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HK620 balance pot bypass

Arny1109

New Member
Hi
Can anyone help? I have an old Harman Kardon HK620 which has duff balance control. Can someone explain how to cut this out of the system as I never used it anyway. Simple terms please as I'm no expert.
Thanks.
 

Diver300

Well-Known Member
If there is something that is stopping one channel from working, or makes it a lot quieter than the other channel, it might not be the balance control.

1588928339624.png

This is the circuit diagram that I found online.

The balance control seems to be a linear stereo 100 kOhm potentiometer. If you can find any linear, stereo 100 kOhm potentiometer that fits, you could use that. Although the diagram has two separate part numbers for VR501 and VR502, the parts list says they are one item. A stereo potentiometer is just two potentiometers controlled by a common shaft.

If you want to bypass the control, you could just fit four 50 kOhm resistors. You would need one from each slider to each end.

1588929282596.png
This seems to be the layout of the balance control. The stereo potentiometer has two banks, one is the three terminal closer to the shaft, and the other is the three terminals further from the shaft.

Each bank has the slider in the middle. If you remove the potentiometer and put four fixed resistors where the red lines are, you will have bypassed the balance control with it set to centre.

Electrically it's quite simple. It's not going to be that easy to get to, and if you take out the potentiometer, there may be nothing to hold the knob on.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
If there is something that is stopping one channel from working, or makes it a lot quieter than the other channel, it might not be the balance control.

View attachment 124900

This is the circuit diagram that I found online.

The balance control seems to be a linear stereo 100 kOhm potentiometer. If you can find any linear, stereo 100 kOhm potentiometer that fits, you could use that. Although the diagram has two separate part numbers for VR501 and VR502, the parts list says they are one item. A stereo potentiometer is just two potentiometers controlled by a common shaft.

If you want to bypass the control, you could just fit four 50 kOhm resistors. You would need one from each slider to each end.
No need to be that 'technical' - simply put a blob of solder between the top and slider of each half of the pot, and leave the pot in place.

Or, if he wants to remove the pot, do the same but place a 100K resistor between top and bottom where each pot was - this is simply to provide a DC path for polarizing any potential electrolytics on either side of the pot. If there are no electrolytics then the 100K's wouldn't be needed.
 

Arny1109

New Member
Thanks
There are six pins. Do the resisters go from the four outer pins to the two middle pins. I want to be sure I do it right.
 

Diver300

Well-Known Member
No need to be that 'technical' - simply put a blob of solder between the top and slider of each half of the pot, and leave the pot in place.

Or, if he wants to remove the pot, do the same but place a 100K resistor between top and bottom where each pot was - this is simply to provide a DC path for polarizing any potential electrolytics on either side of the pot. If there are no electrolytics then the 100K's wouldn't be needed.
It may not be obvious which end the "top" of the potentiometer is, as the top is clockwise for the right channel and anticlockwise for the left channel. Blobbing the wrong end with solder would result in no signal at all.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It may not be obvious which end the "top" of the potentiometer is, as the top is clockwise for the right channel and anticlockwise for the left channel. Blobbing the wrong end with solder would result in no signal at all.
But easily spotted, and obvious what was wrong - and it's no more difficult to spot than deciding which is the slider. It's also dead simple to tell, as the two ground pins will be shorted together, plus they go to ground :D
 

Arny1109

New Member
Hello again,
I did as instructed, just put it back together. Both channels are equal but the overall volume is about 60% less than before. This is what I have done. Any ideas?
 

Attachments

Diver300

Well-Known Member
As far as I can tell from the circuit diagram, I don't think that you have to have the resistors at all if you are putting both sides to maximum. You can just cut the resistors and it will probably be at full volume.

My suggestion with four 50 kOhm resistors was to simulate the balance control being in the middle. With the links and the 100 kOhm resistor, that would give the maximum volume on both sides.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
As far as I can tell from the circuit diagram, I don't think that you have to have the resistors at all if you are putting both sides to maximum. You can just cut the resistors and it will probably be at full volume.

My suggestion with four 50 kOhm resistors was to simulate the balance control being in the middle. With the links and the 100 kOhm resistor, that would give the maximum volume on both sides.
That's only an incredibly small difference, too small to be noticeable (as it was a linear pot).

But as I mentioned earlier, he 'might' need the 100K to replace the pot if it's used to bias electrolytics - we don't have the rest of the circuit to tell.
 

Diver300

Well-Known Member
I found the circuit diagram here:- https://elektrotanya.com/the_harman_kardon_hk620.pdf/download.html

It's not that easy to read, but I don't think that the resistance of the pot will be needed for bias.

One odd thing is that the 100 kOhm balance pots feed 30 kOhm volume pots, which have 1.5 kOhm load resistors. Those loads are so much lower resistance than the potentiometers that the potentiometers are being used more like variable resistors than voltage dividers.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
OK, I've downloaded the circuit now, and as you say the resistors aren't required - so OP, just cut the resistors out, and all will be fine.

The 1.5K resistors are there to make the linear pot into a log one - linear pots are far more accurate across the two halves of the stereo pot, and doing this makes for a better more accurate control.
 

Arny1109

New Member
I didn't see your last posts so I got the correct resistors a couple of days ago, replaced the 100 ohm ones and it's now fine so thanks again for your help. I'd love to know why increasing the resistance increases the volume and taking them out altogether has the same effect but I doubt I'd understand anyway.
 

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