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[API]

Hi.

I am working on a Philips version of the Schoeps CM61 mic.
When I got the mic the lower plexiglass plate that holds the transformer was broken and the transformers somewhat loose.
Today I took it all apart and glued the plexiplate back together and remounted the transformer.
While I had everything apart I replaced the 10uf electrolytic (Bias cap I believe) but I am now wondering about the 1uf/150v (output?) cap that is there as well.

Is it common practice to replace this cap?
From what I understand this is not an electrolytic but some other kind, does anyone know what kind?
The question is though if this cap ages as a electrolytic and needs replacing?
Would like to do that while I have the mic apart since it is a pain to do open it up like this.
And if I should change it, any recomendations for a replacement part?

Another thing I noticed is that the transformer is marked T14/1.
Is this the same transformer as in a AKG C12??

Any help apriciated.
See pics below.....

[klaus]

I am old-fashioned: Replace in a vintage condenser mic only what is provably malfunctioning. Leave the coupling cap be unless you get noise or hum. These are quite sturdy and usually do not deteriorate in any noticeable way.

Yes, the T14/1 was used in C12, C24 and ELA M250/251, as well as in the C28 a and b versions.

[API]

Hi Klaus.

Thanks alot for the advice.
Just as I thought, keep it as original as possible!
I just was not sure what kind of cap this is and if it aged in a bad way or not.

I have not heard the mic yet since I got it broken so I do not know if there are any more troubles.
That is one of the reasons I wanted to change anything that could potentially be a problem now that I have the mic open.

Will report back when it is up and running!

Thanks,
Dennis

[klaus]

I just was not sure what kind of cap this is and if it aged in a bad way or not.

If I read the smeared (+) marking right, it's a polarized electrolytic. Yes, it can fail, but, again, noise/hum will ensue if it ever does. If you are lucky, the date of the cap's manufacture may be printed on the other side, giving you a hint when the mic was made ± a year.

[Jim Williams]

I have a different perspective on this. I find old electrolytic capacitors to not have any redeeming value. They were not intended to last forever and of them will eventually need replacement. Since you already have the mic apart, you need to decide if this is the time or do you want to have to take it all apart again later. It's like what the car mechanic says about changing your oil, "you can pay me now or you can pay me later".

The other benefit is el caps are made much higher performance than they were just a few years ago, if you use some of these newer formulations. The size/package has also been reduced and they are much longer life than older designs. That should have them last another 40 years IF those newer formulations are used. You should be able to fit a 2 uf in there and that will extend the low end response down by 1 octave, even if you don't hear that, the reduction in phase shift should be beneficial.

Just because an old electrolytic cap has not yet failed, most of that age are now in a degraded condition and will affect sonics for several years before outright failure. Not being a gambling type, I would replace it.

[klaus]

I have no argument with Jim's perspective as long as you choose wisely: there can be enormous audible differences in Elco quality at a given value.

For example, I searched long and hard until I found a high value polarized electrolytic bias cap of high audible quality (and commensurate cost). 

If the old cap sounds and works fine, a replacement cap may significantly alter the signature of the mic- being such an integral part of most mic amp designs. So it will take effort and judgement and more effort to arrive at a point of one's liking.

[DigitMus]

It seems to me a sensible thing would be to cover all bases by replacing the cap, but retain the old one in case the replacement has a negative impact on the sound/performance of the mic.

Scott Gould

[klaus]

There you go!

[API]

So, I have had some progress here.

I decided not to change that cap after all, I wanted to listen to the mic first as i was.
So I wired up a cable and checked voltages which were all ok.
Plugged in the mic and measured voltages again and they were perfect and according to the schematic.

But, the mic is very noisy and output seems a little low, maybe a tad dark as well.
I tried three different tubes and it was the same (except for the original tube being very microphonic).
Could it be that this cap needs replacing after all, or could I have a problem with the capsule here??

One strange thing I notices was that the mic cable was quite microphonic as well, if I touched it there was loud noises coming out of the mic.
I have double checked all connections but it could be a bad cable of course (it is a vintage Neumann cable that I rewired).
Or could it be a grounding problem?
I never figured out if these old powersupplies should be rewired with a proper grounded powercord with ground going to chassis.
Is that a standard thing to do when refurbishing a powersupply??

Ok, that was alot of questions but I hope you bear with me here, would love to get this mic up and running.

Thanks,
Dennis

[klaus]

So, I have had some progress here.

...But, the mic is very noisy and output seems a little low, maybe a tad dark as well.
I tried three different tubes and it was the same (except for the original tube being very microphonic).
Could it be that this cap needs replacing after all, or could I have a problem with the capsule here??

When troubleshooting a case like yours, nothing beats substitution, first, on a large scale: amp and capsule, then, once that has been pinpointed, on a more micro-scale (tube? high impedance section? output section? and so on. For example: you may find that indeed you had three bad tubes, unless one came out of a quiet mic.

The first question I always ask when someone reports "noise" is: what kind? The road divides already with that information into: 'capsule related' and 'amp related'.
Is it white noise? Steady state? Periodic discharges? Or: Can the level and quality of noise be influenced by signal (popping a 'P' into the capsule)? and so on.

One strange thing I notices was that the mic cable was quite microphonic as well, if I touched it there was loud noises coming out of the mic.

In my experience, that almost always points to incorrect grounding and or shielding scheme of the mic cable. Mic cables need to be terminated to ground AND shield on both sides of the cable end. That combined ground/shield also needs to be attached to the connector's metal housing- again, on both ends.

I never figured out if these old powersupplies should be rewired with a proper grounded powercord with ground going to chassis.
Is that a standard thing to do when refurbishing a powersupply??

There are plenty of cases of vintage mics where the original AC wiring setup and schematics did not employ chassis ground connections to the third pin (because it had not yet been invented) in household wiring). In all cases, the mics' ground or noise level is not affected by the t(missing) third rail. And, if you were to install it, you would need to consider any ground loop potential in an otherwise unmodified power supply grounding scheme.

[rmburrow]

A little late, but my $0.02 worth:  If you haven't gotten that mic running yet, take a high impedance DC voltmeter and look on the cold side of that 1 uF/150 vdc cap for DC voltage (at the "hot" side of the primary of the output transformer).  There should be none.   If you have steady DC there, it is flowing to ground through the output transformer; that says the output cap is leaky and it should be replaced.  BTW you can probably order (from Mouser, etc.) a 1 uF 150 v mylar cap that fits the space requirements.  The mylar caps have better ESR characteristics.  If the cap in your mic is leaky, the dc in the output transformer primary is probably affecting the mic's sound adversely.  Doesn't take much DC to saturate the core of a small transformer.

[Kai]

...the mic is very noisy and output seems a little low, maybe a tad dark as well. ...
... One strange thing I notices was that the mic cable was quite microphonic

If the cable is highly microphonic you probably have one of these problems:

- a miswired or broken mic cable.
- oxydation on the connector contacts.
- a miswired or (less likely) broken output transformer.

Measure the resistance at the audio output of your setup, XLR pin 2-3.
It should be something around 10-40 Ohms.
If you don't see the above, measure step by step towards the output transformer to find the fault.

If this side is OK (I doubt) check the mentioned output cap like described by rmburrow.
A broken cap would cause low frequency noise.
What if you remove the tube, does the noise stay?
Same applies to the newly installed bias cap (is it reversed maybe, check the voltage?)

Noise can even origin from contamination on the high impedance part of the mic, where the capsule is connected.
Another source of problems is the center contact where the capsule is connected.

Regards
Kai