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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 20th Nov 2019, 2:11 am   #1
Jolly 7
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Default Using a two terminal 1Meg volume control pot

I am currently in the middle of modifying an old radio that uses a small two terminal 1 Meg volume control pot. I am unable to replace it with a three terminal 5K or10K one because of severe space restrictions and the solid metal chassis it's fixed to. I want to be able to connect the pot to an LM386-based amplifier, but all of the designs I've come across use three terminal centre-tap pots. What are my amplifier circuit options using the existing two terminal pot ? I am looking to operate it with 6 to 9 volts.
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Old 20th Nov 2019, 2:24 am   #2
Terry_VK5TM
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Default Re: Using a two terminal 1Meg volume control pot

You'll need to identify the radio better so we can have some idea of how the volume pot is being used in the circuit.
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Old 20th Nov 2019, 2:32 am   #3
Jolly 7
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Default Re: Using a two terminal 1Meg volume control pot

The radio was an old KB valve radio but I have converted it to a TA7642 based TRF receiver after removing most of its innards. The existing tuning cap works great with the TRF circuit.
It also works fine with an external test amplifier fed in to the original 3 ohm speaker. My next step therefore is to fit an internal amplifer which is able to use the existing two terminal 1Meg pot. Not sure if its log or linear.
Maybe that explains it a bit better ?
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Old 20th Nov 2019, 2:55 am   #4
Jolly 7
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Default Re: Using a two terminal 1Meg volume control pot

Here are some pictures of the volume control. The white thumbwheel knob seems to be an integral part of the potentiometer.
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Old 20th Nov 2019, 7:54 am   #5
crackle
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Default Re: Using a two terminal 1Meg volume control pot

If this was an early KB OP21 transistor Rhapsody then there should have been a 3 terminal pot in it. One terminal was not connected.
I have never seen anything other than 3 terminal potentiometers in any of these KB Rhapsody models. I can see 3 terminals in your second photo. Turn the chassis over and look at it from the other side.
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I know it is your radio, but I do wonder why anyone would want to convert a transistor superhet radio to a TRF radio though.

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Old 20th Nov 2019, 10:49 am   #6
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Default Re: Using a two terminal 1Meg volume control pot

The OP did say it was a valve radio. That would be consistent with the 1Meg volume control.

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Old 20th Nov 2019, 11:21 am   #7
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Default Re: Using a two terminal 1Meg volume control pot

There are various ways to use a variable resistor (as opposed to a potentiometer) as a volume control. As the bottom or top element of a pot using a fixed resistor as the other part, depending on whether full volume or zero volume is the more desirable limit at one end. Or you can use it as the input or feedback resistor for an opamp buffer. I'm sure other possibilities are out there too.
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Old 20th Nov 2019, 2:59 pm   #8
Jolly 7
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Default Re: Using a two terminal 1Meg volume control pot

Thanks for all the replies. I managed to see the third leg of the pot clearly, but to do this I had to unscrew it from underneath a metal flap. It had no wires connected to it, left hand one in the picture. A suitable wire will now been soldered on in preparation for connection to my DIY amplifier circuit.
@Crackle: Grateful for your explanation and photo regarding the pot. This radio came to me with multiple cracks, no valves and possibly other parts missing too. Given the state of the radio, as you will gather from the photos, I decided to budget and do what I can as far as my competence allows me.
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Old 20th Nov 2019, 10:00 pm   #9
Jolly 7
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Default Re: Using a two terminal 1Meg volume control pot

Apologies. A correction to my last post. There is a third wire after all. It goes from the pot to the chassis !
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Old 20th Nov 2019, 11:27 pm   #10
Tim
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Default Re: Using a two terminal 1Meg volume control pot

Is it a tone control?
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Old 20th Nov 2019, 11:29 pm   #11
crackle
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Default Re: Using a two terminal 1Meg volume control pot

Quote:
Originally Posted by stuarth View Post
The OP did say it was a valve radio. That would be consistent with the 1Meg volume control.

Stuart
Yes, but I was going on the mention that it was only wired as a 2 contact volume control.
All the KB Rhapsody valve sets, including the OP's PP11 battery only one, and the early transistor ones use a 1M volume pot but the only one wired as a simple variable resistor is the OP21.
But even that used a normal 3 terminal potentiometer, as seen in my photo.
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Old 21st Nov 2019, 12:07 am   #12
Jolly 7
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Default Re: Using a two terminal 1Meg volume control pot

Quote:
Originally Posted by Tim View Post
Is it a tone control?
No, there's only one pot in the radio, so it has to be volume control.
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