Electronics > Metrology

Fluke 5520A - Err 104

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Hello everyone,

I have problem with my calibrator 5520A. Someone just connect something to 20AMPS banana which causes some fire at A7 board (burned PPTC's, surge arrester, some OPAmps etc.). I will repair it later, still waiting for mouser package with spare parts.
Meanwhile I checked other functionalities with this result:
Resistance - OK
Capacitance - OK
Temp (RTD, TC) - OK
Voltage DC - NOK (description below)
Voltage AC - NOK (description below).

Voltage have 2 different problems:
at voltage DC, works perfectly up to 33V range. After 35V or more is dialed, I have Error 104 - hardware relay trip occurred;
at voltage AC (f 50Hz) at every range I have Error 1503 - output current lim exceeded.

Looks like common failure. I have schematics from 5500, it's similar but there are some changes, e.g. 65V+/- supply have zener diodes connected to SCOM, but in 5520A they are connected somewhere I don't know. First issue is that -65V is -80V. I checked all transistors, zener diodes and everything in regulator, but everything is ok there. PCB have more than 2 layers so it's little annoying to follow the route. That's the reason why I need your help. Is there someone, who know something about circuitry in 5520A? Or someone, who had the same issue?

I guess that the problem is at A8 (voltage) board. DDS looks OK - but there may be circuitry for measure current in voltage mode for overloads. Sometimes after pressing operate in ranges >33V calibrator restart itself  :-//

Any idea, note, hint, word, will be very helpful. Thank you so much!

Sooo, first update. +/-65V supplies are OK by now.
There was improvement against 5500 in 65V supplies with AD822 opamp at regulation part, which was dead. I replaced it by LT1013 and ordered AD822.

Good news are that errors still occurs. But first step is done, I'm going to next step.

Following as I have seen this before....And never found a fix.

So I'm here with some news.
With little help of 5500 schematics, I definitely determined source of fault - DDS CH0 and CH1 are not working correctly. Suspected IC's are D/A converters, flip-flops, some of opamps and of course CPLD (inguard CPU). At this point I will try to replace D/A, which may take about 1 week (I must order it from mouser).

Where problem is: voltage error out comes from sense buffer circuitry to first DA (common for DDS CH0 - Voltage channel), to the CH2 of DA to reference (DDS have overall 4 DA channels - 2 IC's, each have 2 channels). Output of this 2nd channel comes to next DA to reference input, which generate output to error amp. Because first DA is not working correctly, there is no correct voltage (especially when A8 board tun on voltage transformer) and this voltage trips the relays (it generates FAULT signal, which stops the program cycle and hardwarw reset all relays).

So that's all yet, I will try to fix this problem and then A7 current board is waiting with several burned components. I'm so excited.

Update #2...

I replaced D/A for DDS CH0 and CH1. Still not working, but it's more "cultured". Err 104 appears only when 1kV is dialed, otherwise we have Err 1503 (output current limit exceeded). When e.g. 35V is dialed, we have 0,6V at the output.
On DDS D/A converters we have N/A state at WR input (for write input must be logic 0 to write, it's negated input). This signal comes from CPLD. But there's a little chance. Both D/A's not working and use separate CPLD output, so there's smaller chance that CPLD is kaput. Both WR commands are in N/A state (CH0 is between 0 and 1, CH1 is high impedance).

At the side of CPLD we have CMOS SRAM (AS7C4098 from Alliance) routed to CPLD. I'm guessing that this is wavetable RAM, where waves are stored for the DDS.
 - I/O's are at high impedance state (because of 3-state logic)
 - RAM configuration is in read mode (according to the truth table from datasheet)
 - row and column selections are working
 - after power-up instrument, CPLD sent words to I/O's to be stored
 - every I/O is at HIz state, so may be common failure
 - this may be irrelevant, but it was soldered, there are stains from flux (but boards are cleaned, when comes from factory).

According to internal logic diagram of SRAM it looks like I/O buffer is not working correctly (or is not working). The biggest problem is that I don't have schematic and 5500 was different, there was more components, but at 5520 looks like it was reduced to CPLD. And I'm guessing that CPLD doesn't need RAM for working - besides that RAM is at HIz state and program is working correctly, so this must be wavetable RAM...

What do you think? May I be right? Anyway, I'm ordering new SRAM from mouser so next update will be here in about 1 week.

Let me point out that every idea, opinion, hint etc. is still welcome. Just don't be shy, friends  ;)


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