Dana 5500 digital voltmeter: need help repairing ohms board FIXED - Page 1
Forum section: electronics / repair

author: valley001
author_rank: Frequent Contributor
timestamp: February 17, 2019, 12:58:07 am
content: Hi All,I am working on this meter, so far everything is working aside from the ohms converter board. I try going through the calibration procedure and I get stuck at the current zero procedure. I attach the 1 meg resistor in between the shorted ground jacks and the signal input jack and there is no response in the display.It will take readings in the 4 wire mode with various resistors, however the readings are not accurate and mostly nonsense. They change between ranges as well.When I first evaluated the board one of the fuses was blown, I did not write down which it was unfortunately. I know it was completely unresponsive before replacing the fuse, now it is partially responsive but not functional (as noted above). I did probe most of the transistors with a diode checker in circuit and so far they seem OK. I am working on getting a card extender so I can properly test under power with a scope, that may be a week away or so.Any ideas here?I attached a schematic of this board, a full manual of the 5524 seems to be available on line and it is very similar. Sorry for the poor quality of the image.

author: Vgkid
author_rank: Super Contributor
timestamp: February 17, 2019, 01:50:56 am
content: Do you have another multimeter to check the current source. When in 4 wire mode , does shorting the sense lines produce a zeroed reading?

author: valley001
author_rank: Frequent Contributor
timestamp: February 17, 2019, 01:53:58 am
content: I do. I did check to see if it was putting out any current at all, and it is. Ill try your suggestion. Also, reading the manual I learned the current should change based on the range selector setting. Ill check that as well.To clarify, the "sense" is the signal input and signal ground correct?

author: Vgkid
author_rank: Super Contributor
timestamp: February 17, 2019, 01:55:30 am
content: You are correct with the sense terminals.

author: floobydust
author_rank: Super Contributor
timestamp: February 17, 2019, 02:06:25 am
content: I too am a bit confused what "no response in the display" means. Is it stuck reading zero on kohms? DCV is working, right. How does it do when you go through the CAL procedure:5.47 KILOHM RANGE CALIBRATION(a) Set FUNCTION switch to KILOHMS. Place Ohms Converter board on extender.(b) Short the following terminals together: +SIGNAL, -SIGNAL, and -OHMS CURRENT SOURCE.(c) Adjust Voltage Zero potentiometer for zero display on DVM.(d) Remove short between +SIGNAL and -SIGNAL and insert 10 Megohm resistor between +SIGNAL and -SIGNAL.(e) Adjust Current Zero potentiometer for zero display on DVM.(f) Connect the input to a 10 Megohm standard and adjust the range potentiometer for a DVM Readout display equal to the value of the standard.(g) Repeat with standard resistors near full scale of the remaining four ranges, adjusting the appropriate controls on the ohms board for a DVM Readout display for each standard value.5.48 FACTORY ADTUSTMENTS5.49. Certain adjustments are considered critical and it is highly recommended that they not be disturbed. These adjustments involve, generally, the selection of FSV (factory-selected values) of components to compensate for temperature drift. It is necessary that an oven be used for these adjustments .5.50. The adjustments considered "Factory Adjustments' are R67 on the Null Detector, R49 on Auto Polarity board, and R42 on the Control Logic board. Procedures for adjusting those controls are given in the following paragraphs. "I believe:F2 is the ohms input fuse, so CR19, CR20 orQ8Q6 E-B junctions would get damaged if a big input overload.F1 is the ohms output fuse and Q13/R43/R45 could get damaged if overload was there.Much of the amplifier section uses high impedances (1M-10M) so small leakage currents will unbalance it. I am wondering if the (factory only adjustment) balance trimpot R30 1MEG is still reasonable, don't touch it until you know if the input stageQ8Q6 is working.edit: it's Q6 not Q8.

author: valley001
author_rank: Frequent Contributor
timestamp: February 17, 2019, 02:18:28 am
content: Quote from: floobydust on February 17, 2019, 02:06:25 amIt fails at D. On the 5500 it is a 1 meg resistor, but when inserted the display stays at 10999.9 and does not change at all. It is unresponsive to changing the current zero pot. If I short + signal input and -signal and -current source the display will go to 0. The procedure is slightly different between these two models. 5500 at d: "Remove short between + signal and and the other two leads (- leads) and insert 1 meg resistor between + signal and the other two leads (- leads)"Quote from: floobydust on February 17, 2019, 02:06:25 amI will check these components more thoroughly. My suspicion is the prior overload may have fried something....hopefully not unobtanium.

author: floobydust
author_rank: Super Contributor
timestamp: February 17, 2019, 03:15:04 am
content: Maybe try something smaller than 1MEG for cal, like 100k or 10k etc. to see what it reads, or use a potentiometer. To look for an offset or span error, or if it has a linear range or is just saturated.

author: valley001
author_rank: Frequent Contributor
timestamp: February 17, 2019, 03:21:43 am
content: I actually did that, and it does. With a 15K resistor used in place of the 1meg per line d, the display will read 1140.5ish kohms. The bottom three digits dither a whole lot but that seems to be the average. No response when adjusting the current zero pot. I am not sure what this could mean.

author: Vgkid
author_rank: Super Contributor
timestamp: February 17, 2019, 06:04:47 am
content: Thats strange, both readings are rather far from expected(below 10% range). So if all of the voltage measurements check out(0.1-10v), then it could be a current source issue.

author: valley001
author_rank: Frequent Contributor
timestamp: February 17, 2019, 05:16:58 pm
content: Quote from: Vgkid on February 17, 2019, 06:04:47 amDo you mean voltage measurements from the ohm current source plugs? I get:1x 6.8v10x 6.8v100x 6.5v1,000x -2.6v10,000x -1.3v

author: Vgkid
author_rank: Super Contributor
timestamp: February 17, 2019, 08:37:58 pm
content: Should have specified that the voltage measurements all check good. The meter can read input voltages correctly.

author: valley001
author_rank: Frequent Contributor
timestamp: February 17, 2019, 10:23:03 pm
content: Yes, the AC and DC voltage measurements are working and track between ranges.How much current should I be able to read from the current source plugs? I only get a few mah.

author: Vgkid
author_rank: Super Contributor
timestamp: February 18, 2019, 02:20:26 am
content: Looking at the circuit(the manual is not clear at all) 10.0ma ,1.0ma ,0.1ma, 0.01ma. I'm reasonably sure on the first three

author: valley001
author_rank: Frequent Contributor
timestamp: February 18, 2019, 02:28:21 am
content: Quote from: Vgkid on February 18, 2019, 02:20:26 amRight, they are nowhere near that. I should look at components down stream of F1 right?

author: Vgkid
author_rank: Super Contributor
timestamp: February 18, 2019, 02:51:58 am
content: What current levels are you getting?

author: floobydust
author_rank: Super Contributor
timestamp: February 18, 2019, 04:12:16 am
content: I think the ohms current source is a compliance voltage of +20V REF through range switches approx. 10MEG, 1MEG, 200k, 20k, 2k going to the +CURRENT output. Short-circuit currents I would estimate are:2uA x10,000 range (10MEG)20uA x1,000 range (1MEG)0.1mA x100 range (200k)1mA x10 range (20k)10mA x1 range (2k)To check if the ohms current source and range switching relays are working, connect another DMM (in mA or uA function) to the 5500 in ohms mode, and see what currents are seen for each range. If you have another meter, I would also check the voltages at the ohms jacks, as I am suspecting that ohms current is being drawn or pulled via the sense lines which might confuse us. Open-circuit (on ohms) I would expect almost 20V out, certainly over 10V.Strange the ohms function seems to take 0-10VDC in and that gets inverted as a negative polarity, then to the A/D.

author: valley001
author_rank: Frequent Contributor
timestamp: February 18, 2019, 04:52:07 am
content: These are the current readings from the ohms current source plugs:1x 3.33 ma10x .35 ma100x .03 ma1,000x -2.5 ua10,000 -.2 ua

author: Vgkid
author_rank: Super Contributor
timestamp: February 18, 2019, 10:25:37 am
content: That kind of looks reasonable, considering it is not a constant current source. Do you have a decade resistor box/enough resistors to measure the voltage created across them. That is for each range.

author: valley001
author_rank: Frequent Contributor
timestamp: February 18, 2019, 04:31:26 pm
content: Quote from: Vgkid on February 18, 2019, 10:25:37 amDo you mean the voltage between a known resistor and the ohms current plug?What is it I am looking to confirm?I do have an extender card on the way, that should make more thorough testing possible.

author: Vgkid
author_rank: Super Contributor
timestamp: February 18, 2019, 04:57:08 pm
content: Yes, do it for every range. I want to see if there is a correlation between the sourced current , and your unstable reading. If the current source reasure ok, then the error lies in the current generator section.(that schematic is hard to follow, discrete opamps.

author: valley001
author_rank: Frequent Contributor
timestamp: February 18, 2019, 05:11:02 pm
content: I apologize, I may have misunderstood the prior requests. I have been taking current and voltage readings with my handheld DMM and reporting those. I have not used the 4 lead test plug installed in the Dana 5500.So, if I now understand, you want:4 lead test plug installed with a resistor at the test clips.A resistor installed in series on the ohms current + jack.Readings from the display on the Dana 5500 through the ranges.Is this correct? I apologize, this is a hobby for me so I do miss some things but am learning.Quote from: Vgkid on February 18, 2019, 04:57:08 pmThe entire Dana manual leaves much to be desired. I have been spoiled thus far with HP manuals, which are very thorough, detailed, and easy to follow.

author: floobydust
author_rank: Super Contributor
timestamp: February 18, 2019, 06:28:05 pm
content: Can you measure the +20V REF.

author: valley001
author_rank: Frequent Contributor
timestamp: February 18, 2019, 06:37:45 pm
content: Quote from: floobydust on February 18, 2019, 06:28:05 pmEverything is packed in tight, so it would be difficult. I guess I could attach a test lead and then reinsert the card. Waiting on card extender. I have assumed the 20v reference is good because it takes accurate DC and AC readings.

author: floobydust
author_rank: Super Contributor
timestamp: February 18, 2019, 08:53:25 pm
content: Yeah, I was wondering why the +20V REF is not showing up at the ohms-converter output. The 5524 manual says it's a 0-10V A/D so I expect more than 6.8V and I thought maybe you could get at the edge-card terminals and wiring and check. But the ref. is feeding everything else, so something else weird is going on.The manual is pretty terrible, making sense of this is a bit of torture.

author: valley001
author_rank: Frequent Contributor
timestamp: February 18, 2019, 10:22:50 pm
content: Quote from: floobydust on February 17, 2019, 02:06:25 amOK, I spent some time looking at these.F1- R43 and R45 are spot on. Q13 seems ok based on my readings with the diode check function. B-E 650mv B-C 650mv reversing the leads I get nothing.F2- Have not sorted out the leads on Q8, it is a multi transistor package I think. Checking CR19 and CR20,they read shorted in both directionslooking at the schematic it appears this may be correct since they are joined anode to cathode on both sides.

author: lowimpedance
author_rank: Super Contributor
timestamp: February 18, 2019, 11:59:59 pm
content: When you say shorted, you mean 0 ohms or not ~0.65V both directions on your DMM diode test ?. Have you lifted one end to check if one or both are short cct ?. These diodes are for protection and should conduct enough to 'hopefully' pop F2 before to much damage is done !. Was F2 intact still ?.

author: valley001
author_rank: Frequent Contributor
timestamp: February 19, 2019, 12:09:30 am
content: Quote from: lowimpedance on February 18, 2019, 11:59:59 pmOne of the fuses was not intact, I dont remember which and did not write it down unfortunately.With the meter in diode check mode it displays as if I short the leads together (.000v) when I test across these diodes, in either direction. In ohms mode they read 0 ohms both directions. These tests are with the diodes in circuit.

author: floobydust
author_rank: Super Contributor
timestamp: February 19, 2019, 01:31:36 am
content: This is with the PC board pulled? That looks like a dead short there- the two-diode clamp should read around 0.65V either polarity.Lift an end or remove each diode CR19, CR20 and if the short is still there, it might beQ8Q6 B1-B2 shorted.If this (short) is with the card in, I would check C7, something with the range relay shielding can to ANA COMMON (ground), or maybe there is a switch that intentionally shorts the ohms-converter inputs when not used.edit: Q6 not Q8 fuzzy text

author: valley001
author_rank: Frequent Contributor
timestamp: February 19, 2019, 01:36:25 am
content: Quote from: floobydust on February 19, 2019, 01:31:36 amThese measurements are with the card removed. Ill lift these out of circuit and test. Hopefully Q8 is not toast, I think its custom Dana part. I do have a parts mule but it does not have the ohms converter option installed.

author: valley001
author_rank: Frequent Contributor
timestamp: February 19, 2019, 01:47:11 am
content: OK, diodes lifted out of circuit:CR20- Reads .65vCR21CR19- Reads 0 ohms both directions

author: Vgkid
author_rank: Super Contributor
timestamp: February 19, 2019, 01:51:43 am
content: Cr21 is toast. Lets hope that is the only thing fried.

author: valley001
author_rank: Frequent Contributor
timestamp: February 19, 2019, 02:05:37 am
content: Q8 tested in circuit with diode checker. Both sides read.65v ish B-E and B-C with positive DMM lead on base.Is it possible the friedCR21CR19 is the cause of the problems with this board?

author: floobydust
author_rank: Super Contributor
timestamp: February 19, 2019, 02:14:58 am
content: Dana 211236 diode "007" I got to a cross being1N3595200mA 150V low leakage.FDH300looks like a possible alternate. It's not a tough diode, to clear the fuse if a big overload.Dana 211083 diode "018" I got to a cross being1N916.The 1N914 or 1N4148 seem fine as an alternate.The Dana diodes are from 1967-68.CR21 is a zener on the schematic, so I'm confused about CR19, CR20 being the clamp diodes?

author: valley001
author_rank: Frequent Contributor
timestamp: February 19, 2019, 02:17:43 am
content: Quote from: floobydust on February 19, 2019, 02:14:58 amThat was me being in a rush, it is CR19 that reads 0 ohms both directions, not CR21.

author: valley001
author_rank: Frequent Contributor
timestamp: February 19, 2019, 03:02:51 pm
content: Could this diode be the cause of the problems?

author: lowimpedance
author_rank: Super Contributor
timestamp: February 20, 2019, 01:44:38 am
content: As Flooby suggests, try a pair of 1n4148 and then retest the function and see if it works. Hopefully yes , if not then further repair is needed.

author: valley001
author_rank: Frequent Contributor
timestamp: February 20, 2019, 02:25:51 am
content: The diode was the Dana 211236 "007," I have a bunch on order. Unfortunately I do not have a large stock of diodes on hand. Would it be best to also change the one that reads correctly, CR20?

author: lowimpedance
author_rank: Super Contributor
timestamp: February 20, 2019, 03:50:35 am
content: Yes. If one was dead the other would always be suspect so best to do both in this instance. With a bit of luck Q8 was spared and the diode kind of did its job.

author: valley001
author_rank: Frequent Contributor
timestamp: February 20, 2019, 08:24:37 pm
content: Q8- Are we looking at the dual transistor directly down stream of the diodes? My photo is grainy and the number is not clear for you all, I apologize. This would be Q6. Just want to be sure I am testing the appropriate transistor. In the pic of the board Q6 is the transistor that shares the binding post with the two diodes bottom right.

author: lowimpedance
author_rank: Super Contributor
timestamp: February 20, 2019, 11:08:53 pm
content: Ahh ok Q6 , my aging tired old eyes couldn't make it out even with glasses and magnifier.Yes with a bit of luck the dual transistor is still functional. Change the diodes and test the meter again.

author: valley001
author_rank: Frequent Contributor
timestamp: February 21, 2019, 03:55:32 pm
content: Completely my fault with the crappy image of the schematic.Q6 tests OK with .63v/.66v B-C/B-E on both sides. Now waiting on diodes, should be Saturday. Fingers crossed.

author: valley001
author_rank: Frequent Contributor
timestamp: February 22, 2019, 05:31:30 pm
content: Well my diodes are stuck on the east coast and I am impatient.I did manage to find a bunch of 1N914 lying around, which as noted seem to cross to the dana 018 diode (though as a novice I am unsure what I should be looking for in the datasheet). Is there any danger using these in place of the Dana 007 diode? They are in the small glass envelope (not sure that matters).

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2019, 05:53:01 pm
content: The 1N914 should be OK for the "018" ones, not much difference there.For the "018" type which is supposed to be more like the 1N3595 it would be a poor replacement, as it may not provide full protection and maybe more leakage. Probably a plain 1N4003/5 could be a better match, though maybe a little more leakage.

author: valley001
author_rank: Frequent Contributor
timestamp: February 23, 2019, 07:30:53 pm
content: I got it working this morning. Replacing the defective diode did the trick, and thankfully none of the custom transistors took a dump. I used two 007 diodes from a parts unit, since my new replacements are stuck in the mail somewhere.I can now zero voltage and current per the manual. Now to get some precision resistors to zero each range.Thanks all for your help with this.