u87i polar pattern issues
« on: July 13, 2020, 05:07:30 AM »
Hi-

I am looking at a friend's U87i- sounds fine but polar patterns are not correct.  I swapped in a known good capsule and the same issue persisted.  I have cardioid at fig 8, and omni-ish at cardioid and omni.  With head removed, I do not see polarisation voltage at pin 5.  Does anyone have any suggestions on where to start with this?

Thanks in advance


Khron

Re: u87i polar pattern issues
« Reply #1 on: July 13, 2020, 06:38:03 AM »
Dirty switches?

With the high impedances involved there, you won't be able to measure much anyway - not directly, in any case.
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Re: u87i polar pattern issues
« Reply #2 on: July 13, 2020, 06:48:46 AM »
I'll check the switches.   I am seeing 40V or so on some of the pins (head removed)

Khron

Re: u87i polar pattern issues
« Reply #3 on: July 13, 2020, 07:39:37 AM »
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

Re: u87i polar pattern issues
« Reply #4 on: July 13, 2020, 07:42:13 AM »
I've been looking at the schematic, just unsure how to test for 47v that is on there at pins  1,2,3,5,6 and how to test those high meg resistors. 

Gus

Re: u87i polar pattern issues
« Reply #5 on: July 13, 2020, 04:59:33 PM »
Was someone in the microphone before you looked at it?

If so check the the correct capsule wires go to the correct connections.
That model 87i needs the isolated backplates to work correctly are the backplates isolated?
There are isolated and non isolated versions of the 87 capsule.

With the head removed and phantom power on you should see about 47VDC at R2 or pin6 to gnd at pin 7

report back after you measure that.

Have you checked the voltages on the circuit to what is on the schematic?

Re: u87i polar pattern issues
« Reply #6 on: July 14, 2020, 10:21:09 AM »
Thanks Gus.
Just checked- Yes, backplates isolated.  43v at pin 6/r2.  Verified connections from 4 elements of capsule connect to corresponding pins on bottom plate of head.  Voltages look good elsewhere in circuit, within a few bolts anyway. 
Still getting omni in cardioid and omni and cardioid at fig 8

So I shouldn’t see 47v at those other pins on schematic?

Gus

Re: u87i polar pattern issues
« Reply #7 on: July 14, 2020, 05:38:56 PM »
Your meter will load the reading. Do you know its input resistance?
I picked pin 6 first because of the lower resistance
You get the voltage to the other pins via R8 a 1meg
so pin 2 will measure the highest
pin 1 the lowest
and pin 3 and pin 5 in the middle
Pin 2 and 5 should measure different

Disconnect the head, pad off, set to Cardiod
meter set to ohms measure pin 2 to 5 on the head should measure open
measure pin 1 to 5
measure  pin 4 to 2
this is to make sure things that should measure open(very high resistance) are open

Re: u87i polar pattern issues
« Reply #8 on: July 15, 2020, 07:57:09 AM »
It’s a Fluke 76.  I was trying to look up input resistance in its data sheet but couldn’t find that figure

No resistance read at those locations that you specified on the head

1meg resistor measures 1 meg and has  43v on one side/38v on side facing the pins.  I see 38v at junction of the 68m resistors. My Fluke is not reading resistance across those but might be out of its range. All voltages measured to ground
« Last Edit: July 15, 2020, 08:00:25 AM by skeptic12 »

Gus

Re: u87i polar pattern issues
« Reply #9 on: July 15, 2020, 04:12:53 PM »


Re: u87i polar pattern issues
« Reply #10 on: July 16, 2020, 04:36:11 AM »
ah sorry, I see it now.  thanks

any thoughts on those last readings?

Gus

Re: u87i polar pattern issues
« Reply #11 on: July 18, 2020, 04:59:27 AM »
So what voltages do you measure at pins 3 and 5 they should measure about the same

You could sum up all of the resistances in series to the pins from the center of the 2.2k phantom resistor and calculate about what voltages you should measure with a 10 meg input resistance meter at the different pins

From the first post did you measure a voltage at pin 3 and no voltage at pin 5 with the head removed you did not post about pin 3.

Re: u87i polar pattern issues
« Reply #12 on: July 18, 2020, 02:16:12 PM »
hi

 this is what I have at pins (referenced to pin 7) with head off

Pin 2- 38v
Pin3- 0v
Pin4- 0v
Pin 5- 4.8v
Pin 6- 43v


maybe I should replace 68M's?
« Last Edit: July 18, 2020, 02:20:15 PM by skeptic12 »

Gus

Re: u87i polar pattern issues
« Reply #13 on: July 18, 2020, 05:56:24 PM »
Check the common node of R3, R5  and  then to pin 3
Do you get a voltage reading at both sides of R5?

Check for continuity from R3, R5 node to the pin with a meter.

Resistor failures are often from overheating from exceeding the power or voltage rating , corrosion related failures or physical damage.



Re: u87i polar pattern issues
« Reply #14 on: July 21, 2020, 12:44:10 PM »
I replaced the 68M resistors and had great joy with all the patterns returning. 

Thanks for your help Gus, I’m trying to wrap my head around the mega and giga resistances in these circuits.


 

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