pucho812

C12vr
« on: February 14, 2019, 12:36:22 PM »
This is a good one as we have the service manual in the tech docs.

I measure 129vdc at test point 5  on the mic which is B+ feeding  the capsule.  I should see 120Vdc. On the adjustable pattern I get from 0vdc to 108vdc
Other test points are a few Volta high as well except the heaters which are 12.6
The mic works,  but in cardioid pattern starts at a nice signal level and after a couple seconds drops in volume.  If I switch patterns  The mic works and sounds pretty good, Omni and fig-8 behave as expected.

Obviously I need to fix the B+  to be 120vdc  as per the design. the capsule looks clean and I have a known good tube.
Could my higher then expected B+ cause my cut out in cardioid  or do I have a capsule issue?
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.


dmp

Re: C12vr
« Reply #1 on: February 14, 2019, 02:54:34 PM »
The low pattern voltage (108 vs 129) tells you too much current is going that way than should be.  10 microAmps (129-108/2.2M). In the mic the pattern voltage only sees ground through the 3 G resistor and that won't sink a microAmp.

Possible someone was modifying it and got solder flux in the high impedance areas?  The loss of bias on the capsule (and sound cutting out) also could be caused by paths to ground around the capsule. 

The capsule biasing grounds the front plate, uses a voltage divider to put 60v on the center, and puts the adj pattern voltage on the back plate. Kind of strange that cardiod is the only one with the problem. For cardiod the pattern voltage is 60, just shutting off the back plate, as they have the same voltage then.  Are you sure the front is working in omni & fig8 and not just the back plate?
Might want to verify the voltage between R8 and R3, which sets the 60v capsule center plate.

As to the power supply check if the 120v zener is fried

pucho812

Re: C12vr
« Reply #2 on: February 14, 2019, 04:30:32 PM »
 The 120 V zener is not fried. I do you measure 129 V across it which is no bueno. When I tested with the power off I get my half volt voltage drop on the diode itself, well a little more then that like .6V.  I will check the resistance and check for solder flux which I did previously and did not find any issues . The mic is been repaired before by somebody else and last time, they made the claim it was a wire to and from the capsule which would be in the high impedance areas. however testing those connections from the capsule to the circuit board seems to be OK.
Hmmm.
« Last Edit: February 14, 2019, 09:37:44 PM by pucho812 »
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

radardoug

Re: C12vr
« Reply #3 on: February 14, 2019, 09:26:11 PM »
Is it cutting out/going low when you speak into it? Thats a sure sign of a contaminated capsule. Very common for the front capsule to be dirty and the back not, as omni/ figure of 8 not used much.

pucho812

Re: C12vr
« Reply #4 on: February 14, 2019, 09:33:48 PM »
Is it cutting out/going low when you speak into it? Thats a sure sign of a contaminated capsule. Very common for the front capsule to be dirty and the back not, as omni/ figure of 8 not used much.

yes and no. for example I could mic a speaker with it and get the same effect or I could clap and mic that with the same effect.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

pucho812

Re: C12vr
« Reply #5 on: February 14, 2019, 10:23:23 PM »
update: no 60V at the capsule.  I measure 60Vdc at r6 on the parallel connction of  r3, r6 and r8, after r6 I have a volt and change.  funny r6 measures correctly on the dmm.
« Last Edit: February 14, 2019, 11:28:53 PM by pucho812 »
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

dmp

Re: C12vr
« Reply #6 on: February 14, 2019, 11:34:54 PM »
you're dmm is pulling down the voltage on the other side of r6. the dmm has an impedance usually of about a meg.
if you have 60 v at the divider, its probably good.

try swapping in a good capsule?

pucho812

Re: C12vr
« Reply #7 on: February 14, 2019, 11:46:06 PM »
you're dmm is pulling down the voltage on the other side of r6. the dmm has an impedance usually of about a meg.
if you have 60 v at the divider, its probably good.

try swapping in a good capsule?

yes I was going to correct my self.  unfortunately I do not have a spare capsule at the moment.  weird though that it just fades out vs cuts out. I am used to cutting out all together when the capsules is dirty biz.
I wired the capsule to the 60VDC at the junction for a test.  I did get a constant volume level but was lower then before, I had no polar patterns switchable  and was super  hissy. it's wired back to normal and fades like clockwork
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

radardoug

Re: C12vr
« Reply #8 on: February 14, 2019, 11:49:57 PM »
In that case, suspect the high value series resistor from the 60V point to the capsule. Not easy to measure without an HP high resistor checker! Just try replacing it with whatever you have thats really high, 30M or more, and see if the fade is gone, but this will cut the bass.

dmp

Re: C12vr
« Reply #9 on: February 14, 2019, 11:53:57 PM »
could the capsule be pulling in because the voltages are too high? 129v b+
i think your zener might need to be replaced


pucho812

Re: C12vr
« Reply #10 on: February 15, 2019, 12:14:05 AM »
could the capsule be pulling in because the voltages are too high? 129v b+
i think your zener might need to be replaced

I don't see why as I get 60V at the junction...
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

dmp

Re: C12vr
« Reply #11 on: February 15, 2019, 10:18:52 AM »
I don't see why as I get 60V at the junction...

I guess the R3 R8 voltage divider is little off and not giving you exactly 1/2 of 129. If you have 60v at the capsule bias, shouldn't be a problem.
So you bypassed R6 and the mic was fine? No fade out?
Then it sounds like the capsule is leaking too much current to ground. Check around C4 also for flux or pathways to ground.

pucho812

Re: C12vr
« Reply #12 on: February 15, 2019, 11:55:42 AM »
this gets better, mods.... someone one did mods on here.

R9 and c6 were removed. r6 has a jumper in place.  r 14 is removed. c7 was changed from 33u to 22uF no other changes found.

they only modded the first half.  it's set to not run the other half of the triode at all.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

dmp

Re: C12vr
« Reply #13 on: February 15, 2019, 12:05:34 PM »
Sounds like a "high noise" mod

Weren't you measuring R6 yesterday?

post a picture of the mic board

pucho812

Re: C12vr
« Reply #14 on: February 15, 2019, 12:13:38 PM »
Sounds like a "high noise" mod

Weren't you measuring R6 yesterday?

post a picture of the mic board

Sorry typo, r9 is jumppered.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

dmp

Re: C12vr
« Reply #15 on: February 15, 2019, 12:20:21 PM »
Looks like they moded it to be more like a elam

Was R7 changed to 1.8k?

pucho812

Re: C12vr
« Reply #16 on: February 15, 2019, 02:20:31 PM »
I will measure/check r7 when I get back to the bench.
But here is the photo you asked about.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

pucho812

Re: C12vr
« Reply #17 on: February 16, 2019, 01:19:49 AM »
Looks like they moded it to be more like a elam

Was R7 changed to 1.8k?

no. 2k7 aka 2.7KΩ
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

pucho812

Re: C12vr
« Reply #18 on: February 17, 2019, 04:08:36 AM »
swapped out the zener and no change, swapped the transistor and no change. my b+ is 126V which I still get 60V at the junction and I still get the fade out in sound.  but 126VDC is o.k. as it is +5%  which the manual says is acceptable.   
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

pucho812

Re: C12vr New
« Reply #19 on: February 19, 2019, 12:23:37 AM »
well hazaa...

I talk with a friend who clued me in one some things.  R6 and c4 are tied to the same edge  pin.  We discussed the capsule. you can rotate the ring.  :(  thats bad.  it means the capsule needs to be replaced.  the B+  and the rest of the VDC is o.k.  so replace the capsule and good to go it seems.

 now what I am not sure of is why r6 jumpers over to a  the solder pad that the edge pin and c4 join at? Answer shared backplate on the capsule in the unit,  part of the mod, I will assume yes.
« Last Edit: February 19, 2019, 04:53:00 PM by pucho812 »
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.


 

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