3102/3207 BBD Question

[lars-musik]

Hi folks,

A friend of mine would like a chorus/vibe pedal with some additional modifications, and we decided to use the Ibanez RC-99 as the basis for this project. I happened to have an old MN3102/3207 combo on hand, so after the PCB arrived, I did some soldering... but there's no BBD output.

I'm unable to detect any clock signal on pins 2 and 4 with my oscilloscope.

The schematic was shared by Dirk Hendrik in another forum. Here's the link to the complete version:

https://www.dirk-hendrik.com/temp/ibanez_rc99.pdf

It seems that other people have successfully built this, but I have a few questions:

VCC (9V) to pin 5 on the 3102 – I haven't seen this setup in other chorus or flanger circuits. Under what circumstances should this be done, and why?

8V supply from the 78L08 – The 78L08 has a voltage drop of around 1.5–2V, which seems problematic in a 9V-powered circuit. However, since the BBD combo only tolerates up to 10V, using a 12V supply isn't an option. Why is 8V required here instead of the 9V line? And if 8V is necessary, wouldn't a diode in series or a voltage divider work just as well?

Schematic shows a PNP labeled as 2SC105 – which is technically an NPN transistor. I placed a PNP here, which seems correct to me.

Maybe the chips are faulty (I can't recall where I got them, so it's possible), or I might have made a different error (the PCB matches the schematic based on a software comparison). Do you notice anything that could be off?

Thanks for taking a look,
Lars

[Mark Hammer]

There are some unanswerable questions here.  A local buddy was recently selling off his RC99, and posted pictures of it with the box and accompanying power wallwart  ( https://www.guitarscanada.com/threads/ibanez-rc99-analog-rotary-chorus-with-box-and-adapter.292700/ ).  The wallwart is marked as an Ibanez unit and provides 9Vdc at 200ma.  The sticker on the bottom of the pedal itself, indicates current draw of 37ma, and also stipulates the part number of the Ibanez wallwart.

Technically, a 78L08 should not function properly with a 9V input, so something seems amiss here.  Dirk indicated he was in Amsterdam when the drawing was made, in 2007.  Was the wall power different, or was the adapter output voltage something higher than the 9V my buddy's pedal shows?  It's just not clear to me how we get from what we see in the unit I linked to, to Dirk's drawing. 

The 2nd generation (MN32xx) Panasonic BBD chips were designed to run off lower voltages.  My ongoing assumption is that this change was in anticipation of batteries wearing out.  Most time-based pedals of that era ran off 9V batteries, and the DC bias voltage that BBDs require was generally derived by dividing down the supply voltage with a trimmer.  As the battery drops from a fresh 9.6VDC to 8 and less, that bias voltage  can become erroneous, and clean performance suffers. Tweak the bias trimmer and you'll hear that it doesn't take very much on either side of the "sweet spot" for the delay sound to get distorted.  By allowing them to run off lower voltage, and using a 5V regulator to turn a 9V battery into a stable 5VDC, the factory bias setting would remain valid until the battery dropped down to around 7V.  And by that point, most of the rest of the circuit wouldn't work properly either.  So audio performance could be made more reliable, and require less frequent installation of new batteries.  This is, of course, a now virtually obsolete concern, with most users relying on power bricks for their pedals.

Is it possible Dirk misread the "8" on the regulator, thinking it was 8 instead of 5?  It IS a simple visual error tomake, and a likely one given the tiny print on those little regulators.  And if his drawing simply traced all connections back to the output of that regulator, rather than actually measuring voltages, that could explain a lot.

If you were to replace the "8V" regulator with a 5V unit, nothing untoward would happen to the 3207/3102 pair, and you may just get the functionality sought.

[lars-musik]

Thanks for the analysis, Mark. There are quite a lot of things, where the 8V are used -including the TL022. I will certainly try to exchange the 78L08 for an 78L05.

Do you have an opinion towards the VCC at pin 5 of the 3102? Why is it there and how does it get along with the 8 (or 5V) that is tied to this pin via the 10K, 5pF and 33k?

[lars-musik]

A quick update/status report:

I replaced the 8V regulator by a 5V one - no difference.

Then I cut the VCC to pin 5 abnd now it vibrates!

Now I have to listen and fine-tune. I wonder how the other folks managed to get this thing working based on the schematic. Additionally I wonder what else might be wrong with it.

[Yazoo]

Oddly enough I've just come from the "other" board and saw your post. I built this pedal and also did not understand how the 7808 could work. I used the LDO version of the 7808 which made more sense. My build works quite happily.

Fake BBDs are a real problem now. I've had two batches of fake MN3007s in the past. It would be worth investing in a new MN3207 just to make sure.

[lars-musik]

Oddly enough I've just come from the "other" board and saw your post. I built this pedal and also did not understand how the 7808 could work. I used the LDO version of the 7808 which made more sense. My build works quite happily.

Fake BBDs are a real problem now. I've had two batches of fake MN3007s in the past. It would be worth investing in a new MN3207 just to make sure.

Now that's interesting. Have you tied VCC to pin 5 of the MN3102 as per schematic? Did you design your own PCB?

[Yazoo]

Looks like Dirk has answered your query - yes the VCC at pin 5 was an oversight on the schematic. I used the pcb Dirk provided and looking at it today, there is no VCC connection to pin 5. I built mine in blissful ignorance and did not pick up on that.