HP 75 memory repair assistance request


I have my 75C now for about 30 years and it still works fine, in general.
A few day ago I got error message <17 RAM is invalid>.
I found out, that the 8k RAM extension module (Part. 82700A)
has a problem with half of its capacity (4k) not working.
When I open it, I find 4 chips CMOS RAM Toshiba TC5516 AF.
A lot of googling gives me the impression that these chips are no longer available.

Can anyone help me kindly how I can get the memory working with a schematic of the memory, a chip source or maybe a spare module 82700A to sell.
BTW: I'm also interested to acquire used 75's.


The RAM chip used in there is a standard pinout 2Kx8 CMOS static RAM so there are many other type that would be suitable replacements. From the datasheet I found online for the TC5516 it would seem to be a 250nS part, so any 2K part with an access time equal to or faster than that would be suitable. For instance I have in front of me a Sony CXK5816 and a Fujitsu MB9416 and looking at the specs for them either one of them would be suitable.

As for a schematic, there only seems to be the 4 SRAMs in the module so I would suspect that all the address and data lines and R/-W are connected to pins common to all 4 chips and then there would be separate -enable lines for each chip. Since there is no glue logic in the module It seems that the -enable for each chip comes from the main logic of the 75, likely from the chip called 'MAR' this chip seems to provide all of the RAM control logic, and the SRAM chips on board are also 2K x 8 so it would seem likely that this chip divides the RAM space into 2K pages and provides -enable lines for each of them. unfortunately the copy of the service guide I found is missing some of the schematic pages so I cannot confirm that.

Since the -enable is supplied by the 75 this raises the possibility of the problem being inside, or perhaps even a connection issue between the module and the main board, did you try reseating it? It is possible, depending on how the 75 'discovers' the RAM that only one chip is at fault. If it reads through the memory until it detects that it does not read back what it wrote and from that assumes it is the end of memory then if the 3rd 2K chip was bad you would still loose 4K.


Thanks, Paul, for your prompt expertise.

I will follow you and examine the substitute chips you mention.
Maybe my problem is, that I'm not familiar with this kind of electronics and the distributors I get my components usually from (e.g. Farnell) do not offer CMOS RAM. At the other side, datasheets are easy to find.

As you recommend, I inspected the board and the edge connector and cannot find any wear - the contacts are bright. I also re-soldered the edgeconnector to consider a cold joint.

The enable signal at the RAM should be detectable with the scope, I guess.


Yes you could see it on a scope but it might be hard to catch as it would not be a regular pulse, a storage scope would be able to capture it or a logic probe. These RAM chips have two -enable lines, pins 18 and 20 but in the 8K module they are tied together on each chip.


One of the most common part numbers for a CMOS 2K*8 static ram is 6116 (originally from Hitachi as HM6116). You want the L suffix for low power, e.g., 6116L-3 which is 150 ns. They should be available from many sources, such as Unicorn Electronics.


Yes I looked there too but they do not say what package they are, for the 8K module the plastic flat pack package is required.


I purchased the HT6116-70S from here (see under heading "RAM's - SMD Type"). That is a 330mil (~8.5mm) wide plastic flat pack (the actual IC that I received is HM6116LFP-3). I used for my PC1251 (which I ended up upgrading to 10k, picture and description - the 2k IC on the left is the 6116 from Futurlec)

Edited: 11 Apr 2011, 5:33 p.m.


The system Sw supports RAM increments of 2K (ref. HP-75 Service manual). The schematics shows the following signals for the J1 connector to expansion RAM:

Signal	Pin		Signal	Pin
------------ ------------
-E6/-E10 3 -E7/-E11 C
-E4/-E8 4 -E5/-E9 D
-W 5 A9 E
A8 6 A7 F
A6 7 A5 H
A4 8 A3 J
A2 9 A1 K
A0 10 R07 L
R06 11 R05 M
R04 12 R03 N
R02 13 R01 P
R00 14 A10 R

The four enable lines are either connected to -E8..-E11 or to -E4..-E9 supporting the discussion about HP-75 originally was planned with 8K RAM onboard.


Very interesting info. I'm asking myself, if the memory controller, which Paul named 'MAR', has some spare enable lines to feed more memory. As far as I know, the Series 80 desktops have 32k at the bottom end and 512k with bank switching.
I have a source now for IDT6116LA45SO chips and try to replace the Toshiba chips of the extension board.


Unfortunately no, it provides 12 select lines for 2K pages of memory, for a total of 24K, that is all that the 75 was designed for. It is possible to install RAM in the front bank selected ports, however memory installed there cannot be used as regular RAM. HP made a device "82713A HP-75 Plug-In Module Simulator" that contained battery backed up RAM. The intent of this device was to assist with the development of ROM plug-ins. I have no first hand experience with this device but it is my understanding that programs and LEX files can be stored in it to free up space in RAM.

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