Houston, there may be a problem .. .. ..

[Musicmanager]

.. .. .. with my Oscilloscope

Hi Guys

If you've been watching the posts recently you may have noticed that I've recently acquired an old oscilloscope. It's an Advance Gould OS4000 and is currently working well. However, I've been systematically stripping, cleaning and re-seating the inside components, PCBs etc to get rid of the piles of dust that had accumulated. I discovered a problem with a large crack in the plastic back panel which I've nearly solved and in the process of doing that I've noticed something that I'm worried may be a problem about to happen.
One of the PCBs, labelled Store and Timing Logic, is populated with a significant number of IC's some of which I cannot see properly but most seem to be 74S74 flip flops most of which show no signs of any problem that I can see, but two of them close to the warm embrace of the PSU have what I thought were discoloured pins but when I tried to scrape a bit I discovered the 'colour' is a dark blue jelly/grease which would clean off. Also alarming, there is a large black ceramic capacitor very close to these two ICs which has bubbles in the surface. There are several of these visible in different areas on the board, none of which have a smooth surface so there may not be problem ? The parts list shows them to be 0.1uf ceramic caps 30v +80% -25%.
I've tried to take a picture to show you but it's difficult without getting the boards out of the unit which I haven't attempted to do yet.

Any advice would be very much appreciated.
Thanks
S

 

[dr pepper]

74act series chips replace some 74 and 74ls series, not sure about 74s, maybe but there will be a fanout restriction.
I'd just chage the 0.1u and see if it still works, if so dont worry too much about it.
Do you have the schem'?

 

[throbscottle]

I wonder if an electrolytic has leaked some time in the past and been repaired. The jelly/grease stuff could be electrolyte. Or it could be high voltage insulation gel that has gone astray for some unknown reason (clumsy maintenance?). Personally I wouldn't worry about the caps, it's probably a superficial manufacturing defect.

 

[Musicmanager]

Hi Guys

Thanks for the help, much appreciated.
Dr P - I do have a schematic but it's an absolutely huge PDF file which is difficult to read and not very well marked to boot. It came with the Service Book I downloaded but is not original, I think it's scanned into the document - rather poorly done.
Throb - there are a couple of large electrolytics on the PSU board which are quite close enough to have done that except that it's a bit strange it isn't anywhere else except the IC pins.
I think when I get the boards out of the unit to clean them I'll pull those 2 ICs and clean the pins and I'll track the caps circuit to see if I can establish their purpose. A small value cap is likely to be smoothing or similar ? As I said above I have a schematic but it's very poorly labelled and difficult to follow. The next job is to take out the power board and see if there are any components underneath the 1/2 inch or so of fluff and dust !

Thanks again Guys

S

 

[Tony Stewart]

1 MHz capacitance meter might be handy to check for flakey caps. Old caps with high surge currents are expected to wear out and test with much higher ESR which then results in excess heat until it exceeds the circuit impedance then the heat loss declines. Logic circuits may continue to work but prone to glitches.

 

[dr pepper]

If the goo smells fishy then it probably is electrolyte.
Caps not allways but often bulge when they go high esr, look carefully, any out of shape are suspect.

You can get component testers for cheap money nowadays, I have one that test most comps and tells you the esr.
Better to build one for repairs as the test voltage can be made less than a silicon junction so you can test caps in circuit.