Wattmeter SIEMENS 7KB 4304 Teardown and repair - Page 1
Forum section: electronics / repair

author: Siriusx86
author_rank: Newbie
timestamp: January 04, 2022, 02:42:35 pm
content: Hello,I have just bought a Wattmeter SIEMENS 7KB 4304 as non functional. And I can not find any information about the device whatsoever.I am looking for documentation or any information about the Wattmeter.So I will try to repair a Wattmeter and document progress here.At first power on the Wattmeter did nothing so seller provide correct informationI opened it and have look inside. There was no obvious issues. All boards are without silk screen or voltage rail information. However there are some test points marked A, B, C and so on. But without manual they are useless. After measurement power supply board I found 3 voltage rails +6.25V +22.12V and -22.98V. So I recap power supply board and voltage stay sames but the wattmeter start shoving some sign of life. Some LEDs come up and also some random symbols on display.There is many Tantalum Capacitors 10uf 35V and 22uf 16V. What do you recommend as suitable replacement for these capacitor ? Can I use Ceramic Capacitors for longer Lifetime ?Thanks in advance for any information about the Wattmeter.

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 08:51:15 am
content: NORMA AC POWER ANALYZER D 5155This wattmeter was produced as Norma D5155 model PRECISION WATTMETER at the same time as Siemens 7KB 4304 model. The interior appearance of both devices is exactly the same. There is no service manual but I have only NORMA D5155 Operation Instructions. In this manual, both operating instructions and troubleshooting tips are given.Tantalum Capacitors always suspiciousI recently repaired this device. The device showed no signs of operation. Mains voltage was coming and fuse was good. 2 pieces 10 µF/35V tantalum capacitors in the power supply had been shorted. Changing them fixed +15V and -15V and +5V voltages.In the capacitor measurements I made, about 10 pieces of 10 µF/35 V tantalum capacitors was found a short circuit. I replaced them with AVX tantalum capacitors. But every time I run device, another 10 µF/35V capacitor was short-circuited at different boards. I changed all of 10 µF tantalum capacitors so as not to leave anything to chance.After some signs of malfunction persisted, I found and replaced a few short circuited 22 µF/16V and 1 µF/35V capacitors at other boards as a result of the measurements.After a long struggle, I have decided to replace the capacitors on all the circuit boards. (More than 100 pcs).I turned on the device. At the end of the initial self-test, it gave a "RAM TEST FAILED" warning.Dead Backup BatteryOn the CPU board, the RAM backup battery 3.6V NiCad had leaked and corroded the copper paths. (You can see this corrosion in the photo). The voltage across the battery was zero volts. I took the battery out and cleaned the printed circuit board thoroughly with isopropyl alcohol. I fixed the copper traces and installed a new 3.6V NiMh battery instead of NiCad battery.I restarted the device. At the end of the initial self-test, it gave a "RAM TEST FAILED" warning again.All calibration data was erased due to the battery voltage dropping to zero volts.According to the description in the operation manual, I turned the "CAL/RUN" switch on the back of the device to the CAL position and then turned the device on by pressing and holding the "RTL" button for about 6 seconds. Device started calibration with message "CALIBRATING CH1", then "CALIBRATING CH2" and "CALIBRATING CH3".The display reads after calibrating completed "TURN KEY TO RUN". I set the switch to RUN. The device is back to normal.I left the measurement and verification processes for later.I added Instruction Manual and some pictures for information.

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 09:11:30 am
content: Here are some pictures;

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 09:28:35 am
content: Pictures cont.

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 09:30:37 am
content: Pictures cont.

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 09:33:41 am
content: Pictures cont.

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 09:39:40 am
content: Pictures cont.

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 09:42:23 am
content: The last pictures group;

author: tbuyuran@tse.org.tr
author_rank: Newbie
timestamp: March 17, 2022, 02:54:37 pm
content: Overall block diagram, CPU board and SMPS block diagram extracted from Insruction Manual.