Output capacitor bad on C12/251 clone?

[tesco_1]

Can anyone give me a quick opoinon on this:

I have a c12/251 clone that can switch between the c12 and 251 voicing, and internal preamp for line operation. its a point to point build, but due to the design it also and limited space in the mic chasis, I believe that the output capacitor in the power supply.

After inspecting the power supply I see that one side of the .47uf capacitor looks slighly burnt (and a few bubbles) as shown in the pics attached.

I dont have a multimeter on me, so I can't test it. But based on pics does this look like a failed capacitor?

 

[Khron]

Not quite sure what to make of that. If a film cap failed (which that, i believe, is), i would expect scorch marks and such. Could that be some sort of glue or resin? And can you be sure that came from within that capacitor?

One thing's for sure though - that is NOT a 47uF film capacitor... What is it even connected to?

 

[tesco_1]

Not quite sure what to make of that. If a film cap failed (which that, i believe, is), i would expect scorch marks and such. Could that be some sort of glue or resin? And can you be sure that came from within that capacitor?

One thing's for sure though - that is NOT a 47uF film capacitor... What is it even connected to?

1. It is straped across the input and ouput of the power supply, (see attached)

2. correct its not '47uf', but a .47uf (0.47) film capacitor, found several links to buy the same type here: Ebay, Another Ebay link, Mouser (I fixed the typo above)

4. If you look at the close-up photo attached, the burn mark appears to come from inside of the cap. The lead looks a little burnt from the inside.

 

[Khron]

That's more credible, but i still don't at all get where and how that's supposed to be connected. Maybe if you had (or drew up) a schematic?

PS: If the output transformer is inside the mic, that is quite definitely not the "output capacitor".

 

[tesco_1]

That's more credible, but i still don't at all get where and how that's supposed to be connected. Maybe if you had (or drew up) a schematic?

PS: If the output transformer is inside the mic, that is quite definitely not the "output capacitor".

The builder is Aco Razbornik. You can find the mic here Studio Tivoli Productions | Audio Mixing, Mastering, Recording, Editing, Restoration (under the products section). I wish I had a schematic, but Aco is Slovenian, and doesnt speak much english. And dealing with his english 'sales guy' didnt yelid and real answers to any of my technical questions.

Looking at the power supply again, the capacitor is attached to the mic input xlr (not output) of the power supply. I assume it is the output cap, as there is nowhere else in a c12/251 circuit (I am aware of) that would call for a .47uf film cap. Also, I dont see an ouput cap anywhere on the microphone, the transformer is housed in a bell chasis attached at the bottom of the mic.

Attached are pics of the Mic and power supply. I attempted to make a bootleg diagram of how the mic is put together based on what I can see.

1. The components are point to point and wrapped around the tube socket. The mic shipped with a chinese capsule, but I swapped it out for a Tim Campbell CT12.

2. I believe the box highlighted in red is the preamp that allows the user to switch the mic between line level and 30+ db.

3. the other switch is the switch that allows the user to select c12 or 251. This switch appears to be connected to a polystyrene 470pf capacictor (which is painted red).

4. The transformer is housed in the bell chasis at the bottom of the mic.


HERE ARE HI RES PICS, as groupdiy downsizes the photos:

 

[LevinGuitar]

I believe that the white/gray component with the printed sheets (that you marked as a "preamp") is the output capacitor. And maybe the 470pf capacitor is the only difference between the C12/251 "voicing"? That small talantum cap could also be a part of the 251 voicing too (3.3uf in that case), but I guess is the tube cathode biasing capacitor, as I don't see more caps

 

[doqmemory]

I believe that the white/gray component with the printed sheets (that you marked as a "preamp") is the output capacitor.

Yes, that is clearly the output capacitor (wired between tube plate and output transformer primary).

 

[Khron]

So is the problem that the OP is encountering, audible? Or purely visual?

 

[Gus]

looks like a Mallory 150

not a fan of the green printed 6072a tubes from the 80s

Power supply build
It looks like at least one bracket on the transformer is not bolted to the case
What 3 trem regulator is used
Are the blue caps mult-section caps, one for the B+ and other for the heater supply?


As LevinGuitar and doqmemory posted
Is the output cap under the folded up cardboard?

It make no sense to place a transformer in the microphone and the output cap in the power supply

 

[tesco_1]

I believe that the white/gray component with the printed sheets (that you marked as a "preamp") is the output capacitor. And maybe the 470pf capacitor is the only difference between the C12/251 "voicing"? That small talantum cap could also be a part of the 251 voicing too (3.3uf in that case), but I guess is the tube cathode biasing capacitor, as I don't see more caps

Yes, that is clearly the output capacitor (wired between tube plate and output transformer primary).

So is the problem that the OP is encountering, audible? Or purely visual?

Thanks you guys for your feedback so far.

So the big grey box is a capacitor... lol. I'm so used to seeing cylindrical K40 style russian output caps that I never even considered that grey box was a capacitor! It makes perfect sense as there are leads coming out of its housing. I was also thrown off because the giant capacitor doesnt have any markings on it, so I had no idea what it was.

1. So if the big grey capacitor is the output cap, then whats with the .47uf cap attached to the mics xlr input?

2. @Khron , So there are not any noticable problems with the signal, as much as I have no reference for how this mic is supposed to sound, especially with the change of capsule. I used to have two of these mics, but the second one was stolen before I installed the CT12 into the current one.

There are a few things that I am curious about:

1. when switching between C12 and 251 voicings, The ouput level of the mic changes. The C12 setting is mics output is lower. When swiched to the 251 the ouput is 5 db louder in contrast. Since I don't have the other mic anymore I can't tell if this behavior is normal.

2. I'm used to hearing C12 mics sound a little brighter, (or maybe leaner) but I've also heard some say the C12 is more neutral. Since there are so many versions and changes to the C12 its hard to pin down what a C12 should sound like. I've also heard Tims Capsule also has more bottom end than other C-12 capsules in current production. I can say with Tims Capsule the bottom end is much larger than with the original capsule. The C12's from Telefunken USA defintely seem to be leaner in the low end. The 251 voicing sounds like expected.

3. The main reason I checked the power supply was the thought that the C12 side seem to be 5 db lower, and I had noticed before there was that .47uf cap on the input of the mic, so I decided to take a look at it, and noticed it had a little burn mark on it.

 

[tesco_1]

looks like a Mallory 150

not a fan of the green printed 6072a tubes from the 80s

Power supply build
It looks like at least one bracket on the transformer is not bolted to the case
What 3 trem regulator is used
Are the blue caps mult-section caps, one for the B+ and other for the heater supply?


As LevinGuitar and doqmemory posted
Is the output cap under the folded up cardboard?

It make no sense to place a transformer in the microphone and the output cap in the power supply

Yep I think your right, the print on the capacitor looks the same as the malory 150's.

Also that's not a 6072a, its a JAN GE 5751.

I have some newer 6072A's from EH, but the 5751 seem to have less distortion in the highs. NOS GE five star 6072's are quite expensive, not sure I want to drop $175 on one at the moment.

 

[Khron]

Thanks you guys for your feedback so far.

So the big grey box is a capacitor... lol. I'm so used to seeing cylindrical K40 style russian output caps that I never even considered that grey box was a capacitor! It makes perfect sense as there are leads coming out of its housing. I was also thrown off because the giant capacitor doesnt have any markings on it, so I had no idea what it was.

1. So if the big grey capacitor is the output cap, then whats with the .47uf cap attached to the mics xlr input?

2. @Khron , So there are not any noticable problems with the signal, as much as I have no reference for how this mic is supposed to sound, especially with the change of capsule. I used to have two of these mics, but the second one was stolen before I installed the CT12 into the current one.

There are a few things that I am curious about:

1. when switching between C12 and 251 voicings, The ouput level of the mic changes. The C12 setting is mics output is lower. When swiched to the 251 the ouput is 5 db louder in contrast. Since I don't have the other mic anymore I can't tell if this behavior is normal.

I'm quite sure the grey capacitor DOES have markings, they're likely just obscured by that label(?) carrying the labeling of the switches and tube.

The mystery of the yellow capacitor would be quickly solved by lifting even part of the schematic of the power supply. I'm quite sure it's nothing overly complicated.

Regarding the level drop - as you yourself admit, how do you (or we) know whether that's normal or not? Is asking its maker not an option?

 

[rock soderstrom]

I suspect the yellow Mallory Cap is part of the PSU, probably in parallel with the last, blue filter electrolytic capacitor of B+.

I have never seen a foil cap that dies/died like that. However, I must admit, it does not look healthy.

Since the Mallory is fairly inexpensive, I would just swap it out with a new one, just to be safe.

It won't change anything about the signal. Your level drop probably comes from the changeable voicing, perhaps implemented via a switchable cathode capacitor, which would explain the different levels. Just a guess...

 

[tesco_1]

I'm quite sure the grey capacitor DOES have markings, they're likely just obscured by that label(?) carrying the labeling of the switches and tube.

The mystery of the yellow capacitor would be quickly solved by lifting even part of the schematic of the power supply. I'm quite sure it's nothing overly complicated.

Regarding the level drop - as you yourself admit, how do you (or we) know whether that's normal or not? Is asking its maker not an option?

1. So you are right again, the grey capacitor does have some markings on it, on the bottom. Unfortunately it looks like Aco removed the printed identifiers (prob with acetone) see attached pic. From previous talks, and looking at his builds, I get the idea that he doesn't want his "secrets" out there, even thought I'm sure it wouldn't be to hard to reverse engineer the circuit.

2. I emailed him yesterday to see if he could give me any insight on the differences in ouput between the two voicings. I'm sure he'll get back to me eventually.

3. So given that the output cap is an unknown vaule, how would I go about changing it if I wanted to> should I remove it and attempt to test its capacitance?

 

[Khron]

Well, desoldering it would be a useful first step Although simply for measuring its value, disconnecting only one of the legs would be enough, provided you can access the other connection as well.

 

[Gus]

That is funny the marking were wiped off. I would guess .47uf to 1uf

I would guess the 251 has a bypass cap across the cathode resistor and the 12 lifts on leg of the bypass cap. rock soderstrom posted the same idea/guess.

A 5751 has higher plate resistance than a 12ay7, so why the 5751?

"I get the idea that he doesn't want his "secrets" out there, even thought I'm sure it wouldn't be to hard to reverse engineer the circuit."
yes it should be easy to reverse it looks like a standard triode plate out circuit unless it is using both triodes
Guesses
2 in || more plate to grid cap and more like a charge amp
plate out to CF
If there is a CF that is most likely is the line level out

The PS build
how are the caps held down?
I think I see a missing bolt for one of the transformer mounts.
Is the steel case used as the what looks to be 3 term voltage regulator?
Are all the voltage connections covered with some kind of insulation at the transformer?