HP8711B VNA power-up failed (alive now) - Page 1
Forum section: electronics / repair

author: mrf245
author_rank: Regular Contributor
timestamp: September 21, 2017, 02:18:05 pm
content: helloI jut picked up HP8711B VNA looks like very clear, but it power up failed.Debug LEDs indicated "8" like picture shows below.I tried to pulled up BOOT ROM, LED shows 8 with a dot and "HALT" LED is illuminating.Referenced to CPU board schematic, use oscilloscopeto check bus, it looks like CPU is reading BOOT ROM continuously, and no other action.Can anyone point out what I should do next?Thanks very much

author: TheSteve
author_rank: Unknown
timestamp: September 21, 2017, 05:48:06 pm
content: I see no ram in the sockets - I thought it needed ram to boot.

author: TK
author_rank: Super Contributor
timestamp: September 21, 2017, 09:35:04 pm
content: The sockets are for adding additional RAM and you need to set the correct DIP switches next to the sockets. The CPU board has DRAM marked as U61 to U68 (The SIP chips).CPU Board schematics (CLIP):http://www.ko4bb.com/manuals/24.3.150.247/HP_HP871x_network_analyzer_Other-A208712-60002CPU.pdf

author: TheSteve
author_rank: Unknown
timestamp: September 21, 2017, 10:38:56 pm
content: I don't have mine anymore. I just know it only had 1 MB of ram even though there was ram in the sockets.

author: TK
author_rank: Super Contributor
timestamp: September 22, 2017, 12:13:23 am
content: I have the 8713B and it shows 4MB DRAM without any modules in the sockets. If I add 4 SIMM modules, it shows 4MB DRAM and 4MB SIMM, for a total of 8MB RAM

author: mrf245
author_rank: Regular Contributor
timestamp: September 22, 2017, 02:26:47 am
content: Thanks TK and Steve. SIMM RAM is not necessary for normal operation except BASIC options.Seller said it worked fine before but one day it suddenly no power-up.I checked battery its OK, and re-plug CPU in set as service manual noticed, nothing.I also set switch beside SIMM socket, it switch on at "1MB" position before. After turn all switch off (set to up position), it still not working.

author: Armadillo
author_rank: Super Contributor
timestamp: September 22, 2017, 02:46:32 am
content: Quote from: mrf245 on September 21, 2017, 02:18:05 pmYou should probe every bits of the Address and Data lines. Objective is to detect if one of the line is stuck.After that, you proceed to the address decoders. Here is where analyser is useful. otherwise manual probe.If there are electrolytics, the former may have changed leaky caps but left the ring of death on the boards.Hope it helps.

author: TK
author_rank: Super Contributor
timestamp: September 22, 2017, 10:28:55 am
content: Do you see anything on the display? My unit had a dead CRT module problem:https://www.eevblog.com/forum/testgear/hp-871xb-vna-crt-module-capacitor-identification-help/msg1239724/#msg1239724Check the fuse on the CRT module.

author: mrf245
author_rank: Regular Contributor
timestamp: September 24, 2017, 01:57:23 pm
content: Quote from: TK on September 22, 2017, 10:28:55 amYes, CRT module is fine. Can you hear "beep" when your VNA power up? I have no heard on this one.

author: TK
author_rank: Super Contributor
timestamp: September 24, 2017, 02:21:54 pm
content: Yes, I can hear "beep".Error code 8 means one or more self-test failed. I think you can find the error by looking at the schematics and following the traces from the diagnostics LEDs.You can find all the schematics (CLIP) here:http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=HP_Agilent/HP_871x_Network_Analyzer

author: TK
author_rank: Super Contributor
timestamp: September 25, 2017, 02:10:49 am
content: From the Service ManualError 8: Test number not used because test status LED powers up with his as default. An "8" on the LED indicates the 68020 was unable to power up and execute basic instructions.

author: mrf245
author_rank: Regular Contributor
timestamp: September 26, 2017, 02:40:02 pm
content: Quote from: TK on September 25, 2017, 02:10:49 amYes I consider it means CPU normal boot failed. Guess BOOT ROM is fine (no use programmer to ensure), maybe shared RAM or other nearby circuit? I will go on to check

author: TK
author_rank: Super Contributor
timestamp: September 26, 2017, 03:42:09 pm
content: Quote from: mrf245 on September 26, 2017, 02:40:02 pmWhat the service manual suggests when you get error 8 is to change the microprocessor 68020.

author: mrf245
author_rank: Regular Contributor
timestamp: September 27, 2017, 02:59:26 am
content: Quote from: TK on September 26, 2017, 03:42:09 pmYes manual said replace 68020 or whole CPU board...This type of 68020 is not easy get from local market. I will try to check CPU and related circuit, or replace CPU if I get one.

author: TheSteve
author_rank: Unknown
timestamp: September 27, 2017, 07:38:06 am
content: I figure there is better odds the ram failed versus the processor.

author: mrf245
author_rank: Regular Contributor
timestamp: October 12, 2017, 04:03:15 pm
content: OK, I update some about repair job.It's finished and working fine now.In fact, not CPU board fault, it's failed in power supply.+15V output is only +4V, and I found there is "pwr-fail"signal in module socket.(shows below)Good module it output 5V, but this fail module output 0V.I have no tracked where this signal goes into CPU systems, but I'm sure if this is low, it will halt whole CPU system from boot, maybe it's protect function, but it mistook me to check if CPU system was failed.So it look like easy. Pull out power module, find +15V output zone, I found rectifier diode (FEP6DT) was shorted.Purchased new one and replace it. Now 15V is normal. Whole 8711B run normally now.

author: TheSteve
author_rank: Unknown
timestamp: October 12, 2017, 06:59:17 pm
content: Very nice! I hope the capacitors are ok, I've seen a few of these supplies with caps that leaked.