Bad 12v line ISSUE 6A with only 5v RAM
Posted: Sat May 01, 2021 6:33 pm
Hi, I ve got this ISSUE 6A TIMEX/PORTUGAL from a CZ2000 (Argentinian rebranded)
Thing is that 12v line is down to about 1v and -5 to about -2v or less (would be more ... nearer to cero) .. but this RAM doesn't need that.
Thing is the LM1889 is not getting feed.. You can see (c) The sinclair research text there so I know ULA, CPU, ROM and at least Lowwr RAM are working.
14MHZ cristal, ULA pin 39 and 32 and CPU pin6 are OK.
ULA PIN 17gives a nice positive nice syncro signal at 15.6Khz.
BUT LM1889 has pins 14,15,16 near 1v and no signal at al at 17,18..
PAL Crystal has no signal as spected.
I've already replaced tr4 and tr5 using some transistors that worked there on other boards like this. But I still have no luck.. I checked pins of Tr4 and neither has 12v, and checked both pins of C74 and has 5v and a bit less the other. I think I will replace C74 and C46..
I the other hand I had to replace Tr7 and the speaker. It was getting nearly 9v from the tr7
Thing is. I am learning with this process (and this forum). So I may have missed something.. .. and where do you recommend to check next..
This one has MCM 4517 low ram IC that only needs +5v being pin compatible with 4116 (this is not common here but not so rare either) Thing is that 12v line is down to about 1v and -5 to about -2v or less (would be more ... nearer to cero) .. but this RAM doesn't need that.
Thing is the LM1889 is not getting feed.. You can see (c) The sinclair research text there so I know ULA, CPU, ROM and at least Lowwr RAM are working.
14MHZ cristal, ULA pin 39 and 32 and CPU pin6 are OK.
ULA PIN 17gives a nice positive nice syncro signal at 15.6Khz.
BUT LM1889 has pins 14,15,16 near 1v and no signal at al at 17,18..
PAL Crystal has no signal as spected.
I've already replaced tr4 and tr5 using some transistors that worked there on other boards like this. But I still have no luck.. I checked pins of Tr4 and neither has 12v, and checked both pins of C74 and has 5v and a bit less the other. I think I will replace C74 and C46..
I the other hand I had to replace Tr7 and the speaker. It was getting nearly 9v from the tr7
Thing is. I am learning with this process (and this forum). So I may have missed something.. .. and where do you recommend to check next..