Keithley 2015THD --- The Case of the Vanishing Capacitors ----

[jpb]

I'm in the process of recapping my ebay 2015THD and I was annoyed to find two extra capacitors C260 and C261 both 47uF 50V that aren't listed on the parts list of the repair manual.

On top of this, the nearby capacitor C146 turned out to be 2,200uF 16V whilst it is listed as 3,300uF 16V so I replaced it with the listed value (as that is what I'd ordered).

BUT - I just looked again at muvideo's teardown:

https://www.eevblog.com/forum/reviews/keithley-2015-teardown/msg114429/#msg114429

and in the sixth photo down labelled

"Digital control, processor and firmware, divided in even and odd bytes:"

I see that on his instrument capacitors C260 and C261 are missing!

I guess this means they decided at some stage to do without them and perhaps, as a related move increased the size of 146 from 2,200 to 3,300 uF.

At the moment I'm planning to replace them as I've ordered the parts (last night) so it seems silly not to use them but it means that perhaps I'll be using an excessively large value for C146 but it shouldn't matter as it is only for smoothing the 5V supply.

My main problem at moment is trying to work out the correct size for replacement screws as almost all my screws are badly corroded - that and trying to clean some of the corrosion off the main case without leaving it in a condition where it rusts again immediately.

[SeanB]

Wash case with rust converter, which will leave black hard oxide spots where the rust was. Then lightly sand with 400 grit carborundum paper, wipe down with a solvent then light coat of grey primer, followed by 2 light coats of a matching colour, likely a machinery grey. That will prevent further corrosion, and screws are easy to buy in small packs online.

[grumpydoc]

My main problem at moment is trying to work out the correct size for replacement screws as almost all my screws are badly corroded

If you mean the small countersunk screws that hold the rear case near the feet then they would appear to be #4-40 1/4"

[SeanB]

Somebody in the USA would likely have to buy them and ship them to the UK, as in the UK it is getting hard to get a large range of imperial screws, but metric is easy. I generally just retap to the next size up metric or drill out the thread and retap to use a metric size thread insert, but I have the taps and drills, so the only cost is the insert that goes in the new thread and the screw. I tend to go for stainless steel screws as I use then a lot in machinery.

[grumpydoc]

Somebody in the USA would likely have to buy them and ship them to the UK, as in the UK it is getting hard to get a large range of imperial screws

http://www.westfieldfasteners.co.uk/ seem to offer a good range in small quantities although it does work out at 50p each if you only buy 10. No affiliation, just googled for the screw size. I found some on ebay for pretty much the same price as well.

[SeanB]

True, but I always had a problem with translating US sizing. Generally I just looked for the nearest metric one and did a conversion, or looked through a big box of assorted old screws to find a match. I learnt in the old days before Google, where you had to find a chart that said a particular one was such a dimension, and then look up a US size and finally go see if the bolt suppliers ( 3 I use locally) either had it or a close equivalent, and in the right material. Often no, so make do with what is available.

[AndyC_772]

Every type of threaded fastener you could wish for:

http://www.namrick.co.uk/
http://www.ahc-camberley.co.uk/

[jpb]

Thanks for the useful information everyone.

My main problem at moment is trying to work out the correct size for replacement screws as almost all my screws are badly corroded

If you mean the small countersunk screws that hold the rear case near the feet then they would appear to be #4-40 1/4"

Actually it is almost all of them i.e.
the small ones holding in the mains filter,
the 7 screws holding the DSP board and the two holding the main board one of which also goes through a transistor heat sink plus a long one holding the metal screen for the analogue bit and the two medium ones that go into the the underside of the GPIB connector and also hold the plastic screen.

In fact the only ones that don't seem to be corroded are the two holding the rear bumper and the two small ones holding the rear end of the case on.

Wash case with rust converter, which will leave black hard oxide spots where the rust was. Then lightly sand with 400 grit carborundum paper, wipe down with a solvent then light coat of grey primer, followed by 2 light coats of a matching colour, likely a machinery grey. That will prevent further corrosion, and screws are easy to buy in small packs online.

The problem is the dozens of threaded standoffs for the screws and also there are several places where the case needs to be bare metal for earth connections, both mains and for shielding. Working around them is a bit messy. I've done a bit of light sanding with 1500 grit wet and dry (I'm rather cautious) and even tried some metal polish.

I think in the end that if I replace the screws I'll probably just live with the light corrosion as if the instrument is used indoors in a reasonably dry atmosphere I don't think it will get any worse.
I may spray it with WD40 - hopefully that won't migrate to the boards when I replace them.

[grumpydoc]

Actually it is almost all of them i.e.
the small ones holding in the mains filter,
the 7 screws holding the DSP board and the two holding the main board one of which also goes through a transistor heat sink plus a long one holding the metal screen for the analogue bit and the two medium ones that go into the the underside of the GPIB connector and also hold the plastic screen.

I'll have a look & see if I can work any of these out.

Though it's a reasonable bet that Keithley - being a US company - used UNC sizes, which seems to be borne out by the rear case screws being #4-40

[jpb]

Actually it is almost all of them i.e.
the small ones holding in the mains filter,
the 7 screws holding the DSP board and the two holding the main board one of which also goes through a transistor heat sink plus a long one holding the metal screen for the analogue bit and the two medium ones that go into the the underside of the GPIB connector and also hold the plastic screen.

I'll have a look & see if I can work any of these out.

Though it's a reasonable bet that Keithley - being a US company - used UNC sizes, which seems to be borne out by the rear case screws being #4-40

That is a very useful lead.

Here are some photos which should give the length and thread pitch. Unfortunately the diameter is not very accurate. I've ordered myself a set of calipers (which I need anyway) and when they come I can get diameters much more accurately.

[jpb]

The smaller screws seem to be 40 TPI (as you've already pointed out grumpydoc) whilst the long shield screw seems to be around 12.5 in 1 cm which is around 32 which is the next pitch down in the Wikipedia Unified Thread Standard table.

This is starting to make sense - thanks for saving me lots of searching.

I'll perhaps wait for my calipers to arrive before deciding the exact diameter.

It's a pity though Keithley don't list the screws in the parts list in the service manual.

Most sizes seem to be #4#40 and the long one I think is #6#32

I hope so as these seem to be the readily available sizes at

http://www.namrick.co.uk/acatalog/UNC_Countersunk___Pan_Head_Machine_Screws_-_Zinc_Plated.html

Though getting the right length may be problematic.