Need Help!- TO-18 package Should the can have measurable voltage?

[CalG]

I have the innards of my Tektronix 465 o-scope all open to discover why there is no trace. It worked perfectly when last used a few months ago.

It seems that there are some troubles with the low voltage supplies.
There are supplies at 110V 55V 15V 5V and -8V on this unit, and they all cascade down from above, so if there is trouble at the 15 V , it may be problems at 55V etc. which seems to be this case.

The trouble appears at the 15 V as NO Voltage at the test point. The 55V TP measures about 53 V and there is a failed 3uf 150 V electrolytic Cap found near the output. Corrosion products and broken lead were easily spotted.

Looking through the troubleshooting guide, some of the circuit transistors are suggested to be tested. Putting the VOM on the metal package of one of the devices displayed the unregulated Voltage in that circuit (22V).

Is that expected? I would have thought the can should be isolated from what goes on inside.

It is an NPN 2N222A

I am looking up the data sheet now.

Comments on "what to look for" in board level electronic repair are welcome!

Tia
CalG

[vger]

Yep, thats normal. take a look at this page....

http://www.interfacebus.com/semicond...ges-TO-18.html

About the middle of the page. Shows the bottom view and the collector is not isolated from the case. In the schematic view it shows the collector connected to the case.

Many many years ago I found out the hard way (ZAP!) that collectors are often connected to the metal case.

Sure would be nice to have an oscope to troubleshoot the oscope....

I've got a Tektronics 466 myself, but I don't have the service manual for it.

If you need it... http://bama.edebris.com/download/tek/465(2)/465.pdf

Thats the service manual... great site for old stuff...

Best of luck,
Steve

[pfred2]

Thanks for the link.

Dang, so close to having one for my scope! I have a 2336 and they have a manual for a 2236. Oh well just my luck. I downloaded it anyways. Its my kind of stuff!

[CalG]

Great comments and good links!

I DO have the full service book on this 465,lucky me! If only I were anything more than a hack with those tiny little electrons. (mass less too, so they won't stay down ;-)

I do need to replace a 3 mf 150 V electrolytic that spewed its innards. It's on the output of 55V supply. That device is even part of the troubleshooting tree. First things first. I'll need to look to the 15V supply after the replacement is soldered in.

Newark has them (the capacitor), but to purchase a $2 part with all the shipping...
I need to get some Phoenix 7 and 9 pin connectors too, but weeding through the catalog is overwhelming! Easily found in Digi-Key, but they don't have the cap! go figure!

I did pull the transistors of this topic for an out-of-circuit test, they are mounted in spring sockets. A-OK there!

Yep, a scope to check a scope is useful, I do have a little single channel unit that I have used to some advantage. But like I said, I wish I knew more.....

More as progress is made

CalG