Flakey HP 70: post modified as RAM Chip identified.

[Geoff Quickfall]

Just received an HP 70 with a bad RAM chip.

The calculator basic functions ( - + x / ) which rely on the stack work. The higher functions ( n i PMT PV FV ) and the memories K and M do retain their inputs. The K register does not auto load with a 12 and anything input in the K manually or the M registers are not stored.

Seems to me that the RAM chip runs the stack and runs the financial functions and memories.

Is this correct? If so, then replacing that chip should resurrect this machine.


Cheers, geoff

[Geoff Quickfall]

Okay, doing some research the following chips are found in the 70:

1818-0116 ROM
1818-0117 ROM
1820-0993 RAM
1820-11?9 (1169 most likely) ARC
1820-1128 clock driver reset
1818-0078 CTC

With the symptoms described above, the inability to retrieve stored numbers in either the financial registers or K and M it would look like I need to replace the RAM chip, any comments. I have a few dud 45s' which may end up being RAM chip donors to get this 70 fully operational.

Or do the symptoms suggest a defunct ROM. Again, a number stored in the financial, K and M registers are not retrievable suggesting a defective RAM. If it was the ROM, then I would be to store and retrieve numbers from the RAM.

Just checking my logic, so tomorrow I will swap the 1820-0993 with one from a dud 45 and let you know.

P.s. Thanks Eric Smith for your chip set descriptions.

[Didier Lachieze]

According to this table the following chips are used on the HP-70:

Code:

part number   models                description    vendor(s)              notes
---------------------------------------------------------------------------------------------- 
1818-0078     35, 45, 55, 70, 80    CTC            AMI, Mostek MK6021P 
1818-0116     70                    ROM, 1024*10   AMI 
1818-0117     70                    ROM, 1024*10   AMI
1820-0993     45, 46, 70 , 81       RAM, 10*56     Mostek MK6036P         replaced by 1820-1393 
1820-1128     35, 45, 55, 70, 80    clock driver and reset, 16 pin DIP     
1820-1169     35, 45, 55, 70, 80    ARC            AMI, Mostek MK6020P

The RAM being the same on the HP-70 as the one on the HP-45 this can be an easy replacement source.

[Geoff Quickfall]

Aha, that's the table I am using!

Tomorrow I will harvest a RAM from a 45 and see if that fixes the 70.

By the way, the stack as well as the 4 basic functions all work, it is jus the higher registers that return a 0 no matter the input.

Thanks Didier.

Geoff

[Harald]

(10-22-2015 06:52 AM)Geoff Quickfall Wrote:  Aha, that's the table I am using!

Tomorrow I will harvest a RAM from a 45 and see if that fixes the 70.

By the way, the stack as well as the 4 basic functions all work, it is jus the higher registers that return a 0 no matter the input.

Thanks Didier.

Geoff

Does sound very much like a RAM problem.
It could be the ROM that partially works and instructions related to the financial functions don't work, but that seems rather unlikely.

I nearly bought a 70 the other day - but couldn't justify spending that much money and ended up loosing the auction...

[Geoff Quickfall]

Interestingly, the entry of all financial knowns works as expected from the entry point of view. Execution of the unknown results in the 'running' display of flashing led segments. The result is an 'error' message most likely a dived by zero error.

So I too am hoping RAM, not ROM.

This was also pricey but came with the following:

The manual in perfect shape
The real estate manual
Guarantees and sales brochures.
Aluminum sticky personal labels (3)
Envelopes,
Original shipping box with a November 73 postmark.
Wall wart
Two battery packs, one new from eBay and the original
Spare bezel (face plate).
No corrosion.
Unopened rear label.

It was advertised as working and looking excellent. The looking excellent was correct. I discovered the non working side of things. In fact this calculator works perfectly as a basic 4 banger with no memory but a 4 level stack. So the seller only tested the basic functions and assumed that meant a working calculator. If you don't know how to use the higher functions then how can you test them?

In any case, I got a mutually agreed substantial refund when I explained the problem. No mess, no fuss. So I am happy either way (working or not) but it would be nice to get it functional and looking factory fresh.

Cheers, Geoff

[Geoff Quickfall]

Well, I picked up a functioning HP 16C and an untested (eBay read for dead) HP 45 in a package for an excellent price. The HP 16C will be a Christmas present for my nephew (programmer).

The 45 WAS to be a donor for an:

1820-0993 RAM chip.

Well wouldn't you know it!!! I got it working after a good PCA and connection clean.

So I took my oldest, rattiest looking 45 PCA and harvested the chip. I now have a functional 45 board that needs a chip, but at least I could insert the chip into the 70 PCA.

And guess what, I now have a fully functional 70 working! It even does Canadian Mortgages, of course that will all change with the new Prime Minister.

So, referring to the faults in the first post, the ROM at power on does insert 12 into the "K" memory which resides in the 1820-0993 chip. The fact the 12 and all other 'memory' function other then the stack pointed to the defunct 0993 chip.

A big thanks AGAIN to Eric Smith and his resource at brouhaha!

best regards all, Geoff

[Harald]

(11-08-2015 01:48 AM)Geoff Quickfall Wrote:  So I took my oldest, rattiest looking 45 PCA and harvested the chip. I now have a functional 45 board that needs a chip, but at least I could insert the chip into the 70 PCA.

And guess what, I now have a fully functional 70 working! It even does Canadian Mortgages, of course that will all change with the new Prime Minister.

Congratulations on getting the 70 to work! 45s are, although very nice calculators, quite cheap. So it makes perfekt sense to sacrifice one to resurect a 70!

Harald