ETI/Powertran/Elrad vocoder

[Magnus Danielson]

From: Hallvard Tangeraas <hall at oslo.online.no>
Subject: Re: ETI/Powertran/Elrad vocoder
Date: Sun, 11 Jun 2000 14:55:38 +0200

> At 21:26 +0100  10-06-2000, Magnus Danielson wrote:
> 
> 
> >From: Hallvard Tangeraas <hall at oslo.online.no>
> 
> >> Anyone built the Powertran/ETI/Elrad vocoder?
>   [...]
> >> Anyway, I'm in the process of building one of these, but could need
> >> some pointers with certain details I'm confused about, so I'd like to
> >> get in touch with someone who's successfully built one.
> >
> >OK, now that you found your way over here... what's your trouble?
> >I am sure that me, Anders and Juergen should be able to solve it...
> 
> Thanks. Nice to be back (after spending a few months trying to figure
> out how to get on the list again, since the old list is apparently
> still working, but isn't in use, which makes it all more confusing!).
> 
> The vocoder....
> It's been one of those "eternal" projects which I'm finally getting back to.
> It's nice that it's built in "modules" as I can check one and one part
> before I fry everything. Actually, all the parts are completed, but I
> never got the input amplifiers to work.
> 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.

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.

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.

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

> Since I soldered/desoldered test wires and the pots, never getting it
> to work, the PCBs have become pretty ugly and damaged, so I've made new
> PCBs and I'm ready to start again. This time I don't want to make any
> mistakes.
> 
> How should we do this?
> I don't have a scanner, but a friend of mine does, and I could scan the
> circuit diagram/PCB layout for those input amps, and make them
> available on a web page of mine, then we can take it from there.
> 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.

I really think that some boards would benefit from a rework on the
layout. However, I am impressed how much and complex functionallity is
living on this single side design without too much jumpers all over
the place.

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.

> If any of you remember any specific problems/confusion regarding those
> input amps, I'd be interested in hearing about them, but it might be
> easier to follow circuit diagrams, then compare with a completed,
> working vocoder.

Oh. The input boards is especially confusing. While it seems simple
enougth the connection points for the jacks are not very clearly
pointed out etc. Anyone building this should really spend some quality
time figuring out exactly how it shall be done.

Also, read the curcuit diagram very carefull when relating to the
breaking plugs, since unless you wire that up correctly, you have
screwed!

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!

Lazy, lazy me! <Standing in the corner of shame>

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

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, like I have, then leave D7
and D8 out or else you will have some hot transformer and a pair of
hot diodes. It was a bit confusing and a bit scary at first, but as I
was contemplating over it I saw the light and just pulling them up
solved the problem. I also had one diode reversed, but that got solved
in the debugging process.

Another tip:
I got myself one of those standard plugs with builtin line
filter. This filter protects in both directions and helps keeping
linebound signals from comming into the powerlines. I wired in the
power switch after it and then after that comes the fuse and
transformer primary side. Now, in retrospect I think I would have put
the fuse before the powerswitch, so if I happent to short anything at
the switch the local fuse would run rather than the 10 A fuse in the
cellar. Thus, that position should buy me a little more protection.

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.

Cheers,
Magnus