K89 / TLM 193 Project

[soliloqueen]

it looks black. maybe it's a spray on coating?

 

[Gus]

I would guess
aluminum anodizing for electrical isolation

I found a link
https://semanoinc.com/anodizing-for-electrical-insulation/

 

[soliloqueen]

I would guess
aluminum anodizing for electrical isolation

I found a link
https://semanoinc.com/anodizing-for-electrical-insulation/

oh! maybe that's why the surfaces that need to be conductive look to have been sand-blasted post assembly rather than before? to remove the layer of oxide from just that area? I should be able to replicate that! thank you for bringing it up!
Here's the full model as-is:

 

[svyet]

This is exciting…

In addition to the anodizing, would there be any benefit (and enough space) to add a thin layer of something soft and non-conductive (some kind of liquid gasket) that could help with capsule resonance, or would that be useless or cause problems?

 

[svyet]

Before getting into the mic to change the coupling capacitors, I’m contemplating a full re-cap so all components are guaranteed within spec and starting with a fresh lifespan. I will measure all the old components anyway, but if I need to pull them to do that, I might as well change them since they are also old. That said, I’m looking for proper direct replacements. All seem to be high-quality C0G except for C1 which I am having a hard time figuring out in general, and of course C2 and C3 being aluminum electrolytic can types.

Any help with all of these would be very much appreciated. I’ve scoured the net and have still fallen short.

- - -

Here is the TLM 193 schematic.

And here is what I’ve documented from the circuit board:

C1
“K5K —4”
(first two characters obstructed) assumed 104 (10e4) based on 0.1 written on schematic. Found possible explanations: Pico Digital, 100000pF / 0.1 uF and alternatively, K5K interpreted as K=Kemet/5=50V/K=+/-10%
Voltage and tolerance rating are not confirmed.

C2 + C3
SME 50V 47 uf (M) 85oC 41 (9)
(Already figured these out and found a few replacements to try)

C4 + C5
“22J [marking] COG100”
(22 pF, J = 5% tolerance, C0G100=100V)
Voltage is in fact 100%?

C6 + C7
“221 J5A”
(220 pF, J = 5% tolerance, J5A (a Murata series which is C0G 50V).
(All but confirmed, particularly voltage rating).

- - -

Would like to know why some might be 100V and some 50V. Is that dependent on its place in the circuit? I assume 50V reflects phantom power, hence why the coupling capacitors are rated as such. Would it be prudent to simply use 100V and lowest tolerance for all (including the coupling caps?), as long as capacitance is correct? I’ve read that it’s a common insurance practice, but I want to be sure. I’m fine with overbuilding.

Is it good practice to also change zener diodes and resistors?

At least the zener in this case, given its apparent relationship with the coupling capacitors (bridging the negative sides of C2 and C3). The zener on the board has “BZY 97C 12V” which seems to be a 12V 1.5W zener (see BZY97C12). When choosing a zener, which specs are rigid, and which ones are flexible (e.g. power dissipation, resistance, leakage current, test current, zener current, etc.)? I can't find an exact match on Mouser based on the linked spec sheet, but I did find two that match except for having a higher power dissipation (3W and 2W, I assume a good thing), lower impedence (both 4 Ohms, I assume also a good thing), and a higher zener current (this one I'm not sure..236mA and 157mA compared to stock 118mA). The one with specs closer to the original is ZY series, so I assume this would simply be an updated direct replacement? Any harm in choosing the slightly higher spec one or does it really matter?

Also curious why they would leave the zener lead so long (see pic in post), i.e. if that would be intentional or not either for electrical or physical reasons (needing to bend it quite a bit to match the lead spacing on the board and needing to keep it long as to not tension things too much - or perhaps helping with heat dissipation?).

Thanks—

 

[Khron]

Would it be prudent to simply use 100V and lowest tolerance for all (including the coupling caps?), as long as capacitance is correct?

... And as long as the higher voltage ones can still be found in the same case size.

Is it good practice to also change zener diodes and resistors?

Why? Unless they've been determined to be the cause of unusual noise, why would you?

 

[svyet]

... And as long as the higher voltage ones can still be found in the same case size.

Ok, and if not, I could simply mount the C0G caps on the other side of the PCB, right? There is absolutely nothing on that side sticking out, and tons of room inside the mic. As for the aluminum electrolytics, I'm sticking with the size and specs of the ones currently on the board. But even if I did want to choose larger ones, there is plenty of room to mount bigger ones horizontally as I've seen done. But it's best to keep the leads as short as possible, right?

Why? Unless they've been determined to be the cause of unusual noise, why would you?

That answers my question But I'm still curious about zener specs for future reference.