The 150 meg ohm resistor provides polarizing voltage to the capsule. If your power supply worked with a known good mic, then you are correct, the problem is in your mic. You need to do a complete check of the wiring point to point. Look for a cold solder joint on the printed board, incorrect connections, etc. Make sure you didn't switch any components by mistake!
Neumann M49 Clone : Build Thread Puck Style (TLM49 Conversion To M49 b-c)
- Thread starter poctop
- Start date
Help Support GroupDIY Audio Forum:
TillM
Well-known member
thank you so much for your help.
The tube get no voltage except for B+.
The resistors are in the right place, I already check it.
I will check all connections.
Maybe I damaged a component, maybe I had not much attention an my soldering iron get at one component with the soldering shaft.
I ask, cause I see a small burning edge on the 560pf styroflex.
I'm still learning and have to understand the circuit and how the parts effects on each other.
The tube get no voltage except for B+.
The resistors are in the right place, I already check it.
I will check all connections.
Maybe I damaged a component, maybe I had not much attention an my soldering iron get at one component with the soldering shaft.
I ask, cause I see a small burning edge on the 560pf styroflex.
I'm still learning and have to understand the circuit and how the parts effects on each other.
Dear Till, I would really like to help but I have no clue what is the problem. It's very hard to judge without watching and measuring the micTillM said:
BTW, output transformer is not connected anyhow with the voltages inside mic, it's separated by 1uf capacitor. Only problem can be that mic does't "give" any sound if the primary is open or shorted, but that will not change any of the usual operating points at the tube side.Let me try to help... You have heater 0v. Connection between PSU and mic can be the reason. If you have heater voltage at PSU side, check does the cable pass it at he other connector side. Then, if it's OK "on cable" please check all the wires inside the mic. For a start check if you wired it correctly from the connector. Again, I'm not familiar with this project but if you have 0v heater than nothing will work, of course.
Best,
Moby
TillM
Well-known member
Well I don’t find the issue.
I first build the 49c pcb.
Yesterday I thought, I try the m49b PCB.
I build it, put a new tube inside, resolder all the cable stuff and so on.
After I was finished, I try the mic and what happend ?! Nothing.
The same issue.
I tried the PSU with my working mic and everything work.
My new mic don’t work, neither the M49c and m49b PCB.
I’ve no clue what’s the problem.
What can I do next ?
I first build the 49c pcb.
Yesterday I thought, I try the m49b PCB.
I build it, put a new tube inside, resolder all the cable stuff and so on.
After I was finished, I try the mic and what happend ?! Nothing.
The same issue.
I tried the PSU with my working mic and everything work.
My new mic don’t work, neither the M49c and m49b PCB.
I’ve no clue what’s the problem.
What can I do next ?
If that mic is not getting heater voltage, it will not work at all. If another mic was connected to the same PSU and cable and worked, then the mic under test has a wiring problem. Make certain heater voltage reaches the mic and visually check the tube heater lights. HT alone won't make that mic work. There was a comment about a capacitor being burnt with a soldering iron. If that cap shorted, that is also a problem.
TillM
Well-known member
Thanks for the help.
I measured with my DMM following.
When I put a 47ohm load resistor in the psu between pin 4 and 7 the heater voltage is 6.3vdc.
When I connect the cable from psu to mic, then I get following.
Measured from Psu ground to mic pcb,
A=22vdc
K=99vdc
H+=22vdc
So way to much voltage on A and H+ for the 6s6b-v tube.
When I measure VDC from HG I get 0vdc on A and K and 98 vdc on K.
The mic behavior like my m49c build that also not work.
The styroflex cap I changed with a new one on the old build.
I don’t get it, my first m49 build works flawless.
I measured with my DMM following.
When I put a 47ohm load resistor in the psu between pin 4 and 7 the heater voltage is 6.3vdc.
When I connect the cable from psu to mic, then I get following.
Measured from Psu ground to mic pcb,
A=22vdc
K=99vdc
H+=22vdc
So way to much voltage on A and H+ for the 6s6b-v tube.
When I measure VDC from HG I get 0vdc on A and K and 98 vdc on K.
The mic behavior like my m49c build that also not work.
The styroflex cap I changed with a new one on the old build.
I don’t get it, my first m49 build works flawless.
Again, I'm pretty sure it's a wiring problem.ln76d said:
TillM
Well-known member
Thanks guys, I will check it again.
The 22 volts for the heater may be the UNLOADED voltage. For test purposes, temporarily connect a 21 ohm (or 20 ohm) 1 watt resistor across the heater terminals and remeasure. Some of these power supplies go to full rectifier output voltage when not loaded. Also not good to plug a mic "hot" into a unloaded supply; 22 volts surge with several thousand uF of capacitor can take out a 6.3 volt heater...
TillM
Well-known member
Thanks rmburrow for your help.
Well, 22v is measured when I use my DMM with a unloaded PSU.
When I put a 47ohm resistor in the psu pin 4 and 7 I can adjust the heater H+ to, let’s say, 6v.
When I connect the mic with the psu I measured on H+ 22v. So it’s like the PSU is unloaded.
When I measure the resistance from my tube between H+ and HG I get 13.5ohm.
So the resistance between the heater terminals is nearly what you suggested.
Well, 22v is measured when I use my DMM with a unloaded PSU.
When I put a 47ohm resistor in the psu pin 4 and 7 I can adjust the heater H+ to, let’s say, 6v.
When I connect the mic with the psu I measured on H+ 22v. So it’s like the PSU is unloaded.
When I measure the resistance from my tube between H+ and HG I get 13.5ohm.
So the resistance between the heater terminals is nearly what you suggested.
TillM said:
Make simple test - if you have installed tube inside microphone, connecct it to the PSU, don't fireup PSU. Check is it heater voltage is down completely (discharged PSU capacitors). Measure resistance of tube heater again, one lead to the heater positive terminal, second lead of multimeter to the ground inside PSU (case if grounded or PCB etc.).
TillM
Well-known member
ln76d said:
I made the test like you suggested.
Heater voltage was 0v.
Then I measured the resistance between PSU ground and H+ on the mic.
It slowly increase the resistance. I really had to wait like three minutes or so, before my DMM stops reading the resistance.
I end up with a resistance of 677 ohm.
The resistance between PSU ground and HG is 691 ohm.
Something is open. Looks like you are reading a capacitor with the increasing resistance. The 13.5 ohms is probably a COLD resistance of the heater...the resistance increases when the heater is powered. Check the heater voltage from the rectifier to the PSU output terminal, then through the cable, and finally to the mic. To avoid damaging a tube, load the heater down with a 21 ohm 2 w resistor (or 20 ohm) and follow the DC voltage. You mentioned you have 22 volts DC at the rectifier output. Follow this through the PSU, the mic cable, to the mic. If you see 22 volts at the mic side but nothing at the resistor, then the heater circuit is open. Could be a bad solder connection to the PC board. Sometimes PC board connections "look" good, but in reality you may have solder on top of PC board varnish and no connection....
TillM
Well-known member
rmburrow said:
Just to be clear, how I've to put the 21(20) ohm resistor in the circuit?
Can I place them in the PSU between pin 4 and 7, or do i've to solder it on the mic pcb between H+ and HG?
How should be the resistance between ground and mic when the PSU is turned off?
You should see the resistance of the tube heater (13.5 ohms cold) to ground if the tube is in place; if you are using the resistor load, you should read the resistance to ground. This assumes one side of the heater is at ground (chassis) potential. Some mic PSU's elevate the heater voltage above ground so I would have to see a schematic of that PSU to make a call. In any event, you should measure the correct heater voltage across the tube terminals if there is continuity.
TillM
Well-known member
rmburrow said:
I use Dan's PSU
https://cdn.groupbuilder.com/groupdiy/u/39511/58d1402a05456.pdf
I don't find a 20 ohm resistor at home, so I first use 47 ohm. I will buy some resistors today.
But with the 47 ohm, i get following.
I put the 47 ohm resistor between pin 4 (Heater) and Pin 7 (Ground). Then I fire up the PSU with Mic connected.
H+ reads 4,58v but K reads also 4,58v. When I adjust the trimmer for K nothing change. (Good to know).
The Tube don't glow.
The resistance between grn and H+ is with the 47 ohm resistor and PSU turned off, 120 ohm.
But also with a 47ohm resistor my DMM starts increasing till it end up with 120 ohm.
Similar threads
