-hp- 339A Distortion Meter:  meter pegging - Page 1
Forum section: electronics / testgear

author: TimFox
author_rank: Super Contributor
timestamp: July 22, 2020, 06:05:09 pm
content: One of my favorite test instruments is an -hp- 339A distortion analyzer (THD meter). The only maintenance I have needed to do is cleaning the rotary switches and the unit seems to meet all its original specifications.However, it has one troublesome habit: whenever I switch mode, the meter momentarily pegs and takes several seconds to come back on-scale. This is very annoying when doing THD as a function of signal level, since that requires switching back and forth between "input level" and "distortion" for the panel meter, and sometimes it's not clear if a pegged "distortion" reading is really pegged, or if I need to wait for it to recover. This also happens when switching the hpf and the two lpfs in and out.Is this behavior normal for this instrument, or is there a repair/fix?

author: TimFox
author_rank: Super Contributor
timestamp: July 24, 2020, 10:04:06 pm
content: Bumping this topic again, after it fell off the page.Can anyone suggest an alternate site to post this question, if no one here can answer?

author: 0culus
author_rank: Super Contributor
timestamp: July 24, 2020, 10:07:34 pm
content: Do you have a service manual for it? Usually the troubleshooting section will offer diagnosis hints.

author: TimFox
author_rank: Super Contributor
timestamp: July 24, 2020, 10:22:12 pm
content: I have the full manual, but could not find anything about this symptom.

author: markce
author_rank: Regular Contributor
timestamp: July 24, 2020, 10:31:00 pm
content: QuoteYou can try diyaudio.com forum, section Equipment & Tools. There is a lot of knowledge on the HP339A there,It has a 87 page topic: 'HP339A distortion analyser'

author: 0culus
author_rank: Super Contributor
timestamp: July 24, 2020, 10:53:03 pm
content: That's good advice...also might be worth checking the power rails for accuracy and ripple. Too much ripple because of a bad filter cap can cause all kinds of interesting problems.

author: bd139
author_rank: Super Contributor
timestamp: July 24, 2020, 10:59:35 pm
content: That might be normal. My HP 400E AC voltmeter did that as well on low ranges. It takes a while to settle. Also some HP power supply metering circuits do it too when you change range.Obviously check around but don't be surprised if there are glaring and annoying things like that in older instruments. It's what gives you time to consume caffeine.

author: bob91343
author_rank: Super Contributor
timestamp: July 24, 2020, 11:59:09 pm
content: Does it happen if you switch to a higher range before changing the meter function?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 25, 2020, 12:35:12 am
content: Hi TimFox and the group,I just checked my 339A.1) The meter is pegged when you first power up.2) switching from OSC LEVEL to distortion causes the meter to peg momentarily.I believe what you are seeing is normal.Regards,Jay_Diddy_B

author: TimFox
author_rank: Super Contributor
timestamp: July 25, 2020, 02:01:28 pm
content: Thanks for the replies. I checked out the thread at diyaudio.com. Several posts indicated that the momentary pegging was a known "feature" of the meter driver circuit. Most of the posts were mods (such as better op-amps) to improve the performance. I routinely get residual distortion (with the built-in oscillator) of < 0.002% (-94 dB), which is adequate for my purposes.Edit: A quick check under ideal circumstances: 3 V at 2 kHz, all three filters (400 Hz hpf, 30 kHz and 80 kHz lpf) in, shielded connection from oscillator to analyzer: 0.0012% (-98 dB).

author: markce
author_rank: Regular Contributor
timestamp: July 25, 2020, 10:09:04 pm
content: Just to be sure: Did you set 'meter response' switch to 'normal'?

author: TimFox
author_rank: Super Contributor
timestamp: July 26, 2020, 02:48:45 am
content: Yes. The other position is "VU", which I don't need.

author: markce
author_rank: Regular Contributor
timestamp: July 26, 2020, 12:25:19 pm
content: Ok, pegging is a lot worse in 'VU' position on my HP339A.

author: TimFox
author_rank: Super Contributor
timestamp: July 26, 2020, 12:57:22 pm
content: I assume that the "VU" position is there for radio-station use in the Input Voltage mode, rather than with Distortion mode.