OPTRO 740 comp / EQ doc's / schemo's ? (WANTED)

[EvLoutonian]

Hi folks.

I have an OPTRO 740 "Audio Processor" here (compressor / EQ unit) which seems to work OK in terms of the nice 10-band inductor EQ section, but there is a compressor bypass switch added on the back (along with attack and release time controls), and I suspect there's an issue with the switch, as I can't get the compression action kicking off so far, at all..

Hoping somebody out there might have some documentation for this unit, or even just a schematic, or anything really!

Cheers!

It looks like this:

front panel:
https://scontent.xx.fbcdn.net/hphotos-f ... 5073_o.jpg

inside:

[EvLoutonian]

Back panel mods:

[mylesgm]

well it looks cool.

[rick]

We had two at fedtival when i was a kid
They were as much a mystery to me then as they are now .

[EvLoutonian]

We had two at festival when i was a kid.
They were as much a mystery to me then as they are now.

Cheers, Rick.

Do you mean how these units behave in terms of compression?
Or getting it to work at all?

.. or what half of that stuff does inside there? ..
(It's certainly pretty chock full of do-dad's!)

[The Mill]

The person who will likely know more than most about this is Melbourne engineer Simon Grounds... don't think he's on the forum but he's a wealth of knowledge about anything to do with Optro.

He's in Melbourne, and if necessary I can give you his email address, but not here... alternatively he might be in the white pages.

[EvLoutonian]

.. if necessary I can give you his email address... alternatively he might be in the white pages.

Thanks Andy! Sounds great. My email address is evanlorden@gmail.com

[The Tasmanian]

Those compressors are most probably Gilbert Multiplier design - a complex compression design that there are no schematics for.
I had 10 of them (from the original channel 9 console), and no tech in the country could work out how they operate, nor find the original designs.

Paul Maybury here on TRock has them now, and has been working with the original tech that co-designed them with Graham Thirkell.
I had this original tech trying to reverse engineer them for over 4 years. Schematics do not exist.
I gave up - if the co-designer could not work them out then it was a lost cause.
Paul has taken over the cause, and is still working with the only person alive that knows the design basics.

Simon Grounds does not have the schematics (but does have a killer Optro console, and info on the mic pre's etc)

Paul Maybury is your only hope here - drop him a line and see where he is at with reverse engineering these compressors.

Best of luck!

[EvLoutonian]

Paul Maybury is your only hope here ..
Best of luck!

Wow! The thlot plickens!

Stating to sound like a scene from Star Wars .. I like it!
You know you're in the right place when that happens.. hehe

I'll get in touch with Paul and see what he reckons..
It sounds like if my compressor is actually not working I might be in trouble.. If it's something simple I might be alright, but otherwise I might just have to look upon it as a nice 10-band inductor EQ unit only.

I did find this brief description in Rob's "On The Bench" (AT66) archives:

WHAT’S EATING THE GILBERT MULTIPLIER?
The Gilbert multiplier is an array of transistors
usually packaged into an IC that is mostly used in
radio and analogue computing. However, a couple of
manufacturers cottoned on to its potential in audio
compressors. Essentially the device mathematically
multiplies two voltages, and if one of those
voltages is an audio signal and the other a control
voltage then a controllable gain device is created.
While similar to VCAs they require a different
implementation. Audix and EMT used multiplier
ICs to create compressors that, while fairly obscure,
are probably quite under-rated."

Thanks for the info Chris the Tasmanian!

[Chris H]

I have one the same (on the outside) and its an opto compressor limiter. That front pic you posted is my comp. I think yours is slightly different. It would be interesting to see in detail.


and an inside pic, and it does look different from yours....?:



....and the opto cell on mine:

[The Tasmanian]

That is positive news Chris. (long time member of the Optro fan club!)
Hopefully its opto design!

Also - I have a pair of ex ABC 10 band graphics only models here that are in my upcoming list to sell.
The low end bands on these inductor EQ's I really like.
PM me if anyone is interested before listing.

[Paul Maybury]

Hello Optro fans.
I don't know anything about the comp/eq's listed here, but as Chris H has shown, they appear to be an opto cell compressor. Which is good news, I would think as they shouldn't be too complicated.
I have a pair of the 203 eq's that appear to be just the eq section of the comp/eq units. They sound great. They also have the amps potted in green and yellow epoxy, as do Even's.
Chris T, I have not gotten anywhere with the 7500 channels and comps.
As my list of things to do gets longer, they stay at the bottom of the list. I don't have much hope of ever getting the comps working, but one day I will definitely get some of the channels strips going. Someday.
Cheers, Paul

[EvLoutonian]

I have one the same (on the outside) and its an opto compressor limiter. That front pic you posted is my comp. I think yours is slightly different. It would be interesting to see in detail.

G'day Chris H.

Good to see the inside pic's of your's.
It really is quite different to mine inside, by the looks of things!
Mine seems to be chock full of all sorts, like a mixed bag of lollies..
Also, more particularly, the EQ slider switches are very different on mine, for example.. just upon initial impressions.

Does your unit have the INDUCTOR EQ assembly mounted on top (to the inside of the lid, or elsewhere)?

How interesting, that we both have the "740" model, but inside they are so different!
Makes me wonder if the basic circuit s the same, or not.. I'll get some more detailed pic's of mine up shortly..

I'll also see if I can get Mat Robins to post some pic's of his unit, and see if we have three different beasts, or perhaps one or the other of ours is an oddball? The cloudy mystery of it all! hehe

[The Tasmanian]

Hey Paul - I still need to pull those edge connectors off the old Optro console for you....
its also on my list that's a moving goal posts story..

[The Tasmanian]

Evan, from what I know of the Optro history, a lot of the equipment was made on a job by job basis for TV stations mainly, and studios.
So designs changed all the time, as each job was determined what was needed.
The old days where a lot of gear was not manufactured in repeat format, the parts would have been made in batches, but the designs were fluid.
Even the 7500 consoles were slightly different designs to each other.

[The Tasmanian]

On further looks at your pic's - they do look very similar to the Gilbert Multiplier boards that Paul now has.

[Chris H]

I have one the same (on the outside) and its an opto compressor limiter. That front pic you posted is my comp. I think yours is slightly different. It would be interesting to see in detail.

G'day Chris H.

Good to see the inside pic's of your's.
It really is quite different to mine inside, by the looks of things!
Mine seems to be chock full of all sorts, like a mixed bag of lollies..
Also, more particularly, the EQ slider switches are very different on mine, for example.. just upon initial impressions.

Does your unit have the INDUCTOR EQ assembly mounted on top (to the inside of the lid, or elsewhere)?

How interesting, that we both have the "740" model, but inside they are so different!
Makes me wonder if the basic circuit s the same, or not.. I'll get some more detailed pic's of mine up shortly..

I'll also see if I can get Mat Robins to post some pic's of his unit, and see if we have three different beasts, or perhaps one or the other of ours is an oddball? The cloudy mystery of it all! hehe

I will take pics showing more detail tonight and post them for you. I would say they are essentially the same but with changes around sections as components or slightly different ways of achieving the same outcome became available, eg different sliders for the eq section.

[The Tasmanian]

Evan, if you can get a pic from on top we may be able to determine more.
Those yellow boards look very much like the Gilbert Multiplier design. (exactly the same colour too)
Maybe Paul could check his boards against a better photo supplied?

[EvLoutonian]

Interesting!

Here are some more detailed pic's of my unit:

IMG_8588 Ev Optro.JPG

IMG_8590 Ev Optro cell.JPG

IMG_8592 Ev Optro.JPG

IMG_8594 Ev Optro.JPG

IMG_8598 Ev Optro.JPG

[EvLoutonian]

Couple more showing additional details of where some wires have been connected in from the back panel where the addition of ATTACK + RELEASE control pot's & BYPASS switch were added to back panel.

IMG_8602 Ev Optro.JPG

IMG_8604 Ev Optro.JPG

Whether these mods are the thing currently causing the compressor not to work, or if it's something else, i don't know..

[The Tasmanian]

Well that card in the first pic looks to me like the Gilbert Multiplier card (from memory)
Maybe Paul can look at his and see - or even take a pic??

[rick]

Ours sat unused in a back room
i plugged them in hopefully a few times
We had another couple of types in our dubbing room
That were not used on anything that was released in the ten years i was there

Optro was a swearword amongst the techs that
Bought and tested the stuff new .

I love how you guys are still digging away at getting them going
I think optro gear falls into the category of
"really clever people using Really clever ideas to make really clever stuff that didnt work anywhere
Near as well as the dumb stuff that worked then and still works today "

Ill just go put my flack jacket on and press post !

[Paul Maybury]

Photo0511.jpg

[Paul Maybury]

Photo0512.jpg

[Paul Maybury]

Photo0509.jpg

[Paul Maybury]

Sorry the photo's are a bit poopie, I only have my laughable and outdated Nokia phone with me today.

[Chris H]

Wouldn't mind betting Gerard McGough did those mods to your comp, and that 2nd pic sure looks like the opto cell.

As for Rick's opinion of Optro.....well it's his forum.......... i suppose he can get away with the odd glib comment

some optroporn? ( there's a new word for the Maquarie Dictionary )
My 2 comps work fine as does my 6 preamps with limiters and inductor eq's
....the opto cell is the long cardboard tube.



and the sidecar of 7500's is working ok too....

[Paul Maybury]

I love my Optro gear, but I'd love it a lot more if there were some goddam schematics....

[The Tasmanian]

Rick - you were in a sheltered workshop at Festival! - vintage neves /pultec's etc, which were definitely made better, and had the backing of a huge enterprise behind them - subsidized by the BBC / UK taxpayer (and Aust!)
Our market was very small, hence the small run of Optro consoles, tape machines etc.

The rest of the plebs (ABC-Channel9) had to generally make do with Aust made stuff - mainly because of the taxes incurred on overseas imported gear were huge (200% on some audio stuff).
Plus they had many studios to fit out with gear.

The Optro's do sound killer.

We should do a run of Optro T-Shirts and send you one!
# probably give those old tech's heart attacks if they caught you wearing one!

[Chris H]

I love my Optro gear, but I'd love it a lot more if there were some goddam schematics....

The story is they all went into the dumpster. Schematics and everything from his Clayton workshop.
A sad story really. Graham was a brilliant man whose business ventures did not bring him the economic security he deserved. With the recording equipment, accoustic design, micro processors and Sontron and Editron editing and syncro systems he covered a a lot of ground as an inovator.

Worth a read:

http://www.aesmelbourne.org.au/graham-thirkell/
click the links at the bottom for some great optro pics and info

http://www.retrotechnology.com/herbs_st ... itron.html

Press release for the comp:

ATN-Optro740-processor-3-sml.jpg

also I cant help thinking this comp, what with the EQ faders and the spectral comp section was the inspiration for the Avalon 747