Hi. 

I had a fully working but very dirty High Speed until I removed the playfield to clean it. On re-fitting I have a clean High Speed but no longer fully working 

I can't get into audits or adjustments with the coin door buttons. If I press the Advance button the pin goes straight into MUSIC TEST and I can't advance out of it. I have to switch the machine off. The buttons were working fine before. 

The ADVANCE and AUTO UP/MANUAL DOWN buttons connect to U14 which I  swapped but no change. I also swapped U56 but misread the poor quality copy schematic and should have swapped U36 which is connected to the MEMORY PROTECT switch. Surprisingly swapping U56 didn't change anything  

I have checked U36 (7404) pins 10 and 11 (MEMORY PROTECT) with the logic probe and it seems to be giving the correct output for a given input, which changes when the switch is pressed as expected.

I can start and play a game OK otherwise. 

The only other IC in play is PIA U51. Any suggestions before I pull the trigger on this chip?

Thanks 

Andy 

Finally enlisted the help of my daughter as I couldn't reach U14 with the logic probe as well as press the coin door buttons.

As soon as I started testing the buttons they started working again, without me doing anything. They continued to work after several power cycles. The next day I was again only able to get MUSIC TEST.

I think it may be an issue with the AUTO UP/MANUAL DOWN button as if its a logic failure I doubt it would be intermittent.

Just realised I haven't finalised this post. I had a loose wire on the diagnostic buttons that must have been making intermittent contact.

It finally broke off the other day. Resoldered and all good.