[SOLVED] HP 34401a - Error 612, 613, 615, 617, 618, 619, 621

[spicytofu]

I started repairing a unit with similar error codes. I have repaired units in the past by replacing U201 as most have done here. However, this one is a no-go. All DCV, ACV ranges work and spot on. DC-I, only the 10 mA and 100 mA are accurate. The 1 A and 3 A ranges are way off. Now lets get into the interesting part: Ohms source.

All opamps have the normal +/- 15 V and 18 V, so voltages and zeners look good. When in ohms mode, no current comes out from the TERMINALS, no readings either. HOWEVER, when I measure the current from the anode of CR202, I am getting the CORRECT currents on all ranges......huhhh? The 9.9 M resistor divider measure good. I measured most of the U102 hybrid but it was not removed from the PCB, anyways I dont suspect U102 to be bad based on the current source is working fine. The only problem is there is no current coming out from the HI terminal. Any ideas? must be a faulty contact or relay?

[HighVoltage]

Did you check the front / rear switch?

[Dr. Frank]

Yes, the front-rear switch might be polluted by its own plasticizer from the housing, but this would not explain the current mode failure.
Anyhow, check front/rear jacks for Ohm, and especially if the 4W mode is working somehow.

As both modes, Ohm and current are affected, K102 is under suspect. It may be stuck, shorting poles 9-13  so that only the low current range is available.
That would imply (or maybe not?) that it would always short the OHMS_CURRENT to GND, via poles 4-8, but you are able to measure the current behind CR202.
Maybe this part of the relay is completely open. Actually, I'm not aware how bistable relays are working mechanically.
Check for continuity on this relay, and also the resistance of the current range shunts, when you switch from 100mA to 1A. The difference 0.1 to 5.1 Ohm should be clearly detectable.

Frank

[spicytofu]

[0quote author=Dr. Frank link=topic=34104.msg4355395#msg4355395 date=1660376375]
Yes, the front-rear switch might be polluted by its own plasticizer from the housing, but this would not explain the current mode failure.
Anyhow, check front/rear jacks for Ohm, and especially if the 4W mode is working somehow.

As both modes, Ohm and current are affected, K102 is under suspect. It may be stuck, shorting poles 9-13  so that only the low current range is available.
That would imply (or maybe not?) that it would always short the OHMS_CURRENT to GND, via poles 4-8, but you are able to measure the current behind CR202.
Maybe this part of the relay is completely open. Actually, I'm not aware how bistable relays are working mechanically.
Check for continuity on this relay, and also the resistance of the current range shunts, when you switch from 100mA to 1A. The difference 0.1 to 5.1 Ohm should be clearly detectable.

Frank
[/quote]

Thanks Frank and HighVoltage. I did see a small PCB trace burnt off at the switch. The trace was so small, I wouldnt have known it was there. I tried connecting back the trace but still had the same errors as the title of this thread. Measuring the voltage of the anode of CR202 I get .55V, and then at the cathode is basically 0V with some uV variations. (if I got my terminology right, cathode is the white line end of the diode, just making sure) I tested the diode and works fine with a FV of .55V

I removed the CR202 and shorted it and still no current comes out of the terminals. (rear and front) Measuring the resistance from the CR202 cathode to LO terminal, I get 9.99 Megs. I tried following the schematic to where the ohms current goes after CR202 and I get lost at all the relay switching. I was attempting to follow the schematics to locate the path where the readback goes into the A/D to see if there are any problems there.

An idea: What if I push a external 1 mA source through a resistor and then connect the HI and LO terminals to read the voltage? Like a 4W mode using two DMMs, but the faulty one would be in 2W mode. Although no current will come out from the HI and LO terminals, I expected it to be able to read the voltage and it will translate into ohms. Then we know it has to be some thing physical blocking the current source.

Update: the 1 mA current source worked. Pushing the correct range current using an external source gives the correct ohms reading on the DMM. Also, 2W and 4W does not work (but does work if you have an external current source). Furthermore, in the DCI, normally I would hear the relay click when it switched to the 1A and 3A range, but there is no click. Measuring the shunt shows 5.5 ohms only, I do not see the 0.1 ohm shunt. Maybe it is a relay problem?

[Dr. Frank]

..

As both modes, Ohm and current are affected, K102 is under suspect. It may be stuck, shorting poles 9-13  so that only the low current range is available.
That would imply (or maybe not?) that it would always short the OHMS_CURRENT to GND, via poles 4-8, but you are able to measure the current behind CR202.
..
Check for continuity on this relay, and also the resistance of the current range shunts, when you switch from 100mA to 1A. The difference 0.1 to 5.1 Ohm should be clearly detectable.

Frank

Thanks Frank and HighVoltage. I did see a small PCB trace burnt off at the switch. The trace was so small, I wouldnt have known it was there. I tried connecting back the trace but still had the same errors as the title of this thread. Measuring the voltage of the anode of CR202 I get .55V, and then at the cathode is basically 0V with some uV variations. (if I got my terminology right, cathode is the white line end of the diode, just making sure) I tested the diode and works fine with a FV of .55V

I removed the CR202 and shorted it and still no current comes out of the terminals. (rear and front) Measuring the resistance from the CR202 cathode to LO terminal, I get 9.99 Megs. I tried following the schematic to where the ohms current goes after CR202 and I get lost at all the relay switching. I was attempting to follow the schematics to locate the path where the readback goes into the A/D to see if there are any problems there.

An idea: What if I push a external 1 mA source through a resistor and then connect the HI and LO terminals to read the voltage? Like a 4W mode using two DMMs, but the faulty one would be in 2W mode. Although no current will come out from the HI and LO terminals, I expected it to be able to read the voltage and it will translate into ohms. Then we know it has to be some thing physical blocking the current source.

Update: the 1 mA current source worked. Pushing the correct range current using an external source gives the correct ohms reading on the DMM. Also, 2W and 4W does not work (but does work if you have an external current source). Furthermore, in the DCI, normally I would hear the relay click when it switched to the 1A and 3A range, but there is no click. Measuring the shunt shows 5.5 ohms only, I do not see the 0.1 ohm shunt. Maybe it is a relay problem?

Hello,
you obviously also didn't read and understand what I have already written: Check relay K102!

It switches the current ranges and as well switches the driving current for Ohm mode, so might explain your double trouble.
As you now additionally describe, that the cathode of CR202 is somehow grounded, this is another confirmation, see schematic!

Please describe in more detail, which signal trace has been burnt! Pictures are welcome!
That could have been another reason for the failure as well, but we need to know, what caused this burning, and what further consequences this might have.

Frank 

[spicytofu]

I was getting to the relay next after my first post. I already checked it and it was stuck, but before I saw your reply. The problem is fixed now, just banged on the relay when I took it out and it is working again. My guess is that a very large current was pushed through burnt some traces and possible arched the relay. The fuse couldnt save it in time. I repaired two burnt traces and everything seems to be working now.

Another note: it seems that the transformer wiring was wrong from the factory. (this unit was never opened) I noticed a very darkened and hot PCB area at the regulators. Checking the AC volts, I saw 28VAC and DCV unreg was 33V (total was 66V!) No wonder the vregs were burning up. I swapped the blk/red and blk/org xformer secondary wires since they were reversed and now I am getting the correct 27VDC and 23VAC at the regulators.