[Milsurplus] RAK Update

[Richard]

A word about the RAK/RAL power supply:  The voltage dropping resistors 
for the VR tube are carbon, mounted underneath that bakelite plate with 
no ventilation, and I haven't seen any that were working.  Further, the 
design has 30 or 40 ma through the VR tube, which is overkill as the 
regulated load is 1/2 mil.  I increased the resistance to pass 5 or 10 
ma through the VR tube. If you use a regulated supply you don't need the 
VR tube as there is an internal resistor to feed the tube.

The RAK and RAL have capacitors plate to ground in the audio stages to 
roll off frequency response above about 1300 cycles, which may be 
necessary for the RAK to keep low frequency RF out of the audio stages.  
They aren't needed in the RAL, and you will notice when the low-pass 
filter is switched out there isn't much difference.  I took them out of 
my RAL which greatly increased audio gain and I had a strong supersonic 
oscillation with 33 volts across the phones.  I added shielding on all 
the low level audio wires and  it went down to 17 volts - still 
oscillating because the plate of the audio output stage was "looking" at 
some low level wiring.  Shielded the tube and all was well, almost.  I 
then had an imbalance in audio and RF gain, and added an audio gain 
control, put it where the AC/DC switch is.  Now it works VERY well, and 
SSB sounds better than in any modern receiver I've used.

Richard, AA1P


On 12/26/2017 09:05 PM, Kenneth G. Gordon wrote:
> 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
>
> ---
>