Repairing a TAC Scorpion Channel Strip (Capacitor Issue)

[thelangston]

Hey friends,

I'm attempting to repair a TAC Scorpion channel strip (S1000). I've procured a schematic and it appears that the pcb is riddled with these tiny capacitors, many of which appear to be blown. They have an underlined "10" on them, as well as what looks to be a tiny dot over the "1". The tops have a black coating. I'm assuming this means 10pF, as the schematic simply read "10P" Are these tantalum caps? I'm having a hard time sourcing them, as I can't seem to find any that look similar.

Any help would be greatly appreciated!

 

[Khron]

Those look like C0G / NPO ceramic capacitors. 10pF sounds about right, most likely in parallel with the feedback resistor (at least the one in the photo you attached) of that opamp. How and why it might've blown, i really couldn't say...

 

[thelangston]

Thanks man. Appreciate it! All the caps like this around the opamp have seen better days...

 

[Walrus]

A picture of that area from the schematic would make it clearer.
Can't see how a feedback capacitor could sink enough power to blow its top!

 

[thelangston]

Here is a bit more info...(schem & real life) there are several spots like this w/ blow caps surrounding TL072 chips. The ones in this picture look healthy compared to the others...

 

[Ike Zimbel]

That is really bizarre. I have NEVER seen a cap in that position fail violently (or maybe even at all...). It kind of looks like someone took a side-cutter to it, but, the black marks on pins 1&2 of the chip look like they could be burn marks...this is just really weird.
AMEK did have a problem with, IIRC, 0.1uf ceramic ps de-coupling caps in some desk...one of the larger studio desks I believe. My understanding of that issue is that they mistakenly received a batch of those caps that were a lower voltage rating than the supply rails, so they would fail short. Matt Syson or Brian Roth could shed more light on this, if they care to chime in. That said, that ain't this...

 

[radardoug]

Perhaps you applied really high power rails, in which case a lot more stuff should have smoked?

 

[thelangston]

That is really bizarre. I have NEVER seen a cap in that position fail violently (or maybe even at all...). It kind of looks like someone took a side-cutter to it, but, the black marks on pins 1&2 of the chip look like they could be burn marks...this is just really weird.
AMEK did have a problem with, IIRC, 0.1uf ceramic ps de-coupling caps in some desk...one of the larger studio desks I believe. My understanding of that issue is that they mistakenly received a batch of those caps that were a lower voltage rating than the supply rails, so they would fail short. Matt Syson or Brian Roth could shed more light on this, if they care to chime in. That said, that ain't this...

Yeah would be grateful for any tips on what to replace these with Not all the channel have this problem, so I’m hoping that it’s due to old caps in the power supply.

 

[thelangston]

Perhaps you applied really high power rails, in which case a lot more stuff should have smoked?

Haven’t made changes to the stock power rails. It came to me like this, so I’m slowly working through the recapping and rechipping process. Could possibly be the result of a power surge?

 

[Walrus]

Unfortunately your picture doesn’t show where the TL072 + input is fed from. The only way that cap can get enough voltage to blow up is by if the ‘072‘s had a catastrophic failure in the past and were then replaced.
The pictures are a bit unclear to me, can’t really make out details of the top of the cap. Looks just like they have been painted black!

 

[Khron]

Looks just like they have been painted black!

The black top is an indicator of C0G / NPO material in through-hole ceramic capacitors, but even those painted tops look "bubbled up" or burnt or something...

 

[Ike Zimbel]

You can replace them with COG 10pf monolithic ceramic caps. Should be a very common part. Are this strips in a working console?

 

[thelangston]

You can replace them with COG 10pf monolithic ceramic caps. Should be a very common part. Are this strips in a working console?

Yes. It's from a working console. All channel pass audio, but they all have their own quirks to them. Would any of these capacitors be a good replacement?

 

[thelangston]

You can replace them with COG 10pf monolithic ceramic caps. Should be a very common part. Are this strips in a working console?

Yeah, it's from a working console. All these channel strips are functional, but to varying degrees of quality.
Any idea on the voltage rating (50v,100v) that would be typical for caps in this type of circuit?

 

[Khron]

50v should be fine, but neither price nor physical size difference should be too significant if you go with 100v.

 

[JohnRoberts]

10pF and 10k ohm forms a pole up in the single digit MHz so even non-ideal dielectric behavior is unlikely to generate audible artifacts.

That said why worry about that just use a suitable 10pf NPO/COG capacitor.

edit- perhaps that cap was damaged by handling... /edit

JR

 

[thelangston]

Thanks John. This is great intel… I’ve been a bit overwhelmed with the capacitor choices on mouser and digikey… wanna make sure I’m putting the right stuff in here

 

[Brian Roth]

AMEK did have a problem with, IIRC, 0.1uf ceramic ps de-coupling caps in some desk...one of the larger studio desks I believe. My understanding of that issue is that they mistakenly received a batch of those caps that were a lower voltage rating than the supply rails, so they would fail short. Matt Syson or Brian Roth could shed more light on this, if they care to chime in. That said, that ain't this...

I heard two different explanations for the random cap failures. In the long run, the best story seems to be just a sketchy batch of caps since the failures were random. They were a 0.1 uF mono ceramic with a pair across each opamp's rails.....so a jillion of them in the desk.

Bri

 

[thelangston]

Really appreciate all the help with this. I have the power supply, master section, busses and most of the channels all recapped and it’s sounding really good with the updates. I do seem to be experiencing a dead channel 10. I’ve tried multiple working channel strips in this section and for some reason it’s not passing audio. All the other channels show clipping when I push them, but this one just has the faintest of sound that comes through it. I’ve taken deoxit to the input (which seems fine and identical to those around it). I thought maybe it could be the fader, but it should at least show the channel clipping (if it’s getting audio) regardless of the fader position. Anybody else run into this?