SOLVED: HP 6227B - slave channel voltage meter slammed right

[Tabloka]

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?

[Swake]

HP 6627B ?

I know of a HP 6627A, but that is 4 channel power supply.

[Tabloka]

Sorry my fault. It‘s a HP 6227B

[Swake]

'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.

[Swake]

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.

[Tabloka]

'Q7 open' means it is damaged as an open circuit.

That'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.

[Swake]

Datasheet for the 2N3740 says min hFE = 30
hFE 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.

[Tabloka]

Could you check the resistance between C and E, it should be very high.

The resistance between C and E of the two are very high. But anyway I order new ones and try it.

[Tabloka]

But anyway I order new ones and try it.

So today I got four new 2N3740 transistors. 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.

[Swake]

(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.

[Swake]

Z1
CA3026

RCA databook: https://archive.org/details/bitsavers_rcadataBoogratedCircuits_48180891/page/n197/mode/2up?q=CA3026

[Tabloka]

You'll likely have to unsolder it to confirm 100% and avoid any interference in the measurement from the rest of the schematic.

I 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 was still hitting the right edge. 

[Swake]

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.

[Swake]

I desoldered the little 12 leads bastard and got following results for testing with DMM in diode mode (see attached file).

Ho, you did it already, but I cannot see the file. Or I overlook it completely.

[Tabloka]

Ho, you did it already, but I cannot see the file. Or I overlook it completely.

Sorry my fault. I forgot to attach the file to my reply. Here you go:

[Swake]

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...

[Tabloka]

Both sides of R36 should really have a very similar voltage.

I 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.

[Swake]

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.

[Tabloka]

Have you put back Z1 the 12pin double comparator? I think it is just fine based on your measurements.

Hello 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.

[Swake]

No problem. Anyhow this weekend I had an overflow of work till very late. Would not have been able to look into it.

[Tabloka]

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 'slave after renew Q6' except TP52. But as you can see in the column 'slave without 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 something in the 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.

[Swake]

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: [Select]

On Figures 7-5 and 7-6, change the circled
test point numbers as follows:
64 change to 66
65 change to 67
66 change to 68
67 change to 69
68 change to 70

You 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.

[HalFoster]

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

[Swake]

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.

[Tabloka]

You could borrow the CA3026 from the good channel or install this one in the good channel.

Yesterday I ordered 4 new CA3026 from a dealer in NL.