Re: [Drakelist] Simple R4B troubleshoot
Don - If the VOM test I described isn't definitive, Richard's procedure will prove one way or the other. IF you find you need a replacement, check with hayseedhamfest.com for a drop-in replacement. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line and TR-4/C Service Supplement CDs Richard Knoppow wrote: - Original Message - From: "Don Rasmussen" To: Sent: Thursday, March 17, 2011 10:53 AM Subject: [Drakelist] Simple R4B troubleshoot Hey guys, The fuse blew out on my R4B today. There are 2 filter caps that I am inclined to check, one is mounted underneath the power transformer, the other is the larger silver can mounted above the chassis. Can these be tested with a simple VOM? What readings to look for - shorted? Also wondering what else to test before tossing another fuse in line. The wiring attached to the NOTCH pot also seems to be disintegrating all by itself - interesting. Thanks, Don Rasmussen Carlsbad, Ca. If the caps are very bad the VOM might tell you but, in general, its not a reliable test. Electrolytics develop _parallel_ resistance, sometimes called leakage. A small amount is normal but it can become quite large, right up to being shorted. The way to tell is to use a variable power supply and a meter capable of measureing _microamps_. Probably any DMM will do it. For an electrolytic the leakage current should be pretty low, perhaps 15 uA. If the cap is bad it will be a lot more. The cap will take a while to charge and for the virtual plate to form so it should be left on the tester for a few minutes and the leakage current watched. Sometimes it will go down, which is OK but if it starts increasing toss the thing. Measuring capacitance takes a bridge or some form of capacitance meter. Bridges are good because they usually indicate _series_ resistance as well as value. Most electrolytics should have more than the marked capacitance value although modern ones are pretty close. Bad ones will measure low. High series resistance also indicates a bad cap _but_ both electrolytics and paper caps can have reasonable series resistance when the parallel resistance is low so both must be measured. It is normal for electrolytic caps to have fairly high series resistance (AKA ESR but also measured as dissipation factor). Sometimes its useful to measure the power factor by measuring the phase shift through the cap. An ideal cap will have exactly 90 degrees shift. Again, one plate of an electrolytic cap is formed chemically by action of voltage on it so measurements taken with only AC may not be adequate. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] Simple R4B troubleshoot
Don - The cap under the transformer is for the 5V supply for the calibrator. Not likely to be the problem. The large silver can is _almost_ certainly the culprit. There are other possibilities of course, but ... You can check with an ohmmeter, start with the Twist-Lok cap terminal with the two white/blue wires on it. Again, this is the 'most likely' spot. Connect you meter from that terminal to chassis. The meter should show a very low resistance at first, and then start increasing. It should read 5 - 10 k ohms, depending upon many factors. If it reads in the low hundreds of ohms, probably bad. Beyond this requires either disconnecting the wiring to the various terminals or an ESR meter. If you decide to disconnect the wires, I would strongly suggest cutting them as close to the terminal as possible. The wire Drake used is extremely sensitive to heat and will melt or 'crawl back' quickly. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line and TR-4/C Service Supplement CDs Don Rasmussen wrote: Hey guys, The fuse blew out on my R4B today. There are 2 filter caps that I am inclined to check, one is mounted underneath the power transformer, the other is the larger silver can mounted above the chassis. Can these be tested with a simple VOM? What readings to look for - shorted? Also wondering what else to test before tossing another fuse in line. The wiring attached to the NOTCH pot also seems to be disintegrating all by itself - interesting. Thanks, Don Rasmussen Carlsbad, Ca. ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] Simple R4B troubleshoot
- Original Message - From: "Don Rasmussen" To: Sent: Thursday, March 17, 2011 10:53 AM Subject: [Drakelist] Simple R4B troubleshoot Hey guys, The fuse blew out on my R4B today. There are 2 filter caps that I am inclined to check, one is mounted underneath the power transformer, the other is the larger silver can mounted above the chassis. Can these be tested with a simple VOM? What readings to look for - shorted? Also wondering what else to test before tossing another fuse in line. The wiring attached to the NOTCH pot also seems to be disintegrating all by itself - interesting. Thanks, Don Rasmussen Carlsbad, Ca. If the caps are very bad the VOM might tell you but, in general, its not a reliable test. Electrolytics develop _parallel_ resistance, sometimes called leakage. A small amount is normal but it can become quite large, right up to being shorted. The way to tell is to use a variable power supply and a meter capable of measureing _microamps_. Probably any DMM will do it. For an electrolytic the leakage current should be pretty low, perhaps 15 uA. If the cap is bad it will be a lot more. The cap will take a while to charge and for the virtual plate to form so it should be left on the tester for a few minutes and the leakage current watched. Sometimes it will go down, which is OK but if it starts increasing toss the thing. Measuring capacitance takes a bridge or some form of capacitance meter. Bridges are good because they usually indicate _series_ resistance as well as value. Most electrolytics should have more than the marked capacitance value although modern ones are pretty close. Bad ones will measure low. High series resistance also indicates a bad cap _but_ both electrolytics and paper caps can have reasonable series resistance when the parallel resistance is low so both must be measured. It is normal for electrolytic caps to have fairly high series resistance (AKA ESR but also measured as dissipation factor). Sometimes its useful to measure the power factor by measuring the phase shift through the cap. An ideal cap will have exactly 90 degrees shift. Again, one plate of an electrolytic cap is formed chemically by action of voltage on it so measurements taken with only AC may not be adequate. -- Richard Knoppow Los Angeles WB6KBL dickb...@ix.netcom.com ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
[Drakelist] Simple R4B troubleshoot
Hey guys, The fuse blew out on my R4B today. There are 2 filter caps that I am inclined to check, one is mounted underneath the power transformer, the other is the larger silver can mounted above the chassis. Can these be tested with a simple VOM? What readings to look for - shorted? Also wondering what else to test before tossing another fuse in line. The wiring attached to the NOTCH pot also seems to be disintegrating all by itself - interesting. Thanks, Don Rasmussen Carlsbad, Ca.___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
