Adding grounded power cable to old VOX Westminster

[Andyzoom]

Hey there. New to the forum! I'm Andy, musician, studio engineer and electronics amateur. Have tools and experience with building, modding and repairing my own gear out of necessity but always learning.

I have a 60's Vox Westminster amp that I want to put a grounded power cable on. If you are plugged into this thing and touch a grounded source like a hot mic or someone else that is plugged into another amp, you get quite the zap. (not life threatening but its not pleasant)

From the service manual it looks like the turret boards and component assemblies have ground points but just no ground from the power cable. It also looks like the line reverse function has a ground point associated with it. Could it be that which ever position the line is in, the lead that isnt feeding the power supply is then grounded?

What I want to do is simply replace the existing power cable with a grounded cable and ground the cable to the chassis.

I know grounding can be a complicated situation and can even cause more problems if the ground is done incorrectly or changes the existing grounding scheme (which this doesn't seem to have!). I'm also a novice at reading schematics so want to make sure I'm not going to mess something up or worse hurt myself!

Thank you for any advice or recommendations.

[Dave H]

What I want to do is simply replace the existing power cable with a grounded cable and ground the cable to the chassis.

Yes, do that and remove the cap between the chassis and the Line Reverse switch (aka death cap)

[SoulFetish]

What kind of tools do you have? Do you have a large soldering iron or crimp tool?

[vintagekiki]

https://music-electronics-forum.com/showthread.php?t=44736
Vox Essex Bass Grounding Question

[Andyzoom]

Yes, do that and remove the cap between the chassis and the Line Reverse switch (aka death cap)

Nice, I'll remove the cap. Thank you.

I have a nice soldering station and other tools but I dont have a crimping tool beyond the crimper on my stripper. What would the crimper be for?

Thanks for the link, kiki, I'll check that out.

[glebert]

Are you going to disable the ground reverse switch? Don't know if it would be an issue but I once had to chase down a nasty hum in a Vox Scorpion where the preamp lead dress would get too near the switch when you closed up the chassis. Open chassis? No hum! Closed chassis? Tons of hum! That was a pain.

[R.G.]

The Thomas Vox Westminister shares a lot of things with the Buckingham, Guardsman, and Beatles. One of these things is a very outdated grounding system. It is fairly difficult to get these things grounded for safety and for low hum. Here's one way, with descriptions of the difficulties and ways around them.

The incoming AC power cord should have a safety ground, of course. This needs to exit the power cable near the cable entry and strain relief. From there it goes to a safety grounding lug. This lug is on the metal enclosure/chassis, and is used for safety ground, nothing else. The metal around the hole should be sanded to bright metal, a screw stuck through the hole, then this stack of parts put on the screw inside the enclosure:
1. Star/toothed washer to bite into the chassis.
2. The ring terminal that is properly crimped onto the safety ground wire.
3. Another star/toothed washer to help prevent the nut from coming loose
4. A matching nut.
The safety ground wire going to the lug is to be made long enough that if you could pull out the line cord in spite of the strain relief, the safety ground wire would break last.
A difficulty with the TV Westiminister/Buckingham/Viscount/Guardsman, and Beatle is that the preamp subchassis is where the AC wire enters, and the power amp chassis is not connected to it via a safety-standard connection. Worse, the power amp chassis sends a speaker output and speaker return out to the back panel, making it "accessible metal" by safety standard, so the power amp chassis also needs its metal frame connected to AC safety ground with a connection capable of carrying 25A of AC current so it can blow any code-meeting breaker. So you need to provide this wire, which does not already exist. Possible solutions are to put another wire into the 9-pin cable connector attaching the two, or to make up a safety ground wire between them that meets the standard. This is arguably tough either way. I hate making up crimped-connector cables.
When you get this put in, you have solved the safety part of the requirements. It may also lower hum a bit. However, the TV "Big Head" amps have some other grounding issues that cause residual hum. One is that the power filter caps are twist lock types which are soldered to the chassis. Good for overall grounding, but the rectifier setup makes this prone to rectifier current buzz because there has to be a wire from the power stage ground (which carries the rectifiers) to the chassi ground on the filter caps. It gets worse if someone has hacked in non-twist lock caps under the chassis. The critical thing is that the power transformer center tap must go directly to the junction of the two first filter caps and no where else. If this is not true, you will get rectifier buzz.
Another issue is that signal jacks and controls are signal grounded to the preamp chassis, and two wires carry signal ground from the power chassis to the preamp chassis. One of these also carries DC power to the preamp.
One of these is the preamp output RCA jack. You can cut the output hum by drilling out the rivets and removing this jack and replacing it with a bushing-mount RCA jack and isolating shoulder washers. Mouser sells both. With the "send" jack disconnected from preamp chassis ground, you need to reference the shield. The power amp (soldered end) of the cable from the power amp ending in an RCA plug probably ought to have the ground connection disconnected on the power amp end, and the preamp jack hard-wired to the signal ground on the preamp board, as well as installing a 10 ohm resistor on the jack from jack signal ground to chassis. This is in line with the shielding practice of grounding the shield at the quiet end.
The power ground sent to the preamp circuits by the 9-pin cable is going to get you some hum that may not be possible to eradicate. However, the indicator lights for on and standby, also eat a lot of current, and the 24-27Vdc supply to them carries ripple. Making these lights NOT use the ground return for the preamp power makes a noticeable reduction in 120hz hum. Sadly, this needs yet another wire put into the 9Pin cable.
It might be better to just assume you're replacing the connectors on the 9-wire cable, get a male/female pair of Mate-N-Loks with 12 contacts and just make the extra wires go in there.
Like I said, it's messy.

[pdf64]

I’m rather dubious that the crimp connections made with typical cheap auto store type crimp tools are in any way suitable for such safety critical circuits. They’re a world apart from professional grade tools.
A mechanically secured then soldered connection of wire to eyelet lug seems a better method for regular small shop techs / DIYers?

[Mick Bailey]

I have some cheap crimp tools that have come my way over the years and also some ex-military tools as well as AMP ratchet tools. The difference in crimp quality is considerable and with the cheap ones it's easy to get a reasonable looking crimp where the terminal will pull right off with a little force. The best ratchet tools I have won't release until the crimp operation is completed and the calibrated force is fully applied. They have a specific and fairly narrow range of wire and insulation sizes that are approved. The plier types stamped from sheet metal are pretty bad and most are far-eastern copies of much better western models. I have some UK and USA ones about three times the thickness of the Chinese ones. Even so, the applied force is not calibrated and relies on operator skill. The jaw area is also not as good as the ratchet types with machined and accurately aligned jaws.

[R.G.]

I did say "properly crimped". That's always a challenge.

I'm doubtful of the typical auto-parts store crimping equipment, too. However, a properly crimped connection - folded, pressed to the metal-distortion point by calibrated crimpers - is as good an electrical joint as a soldered joint, and can be mechanically tougher.

If I didn't have proper crimpers, I'd solder a wire onto a ring terminal which has a solder eye as well as the ring for the screw post. I would prep this properly, so the wire is held into the solder eye by correctly done wrapping, then solder it. It's better than no safety ground, for certain. Solder creeps under pressure, even surprisingly light pressure where the joint can be mechanically stressed. If the solder is there just to keep air out and enhance conductivity, it's probably a good thing, but solder is sometimes used to cover up sloppy joint preparation. A sloppy, low pressure crimp that's been soldered over looks better but this amounts to painting over the defects.