Unwell 800XL - MMU Running Hot

[cwc]

I’ve got an 800XL with (the brilliant) SuperVideo 2.1 modification. The computer which works fine for about 2 hours then has video issues e.g. occasional slight distortion of the picture and also slight interference at the right of the screen which seems to reflect SIO access. I noticed some distortion around the sprites in World Championship Karate. Nothing major, but rather annoying. I opened the 800XL up and found that the MMU (CO61618) is *really* hot to touch when the computer is running. I could find nothing else running hot and couldn't see anything else that was unusual. If I run the 800XL on the desk without case or metal shield the issues do not occur. All the internal Self Tests pass with flying colours. It would appear that the overheating MMU is causing the issue, but could another component be causing the MMU to run hot? Any ideas?

 

[1050]

MMU always does run hot, probably the hottest running chip on the board. Maybe swapping it for a GAL16V8 might result in a reduction of temperature?

 

http://www.atarimax.com/jindroush.atari.org/achmmu.html

 

This site has logic files for both PAL16V8 and GAL16V8, you already have the PAL16V8.

Problem with this approach is the most people don't have GAL16V8 programmer in their back pocket all the time, let alone the chip itself.

 

I don't think another component would cause it to run hotter than it normally does. Would be the first time I've heard of the issue too. Perhaps just the right size piece of a finned heat sink on top of it might help? Sounds foolish but I would want to be sure which part is getting too hot - put it back together only plumb in some aquarium line to the MMU area and the upgrade area. Use an air pump to try to cool one and then the other when symptoms show to see if symptoms go away? Canned air for cleaning computer could work here too.

[+DrVenkman]

You know, coincidence or not, I have an 800XL that I did the fantastic "Quick & Easy Video Upgrade for 800XL" on. It ran great for a month or so, then the hex buffer chip (4050) died. Replaced that and 6 days later the C50 cap blew. Hmm.

 

I wonder if the 800XL's video circuitry is just that susceptible to overheating, or causing nearby components to overheat?

[OX.]

Is there enough clearance from the keyboard to add a simple heat-sink?

[Joey Z]

You know, coincidence or not, I have an 800XL that I did the fantastic "Quick & Easy Video Upgrade for 800XL" on. It ran great for a month or so, then the hex buffer chip (4050) died. Replaced that and 6 days later the C50 cap blew. Hmm.

 

I wonder if the 800XL's video circuitry is just that susceptible to overheating, or causing nearby components to overheat?

Cap is probably unrelated, might have even been why the 4050 blew, if there was excessive AC on a power supply line. Looking at that mod, I see some questionable logic there... I don't doubt it works, but the reasons behind some of the changes don't sound exactly correct. One such change is the decrease in resistance value of the emitter resistor of the luma buffer, to try to match the '75 ohm' impedance of the video line. Well, the atari also has a series resistor on said video line, of 75 ohms... This means the normal method of double terminating a transmission line (which is what I assume is what this resistance change tries to accomplish) becomes more complex in this case. This will also cause more current to be drawn through this transistor, potentially decreasing it's life, or possibly even exceeding it's absolute maximum specs. Another change is the collector resistor for the same amp, is decreased in value, which supposedly 'increases the gain.' In fact, this only increases the maximum peak current output, but does not affect any gain figures, current or voltage.

 

But like I said, if it improves the picture, then it improves it. I'm just not too sure about the justification behind it.

[+DrVenkman]

I don't mean to get too far off-topic here, but that video mod I did looks fantastic. The majority of the improvement seems to be accomplished by pulling out C56 (the "blurring capacitor"), which I did on my second 800XL when I moved my U1MB to that machine. It looks quite a bit sharper than stock, but still not as vibrant as with the rest of the component changes.

 

Anyway, back to MMU issues of the original poster - have you tried running the machine with inside the case with the RF shield removed? Do you get any graphics issues that way?

Edited August 2, 2015 by DrVenkman

[Keatah]

You know, coincidence or not, I have an 800XL that I did the fantastic "Quick & Easy Video Upgrade for 800XL" on. It ran great for a month or so, then the hex buffer chip (4050) died. Replaced that and 6 days later the C50 cap blew. Hmm.

 

I wonder if the 800XL's video circuitry is just that susceptible to overheating, or causing nearby components to overheat?

 

Or maybe the design of the modification is not within specification?

If the mod stresses some other parts, an older machine that has degraded through natural aging would have multiple failures soon.

 

Kinda like giving my gramma a Call of Duty Monster Energy Drink on a summer day!

 

I also have to ask the credentials of the guy selling you the mod, does he have degrees in all the disciplines involved in that mod?

Edited August 2, 2015 by Keatah

[+DrVenkman]

 

Or maybe the design of the modification is not within specification?

If the mod stresses some other parts, an older machine that has degraded through natural aging would have multiple failures soon.

 

Kinda like giving my gramma a Call of Duty Monster Energy Drink on a summer day!

 

I also have to ask the credentials of the guy selling you the mod, does he have degrees in all the disciplines involved in that mod?

 

Um, as the link takes you to an 11 year old AA thread, that was actively updated over the course of about 5 years, I take it you didn't look it over yourself. It's a do it yourself mod that removes a few components, adds a couple others, and adds separate chroma output to the 800XL video jack. It works wonderfully. Now if I replace the C50 cap and it goes sploogy in a month, or takes out another 4050, then I'll have a better reason to suspect it has inherent problems unrelated to the overall age of the machine.

 

At any rate, my backup 800XL is alive and well, so repairing the first one can wait a couple weeks.

Edited August 2, 2015 by DrVenkman

[cwc]

Many thanks for all the replies. As the 800XL seems to work fine with the RF shield off, I am going to try a heatsink on the MMU. Just picked up one of these from Maplin: http://www.maplin.co.uk/p/ic-heatsink-1416-pin-rn69a. Just waiting for adhesive thermal pad to arrive tomorrow. I'll let you know how I get on.

[cwc]

Many thanks to all who responded. An update: I tried a heatsink on the MMU and, on closer inspection of the motherboard, found that L10 (RF choke) looked burned so I replaced it. However, the problems remain. Taking Rescue on Fractalus as an example of how the problems appear after the 800XL has been powered on for a couple of hours, the Lucasfilm title, the animated loading screen, the text on the title screen and high score table are all graphically rock solid. However, when the game goes into cockpit view on the demo mode the graphical glitches (flickering and distortion) occur. I am now wondering whether the issue is restricted to a particular graphics mode and whether the GTIA (it’s a UK PAL machine) or ANTIC may be at fault. I’m still sure that this is an issue triggered by heat so what I think I will try next is removing and reseating the GTIA and ANTIC after cleaning their pins. I will also try 1050’s idea of piping coolant on to suspect components while the 800xl is displaying the symptoms to try and identify the culprit. I will report back in due course. Any other ideas in the meantime?

[1050]

Only other suggestion that I can relate now is to change out your CD4050 for a 74HC4050. CD4050 has a unique input which may be the cause of extra power use and thus heat in the modified circuit and seems to be a trouble spot there, so it appears the mod may indeed effect stock chips after all. HC4050 is faster, better and also CMOS as is the CD4050 without the funky input and possibly without the funky input problems.

 

Today, I found this in passing:

http://electronics.stackexchange.com/questions/141830/cd4050-voltage-leaks-what-can-i-be-doing-wrong

 

Bogus chip alarm is about as real as there is a device designed to fail in every chip made,

 

http://atariage.com/forums/topic/241423-to-mod-or-not-to-mod-that-is-the-question/

 

or better, that the remote control eye on your TV set is really a camera watching you.

 

When I type the phrase used above, I'm always reminded of 'we can build him faster and better...' He missed his calling though, as a mechanic, the six million dollar man wouldn't have needed no stinking impact wrench and he coulda torqued them head bolts up with two arms at the same time !!!

[cwc]

Update: Lifted, cleaned and re-seated GTIA, ANTIC, MMU, CPU, OS and BASIC ROMs but symptoms persisted. Freezing the ANTIC (only) made the issues disappear. The ANTIC runs no hotter than any of the other large chips and is certainly nowhere near as hot running as the MMU, but the ANTIC is indeed the problem. I won't have too much difficulty making up my Best Electronics minimum $20 order now! Many thanks to all who responded.

[Paul Westphal]

I take the rf shields off all the systems I sell. 1- you don't need it, 2- it may short out somewhere, 3- it cools better without it, 4- repairs take less time.