Relays K2 & K3; K2 & K3 Substrate Warping - Hallicrafters SR-150 Operation And Maintenance Manual

3-5. RELAYS K2 & K3
The design of the SR-150 uses the V13 cathode current in receive mode to do four things.
1: In receive mode to reverse bias CR9 to block the output of T6 which feeds V17 in the TX mode.
2: In the receive mode it reverse biases CR2 which kills the heterodyne oscillator signal to the transmit
mixer.
3 &4: It energizes K2 and K3, to enter receive mode. In receive mode the 6AQ5A expects to see around
360 ohms to ground and both K2 and K3 are 780 ohms in parallel equaling 360 ohms.
In a properly functioning ckt the voltage at the cathode of V13 runs 1.5 VDC in transmit and 16 VDC in receive
mode. If the voltage in the receive mode is low then CR2 and CR9 will not switch properly. With low voltage
on the cathode, V13 will not operate properly causing distortion in the audio output. There are three common
failure modes for the relay: 1, contact substrate warping; 2, dirty contacts; 2, contacts loose in the substrate.
I have searched extensively and have not found a suitable replacement for these relays.
In 2011-2012 I got involved in a project to get the original manufacturer to do a build of a number of these
relays. We got some (I didn't get any); he does not want to do any more. However I was able to find the original
specs. So if you want a challenge find a 3PDT relay with a coil resistance of 780 ohms, pull in voltage of 9.0vdc
a drop out voltage of 3.5v suitable for switching RF. Or a 6 to 8 VDC relay with a coil resistance of 180, pull in
at 4.5v and drop out at 1.75 vdc, two of which could be wired in series.
3-5-1. K2 & K3 SUBSTRATE WARPING.
The relay contact gap is .008" and the relay throw is .012". The de-energized contact compression is .002" and
that leaves .002" for energized contact compression (or sometimes referred to as wipe compression). It takes
very little substrate warping in either direction to eat up the .002" of wipe compression. If warping is the
problem it will affect either the TX or the RX. By opening the gap of the NC contacts and closing gap of the
NO contacts the relay can be salvaged. It is rare for the substrate to warp in the opposite direction of the photo
above. If it does then reverse the adjustment procedure above.
By Walter Cates WD0GOF (UPDATE 10/30/2013) 26