[Hallicrafters] SR-150 heads up

[Jim Liles]

The SR-150 is one of the best performing SSB RECEIVERS that the Hal guys 
built.  It's unique in that the design is similar to an SX-115 except that 
it uses a crystal filter rather than the LC 50Kc I.F. system.  The 
transmitter is mediocre until you install a 300ma plate meter in the PS-150, 
reduce the fixed load caps and install a variable load cap.  If you have 
worked with one you probably found a pair of miniature relays that are not 
fun to get to and look cheaper than a sock with a hole.  The first finger 
nail scratch on a blackboard is when you see one of these buggers used for 
the antenna relay.  Your first thought is to get that thing out of there. 
Doing so almost always results in the destruction of the character of the 
radio, and here's why.

The design of the SR-150 uses the cathode current in receive mode to do four 
things.  First, reverse bias CR9 to kill the output of the balanced 
modulator into T6 --- the receive mixer output.  Second, it reverse biases 
CR2 which kills the heterodyne oscillator out to the transmit mixer when in 
receive mode.  Third and fourth, it picks K2 and K3, those tiny relays, to 
enter receive mode.  When in receive mode the 6AQ5A expects to see around 
350 ohms to ground and both K2 and K3 are 800 ohms in parallel equal 400 
ohms.  The problems begin when the antenna relay is replaced with a more 
robust 12 volt relay that has a lower resistance.  It's not easy to find a 
12 volt relay with an 800 ohm coil.  Every SR-150 that has come to this shop 
with a changed out antenna relay has had a low resistance coil.  A simple 
test is to check the 1.5/16 volt line from the 6AQ5A cathode to see if it is 
low.  If it is, verify that you have a good 6AQ5A, the 6AQ5 is not good 
enough.  If that line is low, the diode gates will leak and the relays will 
be slow to return to receive mode which is transferred.  The radio is 
somewhat protected by K3 logic that requires the relay be in the transmit 
position before final bias is lowered.  Finally, those relays are ALWAYS 
warped.  Over time the phenolic warps, moving all of the contacts closer and 
closer until they are all together.  Simply reform them to allow adequate 
gap and movement and you are finished.  A very small dot of super glue 
soaked into the phenolic will prevent further warping.  That is a great 
radio when it's well. Now, let's get out of the box and make that radio even 
more sensitive.  Change V3, a 6EA8 to a 6GH8A and if you wish to take it a 
step further, change V5 to a 6GH8A.  Don't change V17 as you will have to 
redesign the S-Meter bridge.  Have a good time with that radio --- Kindest 
regards Jim K9AXN