Help and Schematics for Bell Electrolabs A.D.T. !!!

[manuel-sevilla]

Hi, I have a problem with my Bell ADT, is a kind of Phase Shifter and the overall modulation is distorted and noisy it seems to be related to the Voltage which controls the CD4006 or the CD4006 by itself.

Any suggestions ?

Does exist any chance to find schematics ?

Are the 20 years old Quartz Crystal Components subjected to Fail ?

and the CD4006 chip ?

Thanks in advance
MF

[Mark Hammer]

That is some rare piece you have there. Congratulations! I certainly hope it is not distorted because of a damaged SAD512D, because those are very hard to find and very expensive when you find them.

Looking at the datasheet for the CD4006, it is likely that this is being used as a divider to take a single master clock frequency, and divide the frequency down before feeding it to the 4 separate SAD512D BBD chips.

The SAD512D is somewhat unique in that almost all other BBD chips require two complementary clock signals, a "tic" and a "toc", to function. The 512D uses one single clock signal, and divides that down internally to provide its own tic and toc.

What may look like quartz crystals (I assume you mean those big silvery box things) are not crystals at all. They are simply capacitors with the leads soldered on the outside, instead of on the inside where we are used to seeing them.

I do see several trimpots on the board, though, and this is also normal for a time-based effect. The BBD chips will require a "bias voltage" at their input. When the bias is a little bit higher or lower than it needs to be, the BBD chips will pass signal, buit it will be distorted. As you move closer to the ideal bias voltage, the signal gets cleaner. As you move outside the ideal bias, whether higher or lower, the chip stops passing signal. It is possible that the trimpot setting needs to be adjusted. It is possible to do this by ear, and will not harm the pedal if you move it around from one extreme to the other. I would recommend adjusting it by ear under the best possible circumstances. So, a clean undistorted input signal, and if you have a way of listening to it through headphones, do that.

That is only a partial strategy, though, first we need to find out which trimpot is the one to adjust.

[manuel-sevilla]

.. Thanks for your reply!

The unit has 4 different Basic Programs selectable by a 4 way switch ( let say 4 different effect Mode ) and a single pot to control the overall deviation ( effect amount).

Does the 4 SAD512D stays for the 4 different programs ?

if yes, as the noise is exactly the same on every program, that suggested me the problem is related by Clocking these 4 SAD512D ..?

I have adjusted the trimpots by ears ... and two of it affects the sound and the noise in dramatic way but no way to find the right spot.

MF

[Mark Hammer]

I do not know where there might be a schematic. If it were possible to provide a better picture of the board, and perhaps both sides of the board, it might be possible to identify where on the board to start troubleshooting.

[Enzo]

Tic and toc, never thought of it that way, I like it.


So you start with a "tic," and then have the interrogator say "Ve haff vays of making you TOC!!"

[manuel-sevilla]

Here's few pictures with board details, front and back.
I can Email full resolution PIC if needed.
Thanks.
Manuel

[Mark Hammer]

I can't see them here at work, and probably won't be able to log on later this evening, so don't interpret that as disinterest. I'll try to devote some time tomorrow evening.

[Mark Hammer]

Just don't flip-flop on your answers, though!

[manuel-sevilla]

Please take it as a joy ! .... no hurry at all, it's a hobby !

Thanks
Manuel

[Mark Hammer]

Okay, I can see the pictures now (thanks!!). I'll spend some time on it this weekend, and see if I can decode the functions of the trimpots.

[Mark Hammer]

Okay, the first thing I can tell you is that the trimpot seen in the lower right-hand corner in http://i629.photobucket.com/albums/u...lADTBoard1.jpg is the bias voltage trimpot. As described before, if you adjust this one you should hear that there is A) no delay sound or effect, B) a very small and distorted delay sound, C) a louder and distorted delay sound, D) a clean clear delay sound, E) a loud distorted delay, F) softer distorted delay, G) no delay sound at all.

Now I have to figure out what the other trimpots are!

Note that, because the unit uses four SAD512D chips it is technically possible to run it on fewer than 4 if, for example, one of them is damaged. A small modification would be needed, but it would run.

One simple way to test whether the chips are still good is to find one of the many pedals that uses such a chip, and substitute one of yours for the chip that is already in the pedal. Many of the older DOD delay-based pedals (chorus and flanger), like this one: DOD 690 Chorus | DiscoFreq's Effects Database or this one: DOD 565 Stereo Chorus | DiscoFreq's Effects Database use that chip. MXR delay-based pedals made in the early 80s do as well, like this one: MXR M-168 Stereo Flanger | DiscoFreq's Effects Database and this one: MXR M-167 Stereo Chorus | DiscoFreq's Effects Database and this one: MXR M-152 Micro Flanger | DiscoFreq's Effects Database used the SAD512D.

Of course, given how rare they are these days, if all 4 of them work, that's about $200 of chips you have there, for any interested buyers!