HP 6237B Power Supply Repair Help - Page 1
Forum section: electronics / repair

author: NapoleonDynamite
author_rank: Newbie
timestamp: February 24, 2025, 03:43:51 am
content: Hello Everybody! I've been working on a project lately, one that has eluded me a few times. It's a HP 6237B power supply - here is the full story:I bought it almost a year ago, from a local guy who said it worked fine, but the power switch had snapped off from the whole unit being dropped on the front left corner, so it would need that replaced. After replacing the switch, I found it mostly worked - both +20V and -20V supplies worked great, but the +18V supply was always stuck at full blast (+21VDC) no matter where the control was set.I found the service manual for free online (https://www.manualslib.com/products/Hp-6237b-5143594.html), and followed the suggested Initial Troubleshooting Procedure. Going through the steps, I stop at Table 5-7, High Output Voltage, Step 2 (Measure voltage at output of OR gate {TP6}) I get only -61mVDC, not either choice of +0.7VDC or -0.7VDC.Then I start going through the schematics, and started looking through the +18V circuit. Because it had been dropped, I tested the control pot out of circuit, and it tested good. Measuring at the cathode of CR42 I would expect the voltage to vary with the control pot for a instant, then stabilize like it does on the working +20VDC supply, but there is no change at all with the control. I then desoldered the input to the voltage control opamp (pin 3 of U3), to test the opamp input circuit. I figured if the opamp had internal issues, then I could adjust the pot and see a corresponding voltage change at either the anode of CR43 or the cathode of CR42, but no dice. The voltage does change from having pin 3 of U3 attached, but the pot still has no effect. For good measure I also checked Q7 and Q8, the series reg and driver transistors - both are good.I am lost at this point. My best guess is that U3 is not working right, but I do not know how to test a opamp out of circuit, and do not know what part would be suitable for a replacement.In regards to the U3 replacement, has anyone had luck with a LM-1558 or LM-1458? A post I found suggests those.Thank you for the help!NapoleonDynamite

author: pqass
author_rank: Super Contributor
timestamp: February 24, 2025, 08:08:49 am
content: With regards to a modern replacement for U3 (CA3458T), seethis Fairchild Databookpage 3-4 where the 1458HC is listed as a direct replacement for the CA3458T.And on the 1458HC detail page 5-183 it says "For single amplifier performance see the uA741 data sheet".So yes you can replace the old-style metal can U3 with a "modern" 1458(commercial) or 1558(military); aka a dual 741 op amp.Just solder half-inch leads to a plastic DIP 1458 and place over the circular pads. The pin order is the same just in a square vs circular package.However, before you troubleshoot, you should have a general understanding how these discrete component power supplies regulate voltage and limit current. Seethis writeup.Main differences between the writeup and your +18V supply are:(a)Your +18V supply uses a PNP series pass transistor vs. NPN in the writeup so voltages, diodes, etc. are reversed, and(b)The writeup uses two voltage references, one for current and one for voltage, relative to positive output and GND terminals, respectively,whereas your +18V supply uses -6.2V and -12.4V only referenced to the GND/COM output terminal.Key info WRT your +18V power supply:(a)The Reference and Bias, the +18V, the +20V, and the -20V supplies have all their GNDs/commons connected at one point; the COM output terminal.Note the triangle ground symbol with the "C" inside everywhere. So most measurements (your multimeter black lead) should be relative to the COM output terminal.(b)The series pass transistor Q7 is defaulted to fully-on (via R54 to Va=-12V). The driver transistor Q8 is defaulted to fully-off (via R53 to -12.4V).So the voltage and current op amps "OR" together (via diodes) whichever op amp wins (is more positive, CV or CC) turns on Q8 enough until Q7 shuts down enough to the desired voltage at the output positive terminal.(c)How CV operation works in detail:1. R41+R42 is a divider where the mid-point aims be 0V (due to negative feedback with the -Vin, pin 2, of U3a being attached to the COM terminal, which is 0V).2. At unit turn-on time, the positive output terminal is close to 0V so that divider mid-point is below 0V.3. U3a sees that -Vin > +Vin so the output will swing to the -12.4V rail which will be stopped by the CR46 diode from affecting Q8.4. Q7 is defaulted to fully-on and Q8 to fully-off, so the positive output will continue to ramp toward maximum (voltage on left side of Q7).5. However, the divider mid-point will eventually go above 0V which will cause U3a output to swing positive which will turn-on Q8 and turn-off Q7.6. But if Q7 is turned-off too much, that divider mid-point will go below 0V again and turn-off Q8 which will turn-on Q7.7. So this negative feedback (via positive output terminal -> divider mid-point -> U3a -> Q8 -> Q7 -> positive output terminal) will turn-on Q8 and Q7 enough to maintain the desired voltage (set via R41 pot).(d)How CC operation works in detail:1. If there is no load attached at the output terminals, then either side of R48 (0.625R) shunt has the same voltage (no drop).2. If there is a 1A load attached then the voltage drop will be 625mV; the left side will be -625mV relative to the right side which is the COM output terminal.3. Z1-I (30K, part of a resistor network)+R46 3K pot allows the wiper to vary between 0V and -1.1V.4. U3b op amp is used as a comparator between left side of R48 shunt and R46 pot wiper setting.5. Normally, under no/low load U3b output will swing to the -12.4V rail which will be stopped by the CR45 diode from affecting Q8.6. If the left side goes below the wiper setting, then +Vin > -Vin so U3b will swing positive which will turn-on Q8 and then turn-off Q7 (current limiting at the positive output terminal).Given the above background, logically start from Q7 and work toward R41 pot.Other things to consider:Given that the unit was dropped, look for cracked/broken solder joints, traces on heavy/loose components like capacitors or those with long leads.High stress/heat components like the series pass transistor Q7 could be shorted.

author: NapoleonDynamite
author_rank: Newbie
timestamp: April 07, 2025, 03:19:51 pm
content: Hello pqass! Thank you for the very helpful reply, I've learned a lot from your response. Here is what I've done after reading everything you sent.First, I checked Q7 and Q8. Both tested good.Then, CR46. Also good.I skipped the opamp (you'll see later).R53, good.R51, good.R52, good.CR43, good.R50, good.CR42, good.R44, good.R42, good.Then I get to R41. In circuit, it tests low, about 6K across the whole track, so I pull it out. Out of circuit, it tests good, about 9.5K. So I reinstall it.This just leaves U3. Since I have no idea how to test a opamp out of circuit, I did this: Pull out U3 and U1 (U1, known working, from +20V supply). Put U1 in U3's spot. Just in case, I also lifted one leg from each diode CR11, CR12, CR31, and CR32 (To cut off all power but bias from +20V supply -now missing the opamp- and -20V supply).Powering it up on my variac, there is no change - U3 opamp likely works. At this point I'm scratching my head again. Then I realize something. In circuit, R41 tested bad across the whole track AND there was no connection between the wiper and R42. Following the middle pin, it goes to a line of vias just above R58 and stops there. There is nothing connected to that via. Then I soldered a jumper between the wiper of the pot and the pin going to R42 and removed the diodes powering Q7, CR51 and CR52, and also removed again U1, now in U3's spot.Now I measured at pin three of both U3's and U1's sockets. Without the jumper at R41, the voltage at U3 is stable and does not change with the front panel output control. Adding the jumper causes the voltage to change, just like on the +20V supply.Then, I reversed everything I changed previously, except I left the new jumper across R41. It now works, even under full load. Not sure where R41's wiper and R42 joined originally, the via where the wiper connection ends showed no signs of solder work.I've since closed up the unit, and placed it on my bench with my other equipment. Over the past few weeks I have been using it and it has not had any issues. Thanks again pqass!