Shure Omnidyne Microphone Hum while touching

[Studiogearlover]

hello guys...I would like to borrow your knowledge on this. I have a pair nice Shure 578S Omnidyne dynamic microphone. When I plug this into my preamp, hums like crazy when its on and while I touch the metal body. I have this user manual and wiring drawings attached, it is wired for balanced output according to the schematic. But if I compare to my new SM57, it is somehow different, and the SM57 does not hum at all.

the 578s pin 1 is not connected to the microphone body if I see this correctly. is this the reason for the huge hum? Do I need to solder the case ground to pin 1? This is used with a vintage tube preamplifier with floating ground ( 2 pin AC cord). Am I same?

Many thanks for your advise!

 

[Walrus]

It's been over 30 years since I last repaired one, so I can't remember exactly how the 578s was wired internally, but the case of the mic should
be connected to pin1 of the xlr. The best place to do this would be by joining pin 1 to the shield connection on the tab of the xlr insert in the mic.
If the pre-amp is wired so that it's pin 1 is connected to chassis that should do the same thing, but as you have hum, it would seem there is a missing
connection from the mic case to pin 1 of the pre-amp.

 

[Studiogearlover]

It's been over 30 years since I last repaired one, so I can't remember exactly how the 578s was wired internally, but the case of the mic should
be connected to pin1 of the xlr. The best place to do this would be by joining pin 1 to the shield connection on the tab of the xlr insert in the mic.
If the pre-amp is wired so that it's pin 1 is connected to chassis that should do the same thing, but as you have hum, it would seem there is a missing
connection from the mic case to pin 1 of the pre-amp.

Thanks so much Kevin. Will do connect the pin1 to the chassis and hopefully this will sort this out. Many thanks for your suggestion!

 

[ccaudle]

The schematic shows pin 1 connected to case ground, there is a line from pin 1 to the cartridge ground, using the same symbol as case ground at the connector shell terminal. Check with an ohmmeter to be sure, but if your current wiring matches the schematic it is probably OK.

The schematic shows the output cable wired as unbalanced, do you have a custom cable that carries the balanced connection through without connecting the cold pin to shield at the cable as shown?

 

[panman]

The data sheet has all the info you need, just that it is not quite clear. As ccaudle said, the mic according to the schematics is wired unbalanced and also high impedance. All you need to do to wire balanced is to solder off the orange wire from the switch-lug and solder it to the lug, where the white wire already is wired to the switch. If there still is hum, then there is another problem around.

 

[Studiogearlover]

The schematic shows pin 1 connected to case ground, there is a line from pin 1 to the cartridge ground, using the same symbol as case ground at the connector shell terminal. Check with an ohmmeter to be sure, but if your current wiring matches the schematic it is probably OK.

The schematic shows the output cable wired as unbalanced, do you have a custom cable that carries the balanced connection through without connecting the cold pin to shield at the cable as shown?

Thanks for the info... I did check the mic's ( I have a pair, both of them hums the same way) and it is wired for balanced low impedance output according to the data sheet. I do not have a custom cable, i have a standard XLR mic cable. Checked with an ohmmeter, the mic case is not connected to the pin 1 at all. So this could be the problem I guess...?

 

[ccaudle]

Does your microphone have the blue wire shown in the schematics from pin 1 to cartridge ground? Maybe the cartridge to case connection has oxidation or something breaking the connection shown from cartridge ground to case.
You are probably on the right track, you could try putting a short jumper from pin 1 to the case tab at the connector and see if it helps. That should be a relatively quick thing to try.

 

[panman]

@Studiogearlover,
You wrote: "I have this user manual and wiring drawings attached, it is wired for balanced output according to the schematic. But if I compare to my new SM57, it is somehow different," and: "and it is wired for balanced low impedance output according to the data sheet".
If I have understood correctly, you say, that your mics are wired like the schematics in the manual. If so, then please understand, that your mics are not wired low-Z balanced, but high-Z unbalanced. That`s how the drawing in the manual shows. You also say it is not wired like your SM57. It should not be, if balanced. There is too much contradiction. So perhaps, please check it out again!

 

[Bo Deadly]

It's balanced either way. But the datasheet says the open circuit output is 20dB lower for low impedance. That means the high impedance is actually quite high. So you really need to re-wire it for low impedance and bump up the gain. So unsolder ORANGE at the switch and solder it to where WHITE is on the switch.

But from looking at the diagram it's not clear if the "CARTRIDGE GROUND" means the cartridge is connected to the "CASE GROUND" or the actual shell itself. If it is but you measure high resistance or an open circuit between the case and pin 1, that means stuff is corroded. I would carefully disassemble all of the metal bits and clean them with a rag with lots of deoxit. Clean the screws. Clean all threaded bits all metal bits that complete the circuit between pin 1 and the case ground. Then I would plug an XLR cable and measure resistance between pin 1 of the unconnected end of the cable to pin 1 inside. Resistance should be zero (0) or as low as your meter goes when you short the probe tips together. Then measure pin 1 to the "CARTRIDGE GROUND" wherever that means. Measure pin 1 to the shell. And for the full path, measure between pin 1 of the unconnected end of the XLR to the shell. Test different bits of metal that are screwed together to make the shell. If anything still measures as an open circuit, I would just solder a short piece of bus wire between the shield lug and pin 1 just inside the XLR. Then repeat measurements to confirm that pin 1 is really solid to the case. That will fix the hum.

 

[panman]

View attachment 84356
It's balanced either way.

Oh sorry, mybad! Thanks for correcting me! I did not see the line in the switch connecting the black wire to pin 2. I see it now.