PDP 11 gear finally moved

Tothwolf tothwolf at concentric.net
Tue Jul 21 20:46:36 CDT 2015

On Tue, 21 Jul 2015, Mike Stein wrote:

> I suspect that the real criterion for whether to shotgun-replace caps is 
> who is paying/getting paid for the materials and labour ;-).

I dunno about that. When I've done commercial boards such as industrial 
process controllers and CPUs for customers with nearly unlimited funds, I 
charged the customer based on an hourly rate. Since I use a vacuum 
desoldering tool, changing out 10-15 aluminum electrolytics on a board 
took me not much more time than 1-2. Most of the time spent on a board 
that comes out of the field is spent on cleaning, testing (before and 
after repairs) and prep, and it only takes a few seconds to pull the 
solder off of a couple of component leads. Replacing aged electrolytics 
wholesale on these types of boards also meant I didn't need to worry that 
the same board would be back on my bench again in the next 3-6 months. 
These days, I'm not taking on any new commercial work though, there was 
just too much demand due to all those shoddy far-east made capacitors, and 
it meant I pushed aside all my own projects.

I guess from a business standpoint, if I had been trying to make extra 
money on boards repeat failing in the field and having to come back in for 
repairs over and over, changing out only 1 or 2 aluminum electrolytics 
would have made sense. That said, industrial process equipment tends to 
run 24/7 and is expected to be 100% reliable. If something shuts down, it 
tends to cost a heck of a lot of money, so I would no doubt have lost many 

> FWIW I'm certainly not about to spend 100s of dollars, not to mention 
> time spent in sourcing and replacing, to replace the caps in systems

100s? Where are you sourcing your components from? The typical board I 
rebuild has a component cost of about $20 or less. Smaller switchmode PSUs 
with a bunch of 10-18mm radials might be closer to $35-50. Larger PSUs 
/might/ cost closer to $100 if they have several large screw terminal 
capacitors in them. All things considered, that isn't very much money in 
today's dollars, and considering the full replacement cost of some of 
these boards (if they are even available), those preventive maintenance 
costs are an absolute bargain, /especially/ if you are doing the work 
yourself on your own time.

> that are running perfectly "just in case"...

How do you -know- they are "running perfectly"? Just because a widget 
itself is functioning, you have no way of knowing if that capacitor is 
working 100% properly /unless/ you actually remove it from circuit and run 
a full battery of tests on it. Simply measuring the capacitance with a DMM 
while a capacitor is in circuit isn't good enough.

Given that a typical aluminum electrolytic capacitor costs anywhere from 
$0.12-$0.15 (4mm or 5mm diameter radials) to about $1.00 (12mm or 16mm 
diameter radial), it also doesn't make much sense to desolder a 20 year 
old part, spend at a minimum 5 or more minutes testing it, and then solder 
it back in. It it much more economical to pull the old part and install a 
new one and be done with it. (You also don't have to worry if the 
desoldering and resoldering process might have damaged the original parts 
end-seals.) That said, I personally pre-test new parts, in bulk, before I 
put them into my stock, so I know ahead of time that I'm installing 
known-good parts.

On many occasions I've cut open old aluminum electrolytics, and the guts 
very much do deteriorate with age. In addition to corrosion of the foil 
(black spots and pitting) and foil to terminal junctions (corrosion), one 
thing I particularly noticed was the more operating hours an aluminum 
electrolytic capacitor had on it, the more its electrolyte and paper 
insulator tended to smell bad compared to an otherwise identical (same 
brand and series) part that had very low hours. These are all clear signs 
of deterioration.

To those who advocate keeping old aluminum electrolytics in service, I 
have to also ask, would you also try to recondition 20 year old NiCd or 
SLA batteries and keep those in service too?

The bottom line really is, if you want something to be as reliable as you 
can possibly make it, you replace old aluminum electrolytic capacitors 
which are outside of their expected service life. If you don't care if 
something fails over and over, or you actually like to have the same 
widget on your service bench year after year, or month after month, you 
just replace 1 or 2.

More information about the cctech mailing list