HP 33120a Function Generator Problem - Page 1
Forum section: electronics / testgear

author: Henry Finley
author_rank: Regular Contributor
timestamp: April 09, 2020, 01:30:02 am
content: I have an HP33120a 15mhz Function Generator here with a problem. When you first boot it up, everything is fine--no errors. But after 2 minutes, the little error rectangle lights up. Then the thing once again shows all the symbols just like it does when it boots up. That lasts a brief second, then you see RS-232 flash on the screen for a tiny instant, then the display shuts down, although the fan still runs. If I shut it off and reboot it, it once again works perfectly. It behaves and puts out everything it is supposes to, but from then on the little ERROR light is on the whole time. From this point on it will have that 625 error problem. but otherwise you can go on about your business using it. I've done what tracing and troubleshooting I can, but the service manual available online free is so low-quality as to be practically useless It mentions a U101 and U901, if I recall, and a couple other items. All these being IC chips. But what nags at me is the greater possiblility that it is some kind of heat problem, or why would it be OK for the first 2 minutes?

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 09, 2020, 02:20:37 am
content: U101 and U901 are opto-couplers that connect the I/O processor rx/tx to the main processor. Error 625 indicates the I/O processor and main processor are no longer talking to one another.Step one is always checking power supply voltage tolerances and ripple. The 5 volt supply “+5VO” supplies the fan and the comms processor. I’d check that for voltage and excessive ripple after its been on for the time it requires to go wonky. The regulator is U1051. It could be something as simple as the fan seizing up and pulling too much current, pulling the voltage down.

author: Henry Finley
author_rank: Regular Contributor
timestamp: April 09, 2020, 05:52:00 am
content: That is a very curious response, particularly since I have noticed the fan being prone to getting a bit noisy.

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2020, 06:05:20 am
content: The fans in these things are pretty crappy. I replaced the fan in mine with a Noctua fan I saw recommended by somebody here and it's FAR quieter.I agree that the power supply is the place to start.

author: Henry Finley
author_rank: Regular Contributor
timestamp: April 09, 2020, 06:22:52 am
content: Hoping (like a fool) for an easy fix, I unplugged the fan. Nope, no deal.Curiously The 4 transistors in the right far back mounted on vertical heat sinks, the inner 2 do not get hot. They warm up a tad to tell me they're working. But the right 2 next to the right side , They get hot enough to burn your finger, pretty quick. THAT can't be right. This thing does not have a set dependable pattern of mis-behaviour. the only thing in common is the display saying "RS-232" on boot up. the little "error" rectangle may or may not be immediately apparent. But the display may immediately shut down leaving the fan running and the 2 transistors getting hotter, or it may keep running perfectly WITH the error. how can you troubleshoot something that won't even mis-behave the same way every time? Some how, my gut feeling is that I am barking up the wrong tree. I'm not nearly as good as you guys. i work on the vintage stuff. I think this thing has a power-supply situation going on.

author: james_s
author_rank: Super Contributor
timestamp: April 09, 2020, 05:02:37 pm
content: I think the RS232 just means that it's configured to use RS232, mine says GPIB address 3 when I power it up and it sounds like yours is rebooting and displaying the powerup messages.I would start by monitoring the power supply voltages and see what happens when it reboots. I remember the heatsinks getting quite warm but I don't think anything got hot enough that I couldn't touch it.

author: J-R
author_rank: Super Contributor
timestamp: April 09, 2020, 06:08:05 pm
content: I have mine configured for RS-232 and restore default settings, so: power on -> full display test -> "RS-232" and sine wave -> default 1KHz sine wave display.

author: bson
author_rank: Unknown
timestamp: April 09, 2020, 08:08:51 pm
content: Check the supply voltages and scope the rails to make sure they're clean. Even if you don't think this is the problem there's no point doing anything else until the supply checks out OK.

author: Henry Finley
author_rank: Regular Contributor
timestamp: April 09, 2020, 10:26:32 pm
content: I intend to check the power supply, but with the quality of the service manual I hnae, this is going to be very hard. "Scope the rails"--i do not understand that expression. I do have a reasonably good ability to operate my Tek 2215 scope. thank you.

author: james_s
author_rank: Super Contributor
timestamp: April 10, 2020, 04:28:20 am
content: Rails = power supply outputs.Scope the rails = Use an oscilloscope to monitor the power supply rails looking for dips or glitches. A digital scope would be ideal here but if there's a problem you'll likely be able to see it on any scope.What service manual do you have? The one I downloaded a while back is excellent quality, the schematic is vector drawn so you can zoom way in and it stays clear and sharp. Actually it seems it's just under the max size threshold so I can just attach it here.

author: Henry Finley
author_rank: Regular Contributor
timestamp: April 12, 2020, 11:15:27 pm
content: Well it looks like I found my main problem,--the fan. Just like another poster on here. I always knew it was a bit noisy, but never considered that as the problem. I think the fan is just drawing a little too much current, and not running fast enough due to bad bearings, and just not pulling enough air through. That, and the fact the the instrument does not have its handle and lays flat (on the end bumpers), and not getting enough circulation. I discovered it with an ordinary window fan on low, pointed at the machine with the cabinet off. I ran for an hour with not a problem. The 2 VR's next to the chassis that WERE getting hot as a firecracker were running cool as a cucumber. My buddy and I checked all those big capacitors and they were running at the edge of the low end of 20% tolerance. I think this is a situation of an old machine with just enough stuff getting close to minimum tolerance and the machine is getting finicky about wanting to set off the error light. Where can I get a good fan for it? One that will move as much air as the current draw allows? Thanks, fellows. I really think it's case closed on this one.

author: james_s
author_rank: Super Contributor
timestamp: April 12, 2020, 11:57:49 pm
content: I used a Noctua NF-A4x20 FLX which was recommended by another member here, I've been very happy with it, it's virtually silent and the performance seems comparable to the original. It's a drop-in replacement, just splice the connector or cut the extension cable that comes with the fan and make up a little adapter.