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Old 25th May 2023, 6:32 pm   #1
Ganradlogin
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Join Date: May 2023
Location: Shirley, Solihull, West Midlands, UK.
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Default Pye P445U Tone Control Puzzle

I've recently been given this 1950's 9-band tabletop radio. I obtained the Pye Service Sheet, recapped it, replaced the output pentode, and bi-passed an OC section of the mains dropper feeding the panel lights. (Used a 1k0 10W aluminium WW resistor bolted to the underside of the chassis).
All sounds good with stations coming in loud and clear on a short length of wire.
But the 5-position tone switch has no effect.
The radio varies from the schematic and the chassis layout top view diagram. It has a large choke in the mains smoothing circuit (not shown in the diagram), and the output transformer only has 2 windings – a tapped primary and a single secondary. The schematic shows an additional feedback winding that connects to the switched tone circuit.
In my case, the feedback connection is from one side of the output to the speaker. There is no additional connection to the other end of the speaker output.
This can't be right as an extension speaker could now only be isolated from the chassis by little more than C32, a 0.25mFd cap. which could provide enough current to be lethal. As the mains input plug was non-polarised the chassis could have been live. (Now corrected with a polarised connection).
So I'm puzzled and wonder if there is an updated schematic anywhere that would help solve the mystery. (Or are there any ideas out there?)
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Old 27th May 2023, 8:48 pm   #2
Ganradlogin
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Default Re: Pye P445U Tone Control Puzzle

UPDATE
I thought the radio was original. No! What was wired as a smoothing choke was the original output transformer. Only 2 of the 7 connections were made (one end of primary to tap). The other end is OC. So the o/p transformer is an addition, with no feedback winding.

The feedback line was connected to the new speaker output, but with no other connection, it was doing nothing, except connecting the speaker wires via a low impedance path to the chassis! A fatal accident extension speakers waiting to happen!

Then I saw this in a post by 'radiomobile' –

"This is a clever tip from the late Chas Miller. If you have a standard output transformer and wish to avoid the cost of rewinding the original, you can fit the standard one in the normal way and use the original as well for the feedback circuit. Simply ignore the o/c primary, leave the feedback winding in circuit, and connect the speaker secondary winding to the secondary of the "new" transformer observing the correct phasing of course."

Bingo! After wiring it up (and then swapping wires to correct the phase) the tone switch works (has an effect) but not as intended.
Volume is reduced on all settings, but much more on a couple of positions.
The volume control peaks at about 70% rotation and then slightly reduces.
When at this peak position, there is some low level motorboating.

I had no idea which winding was feedback and which was speaker output. One measures 12 ohms which I used for feedback. The other is 0.6 ohms which connects to the additional transformer speaker output. I might have them back to front.

If I can't sort it, I might revert to the original connections but with no feedback. It did sound pretty good. However, any other suggestions would be welcome.
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Old 27th May 2023, 10:23 pm   #3
Techman
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Default Re: Pye P445U Tone Control Puzzle

You could always get one of the two transformer rewinders to rewind the original.

Or you could put it back to how it was when you got it and ask yourself whether you really need that tone control.

The last option is to put it back it how it was when you got it and then use that tone control switch to operate a more standard type tone control - have a look at some other radio circuits to give you an idea of how to do this. It'll just involve the switch switching in and out different value capacitors in the appropriate place.

I've not looked at the circuit for that radio, but there's always a way round these things.
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Old 30th May 2023, 10:07 am   #4
Ganradlogin
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Default Re: Pye P445U Tone Control Puzzle

Thanks Techman for the suggestions. I've previously restored an Eddystone 840C communications receiver. It uses the same UL41 pentode output. The tone control is a very simple 100nF connected in series with a 50K pot across the Transformer primary. There is no feedback. I might do the same, but see if I can configure the double wafer switches to bring in 10K steps.
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