[Repaired] EDC 522 DC Voltage Calibrator

[nidlaX]

I'm trying to repair a broken EDC 522 that only displays the 'OVL' indicator on power up (the LED matrix display does not display 'OVERLOAD'). My original plan was to find / request a schematic and service manual, but I would like some feedback on two immediate concerns:

1. Upon opening the unit, I discovered that the sticker covering the Fairchild NMC2764Q EPROM had been removed at a previous point. I immediately went and dumped the contents of the ROM, but I am unsure as to whether or not they have been corrupted. Does anyone have a sample ROM I can use for a comparison? I have a revision A MPU board. It would also be helpful to know the normal behavior of the unit on power up (relay clicks, display behavior, etc.) For what it's worth, the power up fault remains the same on my unit with the dodgy EPROM removed.


2. Compared to grumpydoc's EDC 522, the reference diode on my unit looks super crusty, and I am concerned that someone may have messed with it at some point during the unit's history. Any opinions on my suspicions?


EDIT: A correction: I have a revision A MPU board, not revision H.

[JohnnyBerg]

My first thing would be measuring the reference diode. It looks like a 1N829A, so there should be 5.9 ~ 6.5 V over it.

[SeanB]

Looking like that is about right for the diode. Just measure the voltage across it and see if it matches the label, 6.1744V in your case. If so it is fine, if not look at the opamp as being faulty, as a zener that has been well aged rarely fails.

[Jwalling]

I'm trying to repair a broken EDC 522 that only displays the 'OVL' indicator on power up (the LED matrix display does not display 'OVERLOAD'). My original plan was to find / request a schematic and service manual, but I would like some feedback on two immediate concerns:

1. Upon opening the unit, I discovered that the sticker covering the Fairchild NMC2764Q EPROM had been removed at a previous point. I immediately went and dumped the contents of the ROM, but I am unsure as to whether or not they have been corrupted. Does anyone have a sample ROM I can use for a comparison? I have a revision H MPU board. It would also be helpful to know the normal behavior of the unit on power up (relay clicks, display behavior, etc.) For what it's worth, the power up fault remains the same on my unit with the dodgy EPROM removed.

I've attached the binary. The code revision is 2.07 and the serial # is 19307. In a normal power up, You'll hear one or more relays click, then the unit will count from 0 - 9 while scrolling the digits from right to left. Then it states it's model # and then the GPIB address followed by more relay clicks.
Good luck!

Jay

[nidlaX]

My first thing would be measuring the reference diode. It looks like a 1N829A, so there should be 5.9 ~ 6.5 V over it.

Looking like that is about right for the diode. Just measure the voltage across it and see if it matches the label, 6.1744V in your case. If so it is fine, if not look at the opamp as being faulty, as a zener that has been well aged rarely fails.

Thanks for the info, I'll measure the reference diode!

I'm trying to repair a broken EDC 522 that only displays the 'OVL' indicator on power up (the LED matrix display does not display 'OVERLOAD'). My original plan was to find / request a schematic and service manual, but I would like some feedback on two immediate concerns:

1. Upon opening the unit, I discovered that the sticker covering the Fairchild NMC2764Q EPROM had been removed at a previous point. I immediately went and dumped the contents of the ROM, but I am unsure as to whether or not they have been corrupted. Does anyone have a sample ROM I can use for a comparison? I have a revision H MPU board. It would also be helpful to know the normal behavior of the unit on power up (relay clicks, display behavior, etc.) For what it's worth, the power up fault remains the same on my unit with the dodgy EPROM removed.

I've attached the binary. The code revision is 2.07 and the serial # is 19307. In a normal power up, You'll hear one or more relays click, then the unit will count from 0 - 9 while scrolling the digits from right to left. Then it states it's model # and then the GPIB address followed by more relay clicks.
Good luck!

Jay

Great, thanks for this! There are definitely no such signs of life at power on, looks like the problem will involve the MPU board, power supply, and probably the EPROM. I hope they don't store any calibration data on that thing.

EDIT: I did a quick and dirty comparison between the two ROMs. Empy data sectors look identical and the byte value statistics look very similar as well, so there doesn't seem to be major corruption.

[SeanB]

If you have the eprom out then record the data and then rewrite it to the chip, turning off the blank check. that way any bits that are getting to the edge ( flipping from a "0" to a "1" intermittently or always) will be refreshed back to good levels for a long life again. good for a working prom that has had the label fall off, then put a new label on to keep it dark inside.

[Jwalling]

Great, thanks for this! There are definitely no such signs of life at power on, looks like the problem will involve the MPU board, power supply, and probably the EPROM. I hope they don't store any calibration data on that thing.

EDIT: I did a quick and dirty comparison between the two ROMs. Empy data sectors look identical and the byte value statistics look very similar as well, so there doesn't seem to be major corruption.

There's no calibration data on the EEPROM. Calibration is all done with pots, so no worry there. If you don't see the power on routine I described, it's most likely to be the processor board or power supply. Check the 5V rail, obviously. I have a hard copy manual with schematics. If you need me to scan a part of them let me know, I could probably do it this week sometime...

Jay

[nidlaX]

Great, thanks for this! There are definitely no such signs of life at power on, looks like the problem will involve the MPU board, power supply, and probably the EPROM. I hope they don't store any calibration data on that thing.

EDIT: I did a quick and dirty comparison between the two ROMs. Empy data sectors look identical and the byte value statistics look very similar as well, so there doesn't seem to be major corruption.

There's no calibration data on the EEPROM. Calibration is all done with pots, so no worry there. If you don't see the power on routine I described, it's most likely to be the processor board or power supply. Check the 5V rail, obviously. I have a hard copy manual with schematics. If you need me to scan a part of them let me know, I could probably do it this week sometime...

Jay

Thanks Jay! I took a look at the MPU card and I assume that the 4 edge connector contacts that are shorted together in pairs are ground and power respectively? Do you have a schematic of the mainboard? It would be very handy for checking supply voltages.

Also, it seems that the Hi and Lo load terminals on the front panel are short. I'm not super familiar with calibrators, but I'm guessing that's problem? Next step would be to trace back the output circuit and check all of the devices.

[Jwalling]

Great, thanks for this! There are definitely no such signs of life at power on, looks like the problem will involve the MPU board, power supply, and probably the EPROM. I hope they don't store any calibration data on that thing.

EDIT: I did a quick and dirty comparison between the two ROMs. Empy data sectors look identical and the byte value statistics look very similar as well, so there doesn't seem to be major corruption.

There's no calibration data on the EEPROM. Calibration is all done with pots, so no worry there. If you don't see the power on routine I described, it's most likely to be the processor board or power supply. Check the 5V rail, obviously. I have a hard copy manual with schematics. If you need me to scan a part of them let me know, I could probably do it this week sometime...

Jay

Thanks Jay! I took a look at the MPU card and I assume that the 4 edge connector contacts that are shorted together in pairs are ground and power respectively? Do you have a schematic of the mainboard? It would be very handy for checking supply voltages.

Also, it seems that the Hi and Lo load terminals on the front panel are short. I'm not super familiar with calibrators, but I'm guessing that's problem? Next step would be to trace back the output circuit and check all of the devices.

Don't worry about the output terminal being shorted at this point, that's not the problem and is normal. My EDC 521 does the same thing as does the 522. Concentrate on the processor board. On P1, pins 1,2,26,27 are GND and pins 3,4,28,29 are +5VDC. It may be easier to check +5V across one of the TTL chips, though...

I won't have access to a scanner till mid-week or so, so I'll follow up then.
Jay

[nidlaX]

Great, thanks for this! There are definitely no such signs of life at power on, looks like the problem will involve the MPU board, power supply, and probably the EPROM. I hope they don't store any calibration data on that thing.

EDIT: I did a quick and dirty comparison between the two ROMs. Empy data sectors look identical and the byte value statistics look very similar as well, so there doesn't seem to be major corruption.

There's no calibration data on the EEPROM. Calibration is all done with pots, so no worry there. If you don't see the power on routine I described, it's most likely to be the processor board or power supply. Check the 5V rail, obviously. I have a hard copy manual with schematics. If you need me to scan a part of them let me know, I could probably do it this week sometime...

Jay

Thanks Jay! I took a look at the MPU card and I assume that the 4 edge connector contacts that are shorted together in pairs are ground and power respectively? Do you have a schematic of the mainboard? It would be very handy for checking supply voltages.

Also, it seems that the Hi and Lo load terminals on the front panel are short. I'm not super familiar with calibrators, but I'm guessing that's problem? Next step would be to trace back the output circuit and check all of the devices.

Don't worry about the output terminal being shorted at this point, that's not the problem and is normal. My EDC 521 does the same thing as does the 522. Concentrate on the processor board. On P1, pins 1,2,26,27 are GND and pins 3,4,28,29 are +5VDC. It may be easier to check +5V across one of the TTL chips, though...

I won't have access to a scanner till mid-week or so, so I'll follow up then.
Jay

By P1, are you referring to RP1 on the MPU card? Here are pictures of mine:




As far as I can tell, there is only ~2.6V being supplied to the card from the slot. A schematic will be very helpful to troubleshoot the logic power supply, thanks in advance!

[dacman]

This information is from a 520A, but the 522 is similar.  There is an LM309K in the back (for a 520A) that supplies 5 Vdc.  It usually has a connector.  On a 520A, the right side pin is 0V, middle 5V, and left 11V.  On a 520A, the OVL indicator has nothing to do with the MPU.

[nidlaX]

This information is from a 520A, but the 522 is similar.  There is an LM309K in the back (for a 520A) that supplies 5 Vdc.  It usually has a connector.  On a 520A, the right side pin is 0V, middle 5V, and left 11V.  On a 520A, the OVL indicator has nothing to do with the MPU.

Thanks for the tip. Yes, I assumed the 'OVL' indicator is unrelated to the MPU - it displays even if the MPU board is disconnected.

UPDATE: I measured the LM309K and the left terminal is at ~4.1V relative to ground while the center is at ~2.6V (the right side pin is ground). Time to find out what happened to the input.

[dacman]

The unit may have a blown rectifier.  The LM309K input needs to be at least 2V more than its output.  (If there is a 6800 uF capacitor, check the rectifier that is attached to it.)

[nidlaX]

The unit may have a blown rectifier.  The LM309K input needs to be at least 2V more than its output.  (If there is a 6800 uF capacitor, check the rectifier that is attached to it.)

My thoughts as well. I think this particular type of problem will be easy to diagnose once I get some schematics.

[nidlaX]

As it turns out, the layout of this board is really easy to follow with a good backlight. (I love the thick traces and modular design - it's a shame this earlier revision didn't come with silkscreen labels for all the components).


In any case, something seems to have gone amiss with this 3N255 bridge rectifier. I will start by desoldering it, should I also be concerned about the attached capacitor?


I think it's also a good lesson learned that a visual inspection should be done for both the front and back sides of a PCB on a first pass troubleshoot.

EDIT: Any recommendations for a good replacement part? I don't know whether I should just grab the cheapest 200V 2A rectifier from eBay.

[SeanB]

Looks like it just temperature cycled till it got a dry joint. you can see the other legs have that as well. Replace with a new one ( preferably with a higher current one, KBL10-E4/51 for example) and space the new one off the board by the full lead length ( place a sleeve over the 2 AC leads if you are concerned to keep them from shorting for any reason) so it can dissipate heat better. Looks like it just was running for a long period at near rated current with poor cooling as it was obscured with the other hot bridges.

If you are concerned replace all 5 electrolytic capacitors near there at the same time, they will have been running very hot from the bridge rectifiers dissipated heat and the 5V regulator. If doing that then replace the other 2 bridges as well, or at least resolder the connections to them, they all look solder starved.

[nidlaX]

Looks like it just temperature cycled till it got a dry joint. you can see the other legs have that as well. Replace with a new one ( preferably with a higher current one, KBL10-E4/51 for example) and space the new one off the board by the full lead length ( place a sleeve over the 2 AC leads if you are concerned to keep them from shorting for any reason) so it can dissipate heat better. Looks like it just was running for a long period at near rated current with poor cooling as it was obscured with the other hot bridges.

If you are concerned replace all 5 electrolytic capacitors near there at the same time, they will have been running very hot from the bridge rectifiers dissipated heat and the 5V regulator. If doing that then replace the other 2 bridges as well, or at least resolder the connections to them, they all look solder starved.

Thanks, I'll order some parts!

[JohnnyBerg]

Yeah .. dunno why it is not functioning, so replace all the caps 

[nidlaX]

Yeah .. dunno why it is not functioning, so replace all the caps 

I think the notion to replace the caps is just precautionary.

[JohnnyBerg]

Yeah .. dunno why it is not functioning, so replace all the caps 

I think the notion to replace the caps is just precautionary.

If it ain't broke, don't fix it.

[SeanB]

100Hz ripple and low voltage, with the diode having done a dirty. Think the cap will not be happy now, seeing as you are getting under 3V from the regulator. While the 340 series are rather notorious in failing dead short with age, they do at least have low noise, and you probably will find a lot of 100/120Hz ripple on the output, from a rather well cooked electrolytic. To do the leg like that means it was arcing with a dry joint for a long time, and now the capacitor has cooled down it has gone pretty close to open circuit as well.

[Jwalling]

EDIT: I did a quick and dirty comparison between the two ROMs. Empy data sectors look identical and the byte value statistics look very similar as well, so there doesn't seem to be major corruption.

As far as I can tell, there is only ~2.6V being supplied to the card from the slot. A schematic will be very helpful to troubleshoot the logic power supply, thanks in advance!

Ok, I have just scanned the schematic and manual. It is too big to upload here, so I uploaded both the user manual/ theory of operation and the schematics to KO4BB's website. It'll take a while for it to be available, so I'll send you a PM of where you can download it from my webspace.

Good luck with the repair!

Jay

[nidlaX]

EDIT: I did a quick and dirty comparison between the two ROMs. Empy data sectors look identical and the byte value statistics look very similar as well, so there doesn't seem to be major corruption.

As far as I can tell, there is only ~2.6V being supplied to the card from the slot. A schematic will be very helpful to troubleshoot the logic power supply, thanks in advance!

Ok, I have just scanned the schematic and manual. It is too big to upload here, so I uploaded both the user manual/ theory of operation and the schematics to KO4BB's website. It'll take a while for it to be available, so I'll send you a PM of where you can download it from my webspace.

Good luck with the repair!

Jay

Thanks again, the quality is excellent!

[Jwalling]

Yeah .. dunno why it is not functioning, so replace all the caps 

I think the notion to replace the caps is just precautionary.

If it ain't broke, don't fix it.

There are good reasons to replace aluminum caps before they fail completely. I just finished a repair job on an HP 4195A spectrum/network analyzer for a customer. The thing is loaded with 22uF 50VDC caps on a number of the internal boards. In particular, the A10 board was pulling down on it's +-12VDC rails. The damage to the fiberglass board under the caps from leaking and heating had caused the material to carbonize and act like a resistor. I ended up carving out all the blackened fiberglass until it stopped conducting. Looking at a number of other boards in the unit, this is just beginning to happen elsewhere as well. I probably had to replace 50+ of the damn things all told.

Pretty ugly fix.

[nidlaX]

I tried to swap out the rectifier (D402) today and encountered a slew of troubles. In addition to the already burnt PCB through hole, I also managed to lift the plating around the other three leads when I was desoldering the old rectifier. This seems to be a fairly regular problem for me, so I'm wondering if there's a problem with my desoldering technique. (Iron too hot? Bad idea to use desoldering braid?)

After I thought I'd successfully bodged the new rectifier (NTE169) in, I powered the unit back on and discovered approximately no voltage on the +5V rail. I was quite worried at this point, so I shut the whole thing off and did another visual inspection of both sides.

Now I see at least three (new?) problems I'd previously missed: there is evidence of charring / melting of the enameled winding wire that connects to the black wire (19) on the transformer, one of the resistors (R339?) is burned, and C420 seems discolored (burned? you can see in my picture that one of the leads is an orange color). Also, the AC output points that go into the D402 rectifier are shorted (~0 Ohms).

I'm a little hesitant at this point to continue repairs ad-hoc for fear of further damaging the unit, so I think the next step is to carefully analyze the schematics and ask for more guidance.