ground wire running to the battery box securing bolt that's good.
'Fraid not, as much use as a chocolate teapot ...
Supply is the Black wire from battery -ve. Btw, have you had a look inside the 3-pin plug that connects the Rita to the main harness?
Switch supply from the amp. to the coils is the White/Black wire to (as standard) the timing-side coil -ve terminal.
Return for the amp. electronics themselves is the White/Yellow to (as standard) the drive-side coil +ve terminal. The other White/Yellow on the drive-side coil +ve is the return from the coils series and continues the electronics return to the kill switch, kill switch connects to a White wire to ignition switch terminal #2, Brown/Blue from ignition switch terminal #1 through the fuse to battery +ve completes the 'normal' ignition circuit.
The only connection to 'ground' through the amp. casing is from the internal Zener for spike protection. Only if the amp. is rubber-mounted does the case need a 'ground wire', which should be to the battery terminal itself. If the amp. is rubber-mounted, ever needs spike protection and there isn't a good electrical connection from the "battery box securing bolt" to battery -ve itself, the amp. doesn't have any spike protection.
I'm suspecting the control box is faulty
Seems amazing that the guy was using it as a daily rider, you bought the bike and the amp. stopped working?
Rita is one of the ordinary old EI's that will spark when you turn the ignition off.
+1. Ime, the plugs usually
spark when the ignition circuit is broken (e.g. turn off the ignition switch). Might not happen every time but only if it doesn't happen at all despite making and breaking the ignition circuit several times do you worry about the amp.
When the ignition is switched on it usually throws a weak spark, sometimes immediately and sometimes after a second or two...sometimes not at all..
Ime, the plugs never spark when the ignition circuit is 'made' (e.g. ignition switch turned on); if the plugs do seem
to spark when the ignition circuit is 'made' , that's something like a faulty switch - corrosion, worn contacts, dodgy connection or wire, etc. causing intermittent breaks
as the switch is operated.
What's the rubber cover over the ignition switch like? '79 originally had the same cover as the T160, with a 'slot' in the cover for the ignition key. Even if this hasn't been worn into a hole, rain-water, condensation, washing water, etc. is going to get into the switch especially on a daily rider in NY. Rebuild the ignition switch to better than new
; even before Wassell was allowed to brand its rubbish "Genuine Lucas", I found 'original Lucas' ignition switches better than pattern - the one on my first T160 is the original, the one on my second T160 is the one I replaced a pattern one with in 1982.
Whether you rebuild or replace, before refitting on the bike, drill a small hole in the terminals backing at what'll be the lowest point when it's refitted - lets any water that does get in drain straight out.
Also replace the '79 switch nut and cover with the '82 97-7120 Nut, 97-7123 Ignition key cap and 60-7335 Ignition switch cover.
I touched one of the two wires on the pick up to earth...Each time I earth it, a sizzling blue spark from both plugs
The recognised test is touching amp. White/Purple to (-ve) earth should generate a spark at the plugs, as does passing a screwdriver across certainly the common aftermarket-Rita pick-up; haven't tried the latter on a '79-on standard 5PU?
try drying the box out in a warm cupboard/ airing cupboard, boiler room or such.
might just be damp.
If it starts working again after that, remove the side of the amp. held with four small hex, screws, clean off the sealant, check the wires seal, reseal the side - Rita internals should never be affected by damp.
Otoh, John's reported in the past that the original position of the 3-pin plug means it catches crap off the back wheel. He said there was a modified one, so you might to check what the modified one looks like? Like plug connectors on Japanese bikes, I exclude moisture from my T160's Rita plugs by filling 'em with Vaseline (petroleum jelly) when assembling;
maybe run the box ground straight to the battery pole to cut out any nonsense.
As with any e.i., you can bypass the switches, fuse and wiring in the ignition circuit return by - in this case - a wire from (as standard) drive-side coil +ve direct to battery +ve. You might also want to try a wire from battery -ve to the amp. Black wire, to bypass the standard negative feed wiring.
If it turns out not to be ok, a replacement - based on the earlier and more-reliable 'aftermarket Rita' pick-up - is available from a British seller.