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 1 
 on: Yesterday at 01:08:32 PM 
Started by brucekaphan - Last post by klaus
By no means measure at the tube pins!

I would modify that statement:

1. There is no reason not to measure at the tube socket pins in microphones using plug-in tubes.

2. When measuring voltages on AC701 tubes which do not have pins but use bare filament wires sticking out of the glass of the tube, try to stay away from these wires and measure where the wires are soldered to their (solid) mounting columns.

 2 
 on: Yesterday at 08:30:57 AM 
Started by brucekaphan - Last post by Kai
--even if there's a convenient place to measure filament voltage at the mic PSU,..
I definitely didn’t say that.

... you only want to measure at the tube itself, because everything in-between has resistance which will cause voltage drop.

Also, be very careful to protect not only yourself, but the rare tube inside.
This is why I do NOT advocate to open the mic, instead open the cable connector at the mic’s side to measure.
Voltage drop is along the cable’s resistance only, not between mic side connector and tube.
There’s nothing more in between where a significant voltage drop can appear.

This is why, at the PSU side, Neumann says to set filament Voltage to 4.2 V, with an average cable you can estimate ca. 0.2 V voltage drop.
But we don’t want to estimate.

By no means measure at the tube pins!
Any force on the tube pins can cause a leak and destroy the vacuum inside the tube.


BTW: There‘s a valid way to measure filament voltage at the PSU side with sufficient precision:
• Unplug the mic cable at both ends.
• Insert a wire bridge into pin(holes) 4 to 7 female connector.
• Measure the cable resistant at pin 4 to 7 male connector.
• For each Ohm of measured resistance (typically 1-2 Ohm) add 0.1 V to the 4 V adjustment voltage (filament current is 100 mA).

Example: The measured roundtrip cable resistance is 2.0 Ohm.
2.0 x 0.1 V = 0.2 V
4 V + 0.2 V = adjust to 4.2 V

 3 
 on: December 03, 2022, 08:49:06 PM 
Started by brucekaphan - Last post by soapfoot
I'd just like to add in case it's not 1000% clear--even if there's a convenient place to measure filament voltage at the mic PSU, you only want to measure at the tube itself, because everything in-between has resistance which will cause voltage drop.

Also, be very careful to protect not only yourself, but the rare tube inside. If you end up damaging the rare, pricey, needs-to-be-selected tube, it will be a very, very annoying and expensive fix.

That's why it's important to ensure the heater voltage is correct as soon as is practical.  :)

 4 
 on: December 03, 2022, 07:31:46 PM 
Started by brucekaphan - Last post by brucekaphan
Kai,

Thanks again for your help! I especially like all the reminders to not electrocute myself! I have all-day client-attended sessions the next two days, so it will be at least a few days before I'm able to follow your instructions, but in the meantime, I just wanted to thank you for taking the time to educate me and anyone else who might be looking at the same situation.

Bruce

 5 
 on: December 03, 2022, 05:49:50 PM 
Started by brucekaphan - Last post by Kai
Maybe Klaus has a better method, here’s how I do it:

PREPARATIONS:
• Arrange everything on a table with non-conductive surface: plastic, glass, dry clean wood - NO METAL!
• Unplug mains power at the wall outlet.
• Plug the mic to the PSU - this way the PSU is fully discharged and safe to open.
• Remove and open the female cable connector, the one that goes into the mic.
• Locate the heater voltage control of your PSU.
Usually you have to open the PSU for that, but some have a screwdriver hole opening.
NOT the stepped, course mains voltage adjust for 110-127-220 V, NOT, really!
• Have a fitting, fully ISOLATED small blade screwdriver handy. The ones electricians use for terminal strips, or the fully plastic ones for electronics adjustments.
• Plug the connector back to the mic.
• Connect a DC voltmeter between pin 4 and 7. Use small clips. Double check the meter clips don’t short with neighboring pins. Then double check again.
• Set meter range to read 5 V DC and switch on the meter.
———————
ADJUSTMENT:
• Get everybody out of the room, even pets.
• Plug in mains.
• Put one hand in your back pocket and leave it there through the whole process.
 This is a good safety measure to prevent being electrocuted when something goes wrong.
• After a minute the meter should read a value close to 4 V. Else you’re measuring the wrong pins.
• Adjust to 3.9 - 4.0 V. Turn slowly and watch for changes.
No changes -> wrong control.
• Unplug mains at the wall outlet.

If something doesn’t work as expected, start again with “Unplug mains at the wall outlet”, before locating the error.


 6 
 on: December 03, 2022, 03:57:37 PM 
Started by brucekaphan - Last post by brucekaphan
Kai,

Thanks so much for this thorough reply to my questions. I'm now intrigued by your comment on adjusting the heater voltage. I was unaware that changing cable length would require such an adjustment, and I've never done anything of this nature before. Is making this adjustment something you'd be willing to explain in enough detail so that a relative novice at repairing mics wouldn't stand to do damage?


 7 
 on: December 03, 2022, 03:25:53 PM 
Started by John M Fleming - Last post by John M Fleming
For sale: digital audio cables and impedance-matching transformer.
The 3 items are being sold as a package.  Price (for all): $74.00.
For pick up only, in Garden Grove, CA, U.S.A. 
Please pm me using the forum tools if interested in buying.

Items for sale are as follows:
Neutrik AES/EBU Digital Impedance Transformer Adapter (110 Ohm to 75 Ohm)
Model NADITBNC-FX
Highlights:
* For Long Cable Runs Via Coaxial Lines
*       For Digital Audio Signals
*       Matches 110 Ohm Cable to 75 Ohm Line
*       Input: 3-Pole XLR Female Connector
*       Output: Female BNC Chassis Connector
*       Durable Aluminum Construction
Features
The Neutrik AES/EBU Digital Impedance Transformer Adapter (110 Ohm to 75 Ohm) allows for long cable runs for digital audio signals via low-attenuation coaxial lines. It passively matches the characteristic impedance of a balanced 110 Ohm twisted-pair cable to that of a 75 Ohm unbalanced coaxial line. The adapter is equipped with a 3-pole XLR female input connector and a female BNC chassis output connector.
Key Features


*       Allows long cable runs for AES/EBU digital audio signals via low-attenuation coaxial lines
-

Belden 1800F-XMF-15BE

AES/EBU Female XLR to Male XLR High-Flex Blue - 15 Foot cable

1800F-XMF-15-BE - Overview
Designed to provide the latest in precision impedance and flexibility, these new digital audio interconnects will provide reliable performance with AES/EBU interfaces.
Featuring Belden 1800F "Super Flexible" digital patch cable, these tough assemblies include the best features for crush resistance, French Braid shielding and high density insulation which further reduces capacitance and propagation delay, allowing error-free transmissions over extended distances. Supports all devices that require AES/EBU 110 Ohm XLR to XLR interfaces.


75 Ω Apogee Wyde Eye S/PDIF (RCA) to Word Clock (BNC) Cable, 2.5 m. (8.5 ft.) length

This is a speciality cable terminated with a S/PDIF (RCA) connector on one end and a Word Clock (BNC) connector on the other. 

 8 
 on: December 03, 2022, 12:34:14 PM 
Started by brucekaphan - Last post by afterlifestudios
Glad to hear it! 

 9 
 on: December 02, 2022, 11:50:17 PM 
Started by Eddie Eagle - Last post by Eddie Eagle
A recent TripHop piece I created. Welcome to the hash den
https://soundcloud.com/iamlazerkat/afghan-hash

 10 
 on: December 02, 2022, 07:45:26 PM 
Started by Safari - Last post by Safari
Не знаєте де можна пограти в онлайн казино без ризиків втрати грошей?  Привіт. Пограти в онлайн казино можна де завгодно, але якщо без ризиків то можу порадити тобі цей сайт. Це казино https://mrcasinos.net/igrovi-avtomati/ не є онлайн-казино без юридичного дозволу. Вони є повністю ліцензованим онлайн-казино. Сподіваюся вам сподобатися і ви не розчаруєтеся.

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