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Old 27th Oct 2020, 10:36 pm   #1
Lef de art
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Default Need help with antique replacement volume control.

Hello forum.
I restore one mcmichael 381 radio (1938) but the volume control was replaced from the previous owner.

The original R15 volume control (4 pins) that appears in the manual was 500K and taped in 5K from earthy end.

The new one that has been used is 500k with 3 pins .

So one cable ( from equaliser) was left unplugged.
There is any way to convert the new 3 pin volume control with a 5k resistor and get the same result ?

Sorry for my bad English, i try my best.
Lef
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Old 28th Oct 2020, 12:58 am   #2
Lef de art
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Default Re: Need help with antique replacement volume control.

R15 in schematic
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Old 28th Oct 2020, 7:06 pm   #3
John10b
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Default Re: Need help with antique replacement volume control.

Hi Lef
I find this rather interesting as I donít remember seeing a 4 connection volume control, certainly not in recent decades.
Could you show more of the circuit, it will be interesting to see how itís connected.
Thanks
John
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Old 28th Oct 2020, 7:28 pm   #4
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Default Re: Need help with antique replacement volume control.

Hi Lef
Iím sorry I didnít see your other post in which the Moderator told you not to show pictures of the circuit, oops!
John
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Old 28th Oct 2020, 7:32 pm   #5
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Default Re: Need help with antique replacement volume control.

Showing snips is permitted.

If it helps as in this case to show the point being discussed.

You must not post service data available up top.

The tap is often a form of loudness control.

Cheers

Mike T
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Old 28th Oct 2020, 7:39 pm   #6
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Default Re: Need help with antique replacement volume control.

Hi Mike
Sorry I was late seeing your reply, so I asked the question on new thread.
John
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Old 28th Oct 2020, 7:52 pm   #7
ms660
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Default Re: Need help with antique replacement volume control.

The tap on the volume control is connected to the -ve feedback circuit that starts at the secondary of the audio output transformer part of that feedback circuit is shorted out when the tone switch is set to Normal or Foreign.

Lawrence.
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Old 28th Oct 2020, 7:57 pm   #8
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Default Re: Need help with antique replacement volume control.

The tap in the volume control injects some of the output signal at the speaker to provide negative feedback to reduce distortion. The tap is used so that the feedback is variable so that at high volumes the gain of the output stage is not compromised. Pretty clever for a pre-war set.
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Old 28th Oct 2020, 8:08 pm   #9
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Default Re: Need help with antique replacement volume control.

Thank you Lawrence and PJL.
As I said I donít recall seeing this arrangement, if I have Iíve forgotten, so that is why I was interested to see the circuit, but your explanation is fine, thank you.
John
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Old 28th Oct 2020, 10:08 pm   #10
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Default Re: Need help with antique replacement volume control.

Quite common on many German sets of the '60's. Some had two taps (therefore five terminals).
Les.
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Old 28th Oct 2020, 11:15 pm   #11
Lef de art
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Default Re: Need help with antique replacement volume control.

Quote:
Originally Posted by PJL View Post
The tap in the volume control injects some of the output signal at the speaker to provide negative feedback to reduce distortion. The tap is used so that the feedback is variable so that at high volumes the gain of the output stage is not compromised. Pretty clever for a pre-war set.

Indeed clever for a pre-war radio.
But there is a way to convert the 3 pin potentiometer so i can fit the fourth cable ?
The original 4 connections potentiometer was taped at 5k.
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Old 29th Oct 2020, 6:22 am   #12
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Default Re: Need help with antique replacement volume control.

I wonder if a 1 meg pot with 400k and 10k fixed or similar in parallel would work? don't think a fixed 5k would work as it's in parallel with the pot track. There's one way to find out though, and that's try it. i have the odd tapped pot,not sure if I have a 500k/5k. Is the pot log or lin, what dimensions?

Andy.
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Old 29th Oct 2020, 7:32 am   #13
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Default Re: Need help with antique replacement volume control.

1 think i would just remove the wire ( bottom end)add a series 5k resistor and put the bottom wire on the end off this then i have a 5k tap so i can fit the other wire to the junction .. the extra 5k would not put the circuit to far out off tolerance
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Old 29th Oct 2020, 10:11 am   #14
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Default Re: Need help with antique replacement volume control.

It could be an experiment for conductive paint and fine wire....

Lawrence.
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Old 29th Oct 2020, 12:26 pm   #15
Lef de art
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Default Re: Need help with antique replacement volume control.

Quote:
Originally Posted by Diabolical Artificer View Post
I wonder if a 1 meg pot with 400k and 10k fixed or similar in parallel would work? don't think a fixed 5k would work as it's in parallel with the pot track. There's one way to find out though, and that's try it. i have the odd tapped pot,not sure if I have a 500k/5k. Is the pot log or lin, what dimensions?

Andy.
I believe it is log potentiometer because it is used in volume control.

The original i believe it was about 7 centimetres long.
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Old 29th Oct 2020, 12:33 pm   #16
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Default Re: Need help with antique replacement volume control.

I seem to remember something along these lines in a Philips set, probably from the late fifties - no idea of the model though.
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Old 29th Oct 2020, 12:35 pm   #17
Lef de art
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Default Re: Need help with antique replacement volume control.

In my first experiment i connected the unplugged cable to pin No3 with no results.

If i understand correctly (because of my English) you suggest to connect a 5k resistor in the unplugged cable and to connect it in No2 ?
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Old 31st Oct 2020, 7:03 am   #18
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Default Re: Need help with antique replacement volume control.

It won't,is pin 3 ground? If so it's getting no signal. All R15 is doing is acting as a potential divider, EG two resistors in series, tapping off part of the signal, so if you get a spare 1m pot connect one outer tag to T38, the other outer tag to ground then adjust till it sounds about right. This will alter R15's resistance/impedance to 500k, but it just might work willlittle ill effects.

Andy.
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Old 31st Oct 2020, 10:23 am   #19
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Default Re: Need help with antique replacement volume control.

For those without a schematic....the "ground" end of the volume pot is connected to the bottom end of the 1st AF amplifier's (a triode) cathode bias resistor, from that junction is another resistor that's connected to ground, those two resistors in series set the AGC delay voltage.

You will need a tapped pot that's tapped at the same point or a standard pot which has been modified with a tap. If the manufactures could have done it with a resistor fudge they would have done so to save the expense of a special pot.

Lawrence.
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