What could be the Filter capacitors of this circuit?

bikingbuddha

Well-Known Member
I have no idea how to figure out. What could be the filter capacitors of this amplifier circuit? it is an tube hybrid called Qinpu Q2. I hope to fix the hum / buz it has by replacing the filter caps.

IMG_20180910_132333.jpg
 
Really hard to tell just by a picture of the insides. I would need to see a schematic to say for sure. I doubt that changing the filter caps would solve the problem when and if you find out which ones they are. More than likely the filter caps are the large electrolytic caps I can see. Be sure all your gear is hooked up to the same wall outlet. If not you may have a ground loop problem. You could short out the inputs to see if the noise is internal or from an outside source.
 
Without a schematic it's sort of tough from just a picture .
Semi educated guess .
The bottom right in your image appears to be PS.
It looks like 4 diodes with caps across them .
So I'd guess the 5 caps in that corner.
Looks like a voltage regulator in that area too.
Also wondering about perhaps a cold joint on a ground causing your hum .
Also a guess.
 
Without a schematic it's sort of tough from just a picture .
Semi educated guess .
The bottom right in your image appears to be PS.
It looks like 4 diodes with caps across them .
So I'd guess the 5 caps in that corner.
Looks like a voltage regulator in that area too.
Also wondering about perhaps a cold joint on a ground causing your hum .
Also a guess.

As technicians always say, working without a schematic is like working in the dark. Especially when the circuit is on boards. Point to point tube circuits are pretty easy to figure out but it always helps to have a schematic even with them.
 
Really hard to tell just by a picture of the insides. I would need to see a schematic to say for sure. I doubt that changing the filter caps would solve the problem when and if you find out which ones they are. More than likely the filter caps are the large electrolytic caps I can see. Be sure all your gear is hooked up to the same wall outlet. If not you may have a ground loop problem. You could short out the inputs to see if the noise is internal or from an outside source.

Yep, they are all hooked up to the same electric outlet. The noise starts when tube lights up which probably is the preamp stage. I tried nearly ten to twelve different tubes which are already in use in my other amp and the ones that I have as new. Problem is still there. It sounds much worse in low impedance earphones and earbuds, on my hd650 it's there in the background but not audible as in the others.
 
Without a schematic it's sort of tough from just a picture .
Semi educated guess .
The bottom right in your image appears to be PS.
It looks like 4 diodes with caps across them .
So I'd guess the 5 caps in that corner.
Looks like a voltage regulator in that area too.
Also wondering about perhaps a cold joint on a ground causing your hum .
Also a guess.

The big caps right? Not those tiny green ones at the bottom right corner which goes under the wire.
 
As technicians always say, working without a schematic is like working in the dark. Especially when the circuit is on boards. Point to point tube circuits are pretty easy to figure out but it always helps to have a schematic even with them.
I understand. Right now I searched for schematics of qinpu q2 but there is nothing available online.
 
There's really only a handful all told. They're less than 50 cents a piece. You could change them all for a few bucks.
Pay careful attention to polarity.
Double,,triple check!
 
Assuming that's AC power and a fuse holder looped around in that box? can't be good from hum standpoint... Try checking/cleaning input cables and jacks... Can't offer more without a schematic...
 
Assuming that's AC power and a fuse holder looped around in that box? can't be good from hum standpoint... Try checking/cleaning input cables and jacks... Can't offer more without a schematic...

Good point!
Is It possible to flip those wires to the outside edge as far from the board as possible.
You could also try a tube shield.
 
There is a simple way to test to see if it is a ground loop. You can get one of those cheap plugs that convert a 3 prong to 2 prong. That eliminates the ground to the equipment.

If the equipment is on a power bar or conditioner, you can use that 3-2 plug on that, and the equipment rack will be ungrounded. If you try that, and the hum goes away, then you need to figure out which components are contributing to the problem.

I am not recommending that the plug be the ultimate solution to your problem, or that you run your rack long term that way. It is a very cheap, fast way of eliminating ground loops as the source of the hum. Ground loops are very real, and often hard to resolve, even with 'good' equipment.

It might be a very sensible thing to try before you rip into your equipment. If it is a ground loop problem, it will likely still be there when you are done rebuilding your equipment.
 
There's really only a handful all told. They're less than 50 cents a piece. You could change them all for a few bucks.
Pay careful attention to polarity.
Double,,triple check!

Sounds good. I can change them or get it changed by a professional electronic repairman. They will do a better job in soldering. Before I do all that I just marked the ones that I think are caps on the image. Please correct me if I am wrong.

Just so we're clear I'm talking about round ones.
The green one's are films.
They should be ok.
Leave them be

I just marked the ones I think that are caps. Please correct me if I am wrong.


Use a micrometers . Measure the diameter of ones you're changing.
Make notes of values (uf) and voltage rating

Thank you. I will keep that in mind.

IMG_20181009_211237.jpg
 
There is a simple way to test to see if it is a ground loop. You can get one of those cheap plugs that convert a 3 prong to 2 prong. That eliminates the ground to the equipment.

If the equipment is on a power bar or conditioner, you can use that 3-2 plug on that, and the equipment rack will be ungrounded. If you try that, and the hum goes away, then you need to figure out which components are contributing to the problem.

I am not recommending that the plug be the ultimate solution to your problem, or that you run your rack long term that way. It is a very cheap, fast way of eliminating ground loops as the source of the hum. Ground loops are very real, and often hard to resolve, even with 'good' equipment.

It might be a very sensible thing to try before you rip into your equipment. If it is a ground loop problem, it will likely still be there when you are done rebuilding your equipment.

Sir...! It only has a two pin plug. I manually tried grounding it and it never helped.
 
Good point!
Is It possible to flip those wires to the outside edge as far from the board as possible.
You could also try a tube shield.

Good point. I will try that. I already tried pushing them to the far edge but didn't help. Maybe I should really try to keep them away from the board and see. I will try this before doing anything else.
 
Assuming that's AC power and a fuse holder looped around in that box? can't be good from hum standpoint... Try checking/cleaning input cables and jacks... Can't offer more without a schematic...

I will try all of this and update. I already cleaned the headphone jack, and other inputs but didn't clean the plug.
 
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