Dany Bouchard D-12 Build - Pattern select not working and draining voltage.

[Wordsushi]

Hoping this is an easy fix. I built Dany's D-12 (http://vintagemicrophonepcbkit.com/D-12.html). Great build, but now I'm finding that I have omni on all the time and the pattern selector isn't working.

Things I've tried so far: Replacing all the resistors in the original PSU selector rotary pot with 392K. (no change). Also, replaced the capsule in case that was the issue, still no change. As far as I can tell from the build guide:  (https://www.dropbox.com/s/s1dnqeqs1dso1vc/D12%20Build%20Guide.pdf?dl=0)  my capsule diaphragm and backplate connections are in the correct spots. I also just disconnected just the back diaphragm just to check and all sounds great in cardioid, but I'd love to get the full pattern select working.

If anyone has any thoughts or ideas as to how to fix this, my ears are open.

 

[RuudNL]

To start with, I would measure the resistance between front and back diaphragm.
It happened to me once that the two membranes seemed to be 'connected' internally.
(The manufacturer replaced the capsule. Maybe screws touching each other.)
If front and back membranes are connected through a capacitor, I would check this capacitor too.

 

[Wordsushi]

To start with, I would measure the resistance between front and back diaphragm.
It happened to me once that the two membranes seemed to be 'connected' internally.
(The manufacturer replaced the capsule. Maybe screws touching each other.)
If front and back membranes are connected through a capacitor, I would check this capacitor too.

I'll try. How exactly do I check that resistance?

I've actually already tried an identical capsule with the same result and as far as I can tell, there is no capacitor that connects the two diaphragms.

 

[RuudNL]

Is there a schematic available?
That would make it easier to see what the problem could be.
(I searched, but I could only find a build guide.)

 

[trans4funks1]

Hi Ruud,
I am guessing that you are referring to the idea that that capacitor in position C11 on the original AKG C12 schematic may be shorting to ground as a possible reason why the mic is always operating as Omni. That seems plausible.

Hi Wordsushi,

  If both capsule membranes are at ground potential the mic will remain in Omni.

It seems as if checking for polarization voltage at R16 will be a good first step for trouble shooting. If there is no voltage then perhaps lift C11 and check again. If there is still no voltage then work backwards to the rotary switch.

 

[RuudNL]

OK, so it is the C12 schematic.
In this case C11 (rear diaphragm to ground) could indeed be the problem!
Best way to check is to unsolder the wire connected to the rear diaphragm. Is the microphone cardioid now?

 

[Wordsushi]

OK, so it is the C12 schematic.
In this case C11 (rear diaphragm to ground) could indeed be the problem!
Best way to check is to unsolder the wire connected to the rear diaphragm. Is the microphone cardioid now?

Hi Ruud,
Yes, I actually disconnected the rear diaphragm yesterday and the mic was in cardioid. In your opinion, what does this mean (remember, I'm a noob) and what should I do next to fix the issue?

 

[trans4funks1]

Check for polarization voltage at R16.

Rotate the switch and see if you can get the voltage to raise and lower. 

When the voltage is 0vDC  you should have Omni response.

At +60vDC the rear capsule should become inactive with the membrane and backplate of the condenser at equal voltage, and you will have "Cardiod" response. 

At +120vDC the rear capsule has reverse polarity compared to the front and you should have figure 8 response.

If you can not see the voltage at R16 then go looking for it back towards the switch.

 

[Wordsushi]

Check for polarization voltage at R16.

Rotate the switch and see if you can get the voltage to raise and lower. 

When the voltage is 0vDC  you should have Omni response.

At +60vDC the rear capsule should become inactive with the membrane and backplate of the condenser at equal voltage, and you will have "Cardiod" response. 

At +120vDC the rear capsule has reverse polarity compared to the front and you should have figure 8 response.

If you can not see the voltage at R16 then go looking for it back towards the switch.

Thank you very much, by the way.

Okay, I just tried. No reading on R16 and, as far as I can tell, it is connected directly to the pin 4 connection, marked POL. I also checked at the POL connection at got 0... regardless of pattern switch position... so it appears there's no voltage coming into the pin 4 connection.

So, I'm guessing I should I measure pin 4 on the cable. Question is, is it safe to do this without the mic plugged in with zero load on the PSU?  Also, to check the #4 pin output from the PSU without the cable, should I connect pins 1 and 7 with a 180k resistor first?

 

[Wordsushi]

Also,

I have continuity between pin 4 and R16, and R16, when measured separately, is giving me just a hair over 30m which is right. I've tried with 2 separate 7 pin cables so I'm ruling that out as cause.

I then opened up the PSU, put in the 180k resistor as a load and measured the screw on terminal 4 (pol) of the 5 pin terminal block and also got 0 v. So, it seems the issue is with the PSU somewhere, but I'm not sure what to do next or where to look next.

 

[RuudNL]

On one of the three wires going to the polarity switch, there should be a significant voltage, referred to ground. (120 V or so.)
If not, something on the power supply PCB might be wrong.

 

[Wordsushi]

On one of the three wires going to the polarity switch, there should be a significant voltage, referred to ground. (120 V or so.)
If not, something on the power supply PCB might be wrong.

Okay, so I measured the points on the terminal block going in and out of the rotary pattern select switch and I have 120v on full fig 8 and 0 on Omni, but only 34v in cardioid.  However, on the terminal block next to it, where the wires connect to the 7 pin output jack, I still have 0 on the POL terminal screw no matter what my pattern rotary is set to.

So, I'm wondering why I'm not getting 60v at cardioid and why that voltage is not getting to the POL output point.

 

[RuudNL]

The voltage in cardioid is probably lower than 60 volts, because the voltage divider has a pretty high resistance.
But if there are 8 resistors with the same value between +120 V and 0V, you can be sure that the voltage after the 'fourth' resistor will be +60 V (not loaded).
It seems there maybe is a connection between the rotary contact of the switch and the 'pol' connection missing, somewhere.

If I remember correctly, there is a resistor (1M ?) and a capacitor to ground between the rotary contact of the switch and the 'pol' output. If the capacitor is shorted, the output voltage would always be zero.
This would also explain the low voltage in the cardioid mode, because the voltage divider is then loaded with this 1M ( ? ) resistor.

 

[Wordsushi]

The voltage in cardioid is probably lower than 60 volts, because the voltage divider has a pretty high resistance.
But if there are 8 resistors with the same value between +120 V and 0V, you can be sure that the voltage after the 'fourth' resistor will be +60 V (not loaded).
It seems there maybe is a connection between the rotary contact of the switch and the 'pol' connection missing, somewhere.

If I remember correctly, there is a resistor (1M ?) and a capacitor to ground between the rotary contact of the switch and the 'pol' output. If the capacitor is shorted, the output voltage would always be zero.
This would also explain the low voltage in the cardioid mode, because the voltage divider is then loaded with this 1M ( ? ) resistor.

I just checked under the board and looked at the connection. There is a 1M resistor (R11) but no capacitor, correction yes, there is a 2.2uf cap to ground.

I have voltage on the rotary side of R11, but nothing on the other side (measured above the board against the leads).

Also, I replaced the resistors on the rotary with 392 ohm (which was in the BOM), taking out the 150ohm ones that were originally in there. Noob question: If I go back and replace the 392's with 150s again, would that give me my 60v at straight up Cardioid?

 

[Wordsushi]

I just checked under the board and looked at the connection. There is a 1M resistor (R11) but no capacitor, correction yes, there is a 2.2uf cap to ground.

I have voltage on the rotary side of R11, but nothing on the other side (measured above the board against the leads). So, I will check and see if the 2.2uf cap at C11 is the cause.

Also, I replaced the resistors on the rotary with 392 ohm (which was in the BOM), taking out the 150ohm ones that were originally in there. Noob question: If I go back and replace the 392's with 150s again, would that give me my 60v at straight up Cardioid?

 

[RuudNL]

Resistors on the switch can't be 392 ohm.
There are 8 resistors in series on a voltage of 120 V.
That would mean a total resistance of (8 x 392)  3136 ohm.
On 120 Volts that would mean a current of 38. 3 mA or a total dissipation in the switch resistors of 4.6 Watt!
I think 390 K.ohm would be more likely!

 

[Wordsushi]

Resistors on the switch can't be 392 ohm.
There are 8 resistors in series on a voltage of 120 V.
That would mean a total resistance of (8 x 392)  3136 ohm.
On 120 Volts that would mean a current of 38. 3 mA or a total dissipation in the switch resistors of 4.6 Watt!
I think 390 K.ohm would be more likely!

My bad, yes it's actually supposed to be 392k. On Matador's C12, on which the PSU board looks to be pretty much the same, he's using 402K resistors on the rotary pattern switch.  https://groupdiy.com/index.php?topic=51377.0

The odd thing is that on Dany's D-12, this build, in the build guide there is no mention of replacing the resistors or the switch, though the switch and the 392K resistors are in the BOM.

 

[RuudNL]

Values are not critical, since there is no current going through the wire for the polarisation voltage.
330 K is fine, 560 K is also fine. As long as the 8 resistors have the same value!

 

[Wordsushi]

Values are not critical, since there is no current going through the wire for the polarisation voltage.
330 K is fine, 560 K is also fine. As long as the 8 resistors have the same value!

That's good to know! We can probably rule the pattern selector switch out as the cause.

I'm guessing it's something going on with R11 and/or C11. I hit the solder joints again with the iron and still not getting any polarization voltage to the mic. The leads on C11 are very close to each other, but I made sure the solder joints aren't connected.

 

[RuudNL]

Measure the resistance of C11. (In circuit)
If you measure 0 ohm, you found the problem!