Trying to find old 8k sram chips for HP-75c calculator pod - Page 1
Forum section: electronics / beginners

author: RichardM
author_rank: Frequent Contributor
timestamp: June 06, 2019, 03:08:34 pm
content: Hi allDefinitely 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

author: wraper
author_rank: Unknown
timestamp: June 06, 2019, 03:19:26 pm
content: https://www.ebay.com/itm/1pcs-HM6264FP-15-64kBit-Static-RAM-Memory-HM6264-SO28L-HITACHI/263389473756?hash=item3d533c6bdc:g:q0wAAOSwi5RaNWtUEDIT. this likely will have higher power consumption in standby.https://www.aliexpress.com/item/Free-shipping-10PCS-LOT-HM6264ALFP-12T-HM6264LFP-10-HM6264ALFP-10-HM6264BLFP-10LT-HM6264ALFP-SOP-28-IC/32873394779.html?spm=2114.search0104.3.2.ede13981tbMYNp&ws_ab_test=searchweb0_0,searchweb201602_8_10065_10068_319_10059_10884_317_10887_10696_321_322_10084_453_10083_454_10103_10618_10307_10820_10821_10303_537_10302_536,searchweb201603_52,ppcSwitch_0&algo_expid=f18d8506-affa-4ca3-b79e-98f6a26c6969-0&algo_pvid=f18d8506-affa-4ca3-b79e-98f6a26c6969https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=HM6264ALFP&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=HM6264lFP

author: wraper
author_rank: Unknown
timestamp: June 06, 2019, 03:27:43 pm
content: BTW there seem to be a corrosion under all ceramic caps on the picture. Maybe you don't need to replace RAM at all.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2019, 08:16:33 pm
content: Any 28 pin JEDEC 8k x 8 SRAM should work if it is as fast or faster than the original parts.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 07, 2019, 12:11:31 am
content: 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

author: RichardM
author_rank: Frequent Contributor
timestamp: June 07, 2019, 12:48:43 am
content: 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

author: wraper
author_rank: Unknown
timestamp: June 07, 2019, 12:53:32 am
content: Quote from: RichardM on June 07, 2019, 12:48:43 amThere is some crap around ceramic capacitors.

author: RichardM
author_rank: Frequent Contributor
timestamp: June 07, 2019, 01:09:51 am
content: OK, thanks. I will clean ? Or maybe look at replacing ?Cheers

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 07, 2019, 01:13:19 am
content: 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

author: wraper
author_rank: Unknown
timestamp: June 07, 2019, 01:29:17 am
content: It doesn't look like glue at all. And if it is, there is something wrong with it.

author: wilfred
author_rank: Super Contributor
timestamp: June 07, 2019, 01:33:19 am
content: 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 noglueunder 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

author: Tom45
author_rank: Frequent Contributor
timestamp: June 07, 2019, 01:42:03 am
content: It certainly doesn't look good.Here is a cropped closeup from his original large photo.

author: RichardM
author_rank: Frequent Contributor
timestamp: June 07, 2019, 02:54:45 am
content: Service manual attached. Also a couple of more photos. Looks like corrosion at all similar capacitor locations ?

author: RichardM
author_rank: Frequent Contributor
timestamp: June 07, 2019, 02:55:52 am
content: And a photo of the reverse side.

author: wilfred
author_rank: Super Contributor
timestamp: June 07, 2019, 03:33:44 am
content: 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.

author: Bud
author_rank: Super Contributor
timestamp: June 07, 2019, 04:04:18 am
content: Not corrosion, that stuff is under the resistors too. Probably glue.

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2019, 04:11:08 am
content: Quote from: RichardM on June 07, 2019, 12:48:43 amIt 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.

author: RichardM
author_rank: Frequent Contributor
timestamp: June 07, 2019, 07:46:11 am
content: 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

author: RichardM
author_rank: Frequent Contributor
timestamp: June 07, 2019, 07:58:43 am
content: Quote from: wraper on June 06, 2019, 03:19:26 pmNot sure how to navigate this minefieldWary about incorrectly ordering 64k replacements instead of 8k.What is the difference between ALFP and LFP, also between LFP-15 and LFP-12T etc ? The AliExpress ones are HM6264ALFP-12T and not HM6264LFP-15 as are in the device now. Are they suitable ?Also what type of capacitors are they ?Thanks again.

author: oPossum
author_rank: Super Contributor
timestamp: June 07, 2019, 08:26:21 am
content: 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-SWhere...M designates the manufacturer6264 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.

author: oPossum
author_rank: Super Contributor
timestamp: June 07, 2019, 08:27:48 am
content: Ceramic capacitor.Probably 100 nF, 0805 size

author: oPossum
author_rank: Super Contributor
timestamp: June 07, 2019, 08:29:45 am
content: Breakdown of your original part # HM6264LFP-15HM = Hitachi6264 = 8k SRAML = Low powerFP = SOIC plastic package15 = 150 ns

author: Kjelt
author_rank: Super Contributor
timestamp: June 07, 2019, 02:31:31 pm
content: If you are certain you need those and absolutely can't find any other source I am willing to part from this beauty, but you have to unsolder the ICs your self.https://www.eevblog.com/forum/vintage-computing/pictures-of-vintage-ics/msg1438341/#msg1438341

author: RichardM
author_rank: Frequent Contributor
timestamp: June 09, 2019, 05:09:32 am
content: 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 sayRegardsRichard

author: Kjelt
author_rank: Super Contributor
timestamp: June 09, 2019, 07:19:13 am
content: Quote from: RichardM on June 09, 2019, 05:09:32 amIf you are going to use them it will be postageQuoteAFAIK -15 = 150ns -12 = 120ns etcQuoteI 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.

author: oPossum
author_rank: Super Contributor
timestamp: June 09, 2019, 07:33:48 am
content: Quote from: RichardM on June 09, 2019, 05:09:32 amSmaller numbers are faster. Less time between stable address and valid data.

author: RichardM
author_rank: Frequent Contributor
timestamp: June 09, 2019, 09:01:44 am
content: I will definitely use them, thanks. Can I PM you my details ?Cheers

author: Kjelt
author_rank: Super Contributor
timestamp: July 04, 2019, 03:27:32 pm
content: You have a pm, need your address for shipping.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2019, 04:40:32 pm
content: 8k*8 SRAM chips are common as muck. You can buy them from Digikey or other reputable sellers for a few dollars each. It will cost more to ship an old board than it costs to just buy brand new known working chips.

author: Kjelt
author_rank: Super Contributor
timestamp: July 04, 2019, 04:51:24 pm
content: Besides these are not just srams they are ultra low power versions with uktra low leakage current.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2019, 07:13:38 pm
content: What was considered ultra low leakage current when the HP-75 was made is nothing special today.https://www.digikey.com/product-detail/en/alliance-memory-inc/AS6C6264-55SCN/1450-1037-ND/42345961uA typical standby current, 2.7-5.5V supply range, 55ns access time, 28 pin SOP, current production part and only $2.13 each in single quantity. Am I missing something? Because this looks like a fully compatible and superior part.

author: oPossum
author_rank: Super Contributor
timestamp: July 04, 2019, 11:19:00 pm
content: That looks like a nice universal replacement for 6264 RAMs. Fast, low power, and DIP/SOP/TSOP packages.

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: July 04, 2019, 11:37:15 pm
content: As Kjelt tried to caution RichardM I would also doubt that the RAMs are faulty. I would not eagerly replace them first as they do not just go bad on their own. Only two things commonly kill them, static discharge and an out of range power supply. In either of those two cases it is more likely for a static discharge to go through what ever is connected to the keyboard, likewise a power rail disruption or reversal could easily kill everything not just the RAM.You can measure the resistance of the cruft on the PCB with a multimeter if it is conductive enough.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2019, 12:59:53 am
content: I thought he wanted additional chips to fill blank spots on the board, not to replace suspected faulty parts?I've seen a lot of faulty SRAM chips on old arcade games but those are mostly 2114's.

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: July 05, 2019, 01:47:43 am
content: Quote from: james_s on July 05, 2019, 12:59:53 amOh I see now, both wraper and kjeltthoughtled me to think this was a repair. Kjelt mentioned Richard would have to "unsolder" the chips himself. But richard says he wants to upgrade and that the device is working, my bad for skimming the replies. so ignore previous message. I have never had a badSRAMeven once on any repair.I am just chock full of failtard sauce today

author: RichardM
author_rank: Frequent Contributor
timestamp: July 05, 2019, 02:34:33 am
content: Correct. I am just filling empty spaces. I have used 5 x new 8k chips with 55ns speed and they work fine.https://au.mouser.com/ProductDetail/Alliance-Memory/AS6C6264-55SIN?qs=LD2UibpCYJowQ%252BJQYivAeg==Thanks everyone.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2019, 06:36:14 pm
content: He said he would have to unsolder the SRAM chips from the scrap board he is offering for salvage, not from the HP.I think newer SRAMs are quite a bit more reliable than the ones made in the 1970s and early 80s. RAM is one of the most common failures I encounter on arcade games.