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RangerII meter shunt resistor.




 
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Author Topic: RangerII meter shunt resistor.  (Read 679 times)
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Knightt150
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« on: May 25, 2019, 03:47:50 PM »

Hello: I have a question can I replace the SH3 shunt resistor in my RANGER II with a pair of 10 ohm 2% 2 watt NTE flame proof resistors instead of the carbon resistor the parts list recomends. It would seem to me 2 resistors in series if one opens the meter would not be ruined . I have the 2 10 ohms on hand. What dose everyone think?

John W9BFO
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« Reply #1 on: May 26, 2019, 01:13:14 PM »

airc  there is a 5.1 Ohm resistor used as a shunt for grid or plate current ... a composition resistor that usually goes high in value .... the 2 10 ohm resistors paralled should get you close ... there is a better way that uses a small power supply (battery) and extra resistor and dmm to measure the current that flows and the reading it gives.... want to know more, look up Ohm's law and go from there
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« Reply #2 on: May 26, 2019, 01:56:53 PM »

I think the resistor is 0.51 ohms, not 5.1 ohms. You could use a couple of 0.25 ohm resistors in series. Personally, Id just replace it with a quality 0.51 ohm. Modern components are extremely reliable. Just stick with reputable manufactures and suppliers.

Darrell
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« Reply #3 on: May 26, 2019, 02:18:42 PM »

Hello: I have a question can I replace the SH3 shunt resistor in my RANGER II with a pair of 10 ohm 2% 2 watt NTE flame proof resistors instead of the carbon resistor the parts list recomends. It would seem to me 2 resistors in series if one opens the meter would not be ruined . I have the 2 10 ohms on hand. What dose everyone think?

John W9BFO

Hi John,

If we are talking about the final's control grid meter shunt, it is a 20 ohm resistor that dissipates 1 mW maximum.

So two 1/4W or 1/2W 10 ohms in series is more than enough, see PDF file.

I would replace the 27k ohm grid leak resistor with a new CC or CF 1 Watt since this one dissipates about 600 mW.


Phil - AC0OB

* Ranger II Grid Metering.pdf (101.29 KB - downloaded 25 times.)
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Knightt150
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« Reply #4 on: May 26, 2019, 09:06:00 PM »

Thanks for everyone's input but my main question out of all this would the FLAMEPROOF resistors be ok in that circuit. I am sure the wattage and ohm.s value would work. I thought the flame proof resistors had a coil inside the resistor. With out busting one up to see is this true, and if so would it cause problems in this circuit. The shunt I am talking about is SH3 on the grid of the clamp tube.

Thanks John W9BFO
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« Reply #5 on: May 27, 2019, 01:05:51 AM »

Thanks for everyone's input but my main question out of all this would the FLAMEPROOF resistors be ok in that circuit. I am sure the wattage and ohm.s value would work. I thought the flame proof resistors had a coil inside the resistor. With out busting one up to see is this true, and if so would it cause problems in this circuit. The shunt I am talking about is SH3 on the grid of the clamp tube.

Thanks John W9BFO

John,

Yes, we are talking about SH3.  Smiley

I found a later Ranger II schematic so does the first schematic in the PDF below follow the circuit that you have, or does your circuit follow the second schematic?

I also included some meter protection diodes.

What I was trying to convey was the two 10 ohm 2 Watt resistors are OVERKILL, that is, you only need a 20 ohm 1/2W resistor at the most. If you have an open R14 SH3, then you have more problems than just that one resistor.

You could use a 22 ohm 1/2 Watt resistor with a 10% loss in accuracy, that is, your grid current would only read about 10% higher.

If you have a 22 ohm 1/2W carbon composition or carbon film resistor you can parallel it with a 220 ohm carbon composition or carbon film resistor to equal exactly 20 ohms.

If the resistors that you have are inductive, yes it may cause problems.

The Ranger parts list lists the R14 SH3 as a 20 ohm 1/2W carbon resistor. It does not go to the Clamp tube grid.

R37 (22k or 27k, 1W, depending on the schematic) feeds the clamp tube grid so the total voltage between ground and across R37 and R14 in series feeds the clamp tube grid.

With 2.5 mA of Final control grid current, the clamp tube control grid normally sees about -55.05 volts. If RF excitation fails, the clamp tube control grid goes positive and the clamp tube plate drops the Final's screen voltage across R15 so the 6146's plate current does not become excessive.




Phil - AC0OB






* Ranger II Final Circuit - Later EFJ Schematic.pdf (135.88 KB - downloaded 24 times.)
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« Reply #6 on: May 27, 2019, 12:02:51 PM »

Metal Oxide Film and Metal Film resistors have largely succeeded carbon composition resistors. Yes, they can be used in RF circuits at HF and VHF.   If you have any doubt, simply measure the inductance of a particular resistor.  

[edit] The meter circuit is DC, not AC.  So there should minimal effect on readings from the minimal inductance from any resistor type other than a wire-wound type.  Again, the DC current on the meter will vary quite slowly in real terms and thus the minimal inductance will have little damping action.

It seems this radio myth must be slayed every few months.

[edit] If you want additional meter protection from over voltage, then add back to back diode shunts across the terminals.   If you do happen to have some unwanted AC on the meter lines then add a capacitor shunt across the meter terminals.  Standard stuff.

Thanks for everyone's input but my main question out of all this would the FLAMEPROOF resistors be ok in that circuit. I am sure the wattage and ohm.s value would work. I thought the flame proof resistors had a coil inside the resistor. With out busting one up to see is this true, and if so would it cause problems in this circuit. The shunt I am talking about is SH3 on the grid of the clamp tube.

Thanks John W9BFO
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« Reply #7 on: May 27, 2019, 02:27:04 PM »

Flameproof or Flame retardant resistors have a construction such that they will open before a high temperature or power overload occurs and the resistor coating will not burn or emit incandescent particles under any condition of applied temperature or power overload. These resistors are usually used in high power circuits where a flame would create a fire hazard.

Metal oxide or metal film resistors are usually constructed with a spiral of laser-trimmed metal oxides or metal films and any spiral of metal constitutes a parasitic inductance.  

This parasitic inductance may not affect lower frequency circuits but can have a pronounced effect in circuits reaching into the VHF and UHF spectrum and can contribute to unwanted parasitic oscillations.

If one intends to use metal oxide or metal film resistors one should measure its inductance and examine the circuit capacitance associated with it to make sure parasitic oscillation do not arise.

Phil - AC0OB



 

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« Reply #8 on: May 27, 2019, 09:45:43 PM »

Thanks: For all the work everyone has done, I think I have the info I need to make the decision to use or not to use the 2 resistors I have in stock.

John W9BFO
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