HP 8116A - mysterious gunk and pads inside - Page 1
Forum section: electronics / repair

author: corn11
author_rank: Regular Contributor
timestamp: February 23, 2016, 06:17:19 pm
content: Hello everyone,I've recently purchased a HP 8116A F-Gen with error 42 (should be fixable). While cleaning the unit I've found some strange viscous grey gunk and pads under the microprocessor board.I suspect it might be thermal compound but why is it there ?On the top side of the Board are just two Motorola 68K, RAMs, ROMs and glue logic, neither regulators nor power electronics.I would be glad if someone has a hint for me.Regards and thank you in advance!Cornelius

author: Andy Watson
author_rank: Super Contributor
timestamp: February 23, 2016, 06:25:41 pm
content: The square pads look like they were once "stick-on" rubber/plastic feet - probably used to ensure that the PCB does not contact the case. Over time, the plastic in the feet degrades and reverts to a more liquid form - hence the grey mess inside the case. I think IPA is good for removing the mess.See this repair by Gerry:http://gerrysweeney.com/hp-53131a-frequency-counter-teardown-and-spillage-clean-up/

author: rastro
author_rank: Frequent Contributor
timestamp: February 23, 2016, 08:46:34 pm
content: Gray pads slimming your instrument. I've seen this on both an HP 8116 and HP 8131A.Coincidentally both where manufactured in Germany.IPA is the best solution.

author: Smith
author_rank: Frequent Contributor
timestamp: February 24, 2016, 06:38:47 am
content: I have 2 of them, and both had the slimey pads inside. Like said before, its quite easy to remove with IPA.I replaced the one that holds the bar to the mains switch. Now the mains switch feels like new.

author: corn11
author_rank: Regular Contributor
timestamp: February 24, 2016, 04:58:51 pm
content: Thank you very much for your help!With your hint and after some digging I've found a picture of a 8112A pulse gen with the original feet still intact:https://www.holzleitner.com/el/hp-8112/index-en.htmlNever expected that such ordinary rubber feet can cause such a mess. IPA works really great for removing the gunk!I've cleaned the shield and the first two boards so far but there were as 'Smith' mentioned also feet near the switch bar and on the bottom of the mainboard. So there is plenty of work to do for the next days...@Smith what kind/size of rubber feet did you use as a replacement ?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 24, 2016, 05:16:42 pm
content: Hi,I have repaired a few of these HP8116A Function generators.If I remember correctly E42 is pointing toward one of the three ic on the lower board with heatsinks.These are custom ics used in this generator, they may be hard to get.Regards and good luck !!Jay_Diddy_B

author: free_electron
author_rank: Super Contributor
timestamp: February 24, 2016, 05:17:07 pm
content: OH YES , these are the infamous 3M grey pads. they turn into snot after 5 years. were taken off market.

author: free_electron
author_rank: Super Contributor
timestamp: February 24, 2016, 05:18:47 pm
content: i have several of these asics laying around. don;t need them anymore. i wont be repairing any more of these 'old' machines. i sold my last one couple of months ago. i still have several 'donor machines around. if you need parts ...

author: deepskyridge
author_rank: Regular Contributor
timestamp: February 24, 2016, 10:51:55 pm
content: I recently got a 8112a from Ebay with the grey snot included, use IPA to clean.Mine works fine now.Gary

author: corn11
author_rank: Regular Contributor
timestamp: February 26, 2016, 09:06:08 am
content: @free_electron thank you for your offer !The first mayor problem was the LM337 regulator for the -24V rail with all the legs broken off. I'm going to replace all four regulators and the three power transistors attached to the heatsink because some of them look a bit mechanically predamaged. Hopefully this is the only fault of the unit. I'll report back to you if it works.

author: corn11
author_rank: Regular Contributor
timestamp: March 01, 2016, 03:09:45 pm
content: It's up and running again!Fortunately it was just the mechanically induced problem concerning the LM337 regulator.By the way I'm going to replace the electrolytic caps.Has somebody experience with the Kemet T110 tantalum caps mounted in the 8116A?They're not exactly cheap and I don't want to replace them just on suspicion.

author: Smith
author_rank: Frequent Contributor
timestamp: March 01, 2016, 03:52:59 pm
content: Sorry, I haven't replaced the tantalum caps. I did replace all pots for Bourns multiturn pots. This made it so much easier to calibrate the thing. The old things where horrible.BTW I had one with the broken trigger output, featuring unknown unobtainium transistor pair. One wasn't broke, and I tested it with the Peak Atlas DCA75, and found it to be similar to the 2N3904 as I remember correctly. Replace both, and it works great ever since.The buttons on these things tend to feel sticky and noisy. I rinsed and cleaned all keys with IPA, and after years they still feel as new.Also if you have the sweep option, check the sweep input connector on the back. Mine had corroded and had a bad connection. As a result the low frequencies tend to drop on the output (when not using the sweep anyway). Took me ages, until I found the sweep input circuit on the service manual was connected to the low freq circuit, replaced the connector, and it's working ever since.