PET 6540 ROM operating temperature

John Robertson pinball at
Thu Nov 20 20:33:40 CST 2014

On 11/20/2014, 5:38 PM, Josh Dersch wrote:
> On Wed, Nov 19, 2014 at 11:44 PM, John Kaur <digitgraph at> wrote:
>> My general rule, is if cannot hold your finger on for a count of five, you
>> need fan directly on it (or sink, Robertson said), & experience has told
>> me, that one-two years life!!
>> John Kaur. in AZ where its hot>
>> On Wed, Nov 19, 2014 at 10:39 PM, John Robertson <pinball at>
>> wrote:
>>> On 11/19/2014, 5:47 PM, Josh Dersch wrote:
>>>> Hey all --
>>>> A quick one -- what's the normal operating temperature for the old
>> MPS6540
>>>> ROMs used in the original Commodore PETs?  I noticed last night after
>>>> running mine for ~30 minutes that the ROMs were very hot, to the point
>> of
>>>> being painful to touch.
>>>> The machine is running fine as far as I can tell -- though I haven't
>>>> verified the contents of the ROMs, BASIC runs without any apparent
>> issues.
>>>> Voltages are normal (just a shade under 5V).
>>>> I've not experienced ROM chips that run so very hot -- is this normal?
>>>> Thanks,
>>>> Josh
>>>>   A fan is a good idea on older electronics. the cooler they run the
>>> longer they last. A heat sink epoxied to the top of hot ceramic devices
>> is
>>> not bad either...
> Oh, very much understood.  I don't really want to bandage over a problem,
> though -- if these ROMs aren't *supposed* to be running hot then I'd like
> to know so I can address the issue properly.  If this is normal behavior
> then I'll probably do something to help reduce the heat and prolong the
> (already limited) life of these 6540s.
> Does anyone with direct experience with 6540s have any input here?
> Thanks,
> Josh
Not direct, but you should read/archive those PROM images so you can use 
something like the Commodore collectors use to replace theirs when they die:

These ROMs used from 110 to a max of 150ma @ 5VDC which is 0.75W. Hmm, 
how hot is that to the touch?

John :-#)#
>>> John :-#)#

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