FG-8002 blown why? - Page 1
Forum section: electronics / repair

author: Floopy
author_rank: Regular Contributor
timestamp: June 25, 2020, 04:15:47 am
content: Hello, hello, I recently acquired 7 FG-8002 function generators. All not working... a bit sad at first, but they were cheap so I took them all. They are Keithleys or Helwett Packards, but they work somewhat well.All the units had a similar problem, just a high output signal. I cracked it open, first thing I noticed was the disintegrating rubber feet. Nice and crummy. I traced things back to a burnt LM324. On the first unit I took apart it had some discolored pins I didn't notice until I replaced it, but on the other units the op amps had died in a fascinating fashion. One of them had the plastic blown of the top. I replaced the IC and everything seems to work fine. I don't get anything heating up unusually.Now my question is what could have caused the device to fail? Is it a user error that caused a fail in most of these and they were phased out because they had a pile of them (previous owners, used to belong to a university). Or is it a common failure across the units?-Thanks, at least there are some pictures for those of you who look for those

author: Floopy
author_rank: Regular Contributor
timestamp: June 25, 2020, 04:20:16 am
content: Oops just noticed I made this in the wrong thread. Sorry, hopefully someone will move this to the repair thread

author: bdunham7
author_rank: Super Contributor
timestamp: June 25, 2020, 04:40:32 am
content: Is one of the LM324 sections the output by chance? Perhaps it was backfed a bit too much.

author: Floopy
author_rank: Regular Contributor
timestamp: June 25, 2020, 05:23:10 am
content: Quote from: bdunham7 on June 25, 2020, 04:40:32 amNo, it's very far up from the output, but what do I know. There is a diagram I'll pull it up. The IC in question is U5. I don't understand it completely, but by probing around I can see where things are supposed to flow.

author: duak
author_rank: Super Contributor
timestamp: June 25, 2020, 05:37:16 am
content: One of the best ways to blow a chip apart like that is to exceed its maximum power supply voltage. The LM324 non 'B' version is rated for 32 V or +/-16 V maximum. Looking at the schematic, If a 7815/7915 regulator was bad or oscillating due a failed bypass capacitor, I think this situation could easily happen.U5 could be damaged through the VCF input connector with a high enough voltage. Are there any burnt resistors?PS. I like that schematic! How long did it take to work it out?

author: Floopy
author_rank: Regular Contributor
timestamp: June 25, 2020, 08:33:55 pm
content: Quote from: duak on June 25, 2020, 05:37:16 amI got it online,https://www.eserviceinfo.com/downloadsm/63443/Goldstar_FG-8002.htmlmakes things easier. I checked the voltages and they are all within reasonable limits. I'll keep probing around.

author: bdunham7
author_rank: Super Contributor
timestamp: June 25, 2020, 09:07:22 pm
content: This is marginally readable but may help.schematic.pdf(172.72 kB - downloaded 1342 times.)

author: Floopy
author_rank: Regular Contributor
timestamp: June 26, 2020, 12:01:06 am
content: I like that, thank you I appreciate it a lot!