Hi there, I've only gone and bought another Adret RF Gen, a 740A this time... - Page 1
Forum section: electronics / repair

author: Aston
author_rank: Newbie
timestamp: April 07, 2025, 10:29:40 am
content: As the title says, I've only gone and bought another Adret RF Generator. I know, I know, I'm a glutton for punishment. It's a 740A with all the usual things that needed doing to it, including all of the capacitors being replaced and the backup battery being leaky. These jobs have now been done, however the damage to the CPU board seems to be unusual in as much as the 'acid/whatever' had flown upwards presumably because the unit had been left upside down.Even so, this was a bit of a godsend as it just meant replacing three memory sockets, and everywhere else seemed to have been spared any further damage.Whilst I was there, I confirmed that all the tracks were in good condition and conductivity was fine between them and their corresponding other ends.The problem it still has however is as it was when I bought it with the displays is just showing either "A"'s for random gobbledygook.The yellow LED on the CPU board is on all of the time and the reset line briefly goes to 5v then stays low after power on.I've backed up the Eproms (as I do with all of my units) but I'm not sure if the information is corrupted. It is the CPU boards with 3x 27c32 roms. Firmware version 25-1The power supplies are all fine and set up, there is no ripple on any of the +15v -15v or +5v lines.I have carefully desoldered the remaining chips that were not socketed and checked them with my old eprom programmer and they are all fine, also checked the memory with a 2114 ram checker and it is also fine.It has the extra factory wiring on the CPU board which looks like it builds on an older board design to accommodate later specifications.I've tried the triple chip firmware found somewhere else on the net which was firmware version 25, but it seems to be acting the same way.It's just a guess, but I would think that either the information coming from the Eproms is damaged and it's just going into a loop, or the information isn't getting to the memory. It just looks like it's waiting for something.I popped the board from my 742A in briefly to see if I got anything on the displays and all was fine, but obviously the rest of the unit was probably confused about being sent the wrong data for various stages.Any ideas where I should look at next? Many thanks in advance. Ast...

author: George Edmonds
author_rank: Frequent Contributor
timestamp: April 07, 2025, 07:00:02 pm
content: HiThe CPU reset is active low, in kit using EEPROM’s it is necessary to hold the reset active for a time so as to prevent random false writes to it, so start looking at the EEPROM protective circuit first.G Edmonds

author: DLJ
author_rank: Contributor
timestamp: April 08, 2025, 02:18:11 pm
content: CPU reset should not remain low. I would confirm that it really does, and then determine why.SN3 on the cpu board and PA signal may be worth looking at.Good luck.

author: Aston
author_rank: Newbie
timestamp: April 08, 2025, 07:10:59 pm
content: Thanks gents, will check when I get a chance. I remember checking the PA line and that was ok. On turn on, the reset 5v line pops up briefly (about quarter of a second) and then goes straight back down. I think I tried to disconnect SN3's Q output and manually try resetting the line but that just made more nonsense on the front panel.Also unplugged the GPIA, just in case it was affecting things in some way. At that point it was around midnight, and my brain had turned to custard, so I gave it a rest for a bit.Will try and figure out what SN3's job is, apart from making sure the power supply is ok before switching on and all that, plus why Q9 was added on as an afterthought. But I'm getting ahead of myself.Very grateful for the help and advise, best regards, Ast...