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Amplifier power supply capacitor removal advice needed!

KrisBlueNZ

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That's a point duke37. With the pots turned to maximum, check for any DC voltage across each pot. That is, between one end and the other. This would indicate failure of one or both of the DC blocking capacitors connected to the pot.
 

Chaggy78

Dec 30, 2012
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Hi Thanks for your replies, I just powered up the amplifier with no inputs connected and there is a failry loud buzzing/humming noise in both channels.
I'm not that experienced at testing components so i don't want to go prodding around in there. What do you think i should do? Replace all the capacitors near the 2 volume dials? Most of the capacitors are Axial so would it be ok to replace them with normal style caps?
 

Chaggy78

Dec 30, 2012
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Sorry i meant is it ok to replace axial caps with radials? Is there any way of completely bypassing the two volume pots? What would happen if i removed them then turned on the amp or not a wise idea? Cheeers
 

KrisBlueNZ

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You can replace axial caps with radial caps or vice versa, as long as they're the same capacitance and voltage (or you can use a higher voltage), and in some cases, the same type. Post a photo and/or description of the caps you want to replace and we can advise.

You can remove the pots completely if you want. Connect the connection for the "hot" end (right hand pin when viewed looking into the shaft with the pins pointing down) to the centre pin connection.

Some photos and/or audio files might help. The closer we can be to actually being there, the better we can diagnose the problem.
 

Chaggy78

Dec 30, 2012
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Thanks again, I will try to get a picture of the pcb as soon as possible. After removing the pcb with the pots attached from the amp i noticed that the pots have 4 pins each? 4 pins all in a row. what would the pinouts of these be?
 

Chaggy78

Dec 30, 2012
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Hi I have uploaded 4 pics of the amp for you to look at, sorry about the quality but I used a webcam as I don't have a descent camera:
 

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Chaggy78

Dec 30, 2012
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There is a 3-pin jumper with a little black 2 pin block on two of the 3 pins, next to each pot what do you think these do?
 

Chaggy78

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Here are a few more pics of the pots/circuit board:
 

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KrisBlueNZ

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Re the pots, the fourth pin could be a tap for loudness compensation. Can you take closeups of the area around the pots from both sides? (Component side and underneath.)

Use plenty of illumination (natural light is best) but no direct sunlight, to avoid reflections. Take the component side picture almost directly looking down on the pots but from slightly behind them, facing slightly towards the back of the pots, and take the underside pictures looking directly towards the board.

Also, what are the markings on the backs of the pots? Can you post a picture of them too?

Re the function of the jumper, I wouldn't like to guess. Are there any markings on the board next to them?
 
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Chaggy78

Dec 30, 2012
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I will try to get better pictures, apologies for the quality.
The fourth pin does no seem to link to anything when looking at the back of the circuit board.
On the back of the pots it has: Noble 297Y 20K 8444 printed on them.
I would like to just remove them completely from the circuit if possible.
 

Chaggy78

Dec 30, 2012
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Hi again, i have uploaded a picture of the inside of the pot so we can work out the pinouts.
When the pot is at full volume, the wiper inside is contacting B + C.
When muted (no volume) the wiper is contacting C + D.
I have no idea what pin A does as the wiper never comes in contact with it i think?
so i was thinking if i joined a wire on pcb from pin B to pin C that would allow the amp to operate at full volume without the pot?
Let me know what you guys think,
Many thanks!
 

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Chaggy78

Dec 30, 2012
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Here are two better pictures of the pcb with one pot removed i have labelled the coresponding pins from the pot with arrows to each hole on the pcb.
 

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KrisBlueNZ

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OK, nice work. Good clear pictures and descriptions and information there.

It looks like pin A connects to a tap part-way along the carbon track in the pot. It would be intended for a "loudness compensation" connection, but as you say, it's not connected to anything on the PCB, so you don't need to worry about it.

The wiper (C) and the two ends of the track (B and D) are clearly visible, and the B end is the fully clockwise (i.e. maximum volume) end, based on your description of which pins the wiper connects to when the pot is fully clockwise and fully anticlockwise.

Looking at the PCB track side, I was expecting to see the anticlockwise track end pin, D, connected to the 0V rail of the circuit.

The 0V ("zero volt") rail is usually easy to recognise because it connects to a whole lot of components, and the outer screen braid of any screened wires that connect to the board, and the 0V rail of the power supply, and sometimes, the PCB mounting screws, and it's usually a thick track with large copper areas on it.

The 0V rail is the "no signal" connection point for the whole circuit, so when the pot is turned to that end, the wiper (which is the "output" of the volume control) is connected to the no-signal end, so the amplfier goes fully silent.

So I was surprised to see that all three pot pins have long thin tracks running to them.

It's possible that the track from pin D connects to the 0V rail through a resistor; this would mean that at the fully anticlockwise position, there is still SOME sound audible from the amplfiier. This is done with some amplifiers; turning the volume control fully anticlockwise does not FULLY kill the signal; it just reduces it very greatly.

Anyway... It seems pretty clear that jumpering pin C (wiper) to pin B (clockwise end) will bypass the volume pot and give you maximum volume.

If there's any doubt about pin D, you can connect a resistor of similar value to the pot, i.e. 20K (the closest standard resistor value is 18k or 22k) from pins C and B to pin D will be exactly equivalent to the pot being present and turned fully clockwise. So you might want to add a resistor on each pot position, as well as jumpering those pins together.

I notice that there's some crud around the holes where you've removed the pot. If you want to tidy it up, you can use a cotton swab (here we call them "cotton buds") with firm pressure to clean the PCB. If you have isopropyl alcohol, or any MILD solvent, wet the cotton swab with that first. If the pads have lifted or become less securely attached to the PCB due to overheating when you removed the pot, it's probably best not to rub them hard though.

Finally, bypassing the volume pot and setting the amp to maximum volume may cause problems with background noise. All the noise (buzz, hum, hiss) from the preamplifier stage (the stage before the volume pot, that connects to pin B) will be fed to the next stage (connected to pin C) at maximum volume, and this may mean you hear too much background noise all the time. You can try it and see.
 
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Chaggy78

Dec 30, 2012
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Wow thanks for the very good reply! very useful information is much appreciated indeed!
I was wondering if i could use a top adjustable potentiometer, i found one for sale which looks like it would work, it's a 20K top adjustable pot like the one in the picture below.
 

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KrisBlueNZ

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Yes, but I don't think it would fit into those holes.

Also, trimpots have a "linear law" and volume controls have "logarithmic law" (because the ear's response is logarithmic). If you use a linear trimpot as a volume control, you'll find that over most of the range, the volume doesn't change much, and as you get to the quiet end of the adjustment range, the volume falls off rapidly.

In your case, this might not be a problem, since you want a fairly high gain (high volume setting) anyway. Also, the trimpot you found is a multi-turn one, so it's easy to adjust accurately.

You're welcome :)
 

Chaggy78

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Thanks again KrisBlue for your advice, i think i would prefer to use a 20k resisitor across the holes but don't have a clue how to do it. Could you possible supply a drawing of how to do this?
 

KrisBlueNZ

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I'll describe how to add the resistor. The resistor is a cylindrical thing with a wire coming out each end. (Don't get a surface mount (SMT) resistor!) Cut one wire to about 1/4" long and solder it to pad D. Now you need to solder the other lead to pads B and C (both of them). Try to make a tidy job and avoid bending the leads very close to the resistor body. You can put the resistor on the component side and put the wires through the holes, or you can tack it on the underside.

If you want to replace the pots, yes, you need 20K logarithmic mono pots. Make sure they're physically compatible; the originals are non-standard. You need to fit the standard one (from eBay) into holes B, C and D. Even if the pin spacing is right, the shaft will line up with pad C, whereas on the original pot, the shaft probably lines up half way between pins B and C, because it's a 4-pin component. Also you'll need to check the vertical alignment; you may need to use short wires between the pot terminals and the PCB; this might make it easier to line it up horizontally. Try it and see, I guess.

Edit: Also check that the knob is compatible with the shaft of the new pot.
 
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Chaggy78

Dec 30, 2012
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Thnaks again for that help. What type of resistor should i use? 20k? what voltage?
thanks
 

KrisBlueNZ

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The value isn't critical at all. 18k, 20k or 22k is fine. 1% metal film is best. 1/4 watt or higher.
 
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