entropy continued?

This is where we talk about testing, measuring and repairing things. Sometimes we have to repair the equipment we use to test, measure and repair other things. It's an endless cycle of fixing the broken things we need to fix other broken things.
billshurv
Posts: 290
Joined: Thu Feb 01, 2018 7:07 am

Re: entropy continued?

Post by billshurv » Wed Mar 27, 2019 3:59 am

Thanks I'll take a look. He said in the DIYA thread that he hadn't posted there yet.

Gold
Posts: 521
Joined: Sat Dec 01, 2007 6:20 pm

Re: entropy continued?

Post by Gold » Mon Apr 08, 2019 6:41 pm

I'm working on getting the second T-Filter EQ going. Everything was going smooth until I loaded the first filter boards with IC's. When I powered it up the positive going half of the sine wave was coming up flat topped. After about five seconds the top half of the waveform would come up and look normal. Toggling the tone on and off didn't make a difference but it would do it every time when powering down and up again. This happened in all four opamp positions.

Since there was a time delay I thought a capacitor might be involved. I retouched the solder on the el caps. No dice. Pounding on and flexing the PCB would fix the problem most of the time. I thought it might be a bad solder joint or two. I've been having issues with solder flowing through the double sided boards. I think some of the holes are not big enough for the leads. I went over both the top and bottom and reflowed solder anywhere a component met a trace. Still no dice. Then I thought maybe the LME49720's were oscillating. They weren't getting hot and I couldn't see anything on the scope so I wasn't optimistic. No dice. Same thing with the 5532's I replaced them with.

Then I applied some 99% isopropyl in syringe to the IC sockets. That kind of worked a few times but wasn't reliable. After reflowing the solder yet again I decided to try some Stabilant 22 in the sockets. Bingo! sometimes that stuff can be a miracle worker. I have no idea why these IC sockets/IC's are so troublesome. The sockets are MilMax machined sockets with gold plating. Same sockets that I use on everything. Weird.

billshurv
Posts: 290
Joined: Thu Feb 01, 2018 7:07 am

Re: entropy continued?

Post by billshurv » Tue Apr 09, 2019 3:29 am

How are you bending the IC leads before insertion?

terkio
Posts: 67
Joined: Thu Feb 07, 2019 4:17 am

Re: entropy continued?

Post by terkio » Tue Apr 09, 2019 4:19 am

Bad contacts before, good after contact cleaning spray.
So, there was some contact contamination: Corrosion, oily film, some residu left from manufacturing, storing, handling.
Who knows wether the culprit is the socket or the IC.
Well, you are stuck with those, all you can do is to keep the stabilant 22 handy.

billshurv
Posts: 290
Joined: Thu Feb 01, 2018 7:07 am

Re: entropy continued?

Post by billshurv » Tue Apr 09, 2019 4:58 am

Normally machined sockets are tight enough that you get a good metal to metal contact whatever. IF the pins are aligned properly.

Gold
Posts: 521
Joined: Sat Dec 01, 2007 6:20 pm

Re: entropy continued?

Post by Gold » Tue Apr 09, 2019 9:26 am

billshurv wrote:
Tue Apr 09, 2019 3:29 am
How are you bending the IC leads before insertion?

I usually give the IC a small squeeze. Then I fit the IC pins in one side of the socket and bend the other side until it fits in the socket.

Is there a better way?

billshurv
Posts: 290
Joined: Thu Feb 01, 2018 7:07 am

Re: entropy continued?

Post by billshurv » Tue Apr 09, 2019 10:06 am

There is a right tool for the job. Worth having if you stick a log of ICs into sockets.
Attachments
pinstraightener.jpg

Gold
Posts: 521
Joined: Sat Dec 01, 2007 6:20 pm

Re: entropy continued?

Post by Gold » Tue Apr 09, 2019 5:28 pm

Today one of the four opamps wasn't happy even with the Stabilant 22. I think I'm just going to replace the socket. This board must have been exposed to some debris. Maybe dust from a cutoff wheel? The other three boards aren't doing this. I should just replace all four sockets for good measure.

billshurv
Posts: 290
Joined: Thu Feb 01, 2018 7:07 am

Re: entropy continued?

Post by billshurv » Tue Apr 09, 2019 5:53 pm

Sounds a good plan. Something isn't right there and you need this unit to be reliable to earn you money :)

Gold
Posts: 521
Joined: Sat Dec 01, 2007 6:20 pm

Re: entropy continued?

Post by Gold » Thu Apr 11, 2019 5:42 pm

Umm, I got it wrong. I replaced the IC sockets and nothing changed. A real head scratcher. It turned out to be a general power issue. My power arrangement is a little more complicated than most. I use an 8 pole Buckaneer 400 series connector for power. I run separate wires for Audio Common and Chassis Ground. When I pulled the Audio Common connector It changed the waveform but the signal didn't die. Hmm.

I started testing continuity and realized something was connected somewhere in the grounding arrangement that shouldn't be. I went back to square one. I disconnected every power related cable. Checked the boards for any errant chassis connections. There were none. Then put it back together again checking for continuity and shorts with every new connection. I then checked and In/Out board. When it tested good I tried a filter board. Tested good. Then I tried the other In/Out and Filter board combination. Also worked.

I don't exactly know where the problem was but I'm glad it's fixed.

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