Hi all
Definitely a beginner.
Trying to source compatible 8k sram chips for an old HP calculator (circa early to mid 80s) peripheral. The chips are marked as HM6264LFP-15 and as I understand they are 8k x 8bit 28pin soic. Pic attached. Any help greatly appreciated.
Richard
BTW there seem to be a corrosion under all ceramic caps on the picture. Maybe you don't need to replace RAM at all.
Any 28 pin JEDEC 8k x 8 SRAM should work if it is as fast or faster than the original parts.
Hi,
These are standard 8K x 8 SRAM. However, the 'L' in the part number is used to indicate 10uW in Standby. 10uW is about 3uA at 3V.
Is there a memory backup battery in this product?
If these are powered from the battery, to retain the memory contents when the power is off, then you need to find the low standby versions of these chips.
Regards,
Jay_Diddy_B
Thanks. Yes the device runs off battery power and hence the contents are retained when powered off. Prob not a good idea mixing low standby versions with other versions ?
Not sure about comment on corrosion ? The device functions perfectly, just wanted to add more ram to the empty slots.
Richard
Not sure about comment on corrosion ? The device functions perfectly, just wanted to add more ram to the empty slots.
There is some crap around ceramic capacitors.
OK, thanks. I will clean ? Or maybe look at replacing ?
Cheers
Hi,
This may be glue used to hold the capacitors in place during the assembly process.
Can you take a close up?
Jay_Diddy_b
It doesn't look like glue at all. And if it is, there is something wrong with it.
If it was glue could you confidently infer there are also parts on the other side? If the PCB was single sided *and* there were no glue under other parts eg the ram chips or something outside the picture shown would that point back to corrosion? Are these ceramic or tantalum caps?
not saying - asking.
#HP75c #repair #calculator #PCBcorrosion
It certainly doesn't look good.
Here is a cropped closeup from his original large photo.
Service manual attached. Also a couple of more photos. Looks like corrosion at all similar capacitor locations ?
And a photo of the reverse side.
You're going to need more of those caps anyway for the additional RAM so I would just order replacements as well. They aren't marked as polarised on the circuit pp32.
BTW. What's a kangaroo bus? top of diagram.
Not corrosion, that stuff is under the resistors too. Probably glue.
Thanks. Yes the device runs off battery power and hence the contents are retained when powered off. Prob not a good idea mixing low standby versions with other versions ?
It just means that you will want one of the low standby power versions which are common but mixing them is not a problem. Low standby power versions come in several grades so if you replaced them all with the best ones, standby time could be improved.
Google reveals that the HP-75C/D was code named Kangaroo, and the HP-71B was code named Titan, and were developed concurrently by two “competing” teams of HP engineers. Not sure if there was any Australian input to the code name back then ? Kangaroo was based on the HP-85 desktop computer and its development was suggested and promoted by HP Labs (corporate) and not the calculator division. Titan was inspired by the calculator division as the next generation.
Richard
There aren't 64k SRAM in that package. The next common step up is the 62256. That is 32k and 32 pins.
The general format of part numbers for 8k SRAM is M6264PK-S
Where...
M designates the manufacturer
6264 designates 8k byte (64 k bit)
P is power (nothing for standard, L for low power, LL for ultra low power)
K is package (mfg specific, DIP, SMD, etc)
S is speed in 10s of nanoseconds (12 == 120ns, etc)
Based on the original part number you want something that is low or ultra low power, 150ns or faster, and SOIC package.
Ceramic capacitor.
Probably 100 nF, 0805 size
Breakdown of your original part # HM6264LFP-15
HM = Hitachi
6264 = 8k SRAM
L = Low power
FP = SOIC plastic package
15 = 150 ns
Thanks for the offer. How much do you want for the 3 sram chips ?
Can I ask whether the speed on these (i.e. 15T or 12T) etc is referring to access time (or a better word?) Or actuals speed ? Is 15 "faster" than 12 or 10 ?
Also, do i need to use ceramic capacitors ?
What can go wrong I hear you say
Regards
Richard
Thanks for the offer. How much do you want for the 3 sram chips ?
If you are going to use them it will be postage
Can I ask whether the speed on these (i.e. 15T or 12T) etc is referring to access time (or a better word?) Or actuals speed ? Is 15 "faster" than 12 or 10 ?
AFAIK -15 = 150ns -12 = 120ns etc
Also, do i need to use ceramic capacitors ?
I thought it was a one on one replacement of the hp chips but you want to expand your memory on your device? Then you need to add some ceramic caps probably 100nF to 200nF will suffice since these are very low power.
, but you have to unsolder the ICs your self.