IIfx Running at 16MHz?

[Blib]

I'm trying to restore a Mac IIfx. It had some minor battery and SMD cap leakage which were affecting the power on/off circuit, but I cleaned up, reflowed, added a couple of bodges and replaced C9/24 with tantalum caps. To my delight it now boots and launches MacOS 7.5. Everything seems to work fine - sound, games, diagnostics tests.

But it's dog slow. I think it may be a problem with the CPU clock speed. Apple Personal Diagnostics v1.1.3 states that the CPU is running at 16MHz. Tattletech 2.59 says 12.61MHz (though it confirms that the bus is running at 40MHz). I'm struggling to understand the CPU underclock. I've tried replacing the CPU, FPU and 80MHz crystal with known good parts, but there's no change.

Interestingly, I see that J20 is jumpered on this IIfx, but is left open on my working IIfx. I wondered if this was the issue, but if I remove the jumper, the slow IIfx crashes with bus errors.

For what it's worth, I'm using the stock ROM (tested in the other IIfx), a SuperMac Thunder 24 videocard, a BlueSCSI v2 (have also tried the original HDD) and have tried everything from 4MB to 128MB of known good RAM.

Would really appreciate any thoughts folk may have!

 

[nathall]

How about the other two jumpers, J103 and J106?

This may be of interest: TIL 7446

I remember reading somewhere J20 also has something to do with cache. I think.

 

[Bolle]

I remember reading somewhere J20 also has something to do with cache. I think.

That one disables the 68030s internal caches.

 

[Blib]

Thanks guys! I've been experimenting:

• If I jumper J20 on the working IIfx, then I get the exact same symptoms as the slow IIfx. So I guess that explains the slowness and the CPU readings from APD and Tattletech - seem that it's expected behaviour when you jumper J20 to disable the CPU internal cache
• However, if I try to boot the broken IIfx in 7.5.5 without J20 jumpered, then I get an "error 97" message. Doesn't matter if I disable extensions. Same occurs when I swap in a known working CPU and FPU. If I try to boot into 7.1, it just hangs at the "Welcome to Macintosh" splash.
• To answer Nathall's question, J103 is open. J106 is jumpered. This is the same as on my working IIfx. Removing jumper J106 doesn't seem to change the behaviour of the machine (either when J20 is present or otherwise).

So, it seems that my actual issue is that something on the logic board is borked and causes the system to fall over when J20 is not jumpered (and presumably the 68030 internal cache is enabled). It's not a faulty CPU as I've swapped in a working one.

I'll reflow the CPU socket just in case, but I doubt that's the problem. Any suggestions as to what else to try?

 

[ObeyDaleks]

Can you try using the PSU from the healthy machine?

 

[Blib]

Can you try using the PSU from the healthy machine?

Hi ObeyDaleks - thank you for the suggestion!
Unfortunately swapping the PSUs made no difference