Is my 7 pin XLR cable shorted or is my mic dead?

[The Flying Sloth]

[repost from another forum]

Hi guys,
I pulled out my Groove Tubes GT-67 mic today to compare it to some other mics and noticed that when I left it to warm up the light on the PSU never turned on, unplugged everything, there was power going to it, it turns on when no cables are connected but as soon as I plug the 7 pin cable into the PSU it won't turn on (whether or not there's a mic on the other side). Pulled out the multimeter to test continuity on the cable to see if there was a short and found that pin 2 has continuity with the metal on the XLR connector (that I assume is ground).

Is my cable likely to be shorted or do I have another issue?
Many thanks for the help,

Sloth

[Volbet]

Youre cable is likely shorting out.

The metal casing is shielding and as such shouldn't carry an electrical signal. If there's continuity with a pin, then it might be carrying a live signal. It depends on what 7-pin standard you're currently using.

[The Flying Sloth]

5 hours ago, Volbet said:

Youre cable is likely shorting out.

The metal casing is shielding and as such shouldn't carry an electrical signal. If there's continuity with a pin, then it might be carrying a live signal. It depends on what 7-pin standard you're currently using.

7 pin XLR is standard so far as I can tell so thankyou, if this mic was dead I was going to throw a fit.

[Volbet]

2 minutes ago, The Flying Sloth said:

7 pin XLR is standard so far as I can tell so thankyou, if this mic was dead I was going to throw a fit.

There are a couple of different 7-pin standards, which is why I'm not totally sure. Depending on which standard it is, pin 2 is either carrying volt-, ground or an LED trigger.

 

Either way, shield is carrying a signal it shouldn't. That's probably tripping the surge protection in the PSU.

[The Flying Sloth]

25 minutes ago, Volbet said:

There are a couple of different 7-pin standards, which is why I'm not totally sure. Depending on which standard it is, pin 2 is either carrying volt-, ground or an LED trigger.

 

Either way, shield is carrying a signal it shouldn't. That's probably tripping the surge protection in the PSU.

Just opened one of my other normal XLR mics and pin one (earth) is shorted to the case which contacts the connector on an XLR cable, tested my cable and it didn't seem to short (pin one to shield), so the shield having continuity with ground isn't a bad thing, just that a cable shouldn't display that continuity and we don't know whether pin 2 is even supposed to be ground so yeah, you're right, it's a bad sign but shield carrying ground is by design and not a bad thing on its own.

[The Flying Sloth]

[UPDATE] : Pin 7 is dead as far as I can tell with my multimeter, sounds like it's new cable time lads.
Upon further inspection pin 7 was never soldered to begin with, perhaps pin 7 just isn't required. UUUUUGGGHH, THIS IS CONFUSING

[kasugatei]

According to the PSM1 schematics pin 7 is not used at all. 
pin 1 is ground, pins 4 and 5 are signal - and + while other three pins are power supply for the microphone. 
 

If indeed pin 2 is shortened to the ground that means that low voltage part was in short circuit which could either blow the fuse of the power supply or some components inside. 
 

Since inside of the power supply are potentially lethal voltages I would suggest against opening it. The fuse should be accessible from the outside without opening it. 
Although I have lots of experience working on and servicing tube gear, I am always super nervous when working on a live tube circuit. I got zapped once and it was not fun at all. 
 

That’s what I can tell based on the description because I don’t have a crystal ball to do the remote inspection. 

[The Flying Sloth]

Just now, Niksa said:

According to the PSM1 schematics pin 7 is not used at all. 
pin 1 is ground, pins 4 and 5 are signal - and + while other three pins are power supply for the microphone. 

Yeah, thanks, that's prettymuch what I figured from the schematics on GS and GDIY too, 

2 minutes ago, Niksa said:

If indeed pin 2 is shortened to the ground that means that low voltage part was in short circuit which could either blow the fuse of the power supply or some components inside. 

Pin 2 isn't shorted to pin 1 (ground) but it is shorted to the XLR connector which I usually associate with Shield/Ground in a normal XLR cable though I have no way of knowing whether it is used as such on this cable as this seems to be the original cable the mic shipped with. 

6 minutes ago, Niksa said:

Since inside of the power supply are potentially lethal voltages I would suggest against opening it. The fuse should be accessible from the outside without opening it. 
Although I have lots of experience working on and servicing tube gear, I am always super nervous when working on a live tube circuit. I got zapped once and it was not fun at all. 

I can't say I've been zapped by a tube circuit but I've definitely made mistakes with (thankfully extremely small) transformers as a kid playing around with broken hifi gear so I share your apprehension with working on live circuits. The fuse was the first thing I checked and it seems to be just fine, the PSU still seems to work normally if nothing is plugged in and the LED only disappeared when the cable was introduced. I'd like to assume the previous poster was correct and the (possible) cable short is either harmlessly grounding out or tripping a protection circuit and avoid purchasing a new PSU (or major mic repair) from the designer of the microphone.

To think I thought tube mics were my friend up until this point, it's literally cheaper for me to ship a new cable from Thomann in Germany and pay the $80 shipping than it is to buy one either locally or from Ebay. The $80 gets me up to 30 kilos of freight though so it looks like I might have a fairly large order headed my way fairly soon then.

Sloth
 

[kasugatei]

31 minutes ago, The Flying Sloth said:

Pin 2 isn't shorted to pin 1 (ground) but it is shorted to the XLR connector which I usually associate with Shield/Ground in a normal XLR cable though I have no way of knowing whether it is used as such on this cable as this seems to be the original cable the mic shipped with. 

Ground in electronic circuits is considered everything that is connected to the electrical ground. In this case microphone chassis and PSU chassis are grounded for sure because if live voltage (in this case potentially up to 400V) gets to the chassis of either of those 2 it could harm user.

What could happen is that an wire insulation got broken inside of the cable and is touching wires that it shouldn't.

 

31 minutes ago, The Flying Sloth said:

the PSU still seems to work normally if nothing is plugged in and the LED only disappeared when the cable was introduced

this happens with cable only connected to the PSU without microphone? If so it is a clear indicator of short circuit in the cable.

 

 

Also regarding new cable, companies like Redco carry bulk tube mic cables and quality 7 pin XLR connectors from renowned manufacturers like Neutrik so if you are capable of making your own cables, you can go that way as well.

EDIT: Redco makes build to order cables for tube mics as well for 40$ for 6ft. Longer versions are available for extra $$. They use their own brand of cable and Neutrik XLRs.

[The Flying Sloth]

1 minute ago, Niksa said:

Microphone chassis and PSU chassis are grounded for sure because if live voltage (in this case potentially up to 400V) gets to the chassis of either of those 2 it could harm user.

What could happen is that an wire insulation got broken inside of the cable and is touching wires that it shouldn't.

I'm making the same assumption you are here, though from what I can tell the high voltage line comes through a 1N5380 which is a 120V diode, C13 is only rated for 50V and the 1N4007 near A+ (pin 2) is only a 94V part so (I'm no electrical engineer man, I'm just guessing) it shouldn't be anywhere near 400V, though, an interesting thing is that there is only one resistor different between A+ and the LED+ connection so  it makes sense that a short to ground on A+ would turn the LED off.

10 minutes ago, Niksa said:

this happens with cable only connected to the PSU without microphone? If so it is a clear indicator of short circuit in the cable.

It would be had I not known of load sensing power supplies that won't output unless you actually have something plugged in the other end. I couldn't see anything that looked that complex to me on the diagram but again, I'm no electrical engineer so I try not to deal in absolutes in things I have absolutely no clue about.

12 minutes ago, Niksa said:

Also regarding new cable, companies like Redco carry bulk tube mic cables and quality 7 pin XLR connectors from renowned manufacturers like Neutrik so if you are capable of making your own cables, you can go that way as well.

The connectors on the PSM1 and the original cable are branded Yongsheng which from my understanding is a subdivision of Ningbo Neutrik, I could salvage the connectors and wire my own cable but to be honest I trust my soldering skills just about as much as I trust my surfing (I'm no good at either) and all the Thomann mic cables I've got laying around seem to be exact mechanical clones of the Yongsheng connector (except of course the 4 less pins) and since the only cable I've ever owned that's broken has been boutique with Neutrik I'll give the clone a chance (and at roughly the price of Neutrik connectors alone if it works it will be a steal).

I've sent a message to the original designer of the mic and PSU and assuming he's come to the same conclusion as you and I it may actually be new cable time. I just don't want to risk a new cable allowing something else than may be broken in the mic to blow the PSU and Vice Versa.

THIS IS WHY YOU DON'T COIL YOUR CABLES TIGHT KIDS, EVEN IF THAT'S THE WAY YOU GOT THEM, IT"S A BAD TIME !!

 

[kasugatei]

9 minutes ago, The Flying Sloth said:

I'm making the same assumption you are here, though from what I can tell the high voltage line comes through a 1N5380 which is a 120V diode, C13 is only rated for 50V and the 1N4007 near A+ (pin 2) is only a 94V part so (I'm no electrical engineer man, I'm just guessing) it shouldn't be anywhere near 400V, though, an interesting thing is that there is only one resistor different between A+ and the LED+ connection so  it makes sense that a short to ground on A+ would turn the LED off.

According to the schematic PSU has transformer which has dual secondary coils. One is for tube heater (low voltage) and the secondary is BIAS or high voltage. Unlike traditional tube designs this one has DC regulated heater power supply (for lower noise) and it uses LM317 for voltage stabilization (probably around standard 6.3V ballpark, the trimmer is used to have a correct voltage). All capacitors there are rated for ~50V. Also LED is connected to that low voltage circuit close to the lead which supplies heater voltage on pin 2 so in the case of the short it would stop glowing.

 

The secondary high voltage circuit has 4 electrolytic capacitors rated at 400V. In theory if they are shorted you could get 400V discharge to the closest ground which can also be a human. Capacitor discharge hurts a way more than mains shock, as I learned the hard way.

 

Anyhow, if you know a reputable tube amp tech, that person should be able to check voltages on the PSU and it will be a super easy because compared to tube amps this circuit is a "child's play". Mr. Pittman (have his book) would not be able to help that much remotely without proper measurements I'm afraid.

[The Flying Sloth]

Update : Cable was bung, new cable arrived and everything is fine.

I'll say it again, THIS IS WHY YOU DON'T COIL YOUR CABLES TIGHT KIDS, ESPECIALLY SPECIALISED ONES YOU DON'T HAVE SPARES OF.