Datron Autocal 1061 (Fixed - Thank You for all the help)

[Gyro]

The coil resistance is the same what is written on it. However, there is only 14.5Vdc on the terminals. Not sure whether that is adequate, as per drawing it should be 15Vdc. - Could that be an inadequate power supply?

That's not a problem, most relays close at about 2/3 of rated voltage. The missing 0.5V is probably the drop across the switching transistor.

[dacman]

The relays are actuated by pulsing about 28 Vdc across the relays (for about 10 to 30 ms) then the positive section drops and the relays are held in by the negative supply.  There is about 14.5 Vdc on the coil, vice 15 Vdc,  because of a diode that completes the hold-in circuit.

Test the coil voltage while switching the relay and make sure it is being pulsed to 28 Vdc.

Measure the current path to the front/rear terminals.  The Ohms PCB is a current generator.

Measure contact resistance across the relays.  (I do this by using the schematic and testing at connected parts.)

24V SDS Relay: Actuates at 16.8 Vdc min; Held in by 2.4 Vdc min.

[dacman]

Your symptoms are as if the current path is open.  Check the current fuse.  Even Ohms I+ goes through the current fuse if fitted.  (Without the current option, a link is used vice a fuse.)

[maginnovision]

Are thre ±8V rails critical for Lin Cal? The drop across the diodes is almost 4 volts and the 11V rail is ok which results in ±7V instead ±8. It's literally the only issue I've found on the problem unit.

[Laszlo]

Hi,

Finally, I had the time to take it apart again and checked the fuse the second time. Unfortunately, it was okay
I assumed that if one switch was failing, there might be more mechanical issues along the current path from the front to the rear input relay assembly.
The I+ rings out all the way to the current board, I- does not when I/dc selected.
Therefore, I took the relay board out and did some fault finding.
In my understanding (might be wrong), those relays should operate from +15V dc so thought - yeah let's test it on the bench. Hooked it up to  15V dc, nothing, a slight bodge at most. Increased the voltage to 24Vdc and it clicked. But it still does not work on 15Vdc..

Is there anything special about these relays? I can't seem to find anything when I googled the part number.

Cheers,
Laz

[dacman]

For the Front/Rear relay board, it takes +/- 15 Vdc to run the relays along with the 0V common of the 15 Vdc supplies.  When the Front/Rear switch is set to Front, Q1 and Q6 are turned on which applies -15 Vdc to the coils of RL1 and RL2.  This -15V is also applied to C1 on the side which was near zero volts.  The other side of C1 was near +15V but gets shifted down to near 0.  This turns on Q3 which applies +15 Vdc to the other side of the coils of RL1 and RL2, and the 30V differential across the coils is enough to energize the relays.  (The +15 Vdc from Q3 also reverse biases D3.)  When C1 charges enough to cut off Q3, the relays remain energized through D3 which is now forward biased by the -15 Vdc supply through Q6/Q1 and the coils of RL1/RL2.  When the Front/Rear switch is set to Rear, Q1 and Q6 are turned off and the bottom of C1 is discharged to near zero through the coil of RL2 and D3.

Relays that can replace those are R10-E1P2-S3.2K, R10-E1P4-S1.8K, AZ420-V50-40LUS, and AZ421-V05-10LUS.  These relays have 18V pull-in ratings.  Relays with 21 V pull-in ratings can also be used.

[dacman]

Are thre ±8V rails critical for Lin Cal? The drop across the diodes is almost 4 volts and the 11V rail is ok which results in ±7V instead ±8. It's literally the only issue I've found on the problem unit.

Do you have a thermal imaging camera?  Check C25 and make sure it isn't leaking.  The 8V rails power some of the CMOS and optoisolator ICs.  Try swapping ICs with another unit.  Also, if the analog board is placed in another unit does the problem move with it?

[maginnovision]

No thermal camera. Fault moves with analog board. C25 is a new kemet 20V part though and I selected lowest leakage part(hence the 20V rating). This is also a 1071 analog board.

One thing I had to do for this board the others didn't need was adjust R11/R15 for step 26 of analog board adjustments. Originally there was a resistor in R11 spot and I had to remove it and put one in R15. Are these pick what works or do I need specific value? My TP9 reading is .1mV so the reading is within spec. I'll try swapping some ICs that use those rails.

Checking against a good unit I found the "8V" rail is also unstable. Need to find out if it's the zeners breaking down or something else causing it. Hopefully it's related.

I found out it's caused by an LED on the analog board. I can't find the LED in the schematics or board layout so I'm not sure why it's there but it seems to be original. No IC swapping has helped yet.

EDIT: I've finally got back to this, replaced C9(Digikey P/N: 1572-1352-ND) and C12(Digikey P/N: 399-13034-ND. This is a PPS which are typically direct replacements for PC.) on analog board and it passes linearity calibration step fine.

[Laszlo]

Fixed!

It was the 3rd relay at the rear module. Took it apart, cleaned the contacts and now it works like a charm on all 4 modes. No problem with the accuracy either when I checked it against my calibrated Metrahit multi-meter. Thanks for everyone who helped along the way, I would not have been able to do this without you.

For anyone who had similar error 6 on these meters here is what I had done to rectify the issue;

• Replaced all axial electrolytic capacitors on the PSU and analog board. (They were all dried out without exception)
• Replaced some Tantalums on the Ohms and Analog board.
• Rung out the input all the way to the relays.
• Found 2/4 wire switch broken (replaced it).
• Found relay No3 at the rear not making contact
• Removed relay 3 and 4 and cleaned both.
• Re seated and cleaned Ohm's board IC's
• Re seated all connectors in general

y

[tggzzz]

If only everybody listened, understood, had success, reported and summarised their success.

Good for you!

[maginnovision]

No Datron repair is a success without a PASS screenshot! Good job getting it fixed thanks for the summary.

[Laszlo]

True.

I've done another self-test again to show off with my screenshot and I've got my beloved error message again.
But it still measures everything spot-on. I is well confused now. 

I've also noticed when I first turn it on, the resistance measurements aren't stabilizing, this issue disappears after the instrument was on for like 10 minutes. 
It can't be a mechanical fault surely since it measures. Could it be that its calibration is drifted away, hence the error?

Cheers,
Laz