¿ªÔÆÌåÓýThanks for the help everyone ¨C I ended up tracing it to what appeared to be a dry solder joint on A26K1. It was running the fan but not providing the +15V switched to the rest of the power supply. ? TonyG ? Sent from for Windows 10 ? From: [email protected] <[email protected]> on behalf of Kuba Ober <kuba@...>
Sent: Saturday, November 3, 2018 3:53:01 PM To: [email protected] Subject: Re: [HP-Agilent-Keysight-equipment] 8902A Power Supply issue ?
Exactly what Peter said! Running out of headroom at low line, high load and high regulator¡¯s reference is a bit shameful, so in good designs one makes sure that with everything against them, they¡¯ll still work. Linear regulators waste
lots of power, and that¡¯s that. Personally, seeing something like LM7805 running from 8V unregulated at load is not something I¡¯d expect in a good design: it¡¯ll run out of headroom at 7.2V input, so the minimum unregulated instantaneous (not RMS!) voltage
must be above it in all conditions, with bulk capacitors aged for design lifetime of the power supply. That won¡¯t happen with only 8V RMS input, and even 9V will be too low. Sure, it ¡°will work¡± on the bench, in perfect conditions, but those don¡¯t always occur
in practice.
I don¡¯t know what regulators are in use in the subject instrument ¨C that¡¯s just an example. Cheers, Kuba > 3 nov. 2018 kl. 17:44 skrev Peter Gottlieb <hpnpilot@...>: > > Remember that the supply must have sufficient voltage pre-regulator at the lowest line voltage and the highest load the supply was designed for. Also it must be the lowest voltage of the ripple. > > > Peter > >>> On Nov 3, 2018, at 5:41 PM, Harvey White <madyn@...> wrote: >>> >>> On Sat, 3 Nov 2018 15:41:06 -0400, you wrote: >>> >>> Those seem ok as the unregulated supplies. >> >> I'd think -20 volts and +8 volts might have been a better design. Just >> being conservative without thinking of what they really did. Could >> those voltages be a result of light loads on the bulk supplies? >> >> Harvey >> >>> >>> >>> Peter >>> >>>> On Nov 3, 2018, at 3:36 PM, Tony Goodhew <tony_goodhew@...> wrote: >>>> >>>> Hey All, >>>> >>>> My 8902A stopped powering up and I've just started looking at the power supply. >>>> >>>> I worked through the manual troubleshooting steps and it seems to leave the finger pointing at the regulators (basically no rail other than the +15 came up when I tried the steps and it doesn't appear to indicate a short in any other area). >>>> >>>> Looking at the unregulated -15V and +5V I'm seeing -23.5V and +10V. I understand they're not regulated but that seems a bit far away from the values on the schematic. >>>> >>>> Is this expected or should they be closer to -15V & +5V? >>>> >>>> Thanks, >>>> >>>> TonyG >>>> >>> >>> >> >> >> >> > > > |
