Need help with Circuit

[ZTatZAU]

Thanks 88-
I will check into those capacitors and see what happens.
Yes C5 is a 0.47 uF, 50v, tantalum
Other stuff will keep me pretty busy this week but I will post on my progress with the gyro. Thanks again for the help! ZT

 

[duffy]

Yes! Freeze spray. Excellent way to do it.

A parametric cross to another component means it fits the parameters - current, breakdown voltage and beta being the most important for this one.

The circuit can be simulated, but the value of doing that depends on how many you have to fix. The freeze spray should find your thermally sensitive component.

 

[ZTatZAU]

Shall I say, "Watson come quick!"

Hi Duffy, Hi 88!

I've been away on other projects for a while but got back into the gyro this past weekend. You may recall I had previously replaced the zenor diode CR3 and the coil driver transistor Q7. I was then getting runs of 30 to 40 seconds before the unit would shut down. The coolant spray was hard to direct accurately on one component at a time but did the best I could with folded index card funnels to capture and direct the spray. The spray did enable the gyro to continue spinning but seemed to only continue running at about whatever rpm was present at the time of shut down and spraying. I settled in on and next replaced the capacitor C5 with another capacitor from a spare board. Same results; shutting down after approximately 30-40 seconds.

I then dusted off the oscilloscope to see if I could figure out what was going on and when connecting the probe to the test lead and ground to chassis, I found the gyro continue to speed up and continue to run. Since then, the gyro has run continously with or without the probes attached. Everything seems normal. The gyro spins up and seems to shift into the speed regulating mode (via Q4 & Q5) as I can vary the voltage at the power source and the unit compensates and maintains the desired RPM at anywhere from 10-14 volts. I've since had a couple of 2 hour test runs and one lasting about 6 hrs and everything seems to be operating fine. I can adjust the speed of the gyro with R12 as measured with the oscilloscope.

**broken link removed**

I am still somewhat concerned that the unit continued to shut itself down even after replacing C5 until I connected the oscilloscope test probes. After that it all seems seems to run fine. What could have happened here??? Could a component have been "stuck" in a particular "electronic state" and connecting the test probe grounded something or otherwise "unstuck" something???

 

[user_88]

One possible explanation to the changes that you have observed would be the electrolytic capacitors. These components are prone to change with age, and also with use.
They are constructed with a dielectric material that is sandwiched between two electrodes. The electrodes are often some type of foil. As I understand it, the dielectric material can dry out, or sometimes shrink with temperature and humidity. It is said that the electrolytic capacitor types can sometimes be rejuvenated by allowing them to 'soak' at rated voltage for some length of time.

The non-electrolytic capacitors would probably not be involved in this particular sequence. The application of the oscilloscope probes doesn't appear to have any particular significance ..... maybe just a random voltage stimulus that would have occurred anyway, given a few more hours of powered operation.

 

[ZTatZAU]

Thanks again 88! Your explanation sounds quite reasonable. "Once rejuvenated", would you question the life expectancy of such a capacitor?

 

[user_88]

It might be useful to let the motor run for a few more hours to see if there are any additional developments.

You could replace all of the electrolytic capacitors on that particular board, if you want to achieve a high reliability or confidence level.

 

[duffy]

What electrolytics? I mean, I agree in principle: good idea to replace electrolytics in old circuits if you are trying to get them working (but not so much for really worthy antiques) -

- but in that photo, C3 is polyester, C2 and C4 look like ceramic, C5 and C1 are unfamiliar but if that parts list is any good they're tantalum. Could just replace them all anyway, f'ckit.

 

[ZTatZAU]

Hi Duffy! Hi 88!

Just wanted to thank you both again for your help and let you know that all my testing has been sucessful. The gyro is running fine!

Cory...
I've attached the info that you requested. Good luck with your project!
ZT

 

[user_88]

glad to hear that it is running ....

... 88

 

[duffy]

Glad to hear it's running!