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EM Interference on C64 RAM?

DrAM19

Experienced Member
Joined
Mar 5, 2021
Messages
75
Location
Near Washington, D.C., USA
Hi Everyone,

I am repairing an NTSC C64 breadbin (205407) for a friend and I have encountered a very strange situation.

When I first got it, it displayed a black screen. Checking the PLA in a working C64 of mine showed it was bad. Swapping it with a working PLA did not get me a picture on the screen.

I next noticed the reset line did not release to 5v on startup. I desoldered the reset legs from the CIAs (not socketed chips and I didn't want to cut the legs; I verified they were isolated after desoldering) and confirmed the MOS 7707 Hex Inverter at U8 was bad, so I socketed U8, replaced it with a 7406, and resoldered the two CIA legs.

At this point I got the normal blue startup screen, but had an "Out of Memory Error 0" message, along with some garbage on the screen. Following the errant characters, I started desoldering and socketing RAM one at a time. Each time, one class of junk character disappeared. This continued until I socketed and replaced all 8 4264 RAM chips. (MT RAM unfortunately; and I checked them in a chip tester, all 8 were bad). I then finally got a BYTES FREE message. So now I highly suspect a bad power supply or major static situation.

However this is where a very unexpected symptom appeared.

Sometimes the BYTES FREE message has the wrong value, and sometimes the screen stays black on boot. If I get the BYTES FREE message on screen, and my hand goes near the RAM bank (and I do not touch it), I will get some of the junk characters on the screen again. And they do seem to correspond to which chip I am near (U21 puts a bunch of ! on the screen for example).

Has anyone else ever seen this? What was the cause? My inclination is bad capacitors (C14 is electrolytic, C25-28 and C40-43 are ceramic), but I do not have great evidence to support this.

Thanks!!
 
I had the incorrect memory issue on a 250407 assembly board not to long ago. The RAM was bad. I got a diagnostics cart and it showed 2 were bad, I replaced them and more came up as bad. I ended up replacing the 4 failing chips, and eventually all 8 due to them all being the same brand.
 
Go for the simple explanation first - you have some bad RAM.

If something is 'open circuit' internally, it will be floating and acting as a small antenna.

Dave
 
That's fair advice.

I am waiting for a c64 diagnostic cart in the mail. I have one of Stephan's RCT Pros and tested the RAM before I installed them. All tested good but I'll double check in the tester tonight, and try some other spare RAM I have in stock. And if I haven't figured it out by then, I'll have the diag cart in a couple days.

Good point on a floating open acting like an antenna.
 
Check your socket wiring completely (bottom, and top especially, where it will be hidden). It's easy for a pad/eye to come loose and go unnoticed. It may be barely touching.
 
Well. All 8 MT4264-15s chips in the board tested good. I then decided to try a binary sort to verify this RAM.

I pulled 4 working MN4164-15s out of another board and put them in one of the rows. It did not fix it. So I swapped in the original 4 and moved the MNs to the next row. Still did not fix it. So I installed all 8 MNs I had, and wouldn't you know, that fixed it. Annoying!

Needless to say, all of my MT RAM is now in the garbage.

In the end, this C64 with a black screen had a bad PLA, a bad hex inverter, and 8 bad MT RAM chips. And now it works.

Thanks for the input, everybody!
 
Just out of interest, what was the part number on the MT RAM itself? Of course, there are different speeds etc. associated with different parts. Your MN's are probably 150 ns access time devices based on the part number.

Dave
 
All of the MT4264s I had spare were 150ns (MT4264-15). 2 of the 8 on the board originally were 200ns MT4264-20s. The other 6 were 150ns MTs.

The MT4264-15s I installed from my spares were all from 4th week 1987. These tested good in the RCT, but clearly there was a problem. I had other spare MT4264-15s that tested bad, they don't have a normal date code, but have a number like 408, or 409, or 437 on them.

Also, the bad hex inverter was a MOS 7707 with a date code for 50th week of 83.
 
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