Carvin BX250 micro bass capacitor voltage rating question

[DrGonz78]

Probably a stupid question... But oh well got to just have a second opinion that this will work. The amp symptom is a nasty hiss and every single pot has an effect on the noise, even the active/passive switch. So I narrowed down my scope to everything before that section of the preamp. Looks like C66 (1uf 63v capacitor) on the preamp is measuring 80uf and is obviously out of whack. Anyhow I have this surface mount cap marked 1uf 50s which makes it's voltage rating at 50v. Now the orginal is 63v but the power supply to the cap is only 22vDC. Not good with SMPS type power supplies so just wondering if the cap needs the over rated voltage specification? Will I be okay with the 50v cap? Thanks for putting up with me lol.

[MarkusBass]

The cap is on the 22V power supply rail. It's hard to believe that its capacity has increased to 80 uF. How did you measure it - in circuit? If this is the case, you should know that it is connected with 2R2 resistor to other capacitors and you may be measuring other capacitors and not this one only. I think that voltage rating is not that important (as long as it is over 22V). I suspect that the problem is related to some other component that failed. But in order just to be on the safe side, I would desolder the capacitor and solder even a through-hole capacitor (temporarily). I would rather remove "serial" capacitors - like C122, or C123 and see what happens in this case.

Mark

[DrGonz78]

Well the cap is actually good and just overlooked that C66 is in parallel with C4 so that is why the capacitance read higher. I was measuring other 1uf in the circuit to compare but they were in series. Doh

I asked about voltage rating with smps just out of curiosity if there was a voltage spike or something at power up etc... So that question is good but I am probably over thinking.

Well I guess I need to scope through that part of the circuit to see where the noise is located. I am eyeing that Q19 and will report back later.

[MarkusBass]

If you like risk and adventure, replace Q19 and check results. If you want to know a little bit more before replacing any parts in the amp, disconnect C122 , C123 and report results.
And yes, you are over thinking the problem with voltage rating and SMPS power supply

Mark

[DrGonz78]

So I removed C122 and C123 and the awful hiss has disappeared. I guess that leaves Q19 as a potential suspect for the cause of the hiss?

[DrGonz78]

I reinstalled C122 & C123 and removed the Q9 of the board. The noise is actually still present but not quite as loud. This thing is really hard to work on and smd components are a real pain in the ass. Oh well I will take it slow and see what I can do but this type of amp is just crazy!

[DrGonz78]

Well I signal traced some of the signal paths from the input jack all the way through to after C123 cap. I am not hearing the hiss on the signal trace through any of those areas of the board. I think it is strange that disconnecting C122 and C123 got rid of the hiss almost completely. Still I am unable to trace the location of the noise in the signal path. So tomorrow I will try to just remove C122 to see if that removes the hiss while I have Q19 removed from circuit.

[MarkusBass]

You should remove/add the components one at a time. For SMD components you need special tools. Without the tools you may destroy the PC board.

Mark

[DrGonz78]

You should remove/add the components one at a time. For SMD components you need special tools. Without the tools you may destroy the PC board.

Mark

Yup I need to buy hot air station and get some better magnification methods. So far I have done okay with a very small 10 watt pencil but it still sucks. While you typed that I was just browsing the web looking for prices on those exact special tools. What types do use and what should I avoid?

[MarkusBass]

If you work with SMD boards very seldom, I wouldn't invest any money in specialized tools. I would buy Chipquik: Chip Quik - and solder wick (you probably have it). With these two items you can do everything you need (without spending to much money).

Mark

[J M Fahey]

1) also strongly suspect Q19 ; hiss in general is created by bad *active* components.

2) in these cases, rather than removing caps, I ground some specific points with a short piece of wire ... WAY less invasive.

I'd alternatively ground Q19 gate and Q19 drain, in this case after C123 to avoid loud pops.

This can give you a lot of information in 30 seconds, without messing with the board.

Loved the amp design, classic Carvin Fet design and EQ, and very simple SMPS and power amp.
Way to go.

[DrGonz78]

So I applied JM's grounding technique to my trouble shooting procedure and this is what I get. Grounding out along the signal path all the way up to the very input jack silences the hiss or noise. So effectively grounding signal path before the gate and after the drain on Q19 and Q8 gets rid of my hiss problem. I have attached a picture of the points that grounded out and those are the top four red marks on the cropped schematic image. So I tried to ground out pin 5 of op amp A5b and it made a very loud pop noise.



Edit: Also note that I pulled the input jack board to look for C62 and I don't see that part installed or a place for it. However it is on the schematic.

[Enzo]

With C60, they may have found it superfluous and omitted it. Or since it is so small a value, it could be incorporated in the copper tracery as a "gimick" cap. An examle of that is the PV Classic 30 between plate and grid of first triode. Not on schematic, but look at the layout picture or the board itself.

[Enzo]

Does the hiss remain with a guitar plugged in and its volume control to zero? I see the input jack does not ground the tip, it places a 1k to ground from it.

[DrGonz78]

Actually I think the C60 is this strange little plastic board piece but yeah its not that important. On with the symptoms, with the jack unplugged the hiss is present and master volume will cut out the hiss completely when at zero. Same goes for when you plug a cable in and the hiss actually gets way louder. All of the controls effect the hiss- Gain, Active/passive, all EQ etc. Main point even with guitar plugged in volume off (guitars volume) it is still hissing like mad. Master volume will kill the hiss in every situation. As you insert the cable into the guitar and it touches the tip to ground well that kills the hiss. Just like when I grounded out the signal path in my previous test.