STFM RAM Diagnostics

[Steve]

Hi all,

Who's the plonker who thought he'd rather solder all the RAM directly in and not using sockets.. yes. This would be me.

Well long story short there's a problem with the 512k I soldered in, I get a black screen with white borders, the machine worked fine beforehand. I thought I could isolate the new banks by soldering resistors R71, R72 and R73 to ground but I still get the black screen white borders, so it seems not.

Is there a good way to disable all this RAM I just installed to 100% confirm the issue. Then could I use my fancy new Oscilloscope to find out which chip(s) could be faulty? Thanks

PXL_20210208_163907524.jpg

[exxos]

For fear of stating the obvious, did you actually test the RAM individually in a known machine to make sure each RAM chip actually worked before soldering it in?

[PhilC]

@Steve have you tried booting it eith a diagcart yet?

With serial connected you should get an output that will point you at the direction of the faulty chips. That and it'll keep the ram active and will help with scoping it out.

[Steve]

My diagnostic cart doesn't work. I'm just gonna take it all back out, I was stupid putting it all in in the first place. Sorry.

[DoG]

Connect R71 and R72 to +5V instead of ground. R73 as well if you want but it is not needed.

[rubber_jonnie]

I'm a bit surprised you get indications of no RAM at all.

First off, I must shout the word "Sockets" at you so you don't do it again!

I would suggest that before you start desoldering, you should buzz out the address and data lines on the new bank to see if there are any bad lines.

Similarly check that all of the chips have a good GND and VCC connection.

As for the diag cart, I'd suggest that you hook up to the serial port and see if you can see any output there, it may give you a clue.

Of course the other thing is that you have obviously been handling the board to do the upgrade, which may have jogged the MMU and GLUE, so worth removing, cleaning /tweaking the contacts and reinstalling.

[mst]

You can use YAART.TOS program to diagnose the exact failing address,

yaart020.zip

To disable a chip, you need to desolder the RAS/CAS; but there is a risk

[Steve]

It's okay guys sorry for wasting anyones time... I really shouldn't have soldered it all in without knowing it all worked in the first place - just making myself work for no reason. I've just removed it all and the STFM is back working, I'll just get a 4MB upgrade kit from Exxos instead.

[stephen_usher]

If you fit sockets, you can make the Exxos kit reversable without desoldering. (However, you need to trim the pins and make sure the underneath of the PSU is insulated just in case as it's a tight fit.)

[rubber_jonnie]

Might be worth building a DRAMduino, that way you can test your RAM chips.

I did also build a ZIF to 16 pin adapter that plugs into a socket and let's you quickly swap out RAM for testing.

The DRAMduino is a bit faster for testing.

I've had a couple of mainboards fail when the extra RAM is installed my theory is that the boards were tested and failed for 1meg so got a half.

I managed to get one working, it just needed some bodge wires, the other never worked with the extra RAM.