SOLVED: HP 6227B - slave channel voltage meter slammed right - Page 1
Forum section: electronics / repair

author: Tabloka
author_rank: Contributor
timestamp: September 15, 2023, 05:40:13 pm
content: Hello folks,I don't know what to do next. What has happened so far.One day the slave channel went dead. I checked the F2 fuse and found it blown. Okay maybe an easy repair. I changed the fuse against a new one and since then the slave channel is not adjustable and the meter of the slave channel slammed to the right to over 30 volts. The measured output (slave) is 47.5 volts at the front terminals.As the manual says at 5-62 d. 'I isolated the crowbar from supply by disconnecting anode of CR4' to measure the reference and bias voltages. Step 2 to 6 is no problem and the numbers are quit good but step 1 I get odd numbers. For the master I read +24.7 volts and for the slave -16.3 volts. Both should be +27.5 volts.I connected the common lead to the anode of VR2 and for TP12 I connected the positive lead to the plus of the C6 cap.My next steps were according to table 5-4 (Series Regulator Troubleshooting, High Voltage Condition). For STEP 1 I shorted the driver Q7 from collector (-OUT terminal) to emitter (TP66) and the output voltage decreased as in response b. described. So far so good. I moved further to STEP 2 and shorted the predriver Q6 base (TP64) to emitter (TP65). The result was that the output voltage remained high. The manual says, 'Q7 open'. Unfortunately that doesn't get me anywhere. I checked all the TPs and here is the result:Does anyone have a tip as to where the error could be?

author: Swake
author_rank: Frequent Contributor
timestamp: September 16, 2023, 09:10:56 am
content: HP 6627B ?I know of a HP 6627A, but that is 4 channel power supply.

author: Tabloka
author_rank: Contributor
timestamp: September 16, 2023, 10:48:57 am
content: Sorry my fault. It‘s a HP 6227B

author: Swake
author_rank: Frequent Contributor
timestamp: September 16, 2023, 11:35:00 am
content: 'Q7 open' means it is damaged as an open circuit. With your DMM in diode mode, measure the junctions of Q7 to verify that this thing is still a transistor. You're best to unsolder it to have a good measurement.If you don't measure the 2 junctions (roughly about 0.6 V each) then it is dead.

author: Swake
author_rank: Frequent Contributor
timestamp: September 16, 2023, 11:40:41 am
content: The manual tells us that Q7 is a 1853-0052, and that translates to 2N3740.Always check on the transistor itself if is matching this.Search for 333488487983 on Ebay, that is from a German seller and to me at least seems legit from the pics and reputation.

author: Tabloka
author_rank: Contributor
timestamp: September 16, 2023, 12:16:17 pm
content: Quote from: Swake on September 16, 2023, 11:35:00 amThat's what I thought but I checked both Q7 with a DMM and also with the DCA pro. With the DMM I got TWO junctions with app. 0.6V each (desolderd) and with the DCA pro the results are shown below:Sorry I have trouble to insert images to the post.

author: Swake
author_rank: Frequent Contributor
timestamp: September 16, 2023, 12:58:46 pm
content: Datasheet for the 2N3740 says min hFE = 30hFE of 145 seems high. The increase in hFE over time has been seen in old transistors and then they behave differently. Not saying this is the case here, but it is a sign.Could you check the resistance between C and E, it should be very high.You might try to swap the Q7's and see if the issue flips to the other channel.

author: Tabloka
author_rank: Contributor
timestamp: September 16, 2023, 02:02:04 pm
content: Quote from: Swake on September 16, 2023, 12:58:46 pmThe resistance between C and E of the two are very high. But anyway I order new ones and try it.

author: Tabloka
author_rank: Contributor
timestamp: September 20, 2023, 03:22:05 pm
content: Quote from: Tabloka on September 16, 2023, 02:02:04 pmSo today I got four new2N3740transistors. One of them has a hfe of 47 and this one I soldered to the board instead of the 147.No changes!The output voltage of the slave channel is still 47 volts.So I can say that the Q7(B) is not the problem.Please have a look at the 'bias and reference voltages.jpg' earlier in this thread. The TPs 11, 12 and 13 are so totally different between master and slave. The voltages over the caps C6 and C7 are both fine. What could be the problem? BTW I also substituted the transistor Q6 (predriver) of the (B) channel but also without success.Any suggestions? I'm pretty desperate.

author: Swake
author_rank: Frequent Contributor
timestamp: September 21, 2023, 02:18:01 pm
content: (sorry, bit of a late reply, got lots of work)I'm walking back in the schematic as the Q6 signal must come from somewhere.Your measurements indicate TP60 reads high at 7V. Seems like the issue is even further back. This TP60 signals comes over CR8 (voltage regulation, TP54) and CR9 (current regulation, TP44) Have you checked these diodes? Assuming these are ok, let's check the regulations.Current regulation:TP44 reads high at 2.5V. That triangle thing looks like an operational amplifier comparing TP40 and TP41. These are both slightly off, but let's call that good enough as the other channel has the same measurement. OK, but then why would the output of the comparator be at 2.5 V ? --> there is that other input "M" from the voltage comparator and that is TP53 and measures fine. Another possibility is that this comparator OpAmp is bad.Voltage regulation:TP54 measures high. TP52 (power supply for that OpAmp) is too low. Ow... seems like the OpAmp pulls it down for some reason and I believe that pin is de positive supply for the OpAmp. This is a strong indicator the OpAmp has an issue. The other side of the 20k resistor above TP52 should be at 12.4V and that is fine because you measured TP30 at 12.6V. The negative rail is TP32 and that measure fine too. Input TP50 is slightly low compared to spec, but the other input TP51 compensates for the same amount.Voltage Clamp:TP57 is fine. Don't think there is an issue with the clamp. At 2.6V and over 2 diodes it is not this signal that pulls TP60 to 7.1V .In the schematic notes of the operating and service manual (page 3) there is a diagram for that Z1 device. You should be able to test some of the junctions with your multimeter in diode mode.Between pen 10 (=TP51) and 11 (=TP52) you have a base/collector junction, but you measure exact same voltage (-0.6V). Sounds like a candidate for a short to me. You'll likely have to unsolder it to confirm 100% and avoid any interference in the measurement from the rest of the schematic.

author: Swake
author_rank: Frequent Contributor
timestamp: September 21, 2023, 02:25:35 pm
content: Z1CA3026RCA databook:https://archive.org/details/bitsavers_rcadataBoogratedCircuits_48180891/page/n197/mode/2up?q=CA3026

author: Tabloka
author_rank: Contributor
timestamp: September 21, 2023, 05:45:33 pm
content: Quote from: Swake on September 21, 2023, 02:18:01 pmI desoldered the little 12 leads bastard and got following results for testing with DMM in diode mode (see attached file). As you assumed I checked the diodes CR8 and CR9 at the beginning of the troubleshoting and found that they are 100% okay.I am a little bit surprised about the behavior of the power supply after desoldering the CA3026 for the (B) channel. When I turned on the power supply, the voltmeter wasstill hitting the right edge.

author: Swake
author_rank: Frequent Contributor
timestamp: September 21, 2023, 08:04:17 pm
content: About the PS still going to max.TP63 is at 0.69V now as expected.But TP64 is high at 2.2V, that is something I don't understand as there is only a resistor between those 2 TP and no source. Both sides of R36 should really have a very similar voltage. The resistor is here to limit current into the base of the transistor, and now it is as if the base of Q6 delivers current the other way. A base does not deliver any current, unless the Q has a problem. Or there is some dirt or a (microscopic) short around it that would conduct current towards TP64 side of R36. Please check.Do you know how to check a transistor with your multimeter? The 2 'diode' junctions? You can measure this in circuit. Between base and collector you measure a diode. Same between base and emitter. If one of them is open then the transistor is dead, if it is a short it generally is dead too unless there is a resistance over it which is rare. Of courses the supply must be off.Please check the transistors on the 12pin thingies. For example between pin 4 and 5 you should find a diode. I'd like to try confirming these are ok or not.You could also check all other transistors.

author: Swake
author_rank: Frequent Contributor
timestamp: September 21, 2023, 08:06:08 pm
content: Quote from: Tabloka on September 21, 2023, 05:45:33 pmHo, you did it already, but I cannot see the file. Or I overlook it completely.

author: Tabloka
author_rank: Contributor
timestamp: September 21, 2023, 08:21:06 pm
content: Quote from: Swake on September 21, 2023, 08:06:08 pmSorry my fault. I forgot to attach the file to my reply. Here you go:

author: Swake
author_rank: Frequent Contributor
timestamp: September 22, 2023, 07:15:44 am
content: They all look good. Bit of a high reading, but that might have been a thing in that era.You're in an interesting puzzle...

author: Tabloka
author_rank: Contributor
timestamp: September 22, 2023, 10:24:59 am
content: Quote from: Swake on September 21, 2023, 08:04:17 pmI don't know what happened. I wrote that I substituded the Q6(B) some days before and the original Q6(B) (HP 1854-0071 -> CP4071) was good. Now I found a short between B and C and that is the reason why there is a big diffenrence at the R36 a TPs 63 and 64.I substituded the transistor with a 2N2222 and now the voltages are as shown below. Both sides are similar equal at -4,64 V.

author: Swake
author_rank: Frequent Contributor
timestamp: September 22, 2023, 04:13:40 pm
content: Good!And Q7 still measures ok? (the 2 junctions). And while you are at it, please check Q4 and Q5.Have you put back Z1 the 12pin double comparator? I think it is just fine based on your measurements. After the replacement of Q6 it is also good to restart the debugging from a known situation. So I would put it back.Now that I know Z1 is composed of 3 transistors and is not a 'normal' OpAmps my previous thought process around it is probably invalid.

author: Tabloka
author_rank: Contributor
timestamp: September 25, 2023, 09:14:10 am
content: Quote from: Swake on September 22, 2023, 04:13:40 pmHello Swake, sorry for the very late reply but my wife and I did a fine trip over the weekend.Today I will start with the troubleshooting again. First I will put back the 12 pin Z1 and then I'll start the measurements from the beginning.

author: Swake
author_rank: Frequent Contributor
timestamp: September 25, 2023, 09:48:00 am
content: No problem. Anyhow this weekend I had an overflow of work till very late. Would not have been able to look into it.

author: Tabloka
author_rank: Contributor
timestamp: September 27, 2023, 10:12:48 am
content: After renewing the transistor Q6(B) I checked the voltages again. The results are in the attached file. There is no big difference between the two columns'slave' and 'slaveafter renew Q6' except TP52. But as you can see in the column 'slavewithout CA3026' the voltage at TP52 is the reference voltage from TP30. And I think this should be okay because the CA3026 is desoldered.I was wondering if the CA3026 could be the troublemaker because with the replaced CA3026 the voltage at Z1 lead 11 (TP52) breaks down. So somethinginthe Z1 could cause the problem. What do you think?In the attached file 'Schematic - slave with wrong voltages.jpg' I marked the wrong voltages in red and also wrote the measured voltages next to the TPs.

author: Swake
author_rank: Frequent Contributor
timestamp: September 27, 2023, 06:43:52 pm
content: Your schematic is much nicer than mine. I have one on 3 separate pages.There is a errata about the TP in the manual, have taken this into account?Code:On Figures 7-5 and 7-6, change the circledtest point numbers as follows:64 change to 6665 change to 6766 change to 6867 change to 6968 change to 70You could borrow the CA3026 from the good channel or install this one in the good channel.You could slap 6 NPN's together to replace the CA3026. If it is just to test no need for a PCB. 2N2222 or BC107 or BC546, whatever general transistor you got available actually.

author: HalFoster
author_rank: Frequent Contributor
timestamp: September 27, 2023, 06:46:40 pm
content: As with most troubleshooting, start where things are correct. The -1.3V at TP52 is way off - is the voltage at TP57 2.7V? Could the clamp circuit be hosed?Hal

author: Swake
author_rank: Frequent Contributor
timestamp: September 27, 2023, 08:03:15 pm
content: The TP57 at the output of the clamps measures fine at 2.6V. This indicates it is working as expected.The clamp is connected to TP54 (pin12 of Z1) not to TP52.TP references are valid if the PS is stable. This is not the case. Therefor one has to take into consideration the output voltage of the PS, and this is very high at 47V.The voltage comparator agrees that PS output is high and pulls TP54 as high as it can at 7.7V.TP60 is one diode junction (=0.7V of CR8) away from TP54. I would expect it to be at 7V but it is at 0.9V.Q5 is either working hard or there is a problem with it.Q5 is a PNP transistor and used as inverter.TP64 is the output and is at -4.6V. This matches its function of inverter. Therefor I think Q5 is fine and till that place the PS works.

author: Tabloka
author_rank: Contributor
timestamp: September 27, 2023, 08:14:31 pm
content: Quote from: Swake on September 27, 2023, 06:43:52 pmYesterday I ordered 4 new CA3026 from a dealer in NL.

author: Swake
author_rank: Frequent Contributor
timestamp: September 27, 2023, 08:19:16 pm
content: @tabloka,Have you checked the big transistors Q8 to Q11? If you tell me these are shorted I would believe it. Although your previous test showed these are not...Could you repeat the 5 step high voltage debug procedure from the manual please?

author: Tabloka
author_rank: Contributor
timestamp: September 27, 2023, 08:28:12 pm
content: Quote from: Swake on September 27, 2023, 06:43:52 pmI also read this in the manual and took it into account but the TPs at the schematic are the old ones. This doesn't matter because I checked the voltages excately at the positions where the 'old' TPs should be. So its only an other number but the same place. You can read my tables in accordance with the posted schematic above.

author: Tabloka
author_rank: Contributor
timestamp: September 28, 2023, 10:16:24 am
content: Quote from: Swake on September 27, 2023, 08:19:16 pmI checked the transistors Q8 to Q11 again. There are no short circuits detectable.Quote from: Swake on September 27, 2023, 08:19:16 pmStep 1: response b. 'Output voltage decreases'Step 2: response a. 'Output voltage remains high' - probable cause: Q7 open - I renewed the Q7(B) already but without successStep 3: response a. 'Output voltage remains high' - my test: Q6 and Q4 areNOTshortedStep 4: response a. 'Output voltage remains high' - my test: Q5 isNOTshortedStep 5: response a. 'Output voltage remains high' - here you can read, 'Z1 defective'I will wait for the new CA3026 (Z1) and then I will replace the Z1(B) with a new one. There after I'll do the tests again. I think this should be the next step, shouldn't it?

author: Swake
author_rank: Frequent Contributor
timestamp: September 28, 2023, 10:19:37 am
content: Yes, it is good to confirm and make sure.Compare the 'new' Z1 against what you measured on the 'old' Z1.

author: Tabloka
author_rank: Contributor
timestamp: October 05, 2023, 10:49:45 am
content: I got the new CA3026 from the dealer in NL in a very good condition (NOS) and soldered it to the pcb. NO changes!In the meantime I read something about other PS repairs and found some stuff for the special diode with the HP # 1901-0460. This is a STB523-A and so to say a series of 3 diodes with a Vfwd 1.92V. I desoldered it and checked it with my DMM. The Vfwd is about 1.92V. So here is also not the problem.I'm pretty confused about what to do next.

author: Tabloka
author_rank: Contributor
timestamp: October 06, 2023, 09:48:43 am
content: Above I wrote that I got 4 EA NOS transistors CA3026. Because there was no success with the replacing of the transitor(B) I checked the new (NOS) with my DMM and got following results (see file attached).Does anyone know why I can read voltages from pin 8 to pin 11 and 12 (and voltages from pin 9 to pin 11 and 12)? When I look at the schematic diagram (included below the table) that makes no sense for me.

author: Tabloka
author_rank: Contributor
timestamp: October 06, 2023, 01:12:45 pm
content: After all that days of frustrating work I finally found the failure. I read again the troubleshooting steps at high voltage condition and stumbled again over the phrase "Q7 open". So I checked the emitter track and found no fault. I checked the base track and found no fault. I checked the collector track and found ....... no stable connection to minus front slave. Cross check with the master channel: here are 0 ohms from collector Q7 to minus front master.What is wrong with the slave channel?Barely to see at the bottom side of the pcb mother board a conductor track was burned out.I replaced the burned out conductor track with a wire and now the PS works as fine as on the first day.Special thanks to Swake for the big support!!!

author: Swake
author_rank: Frequent Contributor
timestamp: October 07, 2023, 08:48:48 am
content: It is true that repairs can be very frustrating sometimes, and at the same time so rewarding when you finally find the last issue. I see it as experience, every time you get a little better with it.