now thats down right annoying eh ? (pun intended

)
have u got a copy of the qbert manual with schematics ?
1. print the schematics and stick them together so you have a full page schematic to trace. dow the same with the wiring diagram. these will help follow the following descriptions.
the down right switch is pin 19 on the PCB card edge.
have you checked that the earthing of the switch is also good ? i ask that as vibration coould be enough to stop it working if you have a dry joint (or a crack in a joint of the trace into the buffer IC).
pin 19 is IP40, which connects to B14 via R16.
when down right stops working there are a couple of tests you can do
1. check that R16 is not broken or open circuit or high resistance - with qb turned off, test continuity/resistance - it should be 470 ohms. if open circuit, rpelace or tack a 470 ohm or similar value resistor across r16 and retest.
2. check SIP 2 - that there is continuity/resistance between the JS side of r16 and pin 1 of the sip2 package - it should read 1k. if it is open circuit, you could temporarily tack a 1k resistor between r16 JS side and +5v. see if that makes a diff. look at the board layout to find SIP2
3. check the capacitor c16 (.1uF ) - lift or cut the leg and tack a .1uF monobloc or green cap or ceramic cap and retest.
4. if none of the above make a diff, need to look closely at B14. with QB turned on,
- short the downright switch with some clip leads (card edge pin 19).
- and short down left switch with some clip leads (card edge pin 20).
then test the voltages at IC B14 (74ls240) that buffers the switches.
- downright input is pin 8 of B14, and
- down left imput is pin 13 of B14
then test the voltages on the other side of B14 for each of Down left ande down right. as B14 is an inverting buffer, then with the js switch shorted to ground, the corresponding output pin should be logic HIGH or >2.x volts.
-downright output is pin 12 of b14
- downleft output is pin 7 of b14
if u are not getting a logic high for pin 12 the replace B14. ditto for pin 7.
hopefiully that should fix your issues with downright. dont forget if u haved lifted legs or cut legs, replace or resolder the appropriate component too.