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Old 30th Mar 2008, 6:28 pm   #1
radiozero
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Default Help with circuit needed - thanks

http://homepage.ntlworld.com/richard.../MISC/TRF.html

I'm unsure about the wiring circuit that I have copied from a 1920's homebrew RX I have. The circuit is copied as per the rx, but never-the-less I think the circuit connections are suspect. Can anyone help me sort the wiring/circuit out. Thanks.
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Old 30th Mar 2008, 7:53 pm   #2
Alf
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Default Re: Help with circuit needed - thanks

Hi, hope I can help, point 3 on the transformer would be connected to a grid bias battery (-ve) as to the others, well there's many variations, maybe if you could get hold of a few trf circuits, you'd have a better idea.
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Old 30th Mar 2008, 10:19 pm   #3
PJL
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Default Re: Help with circuit needed - thanks

There seem to be a few things not right.

1. The V2 grid resistor goes to HT- according to your layout.
2. HT2 will need decoupling with a 1uF or so - originally it would have gone to a seperate 'tap' on the HT battery.
3. It's missing a capacitor to couple the anode of V1 to the grid of V2 (leaky grid detector).
4. As Alf says, the point 3 needs to go to a grid bias battery.
5. 1ohm is in fact 1M

Otherwise I think it looks OK...
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Old 31st Mar 2008, 12:21 am   #4
radiozero
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Default Re: Help with circuit needed - thanks

Okay. Point 3 needs either to go to "ground" or to a bias battery. Fine.

The problem seems to center arround the components just above V2. There are two resistors on one plasic component made by "Dubilier". Now I assume there are 2 capacitors in this unit. I think the anode load of V1 is a simple resistor. One capacitor in that Dubilier package must be the decoupling capacitor for that V1 load resistor. True?

Now, where is the grid leak in this set. Is it in the grid of V1 or grid of V2? Is it not in V1 - and V2 is just an AF signal amplifier for the output valve V3? The other capacitor in that Dubilier unit (if there is one) needs to be the coupling capacitor between V1 anode and V2 grid. And what of the other resistor? Is that suppossed to be a terminating load for that coupling capacitor and a means of biasing v2 grid? Should that resistor then, connected to V2 grid - go to ground? Rich
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Old 31st Mar 2008, 12:24 am   #5
radiozero
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Default Re: Help with circuit needed - thanks

To be honest I don't understand that item above V2. It looks like that Dubilier unit is two capacitors, each with a resistor across it.
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Old 31st Mar 2008, 10:57 am   #6
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Default Re: Help with circuit needed - thanks

V1 is a regenerative RF amp.
V2 is a leaky grid detector.
V3 is the audio amplifier.

A leaky grid detector has capacitor coupled to the RF amp and a high value resistance to earth. During positive excursions the grid rectifies the AC signal causing the grid to move -ve to form the audio signal. It won't work if you connect it to a grid bias bettery.

It's a sealed coupling unit? Afraid I don't know much about these and I suspect the content was a 'trade' secret...It should have the connections marked on the unit. Might be an inductor for the anode load, a coupling capacitor and the grid resistor.
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Old 31st Mar 2008, 2:48 pm   #7
radiozero
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Default Re: Help with circuit needed - thanks

Okay, after further inspection that unit above V2 is made by LISSEN, not DUBILIER.

That could mean therefore that it's just a holder for two resistors. The bottom is filled in with pitch, but that does not neccssarily mean there are capacitors inside it.

Now, as you see there are four terminals in that unit above V2. Top left is marked P. Top right LT. Bottom left H.T. and Bottom righ G. As you see these terminals do go to correct destinations. So that unit is probably just a holder for two resistors. I think.
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Old 31st Mar 2008, 3:14 pm   #8
radiozero
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Default Re: Help with circuit needed - thanks

I bet that in that unit above V2 there is a capacitor connecting P. with G. This would represent the coupling capacitor from the anode of V1 to the grid of V2.

Can anyone please confirm that these LISSEN parts had a capacitor in them connecting P. to G? Thanks.
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Old 31st Mar 2008, 6:50 pm   #9
PJL
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Default Re: Help with circuit needed - thanks

Looks like it's all OK then. Before you try it out you will need to check for leakage current in the capacitors. Don't forget to add decoupling for HT2. You'll need high impedance headphones/speaker or an output transformer. The 215P spec is here http://www.andycowley.com/valves/old...ossor-p057.pdf and you need around -7.5V on the grid, 2V for the LT and around 120V for HT1.

That design should do a great job of blocking reception on your other radios too..Peter
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