Vintage Shure Unidyne B Model 515SA.

[Melek]

Hello everyone. So, the wires came off inside my Unidyne B mic, I'd soldered them back on again today, hopefully correctly. The green wire to the terminal with the green paint by it, and the yellow wire on the opposite terminal. I then tried the mic into my Alesis iO 26 interface, and.... maybe I hadn't soldered the wires back on well enough, still, the mic did work, but even with a loud sound, the VU meter only showed up to one bar... -60db. I don't remember if I've used this mic with my interface before the wires came off, but surely this is not right to be at such a low input? I know the preamps aren't very powerful in the iO 26, but the signal should not be this low, right?
I have to re-solder the wires soon, they have come loose, because my soldering is not that good, and the wires are quite thin and not very strong.
Anyway, hopefully someone can let me know about this microphone's performance in modern interfaces in general and whether it's worth even bothering to use it.
I know I've used it before to record, but that may've been with my old interface, a Lexicon Alpha, which probably had better preamps.
Thanks in advance.

[Hugh Robjohns]

Sounds like you either have very poor contacts in the newly soldered joints, one or both of the wires is broklen somewhere else, or you've soldered one or both wires to the wrong places!

Wires don't just fall off inside a microphone -- they have been damaged by someone doing something they shouldn't have been doing... and there could be more damage than what was immediately visible.

hugh

[Tomás Mulcahy]

Try bypassing the transformer to test that it is not faulty (it will also verify that the diaphragm is working). The trafo is the only part of a Shure dynamic that I have ever seen fail. And that was a Unidyne that had been left in a damp leaky shed for years.

[Melek]

Hi again. After all, it just needed a soldering. Still, the output is either a bit low, which I think is just its age, old tech, or it's just the preamps in my audio interface, they aren't the best. Thanks anyway.

jM

[Tim Gillett]

It pays to check out the mic type.

The 515SA is a high impedance model designed with old valve preamps in mind. It generally will sound weak and muffled in a pre designed for low Z mics as modern ones usually are.

You would have been better off with the 515SB which was low Z.

If it's within your abilities it may be possible to bypass the step up transformer altogether and wire the XLR pins 2 and 3 directly to the mic capsule coil outputs. That assumes there is a transformer inside the body. The output impedance may now be a bit too low but probably a better result than with the high impedance output.

Looking in more detail at the Shure website details, the 515SB appears to be the 515SA minus the transformer. So removing the transformer should change it to a 150ohm rated mic, which should be a big improvement.

Tim

[Tomás Mulcahy]

I mistakenly assumed that my 515SB had a transformer. Thanks for the info! I never thought to check Shure's datasheets.

[royleith]

It pays to check out the mic type.

The 515SA is a high impedance model designed with old valve preamps in mind. It generally will sound weak and muffled in a pre designed for low Z mics as modern ones usually are.

You would have been better off with the 515SB which was low Z.

If it's within your abilities it may be possible to bypass the step up transformer altogether and wire the XLR pins 2 and 3 directly to the mic capsule coil outputs. That assumes there is a transformer inside the body. The output impedance may now be a bit too low but probably a better result than with the high impedance output.

Looking in more detail at the Shure website details, the 515SB appears to be the 515SA minus the transformer. So removing the transformer should change it to a 150ohm rated mic, which should be a big improvement.

Tim

I have just completed the repair of a Shure Unidyne B 517SA and it proved to be a disconnection of the transformer windings of both the green and the black wires. The captive-cable 515SA version seems otherwise internally identical to the XLR version. The capsule has a resistance of about 17Ω which requires a 1:9 ratio transformer to get it up to the nominal 150Ω of the low impedance 'B' version.

I have seen videos of 'improved' SM58s with the transformer removed. I wish I had known if they were Hi Z or Low Z. I could have converted my Unidynes to type 'B'!

In the process of testing, I discovered than my XLR to TR 1/4" jack leads are incorrectly wired. If you find that the tip is permanently shorted to the sleeve no matter what the position of the 'On-Off' switch is, or T to S permanently open-circuited, check the XLR end of the cable.

[royleith]

The capsule has a resistance of about 17Ω which requires a 1:9 ratio transformer to get it up to the nominal 150Ω of the low impedance 'B' version.

As you were. The impedance changes by the square of the transformer ratio, so make that 1:3.