EEVblog #620 - Repair: Stanford Research SR650 Programmable Filter

[rsjsouza]

Good videos.

Dave, I feel your annoyance with the botches caused by previous fiddling with the equipment. I recently got a working TRS80 model 100 and had to power it up upon arrival just to be sure the item was as described. After all seemed ok, I opened it up to mainly replace caps and the NiCD battery and found out four loose screws inside!  These were deliberately let loose as the others were tightly in place (fortunately none shorted anything).

Anyhow, I am glad it was a simple fix. In my case I am still chasing an irritating intermittent power up issue (perhaps broken PCB tracks, bad solder joints, etc.)

[Tothwolf]

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.

Possibly...but not using threaded inserts would make it much more difficult to install (and remove) the heatsink. Unlike individual washers and nuts, threaded inserts are also far less likely to work their way loose and rattle around on top of your pc boards. This is especially true for applications such as this where you have a long screw going though the flanges on a fan made of glass filled nylon, since that plastic can over time (vibration) flex and deform just enough to allow the screw to work loose. A small amount of low or medium strength loctite on the threads before installing a lockwasher and nut would help, but then you'd have to either clean the fasteners or replace them any time you removed the heatsink so you could then apply fresh loctite.

These sort of threaded inserts aren't difficult to obtain, and the cost of a Keps nut (or even just a plain nut and external star lockwasher) will cost as much or more than a press-fit insert. Threaded inserts may or may not even be required for the two TO-220 devices, depending on the particular gauge and alloy of the aluminum.

As for installation, I use a PanaPress to install these type of inserts for repairs and small volume stuff but they can also be installed with a hammer and drift, or sometimes even just a washer and screw to pull the insert into place (along with a small amount of oil on the threads to prevent galling), depending on the insert and material.

[Bored@Work]

You did notice that the other edges of the fan and transformer are just hold in place with nuts? So really, the world will not come to an end if you use a few more nuts.

[corrado33]

You did notice that the other edges of the fan and transformer are just hold in place with nuts? So really, the world will not come to an end if you use a few more nuts.

I think he's trying to make the point that if you were using a nut and bolt to mount the packages to the heatsink, and the heatsink looked like the original, there'd be no EASY way to hold the nuts in place on the other side of the heatsink as you tighten the bolts. (It's enclosed.0 Unless you removed the mains filter of course. All in all, it'd be a bit of a pain.

[Tothwolf]

You did notice that the other edges of the fan and transformer are just hold in place with nuts? So really, the world will not come to an end if you use a few more nuts.

How do you propose to get access to the nut at the bottom of the transformer and heatsink behind the large filter capacitor? The bottom nut on the other side where the fan and heatsink meet is also extremely close to a small signal diode and bypass cap. It would be very easy to break those components if you were using a regular nut and washer. (A tip for those having to service this sort of thing, a set of "ignition wrenches" makes things much easier.)

You did notice that the other nuts on the transformer and fan are keps nuts? Even though this is a low-volume production item, they still used keps nuts instead of individual nuts and washers.

Something else to consider are potential metal shards from a lockwasher digging into the aluminum plate. This isn't going to be an issue with the fan's glass filled nylon plastic housing or the transformer's steel laminations.

My point is the designers used threaded inserts for a whole host of reasons. Unless you are doing a failure analysis and re-engineering something due to a faulty mechanical design, it is generally best to stick with what the original designers used since they will have already worked out all the assembly issues and there may have been non-obvious reasons they did something the way they did.

Dave, are U8, Q3 and Q102 part of a PWM circuit for the fan?

[Tothwolf]

You did notice that the other edges of the fan and transformer are just hold in place with nuts? So really, the world will not come to an end if you use a few more nuts.

I think he's trying to make the point that if you were using a nut and bolt to mount the packages to the heatsink, and the heatsink looked like the original, there'd be no EASY way to hold the nuts in place on the other side of the heatsink as you tighten the bolts. (It's enclosed.0 Unless you removed the mains filter of course. All in all, it'd be a bit of a pain.

That too, but they could have also tapped holes in the aluminum plate for the TO-220 devices. Maybe Mark can let us know how his is made?

I did find it interesting how the insulating shoulder washer for the 7805 in Mark's photo is discoloring (from heat) compared to the transistor beside it. I guess it just tends to run hot. It is also interesting to see how they reduced the size of the four heatsinks for the LM317 and LM337 regulators in Dave's filter compared to what they used a few years earlier in Mark's.

[Tothwolf]

Dave, I think this is the correct input filter (Corcom 6J4), but you might want to double check the voltage selections against the datasheet to make sure.

ENG_CS_1654001_CORCOM_PRODUCT_GUIDE_J_SERIES_0611.pdf
ENG_CS_1654001_CORCOM_PRODUCT_GUIDE_0611.pdf
Farnell
Digikey
Mouser

[lewis]

That too, but they could have also tapped holes in the aluminum plate for the TO-220 devices. Maybe Mark can let us know how his is made?

They'd use self-clinching nuts.

[tsmith35]

Dave, I think this is the correct input filter (Corcom 6J4), but you might want to double check the voltage selections against the datasheet to make sure.

I believe the model # is on the filter, but the schematic of the 6J4 and 6J4-2 is attached.
Note 1 says, "Jumper required if only SPST power switch is used"
Note 3 says, "Use only 120V and 240V positions for 2 volt selection units"

[Mark Hennessy]

You did notice that the other edges of the fan and transformer are just hold in place with nuts? So really, the world will not come to an end if you use a few more nuts.

I think he's trying to make the point that if you were using a nut and bolt to mount the packages to the heatsink, and the heatsink looked like the original, there'd be no EASY way to hold the nuts in place on the other side of the heatsink as you tighten the bolts. (It's enclosed.0 Unless you removed the mains filter of course. All in all, it'd be a bit of a pain.

That too, but they could have also tapped holes in the aluminum plate for the TO-220 devices. Maybe Mark can let us know how his is made?

I did find it interesting how the insulating shoulder washer for the 7805 in Mark's photo is discoloring (from heat) compared to the transistor beside it. I guess it just tends to run hot. It is also interesting to see how they reduced the size of the four heatsinks for the LM317 and LM337 regulators in Dave's filter compared to what they used a few years earlier in Mark's.

I imagine that the TO-220 devices are secured using the same type of inserts that were used elsewhere - it makes sense, as to tap the holes would involve another manufacturing process. Also, the aluminium is perhaps a little bit on the thin side for tapping...

As soon as I get the chance, I'll have a look. I'll also try running it for a while to see how hot things get. And I'll look determinedly for date codes, as there is quite a range in there. Might take a day or two, as I have a more pressing job on the bench at the moment.

All the best,

Mark

[tsmith35]

If you want a thread insert in sheet metal, but don't have the means to install it, you can mount a thicker backer plate to the sheet metal using conventional fasteners. Once installed, you can drill & tap the sheet metal and backer. No worries about fasteners pulling out of the sheet metal, since the fasteners are retained by the backer.

Brazing a nut in place is also doable, but you must use care to prevent warping the sheet metal. For something like a TO heatsink, you can probably get by fine by just soldering a nut in place.

[Tothwolf]

If you want a thread insert in sheet metal, but don't have the means to install it, you can mount a thicker backer plate to the sheet metal using conventional fasteners. Once installed, you can drill & tap the sheet metal and backer. No worries about fasteners pulling out of the sheet metal, since the fasteners are retained by the backer.

Brazing a nut in place is also doable, but you must use care to prevent warping the sheet metal. For something like a TO heatsink, you can probably get by fine by just soldering a nut in place.

18ga aluminum generally accepts a 4-40 thread without issue but you can start to run into trouble when you get down to 20ga and thinner (6-32 on the other hand, would be iffy even in 18ga aluminum). Mild steel tends to accept a 4-40 thread down to around 22-24ga, but it will pull out if you over torque the fastener.

A forming tap also works well on thinner metals. I've even used them to repair threads where someone has cross threaded a fastener in thin sheet metal.

Another option is a thread extrusion (which you'll commonly see in manufactured items) where a punch and die are used to form a protrusion on the back side of the metal that can be tapped (with a tap or thread-forming screw). A drift/punch and a scrap of steel with the correct size hole drilled in it can suffice when you need to form a one-off thread extrusion in something.

[Rory]

Probably easier to just use a 4-40 x 1/2" screw fed from the rear of the panel and insulator/washer/locknut on the package tab. 

[Taucher]

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

Thx Dave

[max_torque]

For "one offs" that aren't too critical, i find a nice big blob of a suitable epoxy glue on the back of the thin sheet gives enough thickness to get a decent thread into 

[SeanB]

I do that too, but just lightly grease a screw and use the epoxy to hold a cleaned nut in place as well. When cured you just unscrew.

[IanJ]

Hi all,

So here's the faulty unit I bought off Ebay.

Ebay pic:-


She's in a bit of a state externally......but thankfully internally she's not bad! I see a bent display header which will be the reason behind the dodgy photo on the Ebay ad.......other than that hopefully just a full blown cleanup required if I'm lucky.

With a bit of luck I'll start the video blog repair this weekend.

Ian.

[IanJ]

Hi all,

What a difference a couple days in the workshop makes...........all fixed up.

Ian.

[robrenz]

Awesome job there IanJ! Nice sheet metal skills in straightening out that cover If those bnc's were polished up a bit it would be hard to tell from new.

[IanJ]

Awesome job there IanJ! Nice sheet metal skills in straightening out that cover If those bnc's were polished up a bit it would be hard to tell from new.

Whoa!....not so easy I hear myself saying. I have a fault on Ch.1 whereby enabling the filter causes an offset in the final output......almost as if it's jumping to AC coupled mode. Excellent!.......I didn't want it too easy!

Ian.

[M0BSW]

I watched your video Ian and looking forward to part 2, hope it's on soon.

[Zucca]

+1

IanJ,
Very cool, I can´t wait to watch part 2 of your video.

Any yes, the more complicated the repair is, the more fun we can get out of it.

[Mark Hennessy]

+1 for me, too. Great video, great job...

Funnily enough, I've noticed the "leakage", and considered putting black heatshrink around the LEDs to stop it. I've done the same in the past with DIY projects - heatshrink being less fiddly than black tape. You only need to do every other one, of course...

All the best,

Mark

[Zucca]

I just got another old not working gear from ebay. Those GPIB nuts are always corroded (like in this case), what is the best way to clean them up/stop the corrosion?

[IanJ]

I just got another old not working gear from ebay. Those GPIB nuts are always corroded (like in this case), what is the best way to clean them up/stop the corrosion?

I took them off and buffed them up on the grinder brush. A quick coat of paint and they'll do for me.

Ian.