Does anyone recognize the type and the maker of the following pot? It is the focus control of a Hameg HM605 oscilloscope. The two pin are just for mounting purposes.
Alexander.
Looks similar to the bushless version of Piher PC-16:
http://www.piher.net/sensors/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=26&category_id=1&option=com_virtuemart&Itemid=31With the exception that the 2 mounting pins at the top are wider spaced than the outer 2 pins at the bottom.
You can download the manuals including schematics and PCB-layouts for all Hameg scopes here
http://www.hameg.com/manuals.0.html?&no_cache=1But please note, they do not include schematics in any non-German manuals. So you need to also download the German HM605 manual to get the schematics and PCB-layouts at the end of the manual.
The focus potentiometer in HM605 is part number VR601 and it's placed on the Z-board. It looks like it is connected to -1250V at through a 392k resistor at one side, so you might need a high voltage approved potentiometer to be safe.
But is the potentiometer broken? Usually it's one of the high value resistors in series with the focus potentiometer going open circuit that causes focus problems. That's a common problem with high value resistors. I've worked at a audio and television manufacturer that had a design rule to never use resistors above 1 Mohm (IIRC) for the same reason, because they often break by going open circuit. Something you usually don't learn while studying, but learn the hard way. At some point someone forgot this design rule in a product and it was overlook in design reviews. So this design made it all the way to production. And as a result a lot of products soon came if for repair with an open circuit high value resistor as a result. Actually I don't know if this is only a problem with high value carbon resistors, or if it's also a problem with metal film resistors. Anybody know or has any experience with this?
At least I do know from experience that regular 1/4W 5% carbon film resistors above 1 Mohm often break with a open circuit after only a short while for no apparent reason, so these high value resistors are very unreliable. So if someone needed >1 Mohm in a design, instead two or more <= 1 Mohm resistors in parallel or series should always be used for reliability reasons.