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Products => Test Equipment => Topic started by: iRepairElectronics on October 14, 2019, 03:01:41 pm

Title: HP 8656a Forgets Storage
Post by: iRepairElectronics on October 14, 2019, 03:01:41 pm
Hello All. I just recently purchased a HP8656A Rf Generator off Ebay. It needed a good switch cleaning of data entry switches.  so now that the switches respond i tried storing a frequency and settings, stored to say Key #1. It stored and able to Recall just fine as long as its on.  turn it off and try to recall and its forgotten the storage.
My question, is there a battery hidden in this thing someplace?  |O
Title: Re: HP 8656a Forgets Storage
Post by: wn1fju on October 14, 2019, 05:08:55 pm
My reading of the manual says no battery.  As long as the thing is powered on or in standby, the memory is retained.  Toggle the front panel switch to reset or
physically disconnect and reconnect the power cord and the memories are all lost - in fact, they are all set to the default 100 MHz, -127 dbm setting.

The fact that the fan can remain on in standby was quite a nuisance to a lot of people.  I think HP corrected this in the "B" version.
Title: Re: HP 8656a Forgets Storage
Post by: iRepairElectronics on October 14, 2019, 05:20:46 pm
Hmmm..Seems HP is real good at creating useless features, like HP-IB interface, Continuous fan running, Forgetful Storage.   :scared:
Wondering if there is a way to add a battery somewhere that would retain the memory.  I have been looking at the manual. would be great if i can figure it out.  :-DMM
Title: Re: HP 8656a Forgets Storage
Post by: wn1fju on October 14, 2019, 07:06:56 pm
I just took a quick look at the data sheet for the RAM chips in the 8656A:  MCM68A10P.  If I'm reading it right, it looks like each chip can draw nearly
100 ma.  There doesn't appear to be a low-power data retention mode.  So even if you could wire in some sort of battery, it wouldn't last very long.
Title: Re: HP 8656a Forgets Storage
Post by: iRepairElectronics on October 14, 2019, 08:05:21 pm
Figures.  Then why the heck did they put the store and recall buttons there at all?  :box:
Title: Re: HP 8656a Forgets Storage
Post by: KE5FX on October 14, 2019, 08:24:00 pm
Hmmm..Seems HP is real good at creating useless features, like HP-IB interface, Continuous fan running, Forgetful Storage.   :scared:
Wondering if there is a way to add a battery somewhere that would retain the memory.  I have been looking at the manual. would be great if i can figure it out.  :-DMM

Unfortunately the 8656A is a rare example of a total POS from Hewlett-Packard.  You definitely don't want to judge the rest of their product line by that turkey.  Harsh, but true. :(
Title: Re: HP 8656a Forgets Storage
Post by: james_s on October 14, 2019, 08:49:27 pm
MCM68A10P is a standard 128kx8 SRAM, surely you could adapt something more modern to fit? Some of the Dallas NVRAMs with the internal battery are still available, or you could try to find some parallel 5V FRAMs.
Title: Re: HP 8656a Forgets Storage
Post by: iRepairElectronics on October 14, 2019, 08:56:25 pm
Well Im not judging it by the shortcomings discussed so far. Perhaps there will be others.  However the Build quality and attention to detail is still very good inside of it.
they did as good as they could with whatever was available at the time. I'm sure it still beats the pants off of many lower priced "NEW" Generators found on the market today. :-+
Title: Re: HP 8656a Forgets Storage
Post by: Kean on October 15, 2019, 08:58:54 am
MCM68A10P is a standard 128kx8 SRAM

Um, try 128 bytes (1k bit).

I remember using one in 1984 in the first "computer" I ever sold.  I don't recall why I didn't use 2114's like my previous Z80 designs.
Title: Re: HP 8656a Forgets Storage
Post by: james_s on October 15, 2019, 03:10:10 pm
Whoops you're right, well that's even easier. Slap some DS1220s in there ignoring the extra space or build a little board with a single modern SRAM, battery and a a little address decoding.

ST M48Z12 or the related external battery type are even cheaper, under $10 and should replace both RAMs with minimal effort. It occurred to me though that it will probably also require a ROM change to stop it from initializing the locations where the settings are stored otherwise it will clobber those on startup even if the RAM is kept active. This is a common thing that has been done for a lot of the early arcade games that lacked a provision for saving high scores.
Title: Re: HP 8656a Forgets Storage
Post by: iRepairElectronics on October 16, 2019, 04:32:32 pm
With the DS1220, just wondering how to be able to tie the line for R/W to
WE(Write Enable) is inactive (high) and CE(Chip  Enable)  and  OE(Output  Enable)  :-//
Title: Re: HP 8656a Forgets Storage
Post by: james_s on October 16, 2019, 06:47:48 pm
You might need to use some logic, a hex inverter or quad NAND gate will usually get you what you need. If you're still stuck I can look over the datasheets tonight, it takes a little understanding of how memory and addressing works.