Heathkit IG-102 Earthing The Chassis

[OldTechFan96]

I'm wondering if I should replace the original thin, two core mains lead of my signal generator? Preferably with a three core lead so I can earth the chassis.

I've read about members earthing the chassis on similar Heathkit gear. I though that I better ask and double check.

I've also included a snippet of the power supply.

Thanks

[Silicon]

That circuit may be from the American version of the signal generator.

If you have a UK version, the wiring may be different.

I prefer to connect the metal enclosure of my test equipment to the protective earth of my mains supply by using a 3 core flex.

[OldTechFan96]

Unfortunately I can only find an American assembly guide. Usually British Heathkit models began with 'U' but this one does not. Component wise this sig gen is all American bar two Mullard valves.

The middle lug of the tag board connects to the chassis. This is probably to most convenient earthing point. The smooth wire is live and the rough wire is neutral.

[David G4EBT]

UK versions of Heathkit equipment have the 'U' suffix: RF1-U, IG-102-U etc. The main difference is the primary of the mains transformer: The U.S. version is for 105v - 125V whereas the UK version, for 230V. It was the habit of Heathkit, and a bad habit too, for the two core mains lead to be simply be passed through a grommet in the rear apron of the chassis and tied in a tight knot as a form of strain relief. Over time, the mains lead could be twisted back and forth in use, risking fracture of the connections.

Furthermore, the mains lead is often terminated on a fairly flimsy tagstrip, then from there, to the mains switch, often ganged with a control pot, sometimes with a single pole switch. Thus, even with the equipment turned off, some of the internal wiring is live. It would have been better practice for the mains lead to go direct to a 2-pole mains switch and from there, to the tagstrip. In fairness to Heathkit, we're going back several decades, and even today, there are those who think that anyone who flags up safety issues '('elf 'n safety' as the cynics like to call it), should be laughed off the face of the planet.

As the IG-102 uses an isolation transformer (as does most if not all Heathkit mains powered equipment), it's fine to earth the chassis and I invariably do, but you have to bear in mind that if you're using equipment such as a signal generator on say a live chassis radio (DAC90A etc), if you connect the generator test lead direct to the chassis of the radio, you are earthing the radio chassis (and thus, one side of the mains. The same applies to any earthed test-gear (not just Heathkit) when working on live chassis equipment, so any such connections must be via high voltage capacitors or the radio etc, powered via an isolation transformer. Ride at your own risk!

I've previously written about the shortcoming of the mains lead in a thread about the restoration of an RF1-U signal generator, but I've added two pics below. The first one shows the old knotted 2-core mains flex removed and the new 3-core mains lead in place. I used a cable restraint to firmly anchor the new flex, but the original grommet could have been left in place and a 'P' clip used on the chassis as a restraint to hold the flex in place. The second pic shows the inside of the cable entry. As will be seen, to earth the lead to the chassis, I just fitted a solder tag under the nearest convenient existing nut and bolt.

If - as it most likely does - the IG-102 uses a Sentercel selenium rectifier, it's worth checking the AC input Voltage to the rectifier and comparing it with the DC output voltage. If the output is appreciably lower, the rectifier will need replacing with a silicon rectifier and series resistor. In the RF1-U which I recently restored the DC output from a good rectifier should have been 125V but was only 95V.

Hope that helps.

[FrankB]

I would be concerned about the chassis ground, filament, and B- grounding to the chassis if you add a 3 wire cord to it.

I always plug test equipment and whatever I work on into separate isolation transformers.

(I remember back in tech school the teacher was, well, to be generous, let's just say not the best, and never taught us about "hot" grounds.
I blacked out the entire building 2 times before we students figured it out. Hot chassis TV set and hot chassis Telequipment scope= Blackout, not to mention frying the TV sets power supply).

Also I'd replace the line bypass cap with a proper X-Y type if it's not a ceramic.

[MrBungle]

When I fixed an RF-1U for someone I soldered the centre (chassis) contact on the tag strip to the mains earth. Worked fine. The chassis is floating on these so all you’re doing is ground referencing it. I made sure I made the Earth wire about 4 inches longer than the other two so it was the last to break off.

I was almost tempted to stick an IEC socket on it as there was enough room once the selenium rectifier was evicted but the owner didn’t like that idea.

[David G4EBT]

Originally Posted by FrankB

Also I'd replace the line bypass cap with a proper X-Y type if its not a ceramic.

A good point well made Frank, but they're 1000pF 1000VW disc ceramic caps (four of them) in the mains input filter, so unlike paper caps, aren't prone to failure.

[MrBungle]

They do tend to explode if there’s a transient though so it’s good advice. I’ve seen an exploded one before.

[OldTechFan96]

Thanks for the advice everybody! I'll go ahead and earth the chassis.

Does anybody know how to create an effective strain relief for the new mains lead? The original one can't be used as it not suitable for 3 core lead.

I tried to use large cable ties but they did not grip the cable as tight as I would have liked.

[Herald1360]

The sort in David's picture look perfectly adequate.

This kind of thing

https://uk.farnell.com/pro-power/pp0...train%20relief

Though available in smaller quantities from Ebay sources.....

[MrBungle]

You can get them (usually next day) in single quantities here: https://www.bitsbox.co.uk/index.php?...&cPath=185_191

[OldTechFan96]

I have just ordered a variety of strain relief devices from Bitsbox.

I was prompted to earth the chassis when I tried to use this signal generator in conjunction with my Hameg oscilloscope. There was sparking between the generator's leads and the chassis of the TR82c I was working on. This was with the osciloscope's leads already connected.

[OldTechFan96]

I have fitted a 3 core mains cable and a suitable strain relief. Earthing the chassis has solved the sparking problem I mentioned above.