Regulator mix-up - Page 1
Forum section: electronics / repair

author: McBryce
author_rank: Super Contributor
timestamp: October 10, 2016, 10:56:28 am
content: Hi all,there's a (retro) computer on its way to me for repair, with an extremely strange/interesting failure. The computer normally uses a 7905 to regulate the 5V rail (Yes, a +5V rail, the computer doesn't have a -5V rail). The repairer overlooked this fact as most other computers of the time used a 7805. So he soldered a 7805 in as a replacement!What I was wonder is, what exactly would happen in this case? Will the regulator have fried? Will it have let the full 9V pass through (whether fried or not), or will the rest of the circuitry have got nothing that could damage it?As I've said, I don't have the machine yet, I was just doing some pre-repair speculation and thought that maybe someone here has either come across something similar in the past or knows the internal workings of the 78xx/79xx family well enough to speculate the outcome?McBryce.

author: bktemp
author_rank: Super Contributor
timestamp: October 10, 2016, 11:12:57 am
content: Both regulators have a different pinout:Using a 7805 instead of 7905, input and GND gets swapped. So the negative input voltage is connected to GND and GND is connected to the positive input. So the regulator will see a valid input voltage. That is good, because it will do nothing bad.

author: CJay
author_rank: Super Contributor
timestamp: October 10, 2016, 11:23:32 am
content: Sinclair by any chance?

author: McBryce
author_rank: Super Contributor
timestamp: October 10, 2016, 11:54:44 am
content: Close guess, but the Sinclair uses a 7805 (which is probably what confused the original repairer). The computer in question is an Oric Atmos:http://www.homecomputer.de/images/machines/Oric_Atmos_Large.jpg@bktemp: Thanks for the info! I don't think I've ever used a 7905 in any design, so I wasn't aware of the pinout difference. This may just save me a lot of component swapping.McBryce.

author: StuUK
author_rank: Frequent Contributor
timestamp: October 10, 2016, 12:14:17 pm
content: wow, I remember the ORIC series, that's a rare one....

author: McBryce
author_rank: Super Contributor
timestamp: October 10, 2016, 01:18:17 pm
content: Yup, and I get to keep itI already have an Oric-1, so that completes the Oric collection. Just looking for a Jupiter Ace now (which is so rare that it makes the Atmos look like it was as "rare" as the C64)McBryce.

author: StuUK
author_rank: Frequent Contributor
timestamp: October 10, 2016, 01:51:25 pm
content: Yeah, the ACE was technically better than some of the more popular successors (i.e. ZX81) but Forth was not particularly popular...

author: McBryce
author_rank: Super Contributor
timestamp: October 10, 2016, 02:23:32 pm
content: It's the fact that it came with Forth built in instead of Basic that makes it so interesting.McBryce.

author: CJay
author_rank: Super Contributor
timestamp: October 10, 2016, 02:48:39 pm
content: Had a Jupiter Ace, i bought it from a twitter friend for £25, sadly my ex threw it away along with a lot of my other 'junk'.I keep meaning to build one of the clones as I've got a pile of old CPUs, peripherals and glue from those years

author: McBryce
author_rank: Super Contributor
timestamp: October 11, 2016, 08:56:14 am
content: If you happen to own a ZX Spectrum 48K, there's a ROM on the web (can't find the link at the moment) that converts the ZX into a Jupiter Ace.McBryce.

author: StuUK
author_rank: Frequent Contributor
timestamp: October 11, 2016, 09:21:29 am
content: Quote from: McBryce on October 11, 2016, 08:56:14 amI still to this day have no idea where my ZX 48K went or my BBC Micro Model B

author: McBryce
author_rank: Super Contributor
timestamp: October 11, 2016, 11:30:19 am
content: Have you checked behind the sofa? That's where I usually find things I've lostMcBryce.

author: StuUK
author_rank: Frequent Contributor
timestamp: October 11, 2016, 11:58:29 am
content: if only

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2016, 05:33:45 pm
content: Was cleaning out a shelf today, and found a Spectrum there, in the polystyrene case. Must check if it still works.

author: McBryce
author_rank: Super Contributor
timestamp: October 12, 2016, 11:01:55 am
content: Quote from: SeanB on October 11, 2016, 05:33:45 pmThe keyboard membrane might give you problems, but otherwise they are/were pretty robust. The DC/DC converter fails on rare occassions or if it is a very early model the ULA might be dodgy. Their biggest problem was the unprotected expansion port that goes straight to some CMOS logic. Skidding the computer across the carpet (as kids tended to do) can kill these.McBryce.