DECMate 1 (VT278) - Defective Memory?

[cchhrriiss11]

Hi,

I'm working on a nice DECMate I just got, the whole system (VT278, Keyboard, RX02, DECWriter IV, complete manual set) is in really good shape. I've done some initial testing and it appears that the memory modules are bad. Each time I power the system on, it fails the POST on test number T6 (Main memory read/write). I've removed, cleaned the sockets and reseated each of the chips on the CPU board - including SIP the memory modules, with no improvement. I've also moved the memory modules around and changed the order, with no change in behavior. There was also no change in behavior/activity when trying each of the modules one at a time (thinking that there might be one bad one that is causing the POST to fail).

These memory modules seem to be very hard to find, anyone have any spares or have any tips on the troubleshooting?


Regards,

Chris

 

[AK6DN]

The VT278 schematic is on bitsavers.
Each module covers 4 bits over 4 fields (ie, 16Kx4 using 4 16Kx1 chips), 3 modules (E37,E39,E41) for fields 0-3, and 3 modules (E38,E40,E42) for fields 4-7.
I would try just populating the field 0-3 group with three modules to see if you can find three that work of the six you should have.
Might also check that the +12V and -5V supplies are valid.
If none of the module combinations are working then I might suspect something in the discrete logic in the DRAM controller (ie, a bad 74xx chip).

 

[pkhoury]

Yay, another VT278 owner. I need to start my own thread. I can't get my CRT itself working on mine. Out of curiosity, did you recap yours at all? Also, is a beep supposed to happen on startup like with the VT100/101? Mine reports power and CPU okay (on the rear LEDs).

 

[cchhrriiss11]

Yay, another VT278 owner. I need to start my own thread. I can't get my CRT itself working on mine. Out of curiosity, did you recap yours at all? Also, is a beep supposed to happen on startup like with the VT100/101? Mine reports power and CPU okay (on the rear LEDs).

I've noticed the following behaviour on mine: If the keyboard is not plugged in, the CRT will not power up -- strangely enough...

There's no beep, just a click through the speaker. As for the two LED's, the power light should be on solid and apparently the CPU light should be flashing (if it's solid, it's in an error condition). If the CPU light is solid, you need to look at the keyboard LEDs to decode the failed test:


-Chris

 

[pkhoury]

Flashing CPU LED... almost sounds like a heartbeat indicator like I've seen on some HP 9000 workstations. I don't know where my VT278 keyboard is, so I was using one from my VT101, but I think they're mostly the same electrically, right? I know I've used the VT278 keyboard on the VT101 before.
It's odd that the LED on the keyboard that's lit is only the first. So it would be X 0 0 0 0 0 0 0.

I think it might've done the same thing before recapping, but I'm trying to figure out why another VT101 isn't working after recapping (in another thread).

 

[pkhoury]

So after reading the pocket service guide, it appears my VT278 failed one of the three fatal tests. First appears to be ROM checksum, then RAM and finally making sure the keyboard is plugged in, which could explain why your CRT doesn't power up if the keyboard isn't plugged in.

I think there might be ROM replacements available.

 

[cchhrriiss11]

So after reading the pocket service guide, it appears my VT278 failed one of the three fatal tests. First appears to be ROM checksum, then RAM and finally making sure the keyboard is plugged in, which could explain why your CRT doesn't power up if the keyboard isn't plugged in.

I think there might be ROM replacements available.

Curious what yours has for ROM? Mine has a daughterboard that plugs into one of the ROM sockets and it's labelled as a "ROM Simulator", looks like this:


-Chris