HP 8903A – Need Help with VU Meter Issue and Maintenace  - Page 1
Forum section: electronics / repair

author: Rico75
author_rank: Newbie
timestamp: April 20, 2025, 02:21:13 am
content: Hi everyone,I recently picked up this HP 8903A (yes, I was hunting for a "B" model, but this one will do just fine!). Overall, it seems to work pretty well—as you can see from the photos, the DIST measurement at 1kHz/1V starts around0.0100and then settles to lower values (see pics). Not sure if that initial reading is normal, but it stabilizes.The real head-scratcher is the analog VU meter: after a couple of days of use, it pegs to full scale and stays there. When I power off the unit, it resets to zero. I might have accidentally enabled some function I can’t find—no idea what’s going on. If any of you seasoned HP 8903 experts have seen this before, I’d really appreciate your help!I’ve had it for less than a week (finally found one after ages!), and now I want to give it a proper cleanup and maintenance to keep this beauty running for years. These things are amazing!Thanks in advance!

author: wn1fju
author_rank: Frequent Contributor
timestamp: April 20, 2025, 12:32:10 pm
content: Does sound like something is malfunctioning. All I can suggest is that you look at Schematic #6 in the Service Manual. There is a circuit called "Meter Peg/Off Switch." Has a couple of comparators and FETs. Check that circuit and perhaps that will be the source of the problem. It is fed by controller commands, so hopefully the fault doesn't lie with the controller.

author: Rico75
author_rank: Newbie
timestamp: April 21, 2025, 12:37:13 am
content: Thank you so much for your message and the very helpful advice! I really appreciate it.I’ve already located Schematic #6 (Meter Peg/Off Switch) in the service manual, and I’ll carefully check the comparators, FETs, and surrounding circuitry. Fingers crossed it’s something simple!Since this unit is a veteran (like most HP 8903As these days), I’d also love to hear if you (or anyone else) have general maintenance tips for these machines. Are there any specific components that tend to fail due to age? For example:Electrolytic caps that should be proactively replaced?Relays or switches that wear out and need attention?Known weak points in the analog or power supply sections?I want to give this beauty the care it deserves so it keeps running for years to come. Any wisdom from experienced owners would be golden!Thanks again—I’ll report back once I’ve tested the meter circuit.

author: wn1fju
author_rank: Frequent Contributor
timestamp: April 21, 2025, 11:37:27 am
content: Search the EEVblog forums for "8903" and read through the several topics to get an idea of what people have done with their units.As a general rule, I don't usually start replacing electrolytic (or tantalum) capacitors unless there are obvious signs of leakage, bulging or discoloration. The only exceptions are the gold colored RIFA suppression caps on the incoming AC line (they always need to be replaced, regardless of condition), but the 8903A fortunately doesn't use them. And the 8903A doesn't have a lot of front panel pots or switches that often need cleaning. The usual thing that happens with the HP switches of that era are that the leaf springs inside the switches pop loose. You will feel when this happens and the fix is simply to take the front panel apart and reseat the spring, very easy. Similarly, the power supply is linear, not switching, and is mostly bulletproof.The only thing I did with mine was change one of the internal adjustment pots that sets the noise floor of the unit. It was a very sensitive adjustment and I was having a hard time dialing it in perfectly, so I changed it to a 10-turn pot that made the adjustment easier.After you have found your SINAD meter problem, you might want to run through the performance verification and/or the calibration procedure.

author: Rico75
author_rank: Newbie
timestamp: April 23, 2025, 04:38:59 am
content: Thank you very much for the detailed advice—it’s incredibly helpful!I’ve started inspecting A4 board(Meter Peg/Off Switch circuit)and found that theVR1 zener diode near Q5 is shortedand R78 (nominal 2150Ω) measures only 542Ω– significantly out of spec . I’ll need to dig deeper into the surrounding components, but for now, progress is slow due to work commitments. I’ll post a proper update in a week or two once I’ve had time to test everything thoroughly.Quick question about calibration:What’s the recommended process to perform a full calibration on this unit? Are there specific steps or equipment needed beyond the manual’s procedure? (I want to ensure it’s done right after repairs.)Thanks again for the support—I’ll keep you posted!

author: wn1fju
author_rank: Frequent Contributor
timestamp: April 23, 2025, 12:00:29 pm
content: The calibration procedure is fairly simple for the 8903, just follow the steps in the service manual. I've found the calibration procedure to be a lot simpler than many of the other HP pieces I have. The performance tests are a bit more complicated and thorough. To do everything right, you need fairly good DC and AC sources, DC and AC voltmeters and a frequency standard/counter.That's just my opinion/experience. Perhaps other 8903 owners on the forum would like to chime in...

author: macboy
author_rank: Super Contributor
timestamp: April 23, 2025, 02:47:48 pm
content: Focus on the performance verification aspect of calibration, rather than assuming that you need to perform adjustments.One important adjustment, notch balance (IIRC), is very difficult to dial in exactly. Even just a slight pressure on the adjustment pot can throw it off to one side. Because it is so sensitive, I can see this easily getting knocked out of alignment during shipping, and it is critical to squeezing out the last few counts of residual distortion. I believe this is the adjustment that user wn1fju had referred to earlier as well.In order to measure distortion, the instrument measures the RMS of the input signal, and measures the RMS of the distortion residual (output of the notch filter) and calculates the ratio. These signals will obviously differ by several orders of magnitude. Therefore, pay attention to the verification of the RMS meter in all ranges. It is likely that the lowest range will frequently be used to measure the distortion residual, so any offset or gain error in that range will significantly impact the THD and SNR readings.You will notice that the design does use some RMS-to-DC converter ICs like commonly found in True RMS multimeters. However, these are only used for non-critical things like auto-ranging. For the actual RMS measurement, HP designed its own discrete RMS to DC converter. It uses log/anti-log amplifiers to form an analog multiplier/divider, which is then used to 'compute' the DC voltage equivalent of the RMS of the input signal. One must assume this has better performance than the off-the-shelf converter ICs, or they would have just used another of those in that role instead. If you are interested in old school 'analog voodoo' circuits, then an RMS converter is right in there with the best of them. The service manual has a thorough description of the circuit operation.

author: Rico75
author_rank: Newbie
timestamp: April 24, 2025, 04:35:39 am
content: A huge thank you to everyone for these incredibly valuable insights - this community's knowledge is truly exceptional!Attached are two photos:The A4 board from my unit (actual)The A4 board from the service manualThey appear significantly different in component layout. Could this actually be an 8903B board installed in my 8903A?Any thoughts?Gday

author: srb1954
author_rank: Super Contributor
timestamp: April 24, 2025, 09:17:56 am
content: Firstly, check that the serial number of your 8903A is covered by the manual you have. The range of serial numbers covered is usually indicated on the front page of the manual.A second check would be to compare the PCB assembly numbers in the BOM in the manual against the code etched on the PCB.For instruments such as the 8903A that had a long production life there were often small production changes that didn't warrant the model being raised to a B or C since the changes were minor and didn't alter the performance, features or operation of the instrument. Such changes were usually documented in the manuals with inserts containing errata notices. Check for errata notices applying to your serial number. In the hard copy manuals these are easily recognised as they are printed on yellow paper but a PDF scan may not replicate this colour distinction.

author: wn1fju
author_rank: Frequent Contributor
timestamp: April 24, 2025, 11:56:08 am
content: As srb1954 suggested, there evidently have been several versions of this (and other) boards in the 8903 series. Your board shows number 08903-60117, the service manual for the "A" model lists part number 08903-60001, and the manual for the "B" model shows 08903-60128, 08903-60282 and 08903-60283.Usually, the changes are minor and it is just the layout of the parts that mostly differs. One can usually figure out what is going on by piecing together all the various service manuals you can get your hands on.Section VII of the typical HP service manual lists backdating changes, but since your board has a higher part number than shown in the "A" manual, that isn't going to be much help.HP usually provided yellow change sheets in their manuals. I've downloaded hundreds of HP manuals and I would estimate less than 10% of the downloads included the change sheets. Even when I bought hardcopy HP manuals from manual dealers, they often didn't include them. It's generally not a show-stopper for most common repairs.

author: 1audio
author_rank: Frequent Contributor
timestamp: April 27, 2025, 01:24:22 am
content: You can also get help here DIYaudio.com . There are a number of threads and some experienced users/repair threads. And this should be interesting:https://www.diyaudio.com/community/threads/hp-8903a-audio-analyzer-mods-and-upgrades.295895/page-2#post-4820247