Need VT240 help

[DrCharles]

I recently acquired a VT240 in unknown but long-term-storage condition. After cleanup it worked well for a few minutes, then the dreaded Rifa X-caps blew up. Fixed that.

Terminal was working fine until today, displaying the checkerboard then two different grey bands that would grow upwards from the bottom of the screen, then beep and show the expected VT240 Monitor Error 9 since I have the BNC on the back connected to an old CCTV monitor, and work perfectly after that.

Now it won't finish self-test (displays the checkerboard, keyboard lights go on then off - but then hangs with a blank screen and the "Wait" and "Lock" lights on).

I suspect one of the OTP ROMs has lost a bit, and would like to compare mine to a known good copy. Does anyone have a set they could read?
Of course there are many other places that this complex system could be hanging up. It could be a long slog (burning ROMS for the T11 and 8085 processors to exercise them)...
There are no obvious shorts, burned components, or abnormal power supply voltages.

I found a thread here from 2012 with a very similar problem, but there was no follow-up from the OP as to whether he ever fixed it.

Thoughts?
thanks for any help!

Edited to add: I found a set of VT240 ROMs on MAME of all places... downloaded them, now to figure out how to get the files into my old PB-10 programmer and compare to the live ROMs

 

[DrCharles]

OK. the keyboard is working properly as far as I can tell, data is going in and out, and I even swapped it for the keyboard on my VT220 and the same symptoms persisted.

I just verified all four ROMs on the T11, and the one for the 8085. So far so good.

One interesting finding - two of the lines (DAL3 and DAL1) on the T11 do change states several times, but once the self-test has crashed, they stay high with almost one volt of "wiggle". All the other data/address lines are either high, low or switching between a good 1 and 0.

There are several places that the bus connects, including the ROMs, dynamic RAMs and various latches & bidirectional buffers. I connected a 10 ma VOM between each line and ground (to make sure a low-resistance path (such as in the 'LS245 at E55) wasn't holding it high somehow. All of the DAL15-0 lines requires more than 1.9 ma to bring it to ground (well, 50 mv burden at 250 mv full scale, anyway).

That leaves the unlikely possibility that one of the octal TTL devices has developed a weird internal pathway that only interferes with DAL3 & 1 on some bit patterns, but not all the time. Seems like a zebra rather than a horse.

The T11 spec sheet says that a good logic 0 (<0.4 volt) should be possible with up to 3.2 ma sink... So I suspect the T11 has a couple of bad output pull-down transistors on those lines. Anyone got a spare T11 chip I can buy or borrow? Or send you mine to plug into your board and see if it fails the same way?
thanks.

 

[DrCharles]

I swapped in a T11 that I remembered was on the RQDX3 card in my PDP-11/23+
That didn't change anything though. Nor did piggybacking both 4116's that are on those two lines.
Found the problem by just piggybacking each RAM in turn and on the very last one (16 of 16) the terminal booted and works again