Need help identifying circuit board component

[Bill-kun]

The system is an Epson Apex 100 computer from 1988. The processor is on the center-top of the CPU board, which is placed as an expansion board onto the motherboard.

The system has been unable to power up since I got it several months ago. The power supply seems fine. Having been told that blown capacitors are often the problem, I looked for capacitors. Someone in my circle said this was a capacitor that I should replace.





1. What is this yellow component? It is marked C21 on the board, which seems to imply capacitor. There is absolutely no writing on the component. It does appear to be polarized, however, going by the line across the circle, showing to match the flat side of the component.

2. How do I tell if it is bad or good?

 

[jafir]

Since it’s got a C on the silkscreen I’d say it has got to be a variable capacitor.

 

[jlang]

c21 is a variable capacitor. Not likely to be the problem.
It's used to trim the frequency of the oscillator (it's part of).
Actually I didn't see any electrolytic caps on that board.
So nothing to replace.

joe

 

[Timo W.]

So nothing to replace.

Wrong. Many tantalum caps, which are known to go short sometimes when being old and not powered for a long time. We actually told him that already in another thread.

Check all tantalum caps for shorts. You need to unsolder one leg for each to test. Of course, generally check for a short on the power rail(s) first. If there is none, the caps should still be fine.

 

[Chuck(G)]

You'll always see a trimmer capacitor on 8088/86 PC boards near the 14.3818MHz crystal. This is a signal present on the ISA bus and the idea is to be able to "tweak" the frequency slightly on CGA cards to eliminate/reduce chroma effects. Later CGA and above cards have their own oscillators. The same frequency is used to provide the clock signal for the 8254 counter/timer that, among other things, provides the "beep" and time-of-day, if there is no RTC present.

 

[Al Kossow]

1. What is this yellow component? It is marked C21 on the board, which seems to imply capacitor.

It is a variable capacitor. They rarely fail and would not effect the power supply.

 

[Bill-kun]

Wrong. Many tantalum caps, which are known to go short sometimes when being old and not powered for a long time. We actually told him that already in another thread.

Check all tantalum caps for shorts. You need to unsolder one leg for each to test. Of course, generally check for a short on the power rail(s) first. If there is none, the caps should still be fine.

Thanks. Can we please continue the previous thread? I posted my results of checking for shorts on the power rails twice, but no one responded to say what the results could tell me, so I got stuck:

http://www.vcfed.org/forum/showthre...and-the-3-3-V-orange-wire&p=651189#post651189

 

[Hugo Holden]

Just to add to this thread because it is about trimmer capacitors, the color of it indicates its value.

These little trimmers were made in Blue, White, Red, Green, Yellow, Orange, Brown, Grey and Black.

The yellow one you have is a 6.5pF to 40pF type most likely.

To test a capacitor like this would require disconnecting at least one leg of it from the pcb and using a capacitance meter. However, it is easy to tell of they are ok, if the are in an oscillator, if it is running ok and they adjust the frequency smoothly with rotation (unless they are in a PLL circuit where the frequency might stay stable until it suddenly drops out of lock)

Generally this part is very reliable.

In odd cases people have squirted WD-40 into these, or other fluids, it has a high dielectric constant and significantly raises the capacitance, throwing the oscillator way off frequency and then it drifts over time too. It pays not to let any kind of liquid near a part like this, but various board washing techniques create chaos with this sort of component.

 

[Bill-kun]

Wrapping this up. The yellow component was fine. The blown component was a matchstick-shaped capacitor just downstream of the power supply input. I had to test a dozen or so before I found it. Easily replaced.