[Milsurplus] RAK Update

[Kenneth G. Gordon]

On 27 Dec 2017 at 1:42, Joe Connor via Milsurplus wrote:

> 
> Thank you for the all the help, guys. 
> 
> Here's an update:
> 
> 1. I removed the 38276 tube and threw the switch in the power supply. I would 
> have missed that if you folks hadn't told me about it. 

Good. Now unscrew the tube and put it somewhere safe.

> 2. The prior owner used a terminal-block arrangement to wire the power supply to 
> the receiver. He labeled everything very neatly and clearly. I wish all prior owners 
> were like that!

Snazzy! Well done. The original cable was also very well done. Obviously the fellow you got 
it from didn't have the original cable. It is impossible to get the original cable connected 
incorrectly.
 
> 3. I fired it up on a variac and got weak reception with headphones, but when I 
> hooked it up to an LS-3 speaker, the volume was very good. With only a few feet 
> of wire, the receiver was very sensitive.

Yes, it is quite sensitive. The AF output impedance is 600 ohms, and should give ample 
volume into a set of 600 ohm phones. Perhaps there is something wrong down in the 
phones jack area. There is a fancy RF filter there. Perhaps one of the chokes is open or 
shorted. You will have to take the receiver out of the cabinet and turn it up side down to find 
that small box. You can also connect an external speaker to connections inside the cabinet 
at the back. There should be a round can with an RF filter in it mounted to the back of the 
cabinet.  All this filtering was to keep external transmitters from leaking into the receiver.

> 4. Although the RAK is only supposed to reach 600 khz, I got WFAN (660) at the 
> very top of the highest band. 

Yes. It has overlap on both ends. 

BUT, there is an audio bandpass filter in the circuit all the time which severely limits fidelity 
on AM. Those receivers were never used for AM so that didn't matter. The RAL has a switch 
on the front panel which bypasses that filter. Even with that filter out of the circuit, fidelity on 
AM is still "poor". They were never designed for AM. Only CW.

> 5. Do you use the sensitivity control to control volume?

Yes. The audio gain is set wide-open all the time. There is no way to control it. So the 
"sensitivity" control controls volume. That is actually an RF gain control. It is fairly easy to 
add an AF gain control to the circuit, but there is, essentially, no place to mount it on the 
front panel, except high in the middle of the upper panel...which stinks. it looks terrible 
there.

> 6. I haven't taken the receiver apart yet, but I look forward to doing so. Man, it is 
> heavy. 

Yup. Around 70 lbs....for 6 tubes and no power supply. :-)
 
> 7. Any troublesome caps to keep an eye on? It looks like most are oil-filled.

Only as I mentioned there are some paper or (maybe) mica caps which couple between the 
output (plate) and input (grid) of the two RF amps and the detector. If those get leaky (and 
they do) you will hear "static" with no antenna connected. Those are easily reachable and 
should be replaced with a good silvered-mica job of the same value when necessary.

As long as the receiver has been stored and used upright, the oil-filled jobs should be OK. If 
you see what looks like hardened clear grease around a cap, that probably means it leaked, 
and should be tested.

> 8. Anything else to look out for?

Hmm....can't think of much. The 6D6s are simply a 6 pin version of the 6K7. If you can't find 
6D6s (but they are easily available) you can make an adapter and plug 6K7s in. The 41s 
are essentially 6-pin 6K6s. Same deal there.

> Again, thank you for your help.

You bet. It is SO NICE to hear of another RAK being put back in service. Good on ya! :-)

Ken W7EKB

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