(Yeah, this doesn't sound like a repair... but just wait.)
I have a couple of HP35665A's, one with 2MB of RAM, the other with 4MB. Just out of interest I decided that I would try to upgrade the RAM on one of them to 8MB (the max). When these were sold, memory upgrades were a board swap. 2MB (Std), 4MB, and 8MB were available.
The thing is, these are rather nice pieces of equipment, so I didn't really want to sacrifice on (no matter how good the cause).
I managed to get another 2MB RAM board, and as it has newer firmware, I resolved to start work on my original 2MB card.
This is an original card. The RAM is 2 banks of 8 chips at the middle top of the card.
These are 1Mx1 chips, a 4M card has a single bank of 4Mx1, and the 8M card has 2 banks of 4Mx1.
The boards all have different part numbers, and there are differences other than just the count of RAM chips. However, my 4M board is much newer than my 2M board, so the differences could be due to factors other than function.
My idea is to remove half the RAM and to try to get it to boot up displaying 1MB of RAM. If I can get that far, it seems reasonable that the board supports both 1 and 2 banks of RAM (as would seem likely because the banks are there!). It's also half way toward removing all the RAM so I can replace it.
The difference between 1Mx1 and 4Mx1 is a single address pin, and that pin position, while absent on the 1Mx1 chips, exists on the board, and is also wired up, further evidence that the difference is likely only in the chips used to populate the board.
I started this project when really sick with the flu. This really wasn't a great idea, and although people think I can solder by sense of smell alone, the same does not apply to desoldering.
This board was one of the first HP produced with up to two tracks between 0.1 inch spaced pins. What I didn't realise was exactly how delicate the traces were :-(
Here's the board with the "optional" RAM removed.
And a closeup of the top side of the board.
I really don't want to show you the other side because I would not be overly complementary of someone's skill had they shown me.
That's nasty.
I started from the right, and I only really looked at my work after doing 7 of them.
Actually, the board looks quite a bit worse than it is. The board has a coating on it, so some amount of the nastiness is this being scratched off, but yeah, there's quite a few destroyed tracks.
It turns out there were several problems:
Here is a picture of a fixed desoldering tool:
As mine was delivered, the clear solder receptacle had an open spring pushing against a metal disc, which was then pushing against a filter pad which prevented solder getting into the vacuum tube.
My fix was to replace the spring with a chunk of "solder sponge" , the disc, and finally some cotton wool.
This provides better airflow and made the desoldering dramatically easier.
And here is where the fun started. I had to track down and repair all the broken traces.
Aint wire wrap wire wonderful?
To cut a long story short, after a few goes (and the discovery of a jumper -- marked 1M -- that seems to enable single bank) the result is this:
The device doesn't actually boot up, but that may be because it requires more than 1MB of ram to do so. But it is encouraging!
I have a couple of HP35665A's, one with 2MB of RAM, the other with 4MB. Just out of interest I decided that I would try to upgrade the RAM on one of them to 8MB (the max). When these were sold, memory upgrades were a board swap. 2MB (Std), 4MB, and 8MB were available.
The thing is, these are rather nice pieces of equipment, so I didn't really want to sacrifice on (no matter how good the cause).
I managed to get another 2MB RAM board, and as it has newer firmware, I resolved to start work on my original 2MB card.
This is an original card. The RAM is 2 banks of 8 chips at the middle top of the card.
These are 1Mx1 chips, a 4M card has a single bank of 4Mx1, and the 8M card has 2 banks of 4Mx1.
The boards all have different part numbers, and there are differences other than just the count of RAM chips. However, my 4M board is much newer than my 2M board, so the differences could be due to factors other than function.
My idea is to remove half the RAM and to try to get it to boot up displaying 1MB of RAM. If I can get that far, it seems reasonable that the board supports both 1 and 2 banks of RAM (as would seem likely because the banks are there!). It's also half way toward removing all the RAM so I can replace it.
The difference between 1Mx1 and 4Mx1 is a single address pin, and that pin position, while absent on the 1Mx1 chips, exists on the board, and is also wired up, further evidence that the difference is likely only in the chips used to populate the board.
I started this project when really sick with the flu. This really wasn't a great idea, and although people think I can solder by sense of smell alone, the same does not apply to desoldering.
This board was one of the first HP produced with up to two tracks between 0.1 inch spaced pins. What I didn't realise was exactly how delicate the traces were :-(
Here's the board with the "optional" RAM removed.
And a closeup of the top side of the board.
I really don't want to show you the other side because I would not be overly complementary of someone's skill had they shown me.
That's nasty.
I started from the right, and I only really looked at my work after doing 7 of them.
Actually, the board looks quite a bit worse than it is. The board has a coating on it, so some amount of the nastiness is this being scratched off, but yeah, there's quite a few destroyed tracks.
It turns out there were several problems:
- Don't start something like this when you're unwell.
- The solder appears to be a high temperature type (ROHS?)
- The traces are unbelievably delicate.
- The suction from the desoldering tool was poor.
Here is a picture of a fixed desoldering tool:
As mine was delivered, the clear solder receptacle had an open spring pushing against a metal disc, which was then pushing against a filter pad which prevented solder getting into the vacuum tube.
My fix was to replace the spring with a chunk of "solder sponge" , the disc, and finally some cotton wool.
This provides better airflow and made the desoldering dramatically easier.
And here is where the fun started. I had to track down and repair all the broken traces.
Aint wire wrap wire wonderful?
To cut a long story short, after a few goes (and the discovery of a jumper -- marked 1M -- that seems to enable single bank) the result is this:
The device doesn't actually boot up, but that may be because it requires more than 1MB of ram to do so. But it is encouraging!
Last edited:
