Neumann U87ai needs repair and testing

[moamps]

You have said "everything looks good, more or less" but I am not getting  +\-60v and on the cable I'm getting 48.2v on pin 2 when the xlr cable is inserted it comes down to 45.3v?
So something is obviously wrong here?

Each leg of the phantom power is limited with 6,8k resistor. If your mike draws cca 1mA, and each leg draws 0,5mA we should expect arround 3V voltage sag when mike is connected. So, that IS ok.
Also converter is OK because you got 58 or 59V on diodes what is perfectly fine.
DC points wise, your mike looks OK.
So, you should do measurements on resistor R109 I asked before to be sure that power supply for the amp works. Than you should switch on your scope and find does your Fet amps works or not as I also asked. Then we can go on.

 

[tubestation]

Hello Spence,
If You are talking about the Regulator D 101, C108/109 I suggested a few Posts ago that there must be a Reason why T2 burned. If C 108/109 is leaky or temporaly shorted, too much Current is pulled from T2 ,caused this Transistor to blow. I would doublecheck  or  replace  C108/109. Moamps asked a few Posts ago to meassure both Ends of R109 , I think he did this to see if the desired Current is present. I still think that Your Drainvoltage is too low. How do You bias the FET at the Moment? Not sure If It s  already replaced , If not ,did You re install RS ( 3,3 kOhm) or do still have the 10 k Poti installed ? and If a new FET was installed, did You do at least a rough  Bias Adjustment  for the new FET ?
Greatings,
Lothar

 

[Spencerleehorton]

Hi guys,

Thanks again for all his help you already know your the greatest but I thought I should just give some thanks.

Right my drain voltage is now 8.73v so other side of R109 is 27.56v.
R10 is 27.56v and other side 31.59v
C109 is 31.62v on + side.
R108 is 0v on both sides.

Hopefully this is good enough evidence of what is wrong.

I will try and find a replacement for C109/C108.

Regards

Spence.

 

[moamps]

Hopefully this is good enough evidence of what is wrong.

Nope. All DC readings suggest that, as I stated earlier, DC wise your microphone is OK.
From my experience with U87  microphones there are two or three usual types of malfunctions:
1. capsule is worn, damaged or very dirty
2. switches and contacts in head are oxidized
3. Tantalum or standard capacitors are drained or shorted 

So, to find whats wrong you should do now AC tests, one I suggested earlier, using scope and tone generator.

 

[Spencerleehorton]

Have replaced both C108/109 and no real change?
C109+ is 31.82v, R109 is 8.93v, drain of T1 is 3.87v.
R106/107 both sides are 0v? Along with R108 is 0v both sides?

Spence.

 

[Spencerleehorton]

Hi Moamps,

Just waiting for tesco to deliver some shopping then I'll do the AC test!!
I knew I'd forgotten something!!!

Spence,

 

[moamps]

drain of T1 is 3.87v.

Is drain at 3.87 or 8.73V?

R106/107 both sides are 0v? Along with R108 is 0v both sides?

Thats's fine.

 

[Spencerleehorton]

Drain is at 3.87v when I measure with MM.
Btw I still have 10k pot on RS adjusted so I can see full waveform.

Spence.

 

[Spencerleehorton]

Right I think there is some confusion, I am getting in a muddle it seems.
On the schematic T1 the drain is at the top and source at the bottom correct?
On the pcb it would seem this is the other way round, the silk screen had confused me, as the 2N3819 is the other way round.
On the pcb the top connection goes to the pot (which will be the resistor once set)
The bottom connection when I put the MM on DC volts and twiddle the pot I can adjust it to 11v.
I'm not familiar yet with using the scope to see AC and such yet and might need a little help just to get me started, sorry.
The wave form I'm getting now is much, much better, but unfortunately there is still this popping happening.

Regards

Spence.

 

[Spencerleehorton]

Now i've plugged the mic back in again and nothing works!!!
Are these capsule connections prone to failure? could this be my main problem? apart from my confusion!!!

Spence.

 

[tubestation]

Hello Spence,
You can meassure Drainvoltage at the + Side of C 101 , maybee more easy and save.
I once had popping Noise ( full Level, 0 dB ) coming from a Km 64. in the End it was a faulty Cathoderesistor.that caused this,  due to discharge. Of course Your 87 ai doesn t have a Cathoderesistor, I just want to say that this discharge Problem can have many Reasons, not easy to find and solve.
As  Moamps suggested, clean the Switches and Contacts in the Headbasket  untill no Corrosion is left. If the Capsule You replaced was dirty, rotten, the Change that the Switches and Contacs are affected is  very possible. The Dirt can react like small Capitors that charge and than discharge, causing this Pops, or the Biasvoltage get s  spoiled due to unwanted Crossresistance.
You probably already cleaned the Board properly ? The next Step could be to replace all Tantals , Electrolytics C 107/C110.
Good Luck, keep on , You do it !!
Greatings
Lothar

 

[Spencerleehorton]

Yes I have cleaned everything 6 or 7 times now with 100% IPA.
I have found that without the headbasket I'm not getting popping noises now but when I put the headbasket back on nothing came out either?
So back to square one at the moment!!!
I will take a break then go back out and see what's happening and repeat the checking.

Spence.

 

[tubestation]

Spence,
To get rid of heavy Corrosion  IPA  won t do the Job.
I tried lot s of Liquids to clean corroded Contacs and Switches with unhappy and expensive Results.
What i now suggest is not a Joke but the only satisfactional Result i got is from this Process:
Wrap the Pins with Aluminium Kitchen Foil.. Dunk the Contacts into heavy saturated Saltwater.  ( more Salt than the Water can loose) The holes ( female Contacts ,) can be stuffed  tight with Aluminium Foil  and dunk as well into the Saltwater. Now a chemical Process get  activated , where the Corossion ( Oxidation ) get s reversed. ( The Oxigen Atom gets loose and travels into the Alu and the Saltwater. After a Day, or longer get them out of the Saltwater and unwrap. You will  see a lot of black colored Foil, where the Pins got in contact with the Foil, the Water is blackish as well.Than wash with lots of clean Water and dry. You will be surprised, 
With this Process i even cleaned ( re- newed ) the most dirty, black corroded Switches and Plugs from 70 Years of Oxidation and without the Danger of destroying Plastic or other sensitive Parts, it s only Saltwater.
I can understand You don t believe, try It with a dirty XLR or Tuchelconnector.
I learned this from Somebody who worked in a expensive Hotel. They cleaned there  holy Silver like this.
Greatings
Lothar

 

[Spencerleehorton]

sorry to be a pain but i don't believe it to be dirt, obviously there is a chance it could be but what i could try is hard wiring the capsule connections. but its still weird that the noises where clearly there without the capsule on?
this surely points to somewhere else in the circuit, and when i had the scope on there i could see some sort of noise coming from somewhere else.
could you explain in a little bit more detail of how to look for the AC, i have the signal gen and i have made a mark on 0.1V ac and set to 1khz. i have inserted it at pin 2 and gnd. then using the probe i have put on drain but i'm not really clear on what i should be looking for to be honest with you? as bad as that sounds!!
am i right in thinking i would see the waveform and should see some sort of crackly noise in there as well?
my scope is an old analogue one and does have some faint ghosting happening at the moment so for all i know its not working right!
we have come so far with this so far and it feels kind of like theres not much more to do?

regards

Spence.

 

[tubestation]

Spence,
You can insert the 0,1 V at Pin 2 Ground to Pin 7 , to use the Calibration Input could also be a Way. The Problem could be the low Outputimpedance from the  Generator. Maybee a 1 M Ohm Resistor across + Sgnal and Ground to terminate the Output ?? The Cable from Output Generator to the Pins  should be as short as possible.
Have a read here how  to implement the nessesary Dummy Capitor , when You want to  test without  the Capsule, how to inject  the Testsignal.
http://groupdiy.com/index.php?topic=53550.0
I would look after C110 ( at the - Side ) behause there is no dc  at this Point so should see You See Your 1 kHz  clear on the Scope, If Your Generator puts out a  low THD  Output, and If  the FET Stage works  correct. The Inputimpedance of the Scope  will like the  - 20  kOhm  at this Position,  to look before the FET Stage won t work or is not accurate,  because of the high Impedance, only a Fraction of the Voltage would reach the Input of the Scope, I think. You could also record into Your DAW and Look for a longer Time If the Pops are constant or more chaotic, You can see it better in the Recording  than the shorttime Peaks on the Scope.
I am not sure  If it helps to see the Pops on the Scope or on the Screen .
To replace C 107/110 won t hurt and than You can be sure that the polarised Capitors are not  the Problem. Do You have a magnifying glass ? Observe the Traces If there are lift offs or Corrosion under the Traces.
Do You have a spare 1G Resistor to try ? Sometimes They make discharge Troubles.
My Knowlegde is at the Limit now......
Greatings,
Lothar

 

[Spencerleehorton]

i really appreciate your help, and glad you have stuck it out but like you my knowledge is fading fast!!!
so far i have replaced T1, T2, R103, R104, R105, C105, C107, C108, C109, C110 and still the pops and farts persist?
if i put the probe on R114 i can see the pops and farts, and without the head basket on the noises are still there?
voltages look good, drain at 15.68v sinewave looks symmetrical and i have swapped the 10k pot for a 5k and set the wave to clip symmetrically.
could it be c111 or c112?
the only other caps i havent changed are C106 and C104, could it be them?

regards

Spence.

 

[tubestation]

Spence,
I  miss C 103 ,the Capsule/Gate Couplecapitor on Your List. Could be leaky as well. You already  changed a lot, so to replace the other few Caps could be a Possibility to  encircle the Problem. If You  solved the Problem, You could return the original Parts.
I have a DDR Micpreamp with build in Powersupply , that made Pops and Brutzel Noise. I have another two properly working Ones, exactly the same, even the  same Revision than the noisy One.  They  have two Boards, one for the Powersupply, one for the Amp. I swapped all Cards, even the  Input and Powertransformer ,still Pops and Noise. I resoldered  every Solderpoint on all  Connectors and Cables, than the Problems dissapeared. What I want to say is that it even can be a faulty Solderjoint  that can make big Troubles.
Greatings,
Lothar.

 

[tubestation]

Spence,
If You see the Pops at R 114  that s strange to me. It s the Point where the Supplyvoltage , coming from the Phantompowerresistorbridge , reach the Mic. There should be a clean dc, coming from Outside World.
Stupid Question,
Did You tried the Mic with another Preamp, another Phantompower ?
Greatings,
Lothar

 

[Spencerleehorton]

yes i have tried it with several different pre amps and mixers, all the same.
have tested voltage in XLR cable and 48.3v.
think i'll change C103 again along with C102, i had changed them before but still might be dirty.
if noise then persists i'll look at changing Dr101,Dr102, C113,C114,R115,R116.
should i be able to see a voltage at r108/107? cause im not seeing anything? could this point to C104 being faulty?

Spence.

 

[tubestation]

Hello Spence,
I was sure You tested different Amps, Phantompower, just wanted to be sure.
At the Calibration Input Bu ( Buchse= Plug ) 3/4 no dc is present.,that s the Place where a Testsignal can be injected, so at R7/8 should be 0v dc. If there would be dc than c104 would be leaky. You even could Lift C 104 ,not important for the Funktion of the Mic.
Dr101/102, C113/114 are only a Hf Protection and could even be obmitted. It s not a Zobel Network to satisfy the Outputtransformer.
R 113/114 needs to be be matched within 0,4 %, so to buy 0,1 % Resistors ,If You want to replace,  would be good. When You want to meassure them Yourself, You need a very good Meter, or better a Meassuring Bridge. Don t trust Your standart Meter in this Case, I think. The absolute Resistance Value is not so Important, but the two Resistors should be as equal as possible for the proper Funktion of the Resistorbridge.
Greatings and have a nice Day,
Lothar