HP 6012A Power supply help (Opto isolator replacement) - Page 1
Forum section: electronics / repair

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 22, 2016, 10:26:32 am
content: Hello everyone,My old faithful 6012A stopped working the other week and I have nearly got everything back together except for the opto isolator. I am hoping someone can give me a hand to make sure that I am getting the right one, or at the very least one that will work. Please see below for the things that went wrong, what I have done to fix them, what I have looked at and tried to find.When the power supply stopped working, there was kind of a sizzle sound, then nothing. I quickly turned the power supply off at the switch, then the rear breaker, the wall and disconnected it from the rack.After opening the PSU, the 750mA fuse on the main board was blown, as was a 10ohm carbon resistor (R16 on board A1). This forms part of the soft start circuit which turns the relay on after a few seconds, bypassing the resistors that are there to limit the inrush current to the big capacitors.The circuit itself is pretty simple, bridge rectifier, a few resistors, a transistor, capacitor, zener and a opto isolator. The opto isolator is driven from the control card and turns on the transistor which then turns on the coil to the relay (I have done very little work with transistors before, so please excuse my ignorance on them).The fault turned out to be the electrolytic filter capacitor for the ~165VDC rail had leaked, then scorched a track. The 10ohm resistor is a current limiting resistor for that circuit, and that overheated which then cooked itself and the fuse has blown.Fast forward a week, the burnt part of the PCB has been cleaned up and very carefully dremmel'd out to remove any carbon. The 10 ohm resistor has been replaced with a 1/2 carbon composition 300vdc rated resistor and the capacitor has been replaced with another 22uF 250V (all as per the replacement parts list in the service manual). The transistor was replaced with a MPSA42, the original was a ST48090A. I am pretty sure that the MPSA42 meets or exceeds the requirements of the original ST48090A. The original transistor was checked with a peak component analyser and the leads were all shorted. The way the transistor is fitted to the board was a bit strange, but I made sure that the pins were the same and I didn't get the transistor round the wrong way. I originally looked at the opto isolator and took a gamble on it, thinking I will replace it later if it is stuffed.That brings me to today. Turned the power supply on and I have power, but no relay clicking in after a delay, so the entire power supply is running through the current limit resistors.I carefully measured the various parts on the circuit to get the voltages and the fault is the opto isolator. I am getting 1.15v from the control PCB into the LED side of it. On the DC side (keeping in mind we are up around 165VDC), with the negative of my bench DMM to the negative of the bridge rectifier, I am getting 5.45v at pin 5 of the opto isolator (this is regulated down through a zener (VR1) and a few resistors (R14 and R15)). Pin 4 goes to the base of the transistor and I have 0v (well 0.0015mV, so practically 0v). The emitter of the transistor is going to the negative of the bridge rectifier and the collector is going through another resistor before going to the relay coil. I am seeing 166 VDC at this point because it is coming through the coil.My diagnosis is that the opto isolator is not triggering the transistor, which in turn is not turning on the relay.The problem is that the HP part number for the isolator is 1990-0593 and showing it as a Motorola SOC-156. I have been coming up blank finding a data sheet for this part, and having very little experience with opto isolators, I am not sure how to ensure I am getting the right replacement part or how critical it is for the running of this fairly basic circuit.The information I know / have been able to find outVoltage from the control board to the LED is 1.15V6 pin DIP packagePin 1 / 2 are the LEDPin 3 / 4 are the outputNothing connected to pin 3 or 6From a national stock info page, it is used in USA fighter jets and shows a 60mA If MaxIf someone can please give me a bit of guidance that would be appreciated. If you have questions about part of the circuit, I have done a very crude hand drawn sketch when I was running around with the probes taking measurements. If you need that, I will find out a way to upload a cleaner version that is easier to understand / follow.Thank you for your time.-Chris

author: RobK_NL
author_rank: Frequent Contributor
timestamp: November 22, 2016, 01:39:44 pm
content: That optocoupler is not very critical. In the HP6032 (the 6012's digital brother) they used a 4N35, which is about the most jellybean optocoupler you can get.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 22, 2016, 03:05:02 pm
content: Awesome, I'll give that a go. I noted that they have pin 6 as a external base connection. If that is left floating will there be any issues or should I get one without the base on pin 6 to be sure?I've got to do a farnell order so I'll get a few added onto the order.Thank you-Chris

author: PTR_1275
author_rank: Frequent Contributor
timestamp: November 25, 2016, 11:24:15 am
content: I couldn't get my hands on a 4N35, but I could get 4N38, I took my chances and it worked.The power supply is now back in the rack and happily supplying a lot of power