MayDay, Mayday. Brain cells dying... C414 ULS. New tests done.

I'm working on a AKG C414 ULS with low output seems about 1/2.

The voltages in RED are the ones from the bad mic and BLUE is a working mic of the same exact flavor.

Obviously the element is only seeing half the voltage. I just can't seem to get my head around why. I can test more points for anyone who would like to help...

Can you do more comparison around the converter? Are any of those caps tants that may have shorted? Do the diodes check good? Is the waveform at the transformer Tr.101 the same or different? Is the oscillator working at all?

both of the 7.7VDC readings are low compared to the *note.

What kind of phantom power supply are you using?
What kind of voltmeter are you using?

Do you have a scope?
Do you have a cap tester?

Edit  measure the voltage across R118 use Ohms law and calculate the current.  Look up the specs of the microphone for it's supply current.

It definitely looks like the oscillator either has too low supply voltage or doesn't oscillate properly. What is the voltage at emitter of T106 and gate of T107? Does cranking R117 has an action?

Quick reply:

The waveform looks the same on both mics both primary and secondary of TR101.

Does that eliminate everything upstream of TR101?

I'm checking the diodes and caps on the secondary now...

I'm in a hurry and don't have the time to take look at your readings. But I had a broken C414 EB P48 and after a longish search I found that the output transformer had a broken wire. I also had to exchange the pre-transformer output cap. In the C414 designs the transformer is a crucial part also for the supply voltages, since P48 is sourced through the secondary center tap.

You need to find where the power is going.  Divide up the circuit.  I would break the connection after the place you have marked 7.7VDC.  It has a * and has 14.5VDC with 48VDC phantom(I might use a 15K resistor as a temp load for the amplifier supply part with the amps disconnected).  Both reading you posted are 7.7VDC and that is a difference from 14.5VDC at 48VDC phantom 

Does the shunt reg and DC to DC capsule voltage converter give correct voltages with the amplifier sections disconnected or under a lighter load?  If it does load it with say under 2ma to start 14.5VDC/.002A = 7.25K use 8.2K, check if the voltages drop.

2ma from this link
http://www.akg.com/site/products/powerslave,id,214,pid,214,nodeid,2,_language,EN,view,specs.html

Also <= 2ma x R118 value should equal the voltage drop across R118.

Look for short circuit tantalums, I've found those in 414's before. Just check them all.

jwhmca said:

Quick reply:

The waveform looks the same on both mics both primary and secondary of TR101.

Does that eliminate everything upstream of TR101?

I'm checking the diodes and caps on the secondary now...

Click to expand...

There is not much else left, innit?