Vintage Midas Recording Console - Recapping

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JingleDjango

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Joined
May 19, 2015
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150
Location
Toronto, Canada
Hey, friends. I've been asked to look into recapping the channel strips for this 1970s Midas 24ch studio recording console. We have a bit of an oddity here, as I'm sure many of you are familiar with Midas exclusively as a manufacturer of live mixing consoles. Supposedly a total of 7 of these were produced, this being serial #001. It's quite a handsome desk, but could use a little touching up. I'm checking here on the off chance that one of you might have some experience with a similar console.
I don't have experience with this kind of project or with electronics of this vintage. At this point I'm doing research and moving slowly to avoid rushing into mistakes.

I thought the community here would be interested in taking a peek at this rare beauty and if I'm lucky I might get a little guidance. I have some schematics on paper which I'll scan and upload when I get the chance.
 

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The channels are modular. They come out and slot back in very easily. Gold contacts throughout.
 

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Here is a channel removed from the desk. Everything is hand-wired, which I guess would be the norm back then but I'm not really sure myself.
-We see several TL071cp op-amps, all socketed.
-Each input channel is balanced with Sowter 3360 transformers.
-Axial electrolytics are Phillips brand.
 

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Here is a closeup of one of the boards.
I'm not familiar with these boxy, metallic components. An obsolete capacitor package? Hopefully someone can help me identify these. Anyone?
 

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JingleDjango said:
Here is a closeup of one of the boards.
I'm not familiar with these boxy, metallic components. An obsolete capacitor package? Hopefully someone can help me identify these. Anyone?
"these boxy, metallic components" are Siemens MKH capacitors.  Beside the Philips "tropical fish" capacitors they were very often used in this era because they were cheap - much cheaper than Wima's.

Probably you won´t believe it, but this junk is still available:
http://www.reichelt.de/Siemens-Capacitors-MKH-MKT-/2/index.html?ACTION=2&GROUPID=3156&SHOW=1&START=0&OFFSET=16&SID=14UkvRM38AAAIAAFBhecQ415ffe721c76d708877927a57197cd4a&LANGUAGE=EN

https://www.banzaimusic.com/Siemens-MKH/
 
Note that they, the boxes, are good junk, and do not need to be replaced like the electros do. A waste of time to do so unless it is one of the less than .5% that fail. I have fixed some with a zip tie around in a pinch.
The electros are also in production by BC Vishay,  albeit smaller and monolithic for smaller values.
 
Strange coincidence, but I had one of those Siemens box caps fail in an Aengus graphic EQ I was working on today.  The 500Hz band was not working until I wiggled the cap and it made contact.  Turns out one leg of the cap had broken loose.  I pulled the cap and I was actually able to re-solder the leg.  I tested it on my Sencore cap tester and it was still good.  Re-soldered the cap to the PCB and all was right in the world! 
 
I'll definitely leave those alone then, unless there might be a benefit to replacing there are WIMAs of like values that would match their footprint? I guess if it ain't broke. . . I was hoping to replace the electrolytics with WIMAs or some like-value axial film caps but I need to do more research on what is viable. Someone suggested I could replace all of the electrolytics with film in the audio path, but for DC filtration caps I should replace with modern electrolytics. I'm sure this topic has been covered in the forums before so I'll have to go digging.
 
There is a fine line between "improving things" and losing that "Midas sound".  Put BB's in place of the TLO's and you might not like what you hear.  Put parallel poly caps to the electro's and it's possible to lose the magic.  Do one "improvement" at a time and let everyone involved decide if it is indeed an improvement.  Diminishing marginal returns apply here.

Better to recap the electro's, clean and condition all mechanical connections,  make sure that all switches/pots are solidly working, and clean the knobs/facia.  That goes for the power supply as well.  Then get out of the shop and make music.
Mike
 
love Midas desks though not familiar with this one, if u guys have any spare channels available let me know I might be interested...

good luck and yeah those caps mentioned earlier are pretty decent, even seen in some Neve gear.
 
Siemens caps are ok, i would leave them.
Change electrolytics for axial Philips/BC and since you have opamps in sockets, you can always try one channel with BB and compare to TLXXX
 
BB stands for...? 

I appreciate all of the advice. I'll post updates when I start to move but I'm in no hurry. No sense rushing in before I have a better grasp of what I'm doing.
 
If you didn't recognise the 'boxy' components, or 'BB' then it sounds like you're on the beginning of a fun journey.

Heed sodderboy's wise words about balancing effort against return and lost magic, but should you decide you prefer modern opamps to 1970s ones, here are two notable points from the plethora of advice around for op-amp swappers:

1. quiescent current draw
It's easy to substitute an OPA134 for a TL071, but it draws twice as much current when idling - get the datasheets and look for "Quiescent Current" or "Supply Current", which is usually quoted as max/min/typical as no two chips are the same.  TL071 is 1.4mA - 2.5mA, OPA134 is 4mA - 5mA, the latter being fairly typical for "high-end" audio opamps.

Now calculate worst-case additional load (chips per channel x channel count) and refer to PSU specs (if you have them).  You could easily be eating way too far into your PSU's capacity without realising it until too late (smoke/fire/shutdown).

Two solutions: chunkier PSU, or low-current opamps.  Circuit traces and bus wiring might not be up to carrying additional current safely: caution advised.  The only JFET-input (like TL07x) opamps with v similar current draw and better sonics I found are TI/BB OPA164x (1.8mA - 2.3mA) - but d'oh! they're only available in SOIC so add $2 per chip (for Brown Dog SOIC/DIP adapter), and extra time for soldering the bu%#ers on...

Even though they don't - IMO - "sound good", TL07x are *still* ubiquitous in audio gear because they're stable, measure OK, are cheap as chips :)  and draw very little current.  This means cheaper power supplies, smaller heatsinks, etc.

Replace with OPA1641 and adapter and you're at ~$4.50 per upgraded chip.  For comparison, a TL071 is currently around $0.20 in quantity.

2. oscillation
Replace a TL071 with an AD797 (extreme example, bad idea) and it will sing it's heart out at some MHz.  You *must* have a scope (faster the better), and 1:10 probe to check whether your opamp replacements haven't just created an expensive high-frequency oscillator.  It's like putting a Formula One engine in a Fiat 500: the connections might be similar, but the result could be uncontrollable.

Worse, some part of the circuit - and Aux output, say - might be marginally stable, and only oscillate when something's plugged in, or the opposite - test under as many conditions as you can.

Be organised - make an inventory of all the parts (value and dimensions), cost the various options and test them objectively and subjectively.  Fix a budget, and bear in mind you may need additional tools and materials - scope, magnifier, vacuum desoldering pump highly recommended (you'll have hundreds of joints to desolder and clean and a thumb pump won't do), hot air station if SOIC, wipes, DeOxit...  Create and print a work sheet/tick list for every channel so you don't lose track, and be in no hurry...

Reworking my desk has been far more time consuming than I anticipated - especially the planning stage.  But it's been and educational, enlightening, frustrating, rewarding too.  PSUs recapped, cleaned up, and trimmed, Master and Group modules fully complete, 6 of 32 channels done, and I hope to complete by Christmas.

Go for it!
auto

PS thank your luck your ICs have sockets - mine didn't!
 
Just for the records: don´t fiddle too much with those blue rubber tubes. If you pull them off the wire you will surely break the wire. I´m through this. Midas Pro04 here....

Btw: our member Balijon is en expert with vintage Midas desks. He owns a boatload of them. Drop him a mail if you have specific questions.
 
Hello. I am thinking of buying a similar 1977/78 Midas PR studio/recording “in-line” console, serial nr. 853. It uses an external 30 ch. LED meter bridge instead of internal VU meters. The desk has 20 x PR 03B2 mic/line inputs with 3-band hybrid IC/inductor EQs. And 8 x PR 05C mic/line inputs were added in 1982, when the studio expanded from 16 to 24 tracks. The 24 direct out/tape monitor modules in the bridge are labeled PR30A, and so far I cannot get/find any schematics on these. Does anyone know anything about these custom Midas PR studio consoles, apart from being augmented for recording studio use? How are these in terms of mic pre-amp and EQ quality/usefulness? Thanks & Best Regards from Anders
 

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analogguru said:
"these boxy, metallic components" are Siemens MKH capacitors.  ...
Probably you won´t believe it, but this junk is still available:
http://www.reichelt.de/Siemens-Capacitors-MKH-MKT-/2/index.html?ACTION=2&GROUPID=3156&SHOW=1&START=0&OFFSET=16&SID=14UkvRM38AAAIAAFBhecQ415ffe721c76d708877927a57197cd4a&LANGUAGE=EN

https://www.banzaimusic.com/Siemens-MKH/

Just curious: why do you consider them junk?

Michael
 

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