sqr
File 8 - Setting 4 sqr

Image

Image

Image

Image

Image

Image
bcbox
 
Posts: 587
Joined: Thu May 25, 2006 12:30 am

Re: Component C29 is backwards on all TB-303's... (more inside)

by phono on Tue Nov 30, 2010 6:01 am

seems to affect the resonance considerably, on the examples with little resonance the difference was far more subtle.
Image
x0x #733 Image SH-101 MC-202 TB-303 TR-606 TR-808 TR-909 MKS-50 Juno-106 A-100 Virus-B E-6400 SX-150 Monotron
User avatar
phono
 
Posts: 1502
Joined: Wed May 02, 2007 3:01 pm
Location: forum whore aka 2Cv

Re: Component C29 is backwards on all TB-303's... (more inside)

by antto on Tue Nov 30, 2010 6:35 am

bcbox aha!
i knew there was something wrong with the accent on the x0xb0x
but the patterns you used for this test are very unhelpful
so, we should desolder C29 and solder it backwards?
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)
User avatar
antto
 
Posts: 1361
Joined: Thu Apr 15, 2010 2:21 pm
Location: 127.0.0.1

Re: Component C29 is backwards on all TB-303's... (more inside)

by phono on Tue Nov 30, 2010 6:37 am

antto wrote: we should desolder C29 and solder it backwards?


it would seem so if you want an accurate emulation
Image
x0x #733 Image SH-101 MC-202 TB-303 TR-606 TR-808 TR-909 MKS-50 Juno-106 A-100 Virus-B E-6400 SX-150 Monotron
User avatar
phono
 
Posts: 1502
Joined: Wed May 02, 2007 3:01 pm
Location: forum whore aka 2Cv

Re: Component C29 is backwards on all TB-303's... (more inside)

by antto on Tue Nov 30, 2010 6:49 am

i'll try now
oh damn, desoldering is so hard for me
last time i was desoldering R97 to make a reso boost mod and uhm.. i was looking at R97 but was poking something else on the other side with the iron O_o and was cursing it why it didn't wanna go off (slap myself in the face)

i'm gonna record a reference pattern before i do this
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)
User avatar
antto
 
Posts: 1361
Joined: Thu Apr 15, 2010 2:21 pm
Location: 127.0.0.1

Re: Component C29 is backwards on all TB-303's... (more inside)

by phono on Tue Nov 30, 2010 6:56 am

it can be hard with the x0x, since its a double sided board the holes are plated. Getting the last bit of solder out can be a bit tricky. Since the caps are cheap it might be easier to cut the old one out and then desolder the pins.
Image
x0x #733 Image SH-101 MC-202 TB-303 TR-606 TR-808 TR-909 MKS-50 Juno-106 A-100 Virus-B E-6400 SX-150 Monotron
User avatar
phono
 
Posts: 1502
Joined: Wed May 02, 2007 3:01 pm
Location: forum whore aka 2Cv

Re: Component C29 is backwards on all TB-303's... (more inside)

by phono on Tue Nov 30, 2010 7:02 am

i can confirm that this is also the case on both the 303's here. So it's universal as far as I can tell.
Image
x0x #733 Image SH-101 MC-202 TB-303 TR-606 TR-808 TR-909 MKS-50 Juno-106 A-100 Virus-B E-6400 SX-150 Monotron
User avatar
phono
 
Posts: 1502
Joined: Wed May 02, 2007 3:01 pm
Location: forum whore aka 2Cv

Re: Component C29 is backwards on all TB-303's... (more inside)

by antto on Tue Nov 30, 2010 7:50 am

i did it, and i didn't brake anything O_o
will put the case and knobs back together and re-record the patterns
firs thing i notice now is the resonance level is lower, but i *might* have touched either the reso knob or the R97 pot while disassembling it.. i can't tell
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)
User avatar
antto
 
Posts: 1361
Joined: Thu Apr 15, 2010 2:21 pm
Location: 127.0.0.1

Re: Component C29 is backwards on all TB-303's... (more inside)

by antto on Tue Nov 30, 2010 8:57 am

it seems there is no (or too small) difference in the accent behaviour
the only thing is i needed to boost the resonance (R97) even more to get back to my previous resonance level
this doesn't really sound right
if i set my R97 pot to 10K - it's soo wrong.. i need at least 7.38K there to reach the normal resonance level with the reso knob up

EDIT: here are the recordings (wav files in .rar)
z_C29_original_90-130BPM.rar
z_C29_reversed_90-130BPM.rar
We are here too: irc.freenode.net >>> #x0xb0x
..:: c0nb0x v1.00 ::.. http://forums.adafruit.com/viewtopic.php?f=7&t=29282 (new control app - win/osx)
VCO Tuning: http://forums.adafruit.com/viewtopic.php?f=12&t=24759 (do it the EASY way)
User avatar
antto
 
Posts: 1361
Joined: Thu Apr 15, 2010 2:21 pm
Location: 127.0.0.1

Re: Component C29 is backwards on all TB-303's... (more inside)

by Luap on Tue Nov 30, 2010 12:13 pm

Wow, im surprised this was undiscovered all this time, until now. So well done on finding it.

I listened to all the demo's, and couldn't really tell any difference, except perhaps in File 4, which was marginally different, and even then, it's hard to describe how.

And damn, thats quite a strip down job you are doing on that 303! :shock: What on earth is wrong with it that requires all that work?
Luap
 
Posts: 355
Joined: Wed Jul 08, 2009 6:10 pm
Location: London'ish. UK

Re: Component C29 is backwards on all TB-303's... (more inside)

by Brassteacher on Tue Nov 30, 2010 5:41 pm

Interesting. I've been studying the schematic, and oddly enough I don't think it would make that much difference, at least if we were discussing something that had a brand-new, decent quality cap in that position, which most x0x'x do. However, if we were taking into account a 30 year-old cap, that may not have been the greatest quality, and considering electrolytic caps back then were not as well made, or efficient as the modern ones, there may be a small difference.

I would suggest that if anyone wants to truly test this on a x0x they should somehow find a 1uf, 25V or higher cap that is 20+ years old and put that in place of C29 and repeat your tests.

Rationale: C29 at first appears to be a smoothing cap (filters out any potential noise and other garbage, as well as preventing voltage drops on quick transients) for the +5.333V line going into the buffer that biases Q20 and Q21. If it was a DC-only circuit, it would be quite a difference because C29 would never charge, and at best it would act like a resistor and leak quite a bit of current to ground. However, in this case you have AC voltage coming into the bases of Q21, and while I haven't tried to calculate the AC voltage at that point (I'd rather just stick the probes of my meter there later and measure reality as opposed to theory), I would imagine there's more than enough AC voltage there to charge the cap. Once the cap charges, it can do it's job, which also is to keep that AC voltage from getting into the +5.333V supply. Do caps behave this way? If they didn't, voltage doublers wouldn't work.

Now, in a modern, brand-new cap, this would happen quickly enough to where it really shouldn't make any difference at all really (in theory :mrgreen: ). However, a 30+ year-old cap, that wasn't made as well to start with, and by now probably can't hold a charge well enough anymore to be an effective smoother/filter even if it was biased correctly, I bet it would make a noticeable difference. If it's not filtering correctly, and not shunting all the AC to ground, every +5.333V connection will now have a portion of the signal on it. In some places it may partially cancel out, such as at the collectors of Q26, since the signal is also in coming into both bases. This may explain why the VCO doesn't go crazy under these conditions (or, it may not 8) ). However, unless I haven't counted all the polarity inversions correctly, Q12, Q18, and Q21 don't have that advantage, and well, you can see where they are. :mrgreen:

So, I think it would be worth sticking a really old cap at C29, backwards of course, and then see what happens. Think about one thing, and this same issue comes up in high-end audio circles when discussing vintage equipment, none of us are hearing a TB303 the way it sounded when it was new (unless, as the OP seems to be doing, treating one to a full rebuild, and probably with all new electrolytic caps). In fact, this may very well account for almost all the differences in sound that are perceived between a x0x and a TB303. Most of us are using the same active components, and a lot are probably the same age as the originals, but I don't know of anybody that's built a x0x with 30 year-old passive components that have been used for all those years. As an aside, if someone happens to find 30 year-old electrolytic caps that are NOS, don't even bother, as most likely they would not work at all. Like old cars, they have to be driven :).

I mean, even if I'm totally off base, at least this is food for thought.
Don Taylor, a.k.a. Brassteacher
"Jack of all trades, master of none, proficient at even fewer..."
Brassteacher
 
Posts: 248
Joined: Fri Oct 08, 2010 3:49 am
Location: Kennesaw, Georgia, USA

Re: Component C29 is backwards on all TB-303's... (more inside)

by bcbox on Tue Nov 30, 2010 6:36 pm

So, I think it would be worth sticking a really old cap at C29, backwards of course, and then see what happens.

I've done this as well.
I repeated the test with three different caps.
1. Original panasonic
2. NOS panasonic
3.Sprague 515D-series

Identical results in all cases.
The samples I posted were using the Sprague.
bcbox
 
Posts: 587
Joined: Thu May 25, 2006 12:30 am

Re: Component C29 is backwards on all TB-303's... (more inside)

by Brassteacher on Tue Nov 30, 2010 7:29 pm

bcbox wrote:
So, I think it would be worth sticking a really old cap at C29, backwards of course, and then see what happens.

I've done this as well.
I repeated the test with three different caps.
1. Original panasonic
2. NOS panasonic
3.Sprague 515D-series

Identical results in all cases.
The samples I posted were using the Sprague.


Cool. I noticed in the close up photo of rev1 pcb serial 1532xx that is has a sanyo cap for C29. I've got enough vintage gear lying around that I may be able to find one, and I'll give it a shot in my x0x build along with the other planned experiments. Speaking of which, I noticed in this same photo that all the TO-92 NPN transistors are 2SC1815! :shock: I have a batch of these I was planning to test in the mix anyway, but I have never heard it mentioned before anywhere that an original TB-303 had used them! I was planning on it since my dear departed Juno 6 used the same transistor almost everywhere, with a few 2SC945P mixed in. Time to look at all those photos that you posted again, who knows what I'll find.
Don Taylor, a.k.a. Brassteacher
"Jack of all trades, master of none, proficient at even fewer..."
Brassteacher
 
Posts: 248
Joined: Fri Oct 08, 2010 3:49 am
Location: Kennesaw, Georgia, USA

Re: Component C29 is backwards on all TB-303's... (more inside)

by bcbox on Tue Nov 30, 2010 7:38 pm

And damn, that's quite a strip down job you are doing on that 303! :shock: What on earth is wrong with it that requires all that work?
I didn't accurately describe exactly what I was doing. It was a messed up unit, I was fixing the VCF and VCO and replacing all the electrolytic and tantalum caps. It was a good opportunity to remove all the components from the PCB and get a good scan of it so I could accurately clone the PCB. Much easier than working from the layouts in the service manual.
I noticed in the close up photo of rev1 pcb serial 1532xx that is has a sanyo cap for C29
yes, most all 303's have the old general purpose Sanyo's.
Speaking of which, I noticed in this same photo that all the TO-92 NPN transistors are 2SC1815!
yes! many TB-303's below serial number 200000 have those. They also have 2SC536F, 2SC945 and 2SC1685. The PNP's are always 2SA733AP, although most places aren't critical and I have tested/used the 2SA1015 complement in the digital and some of the audio and control circuits with no change in performance.
bcbox
 
Posts: 587
Joined: Thu May 25, 2006 12:30 am

Re: Component C29 is backwards on all TB-303's... (more inside)

by Computer Controlled on Tue Nov 30, 2010 8:18 pm

Hmm. I'm not hearing a difference in sound at all. But maybe i'll try it to see.
Synths: SH-1 . SH-101 . TB-303 . JX-3P . x0xb0x . uWave II . Alesis Ion . Pro-One w/Synthwood case . MoPho . Monotron

Drum Machines: TR-707 . R-70 . E-MU Procussion
User avatar
Computer Controlled
 
Posts: 715
Joined: Fri May 06, 2005 4:48 pm
Location: Seattle, WA