¿ªÔÆÌåÓýThis is a well-known problem.? The glow-tube voltage reference (V1) may not read high on a voltmeter, but it is ¡°lazy¡± starting up, and during that time the power supply output overshoots.? If it goes high enough long enough, it will activate the ¡°crowbar¡± protection circuit in the 8552, which shorts the rail to ground and pops the fuse. ? The easiest fix is to replace the glow tube with a solid-state reference.? Ed Breya came up with a simple design using a TL431 solid-state zener.? Search the groups.io HPAK forum for topic and photo album titled ¡°141T reference replacement¡±.? Here¡¯s a direct link to the schematic: /g/HP-Agilent-Keysight-equipment/photo/68333/805596?p=created%2C%2C141t+reference+replacement%2C20%2C2%2C0%2C0 ? This mod can be tacked across the glow tube, you don¡¯t even need to remove it. ? Dave Wise ? From: [email protected] <[email protected]>
On Behalf Of Tim Barrett K9VB via groups.io
Sent: Tuesday, February 15, 2022 11:06 AM To: [email protected] Subject: [HP-Agilent-Keysight-equipment] HP141T SA Blowing Fuses ? I was recently given an HP141T / 8552B / 8554B spectrum analyzer by a retired electronics engineer. It looks ine xcellent condition and I believe he would have looked after it.? He warned me that it tended to blow fuses when it had not been powered up for a while and he even gave me a pack of 1 amp fuses. Guess what, he hadn¡¯t powered it up in a while and it just eats 1A fuses ¨C there are two in the IF section both fed by the 141T 12.6v supply. So far blowing fuses is just about all it does though I can see a cursor on the screen since the 141T gets its power before the blown fuse. I¡¯m more familiar with simpler test gear so I need to tread slowly and carefully! ? I had read here that the 82 volt stabilizer tube goes high with age and was hoping that was it. But the voltage across the tube reads 81.9 so it is not the problem. The LV voltage levels are mostly within tolerance (just): +248v is 248, +100v and -100v both read 99 but the -12.6v is only 12. That needs adjustment, but later ¨C it is not the cause of fuse blowing. ? At this stage I¡¯m not sure if the cause is an initial surge from the 141T at power on or if the plug-ins are drawing too much current. Since sometimes both fuses blow in the IF unit, one feeding the 12.6v supply and the other 10V my thoughts are more that the problem lies in the 141T. I have downloaded the service manual from BAMA so I have schematics.??Are there any obvious things I should be checking? ? Tim |
