Advanced Audio Apex 460 Mod Kit Troubleshooting

[greenfendr]

I recently installed the advanced audio mod kit (http://advancedaudio.ca/collections/diy/products/apex-460-mod-pack) 
and I'm not getting any sound coming from the mic.  I tested the mic after I completed the transformer and all the circuit board steps (and it worked fine), but didn't between the capsule, tube, and PSU upgrades.  Power is getting to the tube, but no sound is coming out. 

Has anyone out there done this mod? I don't know what type of voltages I should be seeing. at the capsule I'm reading 1.13 volts between both sides, but none with reference to ground.

Thanks in advance.

 

[aamicrophones]

Hello, sorry you are having trouble with the modification.  I have done over 300 of these and here are some tips.

One of the weak points of the 460 is the lead (wire) from coupling capacitor C4 between the back-plate of the capsule and the input grid (pin 2).

I have dislodged that connection on several occasions while fitting the AK12 capsule.


If you inject a signal into the grid (pin 2) of the 6072a do you get an output from the microphones.

Keep the microphones gain low and only inject about -40dbu.


The AK12 is a dual back plate capsule so both back-plate connections must be connected together at C4 which blocks the dc going into pin 2 of the tube.

You CANNOT measure the voltage at the back-plate because of the 1G blocking resistor.  You should measure about 60v dc where R4 and R5 meet.

A typical DVM will drop 1v for every 20,000 ohms of resistance.


However, you can see if the connections to the AK12 are correct by measuring the capacitance.

If you tie the two back-plates together you will get about 80pf from the back-plates to the front diaphragm and about 80pf from the back-plates to the rear diaphragm.


There should be about 75-80v dc reaching the pin 1 of the plate of the 6072a and about 125v dc on pin 6 of the 6072a and 6.35v dc between pins 4&5 and pin 9 (gnd).


Also, I have repaired some DIY upgrades where C9 and C10 were shorted out with a wire instead of just being removed and this will short the final output of the microphone.

e-mail me at [email protected]  if you have any other quiries about the upgrade.

Cheers, Dave

aamicrophones.com

 

[greenfendr]

Dave,

Thank you so much,  I will check my work against your notes next week and post my findings.  I feel like it may have to do with the Way I have the back plate wired up. but I Don't' have the mic in front of me right now to see.

 

[greenfendr]

Hi Dave,

I took the steps you mentioned. All the wiring connections seem fine.

I don't get audio out when connecting a signal to pin2, but I do get some intermittent clicks if I touch the backplate wires with the probe or my fingers.  as for my measurements

Where R4 andR5 meet I'm seeing 61.2V
Pin 1 = 70V
Pin 6 = 124.3V
and 6.11 V on the heater.

AT the capsule between the diaphragms and the backplates I'm getting a variable reading between 40 and 130 nF. but I'm not sure that I trust my DMM, I've never used it before for capacitance measurements.

I'm going to play amateur electrician here, since all the voltage levels are lower then expected, the capsule isn't seeing the full charge it needs.  Possibly something wrong with the replacement power supply diodes (or more likely the way I installed them?)

Thanks again for your help.

 

[pasarski]

You should have plenty of audio with those voltages. What are the cathode voltages (pins 3 & 8 I suppose)?

Have you tested the mic with the old capsule?

 

[greenfendr]

I didn't have time to check the old capsule yet, but I did measure cathode voltage:

71.7V at Pin 8
1.31V at Pin 3

-M

 

[pasarski]

I didn't have time to check the old capsule yet, but I did measure cathode voltage:

71.7V at Pin 8
1.31V at Pin 3

-M

Actually, I realized I don't know what pin 8 should read, but 70V seems reasonable and 1.3V is fine. The problem is probably the capsule, capsule wiring or a cold joint/wiring mistake somewhere between the tube and the XLR output of the PSU.

 

[pasarski]

but I do get some intermittent clicks if I touch the backplate wires with the probe or my fingers. 

Missed this. Definitely the capsule or the capsule wiring!

 

[greenfendr]

Thanks for the help.

My front diaphragm is showing a connection to the mic chassis , while my back diaphragm is showing to the leg of R9, the backplate shows to the leg of R10.

So from what I can tell, everything is wired right....

Next, I'm Going to see if The old capsule works.. I'm not sure if I'll get to it today though

 

[greenfendr]

And... in the end it was a bad soldering job on my part. the joint at the new C8 came apart on me.  I touched it up and got signal flowing.

I'm actually glad it didn't work out immediately, It forced me to check my work, and I did a ton of reading to try to understand how tube circuits work, 

Thanks for your help everyone!

 

[greenfendr]

And for those who are interested...

Sound Samples!

https://soundcloud.com/greenfendr/sets/apex-460-mod-samples

Recorded on an A&H GS-R24 (SS input), no eq or compression..

This is the stock AAMicrophones Mod,