Dana 5500 digital voltmeter: need help repairing ohms board FIXED

[valley001]

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. 

[Vgkid]

Do you have another multimeter to check the current source. When in 4 wire mode , does shorting the sense lines produce a zeroed reading?

[valley001]

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?

[Vgkid]

You are correct with the sense terminals.

[floobydust]

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 ADTUSTMENTS
5.49. 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 or Q8 Q6 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 stage Q8 Q6 is working.

edit: it's Q6 not Q8.

[valley001]

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.

It 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)"

5.48 FACTORY ADTUSTMENTS
5.49. 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 or Q8 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 stage Q8 is working.

I will check these components more thoroughly.  My suspicion is the prior overload may have fried something....hopefully not unobtanium. 

[floobydust]

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.

[valley001]

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. 

[Vgkid]

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.

[valley001]

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.

Do you mean voltage measurements from the ohm current source plugs?  I get:

1x    6.8v
10x   6.8v
100x  6.5v
1,000x -2.6v
10,000x -1.3v

[Vgkid]

Should have specified that the voltage measurements all check good. The meter can read input voltages correctly.

[valley001]

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. 

[Vgkid]

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

[valley001]

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

Right, they are nowhere near that.  I should look at components down stream of F1 right?

[Vgkid]

What current levels are you getting?

[floobydust]

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.

[valley001]

These are the current readings from the ohms current source plugs:

1x 3.33 ma
10x .35  ma
100x .03 ma
1,000x -2.5 ua
10,000 -.2 ua

[Vgkid]

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.

[valley001]

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.

Do 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. 

[Vgkid]

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.

[valley001]

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. 

(that schematic is hard to follow, discrete opamps.

The 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. 

[floobydust]

Can you measure the +20V REF.

[valley001]

Can you measure the +20V REF.

Everything 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.

[floobydust]

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.

[valley001]

I believe:
F2 is the ohms input fuse, so CR19, CR20 or Q8 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 stage Q8 is working.

OK, 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 directions looking at the schematic it appears this may be correct since they are joined anode to cathode on both sides.