CB55 build guide, layout & schematic

[Circuitbenders]

The build guide can be found at:

Original R1 board - http://www.mediafire.com/download/eysexbvear25ytq/CB55_build_guide.pdf

New R1.1 board - http://www.mediafire.com/view/j66mn4koux7cm79/CB55_build_guide_-_Revision_1_1.pdf

You can download a high resolution copy of the CB55 schematic at:

Original R1 board - http://www.mediafire.com/view/tzl34mlfqf6ii1l/CB55-schematic.png

New R1.1 board - http://www.mediafire.com/view/4nud8imrnrsmhm2/CB55_schematic_-_Revision_1_1.png

The board revision is printed on the solder side of the PCB.

[Mikepep]

Thanks for this - just ordered my board.
Concerning the 0.15uF electrolytic in the accent circuit. Small value elctrolytics were quite popular back in the day, because of the size and cost of film capacitors over 0.1uF, you find lots of them in the 909 for example, and in older car radios too, mainly to  save space.

In my 9090 build I used film caps for most of them, I couldn't hear any difference when I compared the result with using electrolytics.

In this machine, I guess they used the relatively expensive tantalum to get the size and low leakage. I expect a 0.15uF film cap will work fine, I'll try it on mine and see if I can hear any difference.

[Circuitbenders]

Thats the weird thing though, i can't see any reason why they'd use a relatively expensive tantalum there. There doesn't seem to be any benefits to using tantalum over electrolytic there in terms of its properties as a cap. If the designers had the choice of electrolytic or tantalum, you'd think they would have gone for the cheaper option, unless they still had a warehouse full of 0.15uF tantalums left over from some other machine that they wanted to use up.

A film cap would probably work just as well if you can fit it into the footprint of the electrolytic on the board. It just seemed prudent to specify a polarised cap if we weren't quite sure why the original designer had made the choices that he had.

[grenert]

Are the Tone, Volume and Accent pots all log taper?
Thanks!

[Circuitbenders]

Have you read the build guide? It says in the parts list:

TONE        1MA     Tone pot
VOLUME    10KA    Volume pot
ACCENT    500KC   Accent amount pot.

It also says:
'The accent amount pot should ideally be an antilog pot, commonly known as a C curve pot, but a linear B curve pot can be used at a push, or a normal A curve log pot if you use the left hand pin and wire it in to operate backwards'

[grenert]

Thanks.  I did see the parts list, but the A/B designation has not been consistent over time/location:
http://sound.westhost.com/pots.htm#markings
http://www.electronicspoint.com/threads/log-lin-pot-a-b-suffixes-prefixes.95279/

I did not see the comment on the accent pot, so much appreciated that you pointed it out.

[Mikepep]

If you can't get an antilog pot, then a 1 Meg linear, with the anticlockwise end connected to the wiper, and a 1 Meg resistor in parallel will give a reasonable approximation to a 500K anti-log. 

This article is a classic source on pots in general, and shows how to fake certain pot curves.
http://www.geofex.com/article_folders/potsecrets/potscret.htm

Mike

[aerror]

Thanks for this - just ordered my board.
Concerning the 0.15uF electrolytic in the accent circuit. Small value elctrolytics were quite popular back in the day, because of the size and cost of film capacitors over 0.1uF, you find lots of them in the 909 for example, and in older car radios too, mainly to  save space.

In my 9090 build I used film caps for most of them, I couldn't hear any difference when I compared the result with using electrolytics.

In this machine, I guess they used the relatively expensive tantalum to get the size and low leakage. I expect a 0.15uF film cap will work fine, I'll try it on mine and see if I can hear any difference.

How did it work out with the film cap?

Another question: My inductor is polarized and I wasn't quite sure which way I should place it... measurement tells me that positive is towards the cap but just to be sure, is that right?

[Circuitbenders]

What makes you think the inductor is polarised?

Its just a coil of wire. As far as i know it can't be polarised, although it might have some kind of marking that indicates the direction of the winding, which isn't really an issue here. Although i can't see why it would ever be an issue with a 47mH coil, unless you've bought something very weird.

[sensor]

i really would like to trigger the cb-55 with my 808. the point is that it's trigger pulses are about 15V for 20ms.
so how to manage this. should i just put a 5V regulator before the trigger circuit? or is the trigger circuit strong enough to handle the pulses?
thanks a lot.

[sensor]

well i will try voltage dividers.

[Circuitbenders]

Running 15v pulses into the CB55 will probably result in some dubious drum sounds.

Give it a go with a voltage divider, although you could really do with looking at what your 808 is actually outputting on a scope. I don't think its going to be as high as 15v.

[sensor]

i think i give 5v z-diodes a try by just replacing the diodes in the trigger circuit. this should be an allround solution.

[aerror]

What makes you think the inductor is polarised?

Its just a coil of wire. As far as i know it can't be polarised, although it might have some kind of marking that indicates the direction of the winding, which isn't really an issue here. Although i can't see why it would ever be an issue with a 47mH coil, unless you've bought something very weird.

i also thought that a coil may not be polarised but because of the different length of the legs i was unsure. there are no polarization marks on the inductor though.

[netpusher]

This is probably a dumb question, so I apologize in advance.  I don't see any mention of wiring grounds on the trig in's.  That does need to be done right?