[Test-Equipment] Scope info needed

[Brian Clarke]

Hi Fellas,

I agree with Rolynn. I've repaired quite a few TEK CROs and quite a bit of Collins 
equipment. And almost always it's a few tantalum caps that have failed. The later 
7000 series TEK CROs allowed simple diagnosis because of the power 
distribution system used. Measure each of the rail Voltages. When you find one 
is out of spec, leave your multimeter connected to just that rail and go round 
successively removing each board's power connector till the Voltage changes. 
Voila! But, go easy - often the connector also feeds back the Voltage received 
to the regulator board - lifting the connector may also lead to the Voltage rising 
to the unregulated or pre-regulated level, which may take out a few more 
tantalum capacitors - and a jolly good thing too - saves a great deal of time in 
the future.

The problem is in the design. Let me explain:

With ordinary 'wet' electrolytic capacitors, full capacity is reached as you 
approach the rated operating Voltage. However, when tants first became 
available, they were touted for their lower Equivalent Series Resistance [ESR] 
and small size; these two characteristics allowed equipment to be produced 
with a smaller form factor. In English, this meant that the boards were now 
more tightly packed with components. 

Here's the first problem: Tantalum capacitors have the same capacity 
whatever the operating Voltage. The smaller size means that the spacing 
between electrodes is very small and any exceeding of rated Voltage may lead 
to catastrophic failure. However, the operating life is reduced as you approach 
the rated Voltage and as the temperature rises - and the effect is not linear - it's 
a power effect, ie, [VxT]^n. And guess what happens as you approach the rated 
Voltage - the temperature rises - so,  you have a double whammy. But the 
designers and purchasing officers were unaware of this hidden time-bomb.

Here's the second problem: If you try to replace the failed tants with ones of 
higher rated Voltage, there ain't enough room on the high density board. Here 
beginneth the throw-away repair saga and the days of huge repair bills because 
some innocuous component failed.

Solution: Use modern 'wet' electros that are much smaller and have better 
ESR than their predecessors. If necessary, solder them on the other side of 
the board and lie them flat to get short lead lengths.

If you find all this hard to believe, have a look at the Kyocera literature and 
application notes on tantalum capacitors.

73 de Brian, VK2GCE.

Rolynn K7DFW said:
The 7603 is a 100 Mc scope and possibly it's only a 25 cent tantalum that's
failed.