Newbe to GroupDIY needs help with Vintage Mic collection (AKG C-12, C- 24, Telefunken ELA M 251, Neumann U-47, U-64, U-67 & 12 more)

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Ivan K.

Well-known member
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Dec 15, 2021
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Location
Boulder, CO USA
Greetings from the Colorado mountains. I have reclaimed a microphone collection of mine that's been in storage for about 25 years. I've just started to go through them to make certain that they still work, and possibly replace some old parts. Some questions for the working recording engineers and audio professionals;
1. Philosophy: I have started with my AKG C-12 power supply & pattern box. All the PS caps seem fine, but there's a Selenium rectifier and a point contact diode that for reliability's sake, I'd replace, and possibly the caps too. I'd have to adjust the rectifier's output to compensate for lower Vf drop with silicon devices.

The question is, would a buyer in today's market prefer an original kit that works for an unknown time, or an updated, more reliable version? Of course, there will always be arguments about changing the sound even if identical value caps are used and the voltages maintained...

2. I did find a C-12 schematic , but no resistor values for the pattern box. I'd like to check them but most will require desoldering to view their values. They are of the crimped end cap type and I don't want to move them either. Does anyone have these values (and schematic)? Then, I can do a simple resistance measurement.

3. Finally, there is a 2 uF cap from pin 3 to ground (see attached schematic). The top of the cap is in series with a 1M2 resistor to the switch wiper and also goes to one side of the capsule via a 30 M resistor, with a 5000pF cap to ground. I'm not sure I understand what the function of this NP electrolytic cap is and would really appreciate help!

Many thanks,
Ivan
 

Attachments

  • AKG C-12 Schematic .jpg
    AKG C-12 Schematic .jpg
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Selenium rectifiers tend to have a quite high forward voltage drop , why not just sub in a modern silicon diode and a resistor(s) that gives an equivalent output voltage , I see no reason not to increase capacitance values a bit with modern components for a lower ripple voltage while your there , some may argue that point , it should last another lifetimes use with good quality components either way.
 
Thanks for the replies so far. If I were using these mics professionally, I'd want to upgrade everything I could in the interest of reliability--session time is expensive, and changing to a different mic after recording several tracks may well upset the talent! But, I'd hate to change the sound of these mics. I find it unlikely that PS changes that respect the original voltages, and even improve 70 year old capacitor ripple and hum would be objectionable, but there is something to be said concerning " If it ain't broke..."
 
Recently I overhauled a number of KM56/U47/U67 power supplies.
After more than 60 years all electrolytic capacitors had developed an unacceptable leak current, resulting in mains hum and too low voltages.)
(Not to speak about the infamous electrolyt leaking 'Stabilyt' cells, that ruin the PCB...)
 
I'd be interested in what caps you used to replace the cans. To preserve the looks, it would be nice to use replacements, but I doubt that any can type caps will keep up with the 105C and low ESR ratings of modern electrolytics. Possibly fitting new caps under the chassis instead of on top might work if there's room. What did you find?
Thanks,
Ivan
 
I removed the 'screw type' electrolytic capacitors and used modern radial capacitors.
Mounted on a PCB at the same distance from each other as the old ones. (So they protrude through the holes.)
The PCB is under the chassis, a strip of insulation materal is between the capacitors and the chassis, above the chassis.
Functionality and reliability are more important for me than the 'looks'!
(How often do you open the power supply to look at the inside?)
 
If you want a U47 but it doesn't work, you aren't going to cancel a session and tell them to come back in a month while you get it rebuilt. "Vintage" is great until vintage fails, then it's lost time at best, lost customers at worst. For my use, a minimal rebuild would include replacing selenium with silicon and all PS electrolytics. That should not affect the "vintage sound" of anything (unless you count hum as part of vintage sound -- we heard plenty of that in the 60s). From there I would just fix what needs fixing. I used to totally recap gear but I think it does more harm than good, especially on old PCBs that want to disintegrate when they get a whiff of a soldering iron. When you get into the guts of things, doing the least is usually best.
 
I'd be interested in what caps you used to replace the cans. To preserve the looks, it would be nice to use replacements, but I doubt that any can type caps will keep up with the 105C and low ESR ratings of modern electrolytics. Possibly fitting new caps under the chassis instead of on top might work if there's room. What did you find?
Thanks,
Ivan
I use FTcaps can caps, they are a bit expensive,but it keeps the vintage look and value of the micsFB_IMG_1639837818468.jpg
 
Thanks for the mention of FT Caps. Unfortunately, not common in the US. They look nice. Most common here are the Japanese brands like Nicicon United Chemicon or Rubycon. Also, EPCOS, Panasonic, etc.

I agree with RuuDNL & Swpaskett, that reliability outweigh looks! But, we do need to take care with old PCBs and parts.

Thanks,
Ivan
 
3. Finally, there is a 2 uF cap from pin 3 to ground (see attached schematic). The top of the cap is in series with a 1M2 resistor to the switch wiper and also goes to one side of the capsule via a 30 M resistor, with a 5000pF cap to ground. I'm not sure I understand what the function of this NP electrolytic cap is and would really appreciate help!

C20 is a decoupling cap for rear polarization. Helps keep polarization steady.
 
Recently I overhauled a number of KM56/U47/U67 power supplies.
After more than 60 years all electrolytic capacitors had developed an unacceptable leak current, resulting in mains hum and too low voltages.)
(Not to speak about the infamous electrolyt leaking 'Stabilyt' cells, that ruin the PCB...)
RuudNL, did you work on the NKM supplies for the KM56 mics? I found that AA rechargeable batteries replace the leaky "Stabilytes" nicely. The AA cells also hold up the heater voltage in the event of a short duration mains interruption. The HT filter caps hold charge for short duration mains interruptions also. I like the NKM better than the Telefunken Ela M931 supplies which are loaded with selenium rectifiers.
 
I have not worked on a supply with Stabilyt cells so I don't know if what I have read on the web is true.

What I read is the Stabilyt cell were NiCad like cells that did not hold a charge.
My guess is they are kind of like a chemical based shunt voltage regulator, super cap like mix.
I have been unable to find any detailed information on the Stabilyt cells

rmburrow
Keep in mind the AAs will charge and as they approach full charge the voltage will increase to 1.4VDC or even higher. This depends on the charge current and voltage.
Also constant charging when fully charged is not a good way to use NiCads or NiMHs
 
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