Pentium Overdrive 83 speed problem \ VOGONS

Pentium Overdrive 83 speed problem

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First post, by SpectriaForce

Posted on 2017-10-29, 21:52

SpectriaForce Offline

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I use a Pentium Overdrive 83 in an AST Bravo LC 4/33s. The jumpers on the motherboard are set to the P24T setting (also see http://stason.org/TULARC/pc/motherboards/A/AS … -LC-4-100T.html). The BIOS of this pc is very plain jane without any speed settings what so ever. The computer boots fine to DOS and Windows 95. I do have some stability issues in Win95 (memory page faults).

The problem however is that the CPU does not run at full speed. I have used Phil's DOS Benchmark pack. CHKCPU and Speedsys 4.78 both report a clock speed of 33.7 MHz. The multiplier is correct with 2.5x. The bus speed is only 13.4 MHz (should be 33 MHz). CHKCPU also reports that the 'CPU is not in real mode'.

I must admit though that I have removed the very noisy fan from the CPU and replaced it with an additional small heatsink. It does not run extremely hot. I guess this shouldn't be a problem?

What do I do wrong or doesn't my pc fully support this cpu?

Regular 486 cpu's run fine.

Reply 1 of 7, by kanecvr

Posted on 2017-10-29, 22:29

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It's a known issue with the POD - if the fan isn't in proper contact with the three gold pads on the CPU, it will downclock to 33MHz via multiplier to keep from cooking itself. Take the fan off and clean the contacts on the CPU and make sure the spring loaded mechanism on the fan's pins work OK and that they touch the pads. If you don't have the CPUs fan, you won't be able to run it at 83MHz without modification.

Reply 2 of 7, by lazibayer

Posted on 2017-10-30, 03:50

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The OP has replaced the factory fan and heatsink so now mod is the only way.

Reply 3 of 7, by SpectriaForce

Posted on 2017-10-30, 16:23

SpectriaForce Offline

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lazibayer wrote:
The OP has replaced the factory fan and heatsink so now mod is the only way.

I have not replaced the factory heatsink, just the fan with another small heatsink.

@ kanecvr The original fan is very noisy. I have lubricated the thing (carefully disassembled the whole fan) but it still makes too much noise, so it's just worn out. What for modification would make it operate with just a heatsink? I assume it doesn't get very hot anyway, since a regular Pentium 90 for example works fine with a heatsink as well.

Reply 4 of 7, by GPA

Posted on 2017-10-31, 15:55

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The original POD fan pulls "fan detect" pad on CPU to ground. If it is not done, the multiplier stays at 1x, and CPU runs at the bus speed.
You can solder a 5-10 kOhm smd resistor between fan detect and ground pads right on the cpu itself and it will start with 2.5x multiplier.

Reply 5 of 7, by matze79

Posted on 2017-10-31, 16:03

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Hi,

you can repair the original fan,
the ball bearing can be replaced.

i also have done this on my POD 180Mhz MMX

simply use a pencil to connect the two pads is also working.

Pagefaults sound like you have misconfigured your cache behavior (WB/WT..)

https://www.retrokits.de - blog, retro projects, hdd clicker, diy soundcards etc
https://www.retroianer.de - german retro computer board

Reply 6 of 7, by SpectriaForce

Posted on 2017-10-31, 18:43

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Thanks for all help.

I have added some thermal paste on the exposed part of the cpu, added the fan again and now it's pretty quiet (already lubricated it a while ago). I also noticed that when I press the fan when it runs, the noise becomes much less (fan also stops when I press hard enough). It seems the fan simply resonates the noise. I admit, it's a bit of an amateurish modification, but it works for me (so far..). I might do the resistor mod if this doesn't work on the long term. Not sure though with what pads to connect the resistor, the cpu has three gold pads, any hints on this?

Some benchmark shots: