[Milsurplus] RAK-7

[Kenneth G. Gordon]

On 23 Jul 2016 at 15:09, Michael Bittner wrote:
 
> I've read on these lists that some RAK-7 owners are running them on 90 Volts
> B+ from batteries. I assume that, when this is done, the 90 Volts is
> connected to the 180-Volt input on the RAK and that the 90-Volt input is left
> floating. Is this correct? 

Correct. There is a resistor, R-116 in the RAL, R-117 in the RAK, 51K ohms, which is 
connected between the 180 V buss, and the 90 V buss at the detector stage. When you 
connect the 90 V battery to the 180 V buss, the voltage-drop across R-116/R-117 is very low, 
and provides the detector with proper voltage. 

When you connect 180 V to the 180 V buss, AND 90 V to the 90 V buss, that resistor has, 
essentially no effect.

In fact, if your power supply has ONLY 180 VDC output, you can ignore the 90 V DC input 
connection. The receiver won't be as stable as it is when the regulated 90 V supply, but it will 
work just fine. The detector stage then gets its operating voltage from the 180 V connection 
through R-116/R-117.

Since this is so, you cannot connect 90 V to only the 90 V buss without the voltage to the rest 
of the receiver being TOO low. The effect would be the exact opposite of what you would 
want.

Also, if you want to run the receivers on batteries, and want the front panel DC off/on switch 
to work, you have to open two links behind the front panel that bypass that switch for normal 
operation with the original power supply. If that front-panel switch is in operation with that 
original supply, opening that switch will remove the load from the power supply which can 
damage it.

> Also, some have suggested simple AC power supplies for the RAK. I have a
> simple regulated power supply that can provide the 180 Volts. I'm assuming
> that its connection to the RAK's should be the same as for battery
> operation above - correct? 

Yes. Absolutely correct.

And, as I said above, using that power supply, you would simply ignore the 90V connection, 
leaving it open and unconnected.

> By adding up the RAK's plate and screen currents listed in the manual, I get
> 24 mA. Is this current about right for a  small power supply substituting
> for the original monster power supply, or does it need to be more? 

That current is close to correct: the nomenclature plate on many RAKs and RALs states that 
the receiver draws 35 mA at 180 VDC and 1 mA at 90 VDC regulated.

> None of the resistors and VR tube between 180 volts and 90 Volts in the
> original power supply will be in the circuit. 

Correct. The VR tube, an 874, is mounted in the original supply, and the resistors that 
Richard mentioned are there also.

According to the manual, sensitivity of the RAK ranges from a little less than 2 microvolts at 
worst to 0/08 microvolts at best, depending on the frequency to which it is tuned.

Ken W7EKB
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