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STC 4033 & STC 4021 microphones

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STC 4033 & STC 4021 microphones

Postby Matt H » Wed Apr 28, 2010 6:19 pm

Greetings Everyone.
First time posting here.

Does anybody have knowledge that they may wish to impart of how to open up the bodies either of these mics?
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Re: STC 4033 & STC 4021 microphones

Postby Matt H » Thu Apr 29, 2010 2:51 am

4033 is sorted. I unpicked the medium glossy black paint dot (or plastic coat) which conceals the slotted screw head located at the centre on the top of the grille head. This countersunk 1+1/2 inch screw connects the grill to the internal assembly. On other examples of this mic - the silver STC sticker would be found above this screwhead location.

On with the 4021 (Western Electric 630 owners welcome to chime-in). The ball shaped body seems to be a casting of two parts indicated by a dividing seam around the middle. Do I crack the paint seal? - will this twist apart into 2 halves?
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Re: STC 4033 & STC 4021 microphones

Postby Ted Kendall » Thu Apr 29, 2010 11:07 am

Why? I think Coles can still fix these...
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Re: STC 4033 & STC 4021 microphones

Postby Matt H » Thu Apr 29, 2010 5:54 pm

I'd have thought that too Ted .....but sadly to no avail. I've called Coles a couple times in recent years & found them to be less than willing to do with anything other than is manufactured in their current catalogue. This is a pity because they do have the tooling, materials & expertise to at least offer a re-ribbon service for the 4033 - but I have been refused "point-blank" :frown: Coles stand firmly detached from both these models since they never made them under their badge since the early 1970's takeover. I wish COlin & LES would have a change of heart....& I wish there was a loan stock of 4040's placed with their distributors in London for pending customers to try.
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Re: STC 4033 & STC 4021 microphones

Postby Ted Kendall » Thu Apr 29, 2010 11:47 pm

That's a shame - what is the thing not doing?
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Re: STC 4033 & STC 4021 microphones

Postby Matt H » Fri Apr 30, 2010 12:57 am

Sure thing - I've a pair of 4033's with very slack ribbons - speech has that hollow effect of someone talking with a plastic bucket over their head, so Lucasfilms may find them useful for ADR work. The ribbons used in this model differ those fitted in the 4038, in that ....there are around only half a dozen corrugations at the top & bottom of the strip (as can be seen in Beyer & old Altec models). 4038's aluminium strips are corrugated the entire length. Taking up the excess slack can restore the loss in output & improve frequency problems. I've had to do this to some of my other antique ribbons & they can return to acceptable working state. Undoubtedly, freshly corrugated strips would do a better job.

The poorly gloss black 4021 can't reproduce much below 4 or 5 Khz & seems to have a general output drop compared to the other (newer looking - texture finish) I have. There are a couple of good mic technicians that I know of that have resurrected AKG d19s, d20, d25 etc where membrane suspensions have decayed & the debris has lodged itself within the piston or hindered the diaphragm from free movement. I'm wondering to myself whether STC made versions through the course of time with higher output impedances?
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Re: STC 4033 & STC 4021 microphones

Postby Ted Kendall » Fri Apr 30, 2010 7:16 am

Are you sure the 4021 hasn't gone one-legged (open circuit hot or cold wire) - the low output and lack of bass make me wonder...
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Re: STC 4033 & STC 4021 microphones

Postby Hugh Robjohns » Fri Apr 30, 2010 11:19 am

I'd get in touch with AEA in America or Terry Finn of Affinity Audio in the UK (their agent) for advice.

I know they are authorised Coles repairers and have a huge knowledge of repairing ribbons generally. They should be able to tell you how to get in, and what is required to fix the problem.

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Re: STC 4033 & STC 4021 microphones

Postby Matt H » Fri Apr 30, 2010 11:50 am

That's something that will need to be checked Ted.

Last night I delivered the piece to a veteren microphone repair technician. In all his years, he has never had to open up a 4021.

Now I've just referenced my out of print Iliffe Microphones handbook. There's a cross sectional drawing. Also the Stan Coutant microphone website has a drawing too. The text doesn't state how that the upper & lower chambers are brought together but I think a rubber gasket & strong adhesive are involved. It maybe necessary to drop the screws that hold connector assembly within the underside recess.
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Re: STC 4033 & STC 4021 microphones

Postby Matt H » Fri Apr 30, 2010 12:00 pm

Thanks Hugh. You just reminded me that I have to get in touch with Terry anyway ..... also I have to order some RCA mounting assembly parts from AEA. Hope you're keeping well since Maynooth.
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Re: STC 4033 & STC 4021 microphones

Postby ROLO46 » Sun May 02, 2010 5:29 pm

I recently did open heart on my earliest 4033
Sounded very resonant and low level
Took the top wind shield off and the inner , revealead a mesh gag pressing on the ribbon,extracted the mesh but nicked the ribbon !!
Carefully prized the ribbon (corrugated at either end) back between the pole pieces (tricky) and domed the mesh and put it back betwwen the poles.
It worked....
However lowish output
Cleaned (inhibisol) the WE jack plug and socket, this was filthy, OP now normal
Cleaned all my other WE plugs and sockets
Try this on your 4021,they are very relible mics and sound great, but those connectors are trouble ,first balanced plug?
Try Saturn Sound if still in trouble.
I know BBC SM's used to replace STC ribbons with ciggie silver paper
How did they solder it???
The ribbon must be exactly in the poles,getting crenulations right and legth v tricky.
But worth it.
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Re: STC 4033 & STC 4021 microphones

Postby ROLO46 » Mon May 03, 2010 6:25 pm

Just looked again at my 4033a.
The foil is clamped under sealed screws.
I mailed Bob Crowley of Roswellite fame (Crowley and Tripp Ribbon mikes,now owned by Shure)
He says Roswellite unsuitable for STC which use 600nm foil which is v thin.
Fag silver paper too thick, though Im sure they used Players no 6 in Beeb folklore.
Corrugations dificult ,needs to go through a mesher
Roddy Bell of this parish is a ribbon devotee,perhaps he can help.
Looked at my 4021 assembly,it must be glued together
I also have cardioids,hypercardioids and the great stick mic, 4037.
All working perfectly with light airy tops.
Lovely mics. :angel:
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Re: STC 4033 & STC 4021 microphones

Postby Ted Kendall » Tue May 04, 2010 6:06 pm

I think the confluence of fags and ribbon mics in the Beeb was more to do with putting the packet over the back of the mic with a rubber band to put a presence peak in...
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Re: STC 4033 & STC 4021 microphones

Postby Matt H » Wed May 05, 2010 8:30 pm

Nice to hear from you Ted & Roger.

I just heard back from my engineer about my 4021. His comments are:

"Beautifully made with a very delicate diaphragm of extremely thin aluminium. There was a dent in the middle of it and a lot of magnetic dust preventing it moving. There is also a little corrosion resulting in tiny holes in the diaphragm. It now works reasonably well but is 7 dB's quieter than a 'perfect' 4021."

Well, the disintegration didn't come as a great surprise to me since I was informed of the whereabouts of it's previous working locations (you've probably guessed that this was an ebay item)

I asked him about impedance.....& the response is:
"There are no transformers used on the 4021's. The coil impedance is 20 ohms."

Regarding my 4033, I've taken up most of the slack without applying tension that would cause any stretching or further flattening to the corrugations. There was a lot of excess slack - it had a visibly large undulation. I'm handling the ribbon as delicately as "Spiders silk". Still it needs a tweak. I'll try some of that inhibsol - but I've rid of some of the carbon on the pins already swabbing some of that well known household ammonia cream with an isopropyl chaser to get rid of the dry residue. The performance is 90% close to that of my best kept 4033. The WE locking connector could well be one of the earliest designs - mics before that era seemed to have binding posts.

According to Ashley's website he seems to be re-organising things at the moment. Would you think Roddy Bell be ok to PM?

You'd know better than myself but I had read about the ciggie paper trickshot ...& the Swan Vesta's elasticated to the back of the 4038 in a mid 1980's edition of Studio Sound. Same article had a photo of "the mesher". I saw an elaborate looking mini mangler machined in solid brass for sale on internet for around $1500 last year.
On a Chinese ribbon mic I had that self combusted after a it's second year - I tried various wrapper foils and it didn't work for me - everything sounded under-water. Too much material thickness. Maybe it needed the matchbox to boost the midrange resonance as Ted mentioned. Perhaps the foil paper substitute was the only thing that could be done when stranded in the field. (I thought they were "Condor" men in those days). My interpretation of the matchbox concept was to psudo-create a mono-pole pattern situation (i.e instant cardioid).....maybe they loosened off a cravat or handkerchief for wrapping the box with to dampen the reflections.
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Re: STC 4033 & STC 4021 microphones

Postby Pete Thomas Music » Wed Jan 27, 2021 12:12 pm

Just waking this thread up. I have a 4021 that needs repair (seems to be dead). Does anyone know somebody in the UK who may be able to help?

(I'm a bit foxed trying to open it up. I found the screw lurking under the felt at the bottom of the socket, but the two haves still seem to be firmly stuck. Needs an expert!
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