Repair: Rohde & Schwarz / Polarad URE RMS Voltmeter
The shorted C41 is clearly seen in this thermal image, taken within a few seconds after the URE was powered up slowly through a variac.
Following this, all the 1000 µF / 25 V capacitors (Roederstein type EKM) in the three URE units I acquired were replaced with Rubycon type ZLH. In order to start the "autocalibration" routine remove the lid and do not have anything connected to the input. Remove jumper BU33 (marked by the arrow in the picture), press 'SHIFT' once, press '.' three times, and press '4' once. Wait some 20 seconds until the routine completes. Put jumper BU33 and the lid back in place. The routine should be run while the URE has reached operational temperature.
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I got a set of three URE RMS voltmeters that would not power up, one unit branded as Rohde & Schwarz, and two units branded as Polarad. Judged by the serial numbers and the date codes of the components the units appeared to be of the same age, within perhaps two years.
Interestingly, I found that all three units suffered from a case of bad electrolytic capacitors for the digital 5 V supply! This sums up the findings: Unit 1 - Rohde & Schwarz URE. It turned out that one 1000 µF / 25 V capacitor was shorted, and would cause the fuse to blow. I always power up a unit in an unknown condition through a variac, and this time there was clearly an elevated current draw from the mains. I localized a hot spot close to capacitor C41, shown in the picture on the left, using a Flir TG167 spot thermal camera. The TG167 has a limited resolution of merely 80 by 60 pixels, but once again, it proved its value in my lab. I replaced all six 1000 µF / 25 V capacitors with three 2200 µF / 25 V Rubycon type ZLH, and the URE would then boot when powered up. The error message 'Error 8' indicated loss of calibration data. After having replaced the drained lithium battery the "autocalibration" routine was run successfully, and the URE then powered up without error messages. Capacitor aftermath: One short, the 5 others were measured to between 992 µF and 1291 µF, with ESR between 0.06 and 0.19 Ω. Unit 2 - Polarad URE. The total capacitance of the six 1000 µF / 25 V capacitors was reduced so much that the unregulated rail voltage for the 5 V supply was not high enough to let the power watchdog circuitry release the reset signal. I could actually make the unit boot by increasing the mains voltage through the variac from about 230 VAC to 245 VAC. I replaced all six 1000 µF / 35 V capacitors with three 2200 µF / 25 V Rubycon type ZLH. The ripple on the capacitors was reduced from about 785 mV RMS to about 240 mV RMS, the latter figure even during the active state of the URE. The URE could now run down to about 195 V mains voltage before resetting, when wired for 240 V. The lithium battery was not the original, and appeared to be in order. However, as the age of the battery was unknown, it was replaced. Capacitor aftermath: Four practically open circuits, the other two were 936 µF + 0.47 Ω ESR, and 816 µF + 0.64 Ω ESR. Unit 3 - The second Polarad URE. The total capacitance of the six 1000 µF / 25 V capacitors was so low that the ripple approached a staggering 5 V RMS. I replaced all six capacitors with three 2200 µF / 25 V Rubycon type ZLH. The lithium battery was not the original, but was all the same totally drained and had to be replaced. After having replaced the drained lithium battery and running the "autocalibration" routine with success, the URE powered up without error messages. Capacitor aftermath: All six were practically open circuits! The other electrolytic capacitors in the URE, those for the analog +15 and -15 V supplies, the analog +5V supply, and for post-regulator decoupling, are not Roederstein type EKM, but Roederstein type EK. As a precaution, these will eventually have to be replaced. |