Need help with a Vahalla Scientific 2720GS - Page 1
Forum section: electronics / general

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 22, 2025, 07:15:18 pm
content: So I have a Vahalla Scientific 2720GS on the bench that I am hoping to get up and running. I have already rebuilt the power supply and done a full recap of the unit. This cleared the Ato D fault that was happening however I am starting to think that my references might need to be refreshed at this point.Caps have been changed on the reference boards there was capacitor damage that took out a 2212P on one of the references. 2212 was confirmed to be bad. The unit can now see all 8 references however when doing a data recovery the modules go in to service but the unit looses it mind and says the DVM is faulty as well as outputs a low voltage. 7.9285 Vdc on the 10 V setting. All ranges are consistently low.Over the next few hours all but 2 of the references will drop out. When 7&8 are left the DVM fault goes away and the unit starts putting out 10.00000Vdc as expected and shown on a DMM6500.I Have a few other standards in the lab but I am hoping to get this one up and running due to the auto null meter which will be quite useful.Am I at the point where the LM933’s need to be replace as well as the touchup of the oven temps and heaters?When there is no DVM error all tests pass and the unit self checks good just running on a very reduced set of references.Power supply rails are all good and in spec.Zen

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 23, 2025, 04:00:04 am
content: A bit more troubleshooting has happened and it seems that I now for SURE have a bad module that was dragging down all the other references. With the bad module out uncalibrated the 2720GS is outputting 9.95 Vdc much better then the 8 ish volts when the front panel was set to 10.When the bad module is in the unit the DVM faults are present as well as the output is only 2.97 Vdc so it is WAY off.Testing the LM199's out of circuit they are gone. .5V one way open the other not the usual 6.95 Vz that I would expect from a 199.Zen

author: branadic
author_rank: Super Contributor
timestamp: March 23, 2025, 07:19:45 am
content: You can measure the zener voltages in circuit at the 10-way pinheader.https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg4987909/#msg4987909https://www.eevblog.com/forum/metrology/valhalla-2720gs/msg4991971/#msg4991971Starting with an empty X2212P should work to recover the reference board. What are the failure messages of the instrument?-branadic-

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 23, 2025, 01:02:30 pm
content: With the board bad reference board out. Unit needs an external calibration but is outputting 9.95 Vcd in to a 6500 when set to give 10Vdc. Waiting on the 7510 to warm up. Also reference 5 is faulted but LM199 test ok so I expect this to come back with a cal. 2720 is also happy unit passes self test excluding Ref 5.With the bad board in. output is low 7.92Vdc when set to give 10 V as well as a DVM fault. During boot up and self test. Testing the LM199's out of circuit the zeners are gone. They appear to be damaged twords open but not totally blown open. Vz = 0.584 should be 6.95 Vf = overflow should be 0.5when ONLY the bad reference is in place when tons of errors but 2720 will output however asking for 10Vdc output is only 2.95VdcI found a second 2212 that was readable but not write able so calibration was failing as well as data recovery. The 2nd dead 2212 was on the Vref 5/6 board data recovery worked on that one. But REF 5 is to far out and needs to be externally calibrated. Vz is 6.92 Vs 6.95 so I expect a cal can help with that.3rd Dead 2212 was on the external reference board external REF would pass self test but asking it to work would crash the micro-controller and cause a hard lock on the screens as well as needed a power cycle to bring it back. So I am expecting garbage data is making it in to the Micro-controller out of that 2212 but I might be wrong there. This unit does not have the 10Vdc exref it only has the 1Vdc Xref so not sure how useful that is now adays.Zen

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 23, 2025, 03:23:24 pm
content: Also given the intermittent ness of the DVM fault as well as the offset after calibration to which the null meter agrees. I am starting to suspect relay contact resistance issues on the output board.If I read the data sheet correctly I think the relay contacts should be 50 mOhms @ 6Vdc 1A Using the SMU I get 267 mOhms. This might account for some of the voltage offset at the output if I am on the right track.Relay part number is Aromat - S2EB-L2-5VZen

author: Zenwizard
author_rank: Regular Contributor
timestamp: March 27, 2025, 09:20:13 pm
content: New relays are in on the output board of the 2720. I am cautiously optimistic as to the repair of the DVM. System now passes internal checks and is outputting a voltage. No where near the correct voltage but a voltage. I have cleaned the output board with as well as flushed the null meter board with 99.9% iso. This seems to have improved things considerably.I am smashing the 5440B output at 10Vdc in to the null meter of the 2720 and it is saying + 41.65mv off set. This is good news as the outputs of the 2720GS when set to 10.0000 Vdc is consistently outputting 9.954XSo we are getting closer to a working unit.Zen