SM204 transformer (V2545)

[Moby]

Hey guys! In the past few years a lot of people were asking me about possible SM204 (Siemens C12) microphone transformer. People were telling great stories about this microphone, especially for the vocals recording. One of the "mojo" inside is the rare V2545 transformer built by Rohde & Schwarz. It sounded interesting to "see" how it looks and possible to make it again as the aftermarket part. After 2 years of searching around for the "specimen" I finally received one dead, burned and ready for rewinding V2545. So, here we are. I want to share some photos with you and help the owner to put his great sounding microphone in the working condition.
Here is what I received. Unfortunately, in parts, so some more investigation about details is needed.

Bobbin is burned and torn apart.

Windings are burned and some ot the sections are literally welded together so it took me approx. 5 hours to unwind it.
Here it is... Unwinded, and so far I have 80% of the winding data . I still try to figure out the exact ratio but I will (hopefully) receive the measuring results from the working one unit. BTW, is someone here can make it it will be great to double check that important detail.

Stay tuned! Cheers

 

[Murdock]

Whoooop whooop, it started!!!
Is it "simple" secondary over primary? Or are there multiple layers of each?

Thanks so much for doing this!

 

[Moby]

Whoooop whooop, it started!!!
Is it "simple" secondary over primary? Or are there multiple layers of each?

Thanks so much for doing this!

It's not "super complicated" but there is a few suprises. LOL. Generally speaking, typical "German audio transformer" from 50's. Including the crazy M shaped laminations and again obsolette shape-material used. Insulation is of course, textile, and the pressboard hand made bobbin. Time consuming...

 

[geareyes]

Thank you ever so much for detailing your progress and sharing pics & details with the community.

Please do continue to keep us updated.This is very exciting!!

 

[McIrish]

Very cool. Looking forward to seeing how you put this back together again.

 

[Moby]

Very cool. Looking forward to seeing how you put this back together again.

I reconstructed and asembled much smaller and more challenging ones. T14/1, Bv.13...No worries. I still collect all the possible ratio variations. It looks like this one transformer had few versions. So far I discovered 12.3:1 an 14:3:1 versions. I still wait for someone who can measure one more V2545 to be sure about it.

 

[Moby]

Here are the hand made bobbins. Not looking bad

 

[cyrano]

How in heaven's name do you burn a mic transformer like that? Connected to mains voltage or so?

Anyhow, looks like you're doing a great job.

 

[Moby]

How in heaven's name do you burn a mic transformer like that? Connected to mains voltage or so?

Anyhow, looks like you're doing a great job.

LoL. That's why I talk about leaky capacitors. Primary winding literally melted. Also, tantalum failure does this kind of mess. Btw, microphone is not mine, I want to repair this rare transformer, help the owner to repair the SM204 properly and possibly, later, to make the aftermarket replacemeny part.

 

[volker]

How in heaven's name do you burn a mic transformer like that? Connected to mains voltage or so?

Anyhow, looks like you're doing a great job.

I once had a preamp cassette similar to those Telefunken modules for repair. Somebody did exactly that, put 230V on the output side of the transformer. That makes a couple kV on the primary side. The transformer looked pretty burned like in Moby's photos with wires baked together and breaking everywhere. Thanks to the dual bobbin design I was still able to dissect it and get somewhat accurate counts of the windings.

 

[Moby]

I once had a preamp cassette similar to those Telefunken modules for repair. Somebody did exactly that, put 230V on the output side of the transformer. That makes a couple kV on the primary side. The transformer looked pretty burned like in Moby's photos with wires baked together and breaking everywhere. Thanks to the dual bobbin design I was still able to dissect it and get somewhat accurate counts of the windings.

TBH I'm not sure what happened but even +120V DC can make the mess like this since the primary wire is relatively thin.

 

[cyrano]

Every now and then someone brings me stuff from recycling services. Some companies destroy all the gear they send to recycling by putting 230VAC on inputs or outputs. Most of it isn't worth repairing after that. Others just cut off the mains lead.

But sometimes gear survives that and only needs minor repairs.

I've never seen a burnt mic transformer though. Is this one from a mic with an internal tube?

 

[Dark1]

Hey guys! In the past few years a lot of people were asking me about possible SM204 (Siemens C12) microphone transformer. People were telling great stories about this microphone, especially for the vocals recording. One of the "mojo" inside is the rare V2545 transformer built by Rohde & Schwarz. It sounded interesting to "see" how it looks and possible to make it again as the aftermarket part. After 2 years of searching around for the "specimen" I finally received one dead, burned and ready for rewinding V2545. So, here we are. I want to share some photos with you and help the owner to put his great sounding microphone in the working condition.
Here is what I received. Unfortunately, in parts, so some more investigation about details is needed.

Ouch. I doubt the (leaking) output cap could have made such a catastrophic failure/mess? Anyway, Siemens SM204 was supplied via Siemens Austria (aka WSW, Wien); the transformer was probably built by Henry Radio, Wien, like the other large-size transformers in older C12 variations, and not R&S.

 

[Moby]

Ouch. I doubt the (leaking) output cap could have made such a catastrophic failure/mess? Anyway, Siemens SM204 was supplied via Siemens Austria (aka WSW, Wien); the transformer was probably built by Henry Radio, Wien, like the other large-size transformers in older C12 variations, and not R&S.

There was few more similar transformers used inside the SM204 microphone. Besides V2545 made by RS they used the Henry radio 2148 and also the 3044. Strange, they all look the same, or almost the same, so who knows?
Edit: I forget to answer about possible capacitor DC failure (leakage). Imagine that capacitor conduct for example 50V DC and bleed the curent through the 1000ohms .

 

[Dark1]

There was few more similar transformers used inside the SM204 microphone. Besides V2545 made by RS they used the Henry radio 2148 and also the 3044. Strange, they all look the same, or almost the same, so who knows?
Edit: I forget to answer about possible capacitor DC failure (leakage). Imagine that capacitor conduct for example 50V DC and bleed the curent through the 1000ohms .

I think the R&S-made transformer is a myth, they were all most likely made by Henry (form factor, lams etc. all look the same). R&S as a supplier makes no sense I think when they had Henry-made transformers even for prototype production (C2), pre-dating SM204/C23 and C12’s first run… Leakage: yes but I have never seen such high leakage in this type of cap, yet.

 

[Moby]

I think the R&S-made transformer is a myth, they were all most likely made by Henry (form factor, lams etc. all look the same). R&S as a supplier makes no sense I think when they had Henry-made transformers even for prototype production (C2), pre-dating SM204/C23 and C12’s first run… Leakage: yes but I have never seen such high leakage in this type of cap, yet.

Ok let it be Henry. Whatever it is it's time to repair it and possibly make the replacement of the same.
TBH I'm suprised you never seen (measured) high leakage capacitors. PIO and Tantalums are the champions. Some electrolytics also.
Any chance you have a specimen to measure the ratio? I would like to triple check my findings.

 

[Dark1]

Yes, seen leaky paper caps many times, sometimes enough to cause magnetization or move bias points enough to cause trouble, but never to cause such a catastrophe (yet!). (Tantalums are another matter, with so many problems of their own, but I would not mix that topic with C12 et al.) Sadly, I do not have an SM204, sorry (I really am! ).

 

[Tim Campbell]

 

[Moby]

I finally found the appropriate magnet wire to match the original DCR values and here is the re-winded V2545

 

[geareyes]

Amazing!

The owner of this SM204 must be thanking his lucky stars his paths crossed with yours.

Thank you for the continued pics & for your persistence in resurrecting these old gems.

I certainly hope this model of transformer will be added to your catalogue for purchase at some point in the future!