Question about replacing a bias setting variable resistor.

[saabracer23]

Hello all,
So I have an amplifier board from an HK A402 sitting in front of me and I’m wanting to replace these old carbon wipers with some Bourne 25 turners. The originals in the board only uses two leads and the third/middle hole is just soldered over.

The question is, can I just use all three of the leads on the Bournes? As you can see on the back of the board, two of the leads share a trace just as any other amp if replaced these on.

On a side note, I need to moisturize my hands more.





Dan

 

[whoaru99]

In many cases of using a potentiometer to replace a variable resistor it's fine, or possibly even beneficial, to short one end to the wiper. This provides a complete circuit of the full pot resistance rather than open circuit if the wiper loses contact with the track.

However, if you want to put it back just as it was you'd snip one of the end legs and use the other end leg and the wiper leg. Putting it back the way it was is a safe bet.

Which end leg you snip, if you go that way, would depend on which way you insert it relative to which way you'd like to turn it for increase/decrease of bias.

 

[Tom B]

I like the idea of replacing if you must as a 3 leg pot, rather than 2 leg VR. But here is my 2 cents. I only replace bias or DC balance pots/VR's when needed. When needed is when its almost impossible to get the right setting, or excessive drifting when tpping on the pot/VR.
As much as I like the 25 turn feature of the bourns trimpots, they are only rated at 1/4 watt to 1/2 watt (The ones you have appear to be 1/4) The ones you will be replacing have much heavier pins, and the copper wiper is much heavier than the bourns trimpot. While you may not need more than 1/4 watt VR, I like the heavier originals. If you ever opened up a bournes pot, you will see how much smaller they are than original vr's. If I can leave the heavier ones in, I do. If you use a bournes and the circuit requires more than 1/4w the trimpot could heat up and open. "Opening" a bias pot/VR usually results in thermal runaway and fried outputs.
If I do replace originals, I use the trimpot 3296W type, which are rated at 1/2 w
Has anybody ever seen issues relating to bournes 1/4 watt trimpots?

 

[saabracer23]

Thanks guys, so one vote using 2 pins and other for all three, but would prefer to keep originals in place. The ones I have are are the 3296W series.



Dan

 

[Superampman]

In any case, they should be oriented to increase resistance in the same direction as the original spec.

 

[Oldsansui441]

I have used the 25 turn trim pots in amps where there was no trim pot originally fitted (so replacing a fixed resistor) and l have in each case tied the centre pin (wiper) to one of the outer legs. As whoaru99 said this will provide full resistance in the case of a fault instead of an open circuit.

 

[saabracer23]

I have used the 25 turn trim pots in amps where there was no trim pot originally fitted (so replacing a fixed resistor) and l have in each case tied the centre pin (wiper) to one of the outer legs. As whoaru99 said this will provide full resistance in the case of a fault instead of an open circuit.

Great, thank you. I’ll tie it in.

Dan.

 

[pioneervibes]

Old thread, but I'm working on a Sherwood 7100a and need some help sourcing the right multi-turn Bourne trimmer to replace the bias adjust VR seen below. I could use some direction on preferred series, number of turns, adjustment, etc. Any help greatly appreciated!

 

[elnaldo]

I usually replace those trimpots with these single turn trimpots:

 

[tarior]

Always use single turn pots for bias. There is no need for a 25 turn pot there.

Bourns 3386 series are what I use. Look at the datasheet to find the correct one for your application.

 

[pioneervibes]

Thank you everyone! Single turn, PC pin with top slot adjustment?

Here is the list of VR's in my unit. Any others that need replacing? I've seen folks talk about DC offset, but don't see anything below. I'm wondering if "separation adjust" is the same thing.

 

[tarior]

Just the bias trimmers, unless any other trimmers are proven faulty. The main volume/balance/bass/treble controls usually only need a DeOxit bath and they are good to go.

 

[ecluser]

Measure the actual resistance of your bias trim pot because there is a discrepancy between the value on the parts list (1k) and the schematic (4.7k). Ref Sherwood 7100A

 

[pioneervibes]

Measure the actual resistance of your bias trim pot because there is a discrepancy between the value on the parts list (1k) and the schematic (4.7k). Ref Sherwood 7100A

Just measured 700-800 ohms at max adjustment. Parts list accurate and they've drifted?

 

[egellings]

Hello all,
So I have an amplifier board from an HK A402 sitting in front of me and I’m wanting to replace these old carbon wipers with some Bourne 25 turners. The originals in the board only uses two leads and the third/middle hole is just soldered over.

The question is, can I just use all three of the leads on the Bournes? As you can see on the back of the board, two of the leads share a trace just as any other amp if replaced these on.

On a side note, I need to moisturize my hands more.

View attachment 1087162
View attachment 1087163
View attachment 1087164

Dan

What you could also do is to restore the originals by removing them from the PCB and soaking them in a tarnish remover such as the Tarnex brand. Rinse well in distilled water and dry, and they should work just fine. The multiturn pots will work well, but you do not need such a high setting resolution in that circuit.