HP 4192A LF Impedance Analyzer - Help with issue in A1 board source resistors - Page 1
Forum section: electronics / testgear

author: pac1085
author_rank: Regular Contributor
timestamp: October 05, 2024, 08:02:52 pm
content: Hi,First a disclaimer, I'm just a hobbyist with an only a couple of years of electronics experience under my belt. So it's possible I'm missing something very simple due to lack of basic understanding. I've had some good success repairing broken test equipment and could use some help with this HP 4192A I recently acquired.When I received the unit, it would pass zero'ing Open, but it would fail when you tried to Zero short with error E-07.I went thru and executed the performance tests per the manual.Paragraph 4-9 "Measurement Signal Frequency Accuracy Test" - GoodParagraph 4-11 "Measurement Signal Level Accuracy Test" - GoodSkipped the rest since I hit a brick wall when I couldn't set it up for impedance measurements because I couldn't zero short.I also performed 4-21 "Amplitude Accuracy Test", which it passed, after reading 8-113 "Isolation of the VRD Section"After many hours spent in the service manual, I found something in section 8-111, "Isolation of the Range control circuit"In particular, the section that talks about a failure in the source resistor circuit.With a short across the unknown terminals, the test currents were way out of range vs what the manual specified.This led me to the flow diagram on page 8-86. In the 5th column there is a section where you manually set the ZY range and you should see the amplitude of the signal drop from the HCUR terminal. It did not drop when in the 10k or 100k range. The actions suggested checking A1K2, K3, Q19, Q26, U5 and U6 (under the HCUR AMP cover on the A1 board). I tested the relays and could see that the circuit was working correctly when manipulating the range setting on the front panel.During that process, I noticed relay K1 is always on. K1 is only supposed to be on in when the instrument is in amplitude-phase measurement mode. (according to the text on page 8-87)I removed K1 and the relay itself tests OK.With the relay left out, I installed a jumper on the PCB between relay pins 1 and 4, manually bringing the source resistors into play, so the unit should function correctly in impedance measurement mode now.It fixed the problem. I can now zero open, zero short, and make impedance, capacitance, etc measurements with no issues (and they are roughly the same readings im getting on my 4274A)Now I need to figure out why K1 is always stuck on. It is a normally open relay. This is where I need some help.The manual says to check the relay K1, its driver transistor, a 2n3904 (Q8) and the optoisolator U2. The relay test OK, the transistor tests ok on the curve tracer, I tried another one just in case. I have no experience with optoisolators and I'm not really sure what I'm looking for.I installed an LED in place of the relay coil, and it does appear the commands from the front panel are making It thru the optoisolator and firing the driver transistor. The intensity of the LED changes but it never shuts off completely.Here are the voltages from the bottom half of U2 (pins 3,4,5,6), which is the section used to drive Q8.Impedance Mode (K1 should be off) ||| Amplitude-Phase Mode (K1 should be on)Pin 3 3.414 ||| 4.85Pin 4 4.866 ||| 4.86Pin 5 -15.81 ||| -16.06Pin 6 -4.89 ||| 0.65It appears that the relay coil pins are still getting 9-10v even when it's supposed to be off.What else should I check with regards to this optoisolator? It's HP P/N 01990-0577. I spent a while doing a parametric search on digikey and can't find any modern comparable transistor output parts. Mostly because the anode/cathode is in a different orientation, but I notice there are a few resistors in the HP opto that I don't see in the modern ones. Not sure what other specs to look for other than 50mA (which is what the description of the part says in the HP manual). I'm not sure what type of signal is supposed to be going into the opto on pin 3, is it possible that the front panel isn't supplying enough signal to fire it fully?Any help greatly appreciated. Thanks!sorry, can't figure out why that last pic is sideways. it's not on my pc/phone, only when I upload it here...

author: coppercone2
author_rank: Super Contributor
timestamp: October 05, 2024, 08:23:44 pm
content: measure current going into opto its a gain device, it sounds like one of power supplies has not enough juice.led is very low current compared to relay, not good choice for test. a bulb maybe

author: pac1085
author_rank: Regular Contributor
timestamp: October 05, 2024, 08:34:37 pm
content: So the current across R9?I selected the dropping resistor for led to match the current I was seeing the relay pull on the bench psu, so should be similar minus the inrush when it turns on

author: coppercone2
author_rank: Super Contributor
timestamp: October 05, 2024, 09:07:57 pm
content: the light input is proportional to the current that flows in the isolator. you need to see if their getting biased correctly.it has a current gain spec. it might need a good 10-20mA to fire that relay properly. is it getting that?

author: coppercone2
author_rank: Super Contributor
timestamp: October 05, 2024, 09:24:32 pm
content: like, in these situations you need to start measuringmiliamps. it means some workthe easy method of just checking voltage level is not enough

author: pac1085
author_rank: Regular Contributor
timestamp: October 13, 2024, 03:06:31 am
content: It ended up being a bad optoisolator, A1U2. I replaced it with Broadcom HCPL-2531 and the instrument is now fully functional.