Metrix 920C Signal Generator, 1960s, Some Notes - UK Vintage Radio Repair and Restoration Discussion Forum

Radio1950 · 5th Jul 2018, 5:37 am

Metrix 920C Signal Generator, 1960s, Some Notes.

I recently used my old analogue Metrix (French, and valve) sig gen whilst testing a JRC NRD515 coms receiver, as it can easily and quickly tune across a frequency range.
My prime sig gens are Marconi 2018 and 2022, and they cannot do this.

The NRD515 RX is another story.

The Metrix had low RF level, and moderate hum on the RF signal.

I found that the 6X4 rectifier V1, in half wave mode, had low emission, and I did not have a spare.

Filter cap C2 was very low capacity as well.

I actually use this sig gen and prefer it to be functional, so I am not so bothered with originality issues.

So I wired a 1N4007 in lieu of the 6X4, added a “compatibility” resistor, and a new filter cap across C2.
All is now well with the world.

But the purpose of this post is not to tell you how to do this mod, as it is only a minor job, but rather to pull a few bits of info together into one post, for possible assistance to others in the future.

Mod details if required.
1 – turn V1 permanently “off” by rewiring heater wire on pin 4 to pin 5 (NC). you can leave V1 plugged in.
2 – add new 1N4007, inter alia, anode to V1 pin 1, cathode to pin 2.
3 – add new R200 5W from V1 pin 2 to C2 terminal (used as tag only)
4 – add new filter cap from C2 tag to earth, suggest 47M 450VW.
5 – V dc at junction R2 and C1 is now approx 135 Vdc, ripple < 50mV.

Previous mods from over the last 25 years –
1 – replace some mains wiring, original had thin insulation
2 – replace input mains connector, and Metrix “mains filter”, with IEC type.
3 – insert 0.1 Mfd cap in RF output to make it AC coupled.
4 – place RF output variable “attenuator” pot P2 inside the sig gen case on a metal “U” support, and use insulated rod for knob spindle extension.
5 – replace original RF Output connector (unusual Metrix proprietary) with silver plated BNC type.

Other Notes –
I had been previously told by our Test Equipment Lab that these sig gens leak RF somewhat through the mains cable, and even from the RF pot shaft, which is why I moved that pot inside.

The RF Output cal is approximate only, and has been set up by Metrix to be cal for high impedance RF loads.

I have an original Metrix Manual, but most info is available for download from the web.

As an interesting aside, I trained in radio and electronics using this exact type of sig gen in the 1960s, and the back of my unit is even marked with our old Radio School name!! We also used its big brother, the Metrix 931. Nostalgia rules!

There are just some things which can be done on an old type variable sig gen that cannot be achieved with a push button entry synthesized unit.

regards to all, mike

Radio1950 · 7th Jul 2018, 7:50 am

By popular request.

Metrix 920C.
Factory Operating and Service Manual in English ex Metrix 1967.

5th Jul 2018, 5:37 am	#1
Radio1950 Hexode Join Date: Apr 2017 Location: Buderim, Queensland, Australia. Posts: 428	Metrix 920C Signal Generator, 1960s, Some Notes Metrix 920C Signal Generator, 1960s, Some Notes. I recently used my old analogue Metrix (French, and valve) sig gen whilst testing a JRC NRD515 coms receiver, as it can easily and quickly tune across a frequency range. My prime sig gens are Marconi 2018 and 2022, and they cannot do this. The NRD515 RX is another story. The Metrix had low RF level, and moderate hum on the RF signal. I found that the 6X4 rectifier V1, in half wave mode, had low emission, and I did not have a spare. Filter cap C2 was very low capacity as well. I actually use this sig gen and prefer it to be functional, so I am not so bothered with originality issues. So I wired a 1N4007 in lieu of the 6X4, added a “compatibility” resistor, and a new filter cap across C2. All is now well with the world. But the purpose of this post is not to tell you how to do this mod, as it is only a minor job, but rather to pull a few bits of info together into one post, for possible assistance to others in the future. Mod details if required. 1 – turn V1 permanently “off” by rewiring heater wire on pin 4 to pin 5 (NC). you can leave V1 plugged in. 2 – add new 1N4007, inter alia, anode to V1 pin 1, cathode to pin 2. 3 – add new R200 5W from V1 pin 2 to C2 terminal (used as tag only) 4 – add new filter cap from C2 tag to earth, suggest 47M 450VW. 5 – V dc at junction R2 and C1 is now approx 135 Vdc, ripple < 50mV. Previous mods from over the last 25 years – 1 – replace some mains wiring, original had thin insulation 2 – replace input mains connector, and Metrix “mains filter”, with IEC type. 3 – insert 0.1 Mfd cap in RF output to make it AC coupled. 4 – place RF output variable “attenuator” pot P2 inside the sig gen case on a metal “U” support, and use insulated rod for knob spindle extension. 5 – replace original RF Output connector (unusual Metrix proprietary) with silver plated BNC type. Other Notes – I had been previously told by our Test Equipment Lab that these sig gens leak RF somewhat through the mains cable, and even from the RF pot shaft, which is why I moved that pot inside. The RF Output cal is approximate only, and has been set up by Metrix to be cal for high impedance RF loads. I have an original Metrix Manual, but most info is available for download from the web. As an interesting aside, I trained in radio and electronics using this exact type of sig gen in the 1960s, and the back of my unit is even marked with our old Radio School name!! We also used its big brother, the Metrix 931. Nostalgia rules! There are just some things which can be done on an old type variable sig gen that cannot be achieved with a push button entry synthesized unit. regards to all, mike Attached Thumbnails