Agilent/HP 33120A Function Generator Repair - Page 1
Forum section: electronics / repair

author: Shay
author_rank: Regular Contributor
timestamp: November 24, 2022, 01:18:27 am
content: I've bought a second-hand HP 33120A Function Generator.Powering it up for the first time, the display displayed all the letters and digits, then it beeped and a second later, there was nothing on the display.I opened it and started playing with the contacts. I noticed that the connector on the main PCB that has a ribbon cable that goes to the display LCD, didnt have good contacts, as playing with it sometimes returned the display to normal and made function generator almost working perfectly. (TLDR: this isn't the issue).After powering it up successfully, I noticed the "ERROR" box was displayed (I believe it was error 772, which isn't displayed on the service manual). Connecting the DMM+scope I noticed that the ampltiude isn't correct at all - it is off by a few hunderes millivolts. at worst, the generator is showing 20V and I only get 8v output. Tried to calibrate it, but the output voltage was too low and the generator didn't allow me to set it correctly (you couldn't go below a certain voltage, e,g on the 5.5V settings the minimum measured voltage was 5.1V but i measured below that).To solve the issues, first I wanted to fix the issue with the display connector. so I simply removed the connector and soldered the ribbon cable wires directly to the board. After doing in, the unit is stuck at displaying all the digits for a split second, and then beeping and showing nothing. Playing with the ribbon cable this time did nothing.All the power rails have the correct voltage on them, expect the LM2940-5 - for some reason it outputs 4.6V. Replacing it with 7805, and the output voltage is 5V again. but after a few minutes, it also dropped to 4.6VSomething is making the voltage drop, I am not sure what is causing that - since sometimes I read 5V and then it randomly drops to 4.6V.U1006 MC3423 - outputs 1.8V while the input to the IC is 5V. so clearly its faulty - I injected to the output pin of U1006 (which is also connected to the RST pin of the main uC) 5V, but it does nothing.I've seen this thread:https://www.eevblog.com/forum/repair/hp-33120a-function-generator-power-on-problem/and it seems like the issue for them was U103 or U105. However since I did manage to get the unit working when playing with the cable before I soldered it directly, I am not sure if a faulty ASIC or SRAM could cause it.Would like any help or advice for fixing this unit.Thanks!

author: R-1125F
author_rank: Regular Contributor
timestamp: November 24, 2022, 03:53:15 am
content: I repaired one of these last year. I don't know if you have the same problem, but check the voltages across the zener diodes, such as CR701, CR702, and CR401. I had a problem with those.Does the output waveform look correct?

author: Shay
author_rank: Regular Contributor
timestamp: November 25, 2022, 01:30:25 pm
content: Quote from: R-1125F on November 24, 2022, 03:53:15 amJust checked all the zener diodes on board and they're good, expect CR401 which I can't check reverse bias on it (not enough voltage to drop 3V3) but it has a correct foward drop of 0.6V.Could it be an issue with the display board? the unit worked almost OK (Expect the output under-voltage) when I did play with the front panel cable, and after removing the connector from the main board and soldering the ribbon cable wires directly to the main board, the issue is now consistent.Somehow playing with the contacts of J101 front panel connector, made the unit sometimes work beforehand.When I did manage to get the unit to work before, I checked on my osciliscope the output waveform and it looked very sharp and stable, no jigger, everything looked as it should.

author: R-1125F
author_rank: Regular Contributor
timestamp: November 25, 2022, 09:42:21 pm
content: Unless you made a mistake soldering the front panel cable to the board, I don't see how it could cause the problem. The obvious thing to do is to disconnect the front panel and see if the voltages come back to normal.Error 772 is on the user manual, Nonvolatile system memory checksum failure. Why they didn't include all of the error codes on the service manual is a mystery.

author: jwet
author_rank: Frequent Contributor
timestamp: November 25, 2022, 09:49:13 pm
content: I don't know about the display issue- seems like its just connectivity, careful wiring. Haven't deal with that. The fact that you had it working recently is encouraging.On the output issue. I repaired mine a few years ago when it had an output problem- it got back driven by a DC source which took out the output amp. Putting 15v P-P in into a 50 ohm load with DC offset takes some pretty stout stuff especially up at 15 Mhz. (2+ Watts). The output is all discrete and well designed. The HP service docs were good. Ended up replacing the final output Q's and their drivers with exact replacements. Increased heat sink size on finals. My symptoms were similar- low output.

author: Shay
author_rank: Regular Contributor
timestamp: November 25, 2022, 10:59:54 pm
content: Quote from: jwet on November 25, 2022, 09:49:13 pmCheck the output transistors, they are all OK. also the capacitors on the output are OK. haven't checked the pass transistors drivers. this is next after I fix the display issue.Quote from: R-1125F on November 25, 2022, 09:42:21 pmThe regulators voltage and the U1006 voltage is very weird, sometimes it is the correct voltage, and sometimes not. However, this is not the main issue I believe, as bypassing U1006 and replacing the 5v regulator didn't fix the issue. I tried de-soldering the front panel wires and re-attaching them. didn't help.Nonvolatile system memory = EEPROM is bad (?). Means I am out of luck basically?

author: R-1125F
author_rank: Regular Contributor
timestamp: November 26, 2022, 03:11:21 am
content: U1006 holds the processor in reset (PONRST* signal) until the +5V is fully turned on . If the +5V supply does not exceed +4.6V, the processor will not state. Monitor pin 1 of U1006 with an oscilloscope to be certain it is turning on. If it is low or keeps reseting, the processor will not start.

author: Shay
author_rank: Regular Contributor
timestamp: November 26, 2022, 08:43:00 pm
content: Quote from: R-1125F on November 26, 2022, 03:11:21 amIt is indeed constantly low at 2V. I removed U1006 from the board and provided +5V to the RST pin from one of the 5V regulators. I check with my scope the RST pin now and it is held constant at 5V, but nothing works.

author: R-1125F
author_rank: Regular Contributor
timestamp: November 27, 2022, 07:31:31 pm
content: Did you check all power supply voltages with respect to analog and digital ground?If they are all good, check for activity on the address and data pins on the micro controllers, U102 and U1101. If there is no activity, check the power reset lines.

author: Shay
author_rank: Regular Contributor
timestamp: November 27, 2022, 11:31:25 pm
content: Quote from: R-1125F on November 27, 2022, 07:31:31 pmI checked all the PSU voltages with respect to E-GND. the voltages are almost spot on, so I believe thats OK.I desoldered the VFD Display, in order to get access to the componets under it. I have checked the voltage supervisor 34064, and the 78L05 regulator. They are all perfectly working.How can I check for activity on the address & data lines? what pins should I check?Thank you very much for your help!

author: R-1125F
author_rank: Regular Contributor
timestamp: November 28, 2022, 03:15:37 am
content: The easiest place to look U102. See the schematic at chips U102. Using an oscilloscope, first check for the 12 MHz clock signal at pins 66 or 67 of U102. The address pins are labeled AD and start at pin 60. If you see activity. This will be at 0 and 5V logic levels. If you see activity anywhere the microcontroller for the main board is working.The front panel will be a little more difficult. Using an oscilloscope, check for the 12 MHz clock signal on U1102 pins 20 or 21. Then, using an oscilloscope, check for activity starting at pin 2 and work your way around the chip. If you see activity anywhere the microcontroller for the front panel is working.

author: Shay
author_rank: Regular Contributor
timestamp: November 28, 2022, 10:45:26 pm
content: Quote from: R-1125F on November 28, 2022, 03:15:37 amAll the crystal oscillators are fine, producing 12Mhz sine wave.I have checked U102 and U1101 address and it seems like there is some activity on U1101 and U102 pins. However, on he address pins of U102 there is only 0V or 5V constantly. There is data or activity. Is that OK?I noticed the output stage trannys is also getting really hot doing nothing, so I checked every component that was close to the heating transistors. Everything is OK.

author: R-1125F
author_rank: Regular Contributor
timestamp: November 29, 2022, 03:30:23 am
content: I am not certain about the address lines. The output transistors will get hot. Did you look at the output transistors with your oscilloscope to make sure that there was no output?You cold also use your oscilloscope to check if there is data going to the display.