HP Spectrum Analyzer 141T no trace   - Page 1
Forum section: electronics / repair

author: aqibi2000
author_rank: Regular Contributor
timestamp: November 13, 2021, 10:12:44 pm
content: Just acquired this and I have no trace.Where can I start with the diagnosis, intensity and focus potentiometers rotate smoothly.Thanks

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 13, 2021, 10:38:40 pm
content: Read thishttps://www.eevblog.com/forum/repair/hp-141t-crt-restoration/msg3774392/#msg3774392On my post #5 I put some expanded details about following the service manual, along with 2 common failures.One is the failure of the 82 Volt glow tube reference, and the other is the flashover of the 6600 V storage supply which damages components on the pulse board.Removing the left side cover exposes the boards associated with the 'scope.Before starting, remove both the RF and IF modules until the above 2 possible problems are verified as OK, and all the DC power supply rails are tested and OK.But the 'scope needs the modules to get a trace, so you have to refit them after the above 2 problems are fixed.Good Luck!

author: factory
author_rank: Super Contributor
timestamp: November 14, 2021, 06:41:28 pm
content: Have you read the manuals? I note the scan mode switch is set to "single", try setting it to "internal" and the scan trigger switch to "auto".Also beware of setting the intensity to high, it's very easy to damage the storage CRT, especially when set to non-storage mode.David

author: aqibi2000
author_rank: Regular Contributor
timestamp: November 14, 2021, 08:49:08 pm
content: This is my first timer ever using a spectrum analyzer, only know my way around only use to oscilloscopes.Did change the scan mode and trigger to int and auto but no avail.I only played with the intensity to see if I could get a beam to show up.I’ve had a read of post 5 and need to spend more time understanding which board the voltage regulator is mounted on since I’m not familiar with this chassis.I can’t find figure 8.10 in the service manual to check the voltage regulator mentioned:https://xdevs.com/doc/HP_Agilent_Keysight/HP%20141T%20Operating%20%26%20Service.pdfI do notice some liquid on the hv caps, I’m thinking this is water to be honest, bought the unit off of a Gardner who did a house clearance.A few pictures attached.Thanks both

author: aqibi2000
author_rank: Regular Contributor
timestamp: November 14, 2021, 09:32:49 pm
content: Found a better service manual.http://ftb.ko4bb.com/manuals/85.255.233.56/HP_141T_Service.pdfAt a quick glance I’ve only spotted one power resistor slightly discoloured.Excellent demo for checking the 4 low Voltages and the HV:https://youtu.be/g3yog2r8yaMIs figure 8.11 the layout of figure 8.10?

author: factory
author_rank: Super Contributor
timestamp: November 14, 2021, 10:47:16 pm
content: Yes that is the right layout, most of the PCB is hidden under a cover. Make sure the manual covers the part numbers on the PCB, as they kept revising the design. I would normally say to check it covers the serial number prefix, but I have an older 141T that HP replaced the PSU board with a much newer one.I'm seeing another cooked resistor above the fuses.Another common failure with the 140 & 141 series are the voltage divider resistors either side of the power supply adjustment pots.You can remove the plug-ins until the power supply rails have been checked and repaired, if faulty.Best to clean & dry any areas that are wet too, is it possible the seller moved it when it was raining? there is nothing to stop water getting in through the holes in the side panels.David

author: aqibi2000
author_rank: Regular Contributor
timestamp: November 30, 2021, 10:42:03 pm
content: I finally got around to looking at this and found the 32mm glass fuse of the -12.6V rail blown, the other 3 low voltages are fine.Just changing the fuse brings the whole rail down to 0.1V but does now blow again. With the new fuse removed there is 24.8V available on inside.Diagram of the area I’m interested in, don’t like the fact that my psu layout does not match the service manual.Any thoughts how to go about testing this? Normally I would pull the transistors and test out of circuit and measure the resistors with one leg out of circuit but it looks like a very delicate/fragile procedure as well as time consuming.

author: Runco990
author_rank: Frequent Contributor
timestamp: December 01, 2021, 12:48:23 am
content: I'd start by checking C9 for a short. If not, keep going up the rail and check for shorted capacitors. This is the most COMMON failure before you start pulling transistors.

author: factory
author_rank: Super Contributor
timestamp: December 02, 2021, 07:51:26 pm
content: Yes check C9, it's metal axial hermetically sealed solid tant (Sprague 150D), an expensive part which rarely fail.Also note the plug-ins can blow the -12.6V fuse in the mainframe, picked up a faulty 8555A today with a note on it to stating it does this.If you need some decent diagrams, I could take some pictures from the appropriate manual, got two here, one covers 1973 and another that covers up the the 1980's, look under the high voltage cover and see what part number is on the PCB.David

author: aqibi2000
author_rank: Regular Contributor
timestamp: December 02, 2021, 07:52:36 pm
content: 18 ohm short in the rail, C9 isn’t the cause

author: factory
author_rank: Super Contributor
timestamp: December 02, 2021, 08:18:30 pm
content: The resistance would need to be lower than that to blow the 3A fast blow fuse (F4) on the -12.6V rail.David

author: aqibi2000
author_rank: Regular Contributor
timestamp: December 02, 2021, 09:48:34 pm
content: I do agree with your logic, 18 ohms is the lowest impedance there is measurable unpowered, some failed components can be reactive wherein they decrease in resistance with increased temp or frequency (in this scenario the voltage is a half wave rectified output from a centre tapped transformer so therefore the is a frequency component), could be a factor Xc=1/2*pi*f*C? Time to bust out my ESR meter, got a Capacitor Wizard and a EDS-88A. I feel it is a high esr capAs before, with a working fuse inserted the 18 ohms is enough to bring the 25v down to 0.1vzAside from this, unplugging this one wire brings the rail impedance up to 468 ohms.thanks btw David

author: m3vuv
author_rank: Super Contributor
timestamp: December 03, 2021, 12:12:49 am
content: this brings back memorys,about 8yrs ago i was at newark hamfest,i was on the lookout for a rectum paralyser to setup a messed with ts-430,saw one on a stand with a few sa's on it,they were all £100 mark,anyway i was hovering deciding what to do and the owner of the stall went off too the loo leaving his lad in charge,so i offered 25 quid for the sa,he said no its got to be £35,i said but it may not work,he said no they are all tested working to spec,i thought it best i pay him then legg it before the owner got back from the loo,well i tried to pick it up,i thought it was welded to the bench,weighed a ton!,got it back to my mates van,aligned the radio with it then sold it on ebay for £180,happy days,good luck with the repair.

author: W3SLK
author_rank: Contributor
timestamp: March 20, 2022, 04:06:11 pm
content: First post here and resurrecting this thread. I've posted about my trials and tribulations with my 141T(s) on another forum board. However, there is one issue that I haven't been able to resolve or find an answer to. I was having the proverbial issues with the fuses blowing on my 8552B unit. It was suggested to remove the 'neon regulator' tube and swap it with a pair of zeners because the time for the tube to fire causes the crowbars to activate on the LVPS in the module. Prior to doing that I had gone through the main frame and attempted to adjust all the voltage rails. Of interest was the +248VDC rail was +293VDC, and its associated adjustment POT was marked '290VDC', and the HV going to the tripler was 2835VDC as opposed to 2350VDC. I reduced the HV to 2350 but can't get anywhere close to +248! Was there a change made to raise both? I haven't found anything to support it. And now i don't have a trace, although I am sure my IF module is working, (somewhat!). If anyone can help I would certainly appreciate it! TNXMike(y)/W3SLK

author: factory
author_rank: Super Contributor
timestamp: March 20, 2022, 07:43:40 pm
content: All the diagrams in the 1982 manual, including the updated PSU board have a +248V output.What is the serial number prefix of your 141T?Here is the updated PSU board in my 973 prefix 141T, clearly at some point when it was repaired by HP they gave up on the original PSU board & replaced it.The common problems with the PSU boards are; the precision resistors drifting with age, they can to a pain to find and the neon ref tubes can fail with age, causing the voltage across them to rise.David

author: W3SLK
author_rank: Contributor
timestamp: March 21, 2022, 12:54:52 am
content: S/N 1850A 18458. Mine shows R-3 & R-4 extended off a bulkhead that separates it from the HVPS. A picture is worth a thousand words, let me see if I can upload a photo of mine:

author: factory
author_rank: Super Contributor
timestamp: March 21, 2022, 09:08:06 pm
content: The manual changes for prefix 1850A in the 1982 manual, gives part no 00141-66524 for A2, this has values of 49.9k for R10 & 100K for R12, these match what I can see in your picture.I would check those, but the problem could be elsewhere, for example I've had problems with the small plastic cased transistors failing in numerous HP PSU boards, concentrate on fixing the +248V supply rail first.Attached are some diagrams for the 00141-66524 PSU board, hope they help and you don't find anymore things that have been bodged by a previous owner/user.Mine differs as it's an older 141T that got modernised by HP at some point, those divider resistors & POTS for the voltage adjustments were changed for higher values, wonder if they knew they were a common failure.Being an older 141T it also has the extra parts for the calibrator circuit, this was only used with oscilloscope plug-ins, I see those parts are not populated on your PSU board, the POT labelled "CAL" will do nothing.David

author: W3SLK
author_rank: Contributor
timestamp: March 22, 2022, 10:17:45 am
content: Thanks David. Like I said I have been bouncing back and forth between forums searching for answers. I'm going to keep pounding away. I'm the type of person who likes to share information so that others can learn from my experience.David said:QuoteI was going to ask you about that. I see where they removed the gas regulator and added the zener diodes! I hope I no longer have to worry about the issues of blown fuses on the rear of the 8552B!

author: W3SLK
author_rank: Contributor
timestamp: March 28, 2022, 10:22:47 am
content: OK as an update. I changed the mainframe I was using. The voltages were all over the place and I wasn't getting anywhere with it. I am now using a mainframe with a less than aesthetic pleasing chassis, (rear corner is pushed in). However the voltages are where they are supposed to be, the CRT I know has a good trace, and it worked well before the deflection amp smoked! I replaced the zeners in it and all the voltages are where they should be. I installed the 8552B and 8553 and reset the voltages, (they just needed slightly tweaked). Now the modules work together like they should. But I still have no trace. I can see a faint 'trace of a trace' at the base of the CRT but no matter how much I tweak the vertical/horizontal gain and position I still can't get anything where it should be. The last trick in the bag is to use the 8552B and replace the deflection amp board with the good one.

author: factory
author_rank: Super Contributor
timestamp: March 28, 2022, 06:21:12 pm
content: How long ago was this all working, if it's been many years then the switches contacts might be dirty, I've certainly had problems with them, but usually intermittant.Quote from: W3SLK on March 22, 2022, 10:17:45 amThey only usedoneZener in place of the voltage reference tube, it is a 1N938 temperature compensated type at 9V, they did change quite a bit of the circuit around it too.David

author: W3SLK
author_rank: Contributor
timestamp: March 29, 2022, 02:49:02 pm
content: David said:QuoteI last used it seriously around 2011. I lit it off again in 2015 and that is when the board smoked. I have cycled the switches and they all seem to work. However I won't know nothing until I definitively establish a trace.This board I changed the VR01 out by using the holes left by the 'stabilizer loop.' I connected the zeners together under the board. I am going to try the other IF amp in hopes I can establish some trace and I can figure out what is exactly bad.ADDENDUM:I was able to successfully swap the other IF module and now I have a working spectrum analyzer again. I put the panels back on with confidence I won't be needing them again and connected the tracking generator. Everything was going well until the tracking generator died. It is a power issue but I need to find exactly where. Another Russian doll opened!

author: W3SLK
author_rank: Contributor
timestamp: April 11, 2022, 03:40:15 pm
content: I was able to get the 8443A tracking generator operating again but it was very trying. I went through the rectifier and sense amplifier boards semiconductors. I found a bad zener diode on the sense amp board. It started to work, then it quit again. I learned that the +24VDC rail was going to 4VDC! I systematically removed components from the circuit, (either by removal or isolating pins) until the +24VDC returned. I found a bad transistor that was only discovered by removing it and replacing it with another in circuit. Throughout it all this has been a journey with some unusual problems: a shorted resistor, an open zener diode, a transistor that statically showed good both in and out of circuit. Ufffff!!!