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 1 
 on: Today at 03:50:35 pm 
Started by duskb - Last post by Derek Reese Music
Hello Klaus, do you have a link to these M3 screws ? i have looked online and found the right size, but they have a flat head which means they will be sticking out which doesn't look nice.

 2 
 on: February 18, 2020, 10:36:38 am 
Started by Eddie Eagle - Last post by Eddie Eagle
new 2 spring replacement was only $40 so it was an easy fix to replace.

 3 
 on: February 16, 2020, 09:48:20 pm 
Started by RuudNL - Last post by klaus
Brilliant summary.
Thank you!

 4 
 on: February 16, 2020, 08:25:16 pm 
Started by RuudNL - Last post by Kai
Evaluating the health of capacitors under working conditions has never been easy (at least no for me):
Simulating work i.e. what happens when a stressor is applied, does not compute under static capacitance test conditions (with a meter).

I'd gladly take some lessons from someone who knows how to improve capacitor testing under real-world conditions, preferably while they are still installed in the circuit.
A universal way testing capacitors in-circuit does not exist.
There are too many different purposes a cap can serve inside a circuit.

And there are too many parameters that might be wrong in a broken or half-broken cap:
- loss of capacitance
- loss of isolation, current leakage, shortcut
- loss of voltage capability, temporary or permanent (kind of similar to above, but different symptomatic)
- increase of series resistance
- and the easiest to identify: physical leakage of acid from electrolytics.

Audio coupling caps are relatively easy to examine in an active circuit:
measuring the AC voltage across the cap (best done with an oscilloscope) should result in very low to zero values, except for very low frequencies.
DC leakage shows up by residual DC on the output side, e.g. at the input of the Xformer of a mic it should simply be 0mV.
In general, if a DC voltage inside a circuit is off, caps are usually under the first suspects.

Similar applies to power supply filter caps, you should be able to observe the usual rounded ripple wave form, else they are dried out.
When into this, it's time to check if the frequency of the ripple is consistent with the type of rectifier used, often one of the rectifier diodes is broken which shows up here.

Oscilloscope (non grounded like the Fluke handhelds) + signal generator are the best tools to trace and measure signals in an audio circuit, and with a bit of thinking it's mostly possible to find the primary suspects.


Having a vacuum desolderer makes the next step easier, external component testing:
https://smile.amazon.de/Regelbare-digitale-Entlötstation-ZD-915-ESD/dp/B00T6FYP1A/ref=sr_1_5?__mk_de_DE=ÅMŎÕÑ&keywords=Entlötstation&qid=1581899463&sr=8-5

For component testing I acquired this, and it's astonishingly good to work with at that price:
https://smile.amazon.de/gp/product/B07C5LC486/ref=ppx_yo_dt_b_asin_title_o03_s00ie=UTF8&psc=1


For testing of components that need higher voltage I have a very, very old Bruel&Kjaer 2423 Mega-Ohm-Meter that can apply up to 100V (up to 1000V with external feed) to a cap and exactly shows the leakage/isolation value up to 10 Tera-Ohm (!), even on foil caps, capsules, cables, everything.
If a foil-cap passes this test, it's clearly not broken.


If I want to go one step further, I can measure the harmonic distortion of a cap down to -155dB or 0,00000002%.
There is no cap (except some special WIMA types) that does not show distortions in this test, -120dB to -140dB is the common range.
The measurement is at the edge of physics and time consuming, not an every day task.


Final thought:
Of course it's often easier just to replace a suspect part, but if you work on historic originals that's not always an option.
Having the capability to thoroughly test a component is mandatory in such cases I think.

 5 
 on: February 16, 2020, 04:41:59 am 
Started by RuudNL - Last post by RuudNL
About the capacity of the CK28 capsule:
According to BBC Report No. L-048 ("A.K.G. Electrostatic microphones type C26 and C28") dated December 1961: "the type CK28 capsule has a capacitance of about 25 pF".

 6 
 on: February 15, 2020, 07:43:53 am 
Started by RuudNL - Last post by klaus
Evaluating the health of capacitors under working conditions has never been easy (at least no for me):
Simulating work i.e. what happens when a stressor is applied, does not compute under static capacitance test conditions (with a meter).

I'd gladly take some lessons from someone who knows how to improve capacitor testing under real-world conditions, preferably while they are still installed in the circuit.

 7 
 on: February 15, 2020, 05:25:11 am 
Started by RuudNL - Last post by RuudNL
Yes, of course I measured the voltage under load!

Problem is solved now. Because I didn't trust some components, I replaced R1, R2, R3 and C2 (10nF).
After this, during a quick test, my voice sounded much more natural than before, no more 'telephone sound'.
A measurement showed a tremendous improvement: level at 40 Hz had increased by 10 dB...
It is still not clear what has caused this problem, because the 10 nF capacitor that I removed measured the correct value...
The only thing to replace now is the nuvistor, because the microphone seems noisier than it should be.
But that should be no problem, because a couple of NOS nuvistors are already ordered.

 8 
 on: February 14, 2020, 05:38:11 pm 
Started by RuudNL - Last post by afterlifestudios

When I checked the +HT of the (not official) power supply, I measured a dangerously high voltage of 167 Volts!
This means that the polarisation voltage will be in the order of 83.5 Volts, instead of the desired 60 Volts.
So: the first thing I am going to do is to reduce the +HT voltage to +120 Volts.

Measured under load (microphone connected), right? 

 9 
 on: February 14, 2020, 03:06:20 pm 
Started by RuudNL - Last post by klaus
You indeed forgot to mention that small little detail: bad voltage from power supply!
So much for a rigorous troubleshooting protocol.

Best of luck, and please keep us posted.

 10 
 on: February 14, 2020, 02:51:44 pm 
Started by RuudNL - Last post by RuudNL
In the meantime I have done some more tests with the AKG C28.

- Although the capsule capacity (~18 pF) might be a bit low, the capsule performs normal on an AKG C451 body, with a full low end.
- When I inject a signal to the capsule input (the center connection) of the C28 body, through a 15 pF capacitor, I measure a frequency response that is only 1 dB down at 40 Hz.

But...

When I checked the +HT of the (not official) power supply, I measured a dangerously high voltage of 167 Volts!
This means that the polarisation voltage will be in the order of 83.5 Volts, instead of the desired 60 Volts.
So: the first thing I am going to do is to reduce the +HT voltage to +120 Volts.

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