FG-8002 blown why?

[Floopy]

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 

[Floopy]

Oops just noticed I made this in the wrong thread. Sorry, hopefully someone will move this to the repair thread 

[bdunham7]

Is one of the LM324 sections the output by chance?  Perhaps it was backfed a bit too much. 

[Floopy]

Is one of the LM324 sections the output by chance?  Perhaps it was backfed a bit too much. 

No, 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.

[duak]

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?

[Floopy]

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?

I got it online, https://www.eserviceinfo.com/downloadsm/63443/Goldstar_FG-8002.html  makes things easier. I checked the voltages and they are all within reasonable limits. I'll keep probing around.

[bdunham7]

This is marginally readable but may help.

 schematic.pdf (172.72 kB - downloaded 1177 times.)

[Floopy]

I like that, thank you I appreciate it a lot!