EEVblog #620 - Repair: Stanford Research SR650 Programmable Filter

[Stonent]

I was hoping for more of a inspection of the guts "tear-down" style, maybe a follow-up with some more details on the theory of operation?

[Taucher]

I really like that you did put the good old Rigol scope back to work -
but still wondering why not even a peek with the Flir thermal camera.

I know you already know that such issues are getting instantly visible with that equipment - even heated traces etc. - so why not use it as USB connected capture device and get perfect 9fps video
Even bad ESR caps could start heating up etc ... and for anybody without a thermal cam it's still a valueable (visualized) lesson what can heat up etc.

Suggestion: Fundamentals friday on LM78xx junction temperature calculation? 

[nitro2k01]

Part 2 of the repair is up!



I think the fact that you removed the lid may have also contributed to the heat problems. Maybe time to bring out the VFC500 and systematically measure the temperature with and without the lid, and at 230V and 240V set voltage. Though the voltage selection switch board clearly says 240 V, so you would think they would have designed for that.

And I'm curious how exactly the heatsink went missing. My theory would be that first the input filter broke. Especially if it looked like that from the factory, with electrical tape etc. (That part is suspicious as well.) And then someone nicked the heatsink from the "broken" unit to use it with something else.

Suggestion: Fundamentals friday on LM78xx junction temperature calculation? 

Seen this old episode? It may not have been uploaded on a Friday and it may not be about LM78xx specifically, but it may cover enough of what you're asking for.

[moemoe]

Regadring the packing, I do not know if it this is the case, but for a product we manufactured once, there were 3M "instant" packing products that were a tiny bag with two compartments filled with two chemicals inside. When you need to pack something you just break a couple of bridges between them and mixed them by hand. Than you place it into the box, your instrument inside, another on the top and the thing grows as a foam inside the bags taking the shape of the instrument.

They are available in multiple variants, the one you mentioned with the two components, a machine filling and sealing the bags for you, but still outside of the packaging, or the version Dave mentioned: http://www.sealedairprotects.com/ap/en/products/foam_packaging/instapak.aspx

[Taucher]

Suggestion: Fundamentals friday on LM78xx junction temperature calculation? 

Seen this old episode? It may not have been uploaded on a Friday and it may not be about LM78xx specifically, but it may cover enough of what you're asking for. #105 -  Electronics Thermal Heatsink Design Tutorial

Surely - seen it a long time ago

[Mark Hennessy]

With apologies for the lighting, perhaps this picture of my unit will help?

As you can see, you are missing a large aluminium heat sink. Look closely, and it also explains why the nuts appear to missing from the mains transformer.

My unit also has larger heat sinks on the analogue supply regulators. I haven't measured the temperature of these when running, but am happy to do so if you like.

Hope this helps,

Mark

[EEVblog]

With apologies for the lighting, perhaps this picture of my unit will help?
As you can see, you are missing a large aluminium heat sink. Look closely, and it also explains why the nuts appear to missing from the mains transformer.

Oh wow, thanks, that explains everything!

[lewis]

Dave, I found myself screaming at my monitor THERMAL IMAGING CAMERA! THERMAL IMAGING CAMERA! THERMAL IMAGING CAMERA!

Poke it in there, you'll see all sorts. It's the first thing we do when we're troubleshooting anything

[Dave Turner]

Man - if that's the EEVblog curse - can I have it too?

To score a non-working unit for $100 and get it working that simply 

[max_torque]

Are you being a bit harsh on the designers of that PSU Dave?  I mean, it's like over 20 years old, and still working, even after being run with the large heatsink missing!  So, yes, those VRs are getting to 70degC, but so what. I'd say it would be a lot more annoying to have an instrument that drops out when the mains voltage dips a tad etc!

Also, for instruments like this, you often can't control the environment they are run in.  Yes, it's nice to think they might all be running in 25degC ambient air conditioned labs, but i bet a load of them end up in equipment racks, getting baked by large power hogs next to them etc. And for that, you just can't beat having a proper fan to boost cooling way beyond that provided by natural convection etc!

[EEVblog]

Dave, I found myself screaming at my monitor THERMAL IMAGING CAMERA! THERMAL IMAGING CAMERA! THERMAL IMAGING CAMERA!

It was at home. I was too lazy to drive back to get it.

[EEVblog]

Man - if that's the EEVblog curse - can I have it too?
To score a non-working unit for $100 and get it working that simply 

Yeah, in that respect I'm not complaining. I could flip this thing for 10 times what I paid for it. But because I'd never get another one (or any programmable filter) at this price again, it's a keeper.

[Taucher]

Dave, I found myself screaming at my monitor THERMAL IMAGING CAMERA! THERMAL IMAGING CAMERA! THERMAL IMAGING CAMERA!

It was at home. I was too lazy to drive back to get it.

FAIL 

... get them to supply you with a secondary one - maybe the new one for smartphones

[jancumps]

FAIL 

... get them to supply you with a secondary one - maybe the new one for smartphones

Or get one of them Mµs !

[miguelvp]

FAIL 

... get them to supply you with a secondary one - maybe the new one for smartphones

Or get one of them Mµs !

[corrado33]

As you can see, you are missing a large aluminium heat sink. Look closely, and it also explains why the nuts appear to missing from the mains transformer.

How about a video of dave manufacturing a heatsink to match the one in the picture above? Bending sheet metal is pretty simple, even without a dedicated metal bender.

A large heatsink that spanned both of the packages back there was my first guess since only the regulator had the electrically insulating pad on it.

[edpalmer42]

As you can see, you are missing a large aluminium heat sink. Look closely, and it also explains why the nuts appear to missing from the mains transformer.

How about a video of dave manufacturing a heatsink to match the one in the picture above? Bending sheet metal is pretty simple, even without a dedicated metal bender.

A large heatsink that spanned both of the packages back there was my first guess since only the regulator had the electrically insulating pad on it.

I noticed that, but the mounting tab on a regulator is grounded so electrical insulation isn't required.  But if you look at Mark Hennessy's picture you'll notice that both devices use Sil Pads and insulating washers.  The transistor typically needs it for electrical isolation.  The regulator might need it for thermal performance.

Ed

[pickle9000]

One thing you can say about Dave's unit is that it was scavenged. Probably by the seller to get a couple other units up and running.

- missing heat sinks
- missing screws

Luckily it was not worse. To be honest I'd probably do the same.

 

[corrado33]

I noticed that, but the mounting tab on a regulator is grounded so electrical insulation isn't required.  But if you look at Mark Hennessy's picture you'll notice that both devices use Sil Pads and insulating washers.  The transistor typically needs it for electrical isolation.  The regulator might need it for thermal performance.

Ed

I said it was my first thought, not necessarily the correct one  Although in this case, the thought was correct, but with incorrect reasoning, so therefore I was still wrong. 

Also, when I've seen similar packages lined up in a row close together like that, I think shared heatsink. Otherwise it would have been designed to accommodate a larger heatsink for the regulator. If it was my repair, I probably would have ended up throwing sil pads on both of them and put a large heatsink there anyway. I mean, is there a downside to doing that?

Either way, interesting to watch dave with the repair.

We recently cleaned out one of our labs that had a lot of old stanford research stuff in it. Maybe I'll see if I can snag one and send it off to Dave. I really have no use for them (neither does our lab). I think they're... lock-in amplifiers and boxcar averagers.

[Tothwolf]

Darn, Mark beat me to it.

I wonder if Stanford Research could supply a replacement heatsink or at least a drawing for the original part? With a drawing, a good sheetmetal shop could fabricate one easily, but you'd still have to source the threaded inserts (4-40 for the two TO-220 devices and 8-32(?) for the transformer and fan screws). Don't forget the plastic insulating/shoulder washers for the two TO-220 devices though

It looks the thermistor for the fan speed control is right in front of the bottom edge of the fan. Maybe the fan will slow and quiet down once the cover is on and there is better airflow?

[tsmith35]

Grab a new Corcom 6J4-2 filtered input module while you're fixing things. Now owned by TE Connectivity...

[daveshah]

One thing you can say about Dave's unit is that it was scavenged. Probably by the seller to get a couple other units up and running.

- missing heat sinks
- missing screws

Luckily it was not worse. To be honest I'd probably do the same.

More likely, in my opinion, they removed the heatsink in an attempt to troubleshoot why it wasn't turning on, and forgot to put it back on.

[RupertGo]

But what was the function of the banana at 20:03 in the first video?

[Bored@Work]

but you'd still have to source the threaded inserts (4-40 for the two TO-220 devices and 8-32(?) for the transformer and fan screws).

I think a DIY heatsink wouldn't mind if you just use washers and nuts.

[retrolefty]

One thing you can say about Dave's unit is that it was scavenged. Probably by the seller to get a couple other units up and running.

- missing heat sinks
- missing screws

Luckily it was not worse. To be honest I'd probably do the same.

More likely, in my opinion, they removed the heatsink in an attempt to troubleshoot why it wasn't turning on, and forgot to put it back on.

 I agree, heatsinks rarely go bad and need to be savaged from a bone-yard unit.