Marconi 2018 RF Sig Gen Crowbar for +5V PS

[Radio1950]

Marconi 2018 +5 Volt DC Supply Crowbar.

Spurred on by comments from contributor Mr Bungle, who had a previous “most unfortunate experience” with a M2018, I have installed a simple classical overvoltage crowbar on the +5V DC output of my sig gen.

refer https://www.vintage-radio.net/forum/...d.php?t=157149



Standard basic circuit with no frills, TYN816, 16A 800V TO220 SCR, a selected zener 6V2 1W, a 1K resistor and a few 0.1 uF caps, all of which is mounted with fuse holder on a bracket attached to the rear panel behind the attenuator, using existing holes.
Photo.

The circuit was bench tested to fire at about 6.4 V which is below the TTL Supply Vmax spec.

The fuse chosen is an M205 5A QA, as the holder is electrically robust, and a range of quality fuses is easily available.
The circuit quiescent current is 2.05A, and the V drop across the fuse is 45 mV.

The crowbar fuse is installed between the output of the +5V LM350K on the rear heatsink, and the three +5V supply lines.
Refer Marconi circuit.

A bit of work required here, as there are three separate wires fed from a star solder lug on the regulator case(+5V OP), and I bunched them together and soldered them to my fuse feed.
Just to be safe, I used four separate pieces of heatshrink here.
Use good 10A cable, short as possible, good solder connections.
Don’t disturb the TO3 regulator insulated washers.
Photos

(The other M205 5A fuse in the photo is an extra precaution, and placed in the output of the +9V UNREG) bridge.

After all work, the voltages were all tweaked and checked.
Sleeping better at night now.
.