ETI/Powertran/Elrad vocoder

[Hallvard Tangeraas]

At 18:02 +0200  12-06-2000, Magnus Danielson wrote:

>From: Hallvard Tangeraas <hall at oslo.online.no>

>> There's something weird with the circuits -as far as I can remember
>> there was some confusion about the placement of the pots (there were
>> some extra connection points, which made it very strange).
>
>Ah. Even if you did not say *which* pots, here is the secret:
>
>If you look really carefull you will see that the pots on the photos
>have metal supporters. These supporters have their own holes and
>solder points. Now, somebody has been smart and use these as ground
>connection between parts of the PCB. Thus, if you look at the PCB each
>pot have a hole frame like this:
>
>O       O
>
>O O O O O
>
>Where the 4 corner holes are the metal supporter connection and the
>three middle onces is the actual pot ends and tapper in the middle.

I understand, but it's still confusing.
I've had a friend scan the PCB/component placement as well as the
circuit and plan to colour-code the connection points and upload to a
web page, so we can take it from there.
Currently I'm having a hard time comparing the schematic with the
actual PCB. I'm going through every component, and have covered a few
connection points, but I'm still confused. I need some help.
Do you have all the diagrams there?
(it's a little hard to explain if you don't ;-)

OK, here we go....

1) Looking at the schematic, there are some discrepancies...
To the left side you have "IC2a" and "IC2b". This is the 1458.
There are no pinouts, but I've looked the IC up, and looking
at "IC2a", "+" is pin 3 and "-" is pin 2.
The strange thing is that in the actual PCB, *pin 2* seems to go to the
middle connector of "RV2" pot. What do you make of this?
It should be pin 3 according to the schematic.

2) The jacks...
Looking at the schematic again, you have SK1 at the very left, then SK2
a little to the right/top of it. Then there's a jack-switch a little to
the
right/under SK2.
Is this part of SK1 or SK2?



>On some PCBs there is explicit usage of the supporters for ground
>connections so if you do not have the matching pots you are required
>to wire these by hand.

I sort of figured that out last time, the biggest problem knowing where
to connect the wires.


>This thing confused me quite alot also, but when I discovered this
>solution it all became clear to me. Let me tell you that it did cost
>me quite some time to do the wiring.

Me too, and damaged PCBs from all the soldering/desoldering.
This time I have fresh PCBs, and I'm going to figure out the
connections *before* soldering!


>Now, add this fact to the not allways so clear component placement
>indications in the article and you have a good basis for confusion.

Agreed. It really is a bit too messy.


>> And perhaps I should just get my equipment out of the closet and start
>> building those PCBs again... any other suggestions?
>
>Ugh. I hope we are only speaking about the input boards.

Yes, fortunately :-)
The other PCBs are OK.


>One confusing part is that some of the busses over the analysis board
>is cut and should only operate over some of the channels. It took some
>thinking to figure out how to do that. The trouble is, the info is
>there, but it is a bit too confusing.

Hmmm.. I can't remember having any problems with that board. I
completed it several years ago, but we can compare notes when I'm done
with the input amps and I'm ready to connect everything together,
hoping it won't blow up or anything!
I spent 2 years or so finding the PCB layout for the main board!!!!!

ETI didn't have it (you were supposed to order it from Powertran, who
designed the circuit and also sold the kit), but they had gone out of
business, and ETI couldn't help me.
Someone on the net (might even be on this list) directed me to the
German magazine "Elrad", and a few weeks later I finally got the PCB
layout!


>I for one has trouble with my input boards, I have somehow modified
>one to function but naturally failed to duplicate this mod over to the
>other board (they are equal designs!). Now, I can only blame myself
>for not spending the quality time when my head is fresh enought, so
>this is the main reason I haven't got my Vocoder operational!

But why not just look at the modified PCB and compare it with the other one?
I'm very interested in hearing about it.
So you actually have one fully working input amp board then?


>OK. I really should get it done and continue the trimming and setup job.

Same here. I've spent so much time and money on this project, and I'm
still not done. I miss having a vocoder, after having sold my Roland
VP-330, building this one to replace it.

>Oh, tip:
>If you dont have a transformer with a separare 7.5V coil, but rather a
>pair of 7.5/8 V tappers in the 15V coils
  [.......]

I modified it slightly to accomodate for a different transformer I
believe, but I can get back to this later. The input amps are enough
for the moment ;-)


>Another tip:
>I got myself one of those standard plugs with builtin line
>filter.

AC power plug you mean?

>Anyway, what I think that this design should benefit from where a more
>comprehensive description of how to build, how things functions, how
>to wire things up etc. Add to that a rework of the schematics and
>possibly also of the PCB and building it would go much smoother.
>It is not that the articles are bad, but they where forced to compress
>it into a small number of pages.

I think you're right.
I've put a lot of work into this project, and along the way, asking for
help I've instead received lots of requests about where to get the
diagrams, more information etc. as they want to build one themselves!
As far as I can remember, someone scanned the whole project and made it
available on the web. If I succeed in completing mine I want to add
information about these input amps and other things that can be
misunderstood easily.


Hallvard
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