HP 8012B Pulse Generator -- To Hot?? - Page 1
Forum section: electronics / testgear

author: PwrElectronics
author_rank: Regular Contributor
timestamp: March 09, 2025, 01:34:36 pm
content: Some years back, I acquired this generator but other than a brief check I haven't used it. I believe it dates from the '70s as I remember the electronics tech school I attended in the mid 80s had some of these in the lab. There is a sticker on it from '97 when it was put in unused storage someplace before I got it probably 10yrs later.Last week, I needed to use it and thus it was powered on for a couple hours. I noticed the back panel was getting really hot. As in can't keep your hand on there for more than a few moments. I pulled the side cover and there are 2 larger transistors (power supply most likely) attached to the back as a heatsink. Thermal camera had those at something like 75C (50C rise). Also, a pair of 3W resistors on the board were about 120C. The E caps did not seem to be hot.Other than that, the unit seems to work OK. Is this normal for this?

author: wn1fju
author_rank: Frequent Contributor
timestamp: March 10, 2025, 04:52:02 pm
content: Seems high to me.I turned my 8012B on for about 45 minutes, set to square wave, internal load, maximum amplitude, mid-range frequency and let it cook. The two pass transistors on the back panel, measured with a thermal camera pointing at them from the inside of the unit, were about 50C. Similarly, the two current sense resistors measured about 50C. The back panel itself was moderately warm - I could keep my hands on the panel indefinitely if I wanted to.One would think that things couldn't get too hot. I measured about 0.3V across the 0.51 ohm current sense resistors, so that's about 0.6A current (on each of the supplies). So the resistors are only dissipating about 0.2W. The most dissipation they would have is when the 723 regulator goes into current limiting (and this occurs at approximately 1.4A), so the dissipation would be 1W. Similarly, the input voltage to the pass transistors is about 25V (or -25V for the negative supply) and with a 0.6W draw, the transistors are only dissipating about (25-17)*0.6 = 4.8W.I would be surprised if these dissipations are high enough to cause temperatures like you are experiencing. Just my opinion.Incidentally, if you can measure the power draw from the AC line, I measured just under 40 watts for my 8012B.

author: PwrElectronics
author_rank: Regular Contributor
timestamp: March 10, 2025, 10:29:48 pm
content: Thanks for the reply. I will see if I can spend some time checking the things you mention next weekend.Didn't seem right to me the back would be on the pain threshold. I actually have 2 of these and the other one has some problems as I recall but its worth powering it up to compare too.I have a pdf of the manual somewhere in my files as well.

author: AndersJ
author_rank: Frequent Contributor
timestamp: March 10, 2025, 10:51:36 pm
content: Could it be set to a too low mains input voltage?Such as set to 115V, but you run it at 230V.

author: Kean
author_rank: Unknown
timestamp: March 11, 2025, 03:54:48 pm
content: Quote from: AndersJ on March 10, 2025, 10:51:36 pmI was thinking more like the AC selector is set for 100V (Japan) vs 120V (US)...Even if set correctly, I'd start by checking the unregulated DC voltages.Some possibly useful info athttps://www.barbouri.com/2022/05/14/hewlett-packard-8012b-pulse-generator/

author: PwrElectronics
author_rank: Regular Contributor
timestamp: March 16, 2025, 05:07:11 pm
content: So, I looked at the power selector and its confusing and does not match the manual I downloaded a pdf scan of a few years ago:My unit (both of them):Both switches are on the left positionThe manual:The diagram in the manual does not match the layout on the unit so I am not sure what I am looking at.Checked the +/-17V supply rails and they seem OK. Also, input to the NPN pass transistors is ~26-27V each. Voltage across the 0.51 ohms resistors is .37v for the + supply and .27v for the - supply.I did notice the DC offset for the signal output will not go as negative as positive. Also, the double pulse function is not working.Actually, I think this is OK but offset max is about +/-6V and I didn't have the DP settings right.I checked the other unit and that one may have other issues too.