[Repaired] EDC 522 DC Voltage Calibrator - Page 1
Forum section: electronics / repair

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 05, 2015, 10:31:29 am
content: 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 togrumpydoc'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.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 05, 2015, 10:34:55 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 05, 2015, 10:50:46 am
content: 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.

author: Jwalling
author_rank: Unknown
timestamp: April 05, 2015, 11:13:59 am
content: Quote from: nidlaX on April 05, 2015, 10:31:29 amI'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

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 05, 2015, 11:37:06 am
content: Quote from: JohnnyBerg on April 05, 2015, 10:34:55 amQuote from: SeanB on April 05, 2015, 10:50:46 amThanks for the info, I'll measure the reference diode!Quote from: Jwalling on April 05, 2015, 11:13:59 amGreat, 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 05, 2015, 11:50:06 am
content: 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.

author: Jwalling
author_rank: Unknown
timestamp: April 05, 2015, 12:46:21 pm
content: Quote from: nidlaX on April 05, 2015, 11:37:06 amThere'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

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 05, 2015, 01:22:25 pm
content: Quote from: Jwalling on April 05, 2015, 12:46:21 pmThanks 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.

author: Jwalling
author_rank: Unknown
timestamp: April 05, 2015, 04:22:33 pm
content: Quote from: nidlaX on April 05, 2015, 01:22:25 pmDon'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

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 06, 2015, 02:15:11 am
content: Quote from: Jwalling on April 05, 2015, 04:22:33 pmBy 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!

author: dacman
author_rank: Frequent Contributor
timestamp: April 06, 2015, 02:36:58 am
content: 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.

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 06, 2015, 02:57:14 am
content: Quote from: dacman on April 06, 2015, 02:36:58 amThanks 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.

author: dacman
author_rank: Frequent Contributor
timestamp: April 06, 2015, 04:00:12 am
content: 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.)

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 06, 2015, 06:21:28 am
content: Quote from: dacman on April 06, 2015, 04:00:12 amMy thoughts as well. I think this particular type of problem will be easy to diagnose once I get some schematics.

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 06, 2015, 06:51:25 am
content: 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 frontandback 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.

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2015, 07:01:57 am
content: 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.

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 06, 2015, 07:48:30 am
content: Quote from: SeanB on April 06, 2015, 07:01:57 amThanks, I'll order some parts!

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 07:52:21 am
content: Yeah .. dunno why it is not functioning, so replace all the caps

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 06, 2015, 08:33:03 am
content: Quote from: JohnnyBerg on April 06, 2015, 07:52:21 amI think the notion to replace the caps is just precautionary.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: April 06, 2015, 09:46:38 am
content: Quote from: nidlaX on April 06, 2015, 08:33:03 amIf it ain't broke, don't fix it.

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2015, 11:27:25 am
content: 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.

author: Jwalling
author_rank: Unknown
timestamp: April 07, 2015, 10:34:13 am
content: QuoteOk, 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

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 08, 2015, 12:32:12 am
content: Quote from: Jwalling on April 07, 2015, 10:34:13 amThanks again, the quality is excellent!

author: Jwalling
author_rank: Unknown
timestamp: April 08, 2015, 10:27:05 am
content: Quote from: JohnnyBerg on April 06, 2015, 09:46:38 amThere 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.

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 09, 2015, 02:45:43 am
content: 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.

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 09, 2015, 05:44:02 am
content: Well good news, my de-soldering is less incompetent than I thought, the plating around the other three through-holes is still ok. Looks like I didn't clean it vigorously enough...

author: jlmoon
author_rank: Unknown
timestamp: April 09, 2015, 04:22:41 pm
content: Hello,I have been following this thread for a bit. Might I suggest you measure the voltage at the junction of R334 and R335. These are precision (individually selected per unit) 16.681K resistors. Be real careful doing this because there could be - or + ~120Volts at this point. This is the feedback reference from the output relay switching arrangement that sets the feedback loop up based on the front panel voltage range selection. These instruments can be quite a trick to diagnose unless you understand the grand scheme of things and how they're supposed to function. When you first power the unit up the display should go through a display diagnostics displaying the model number and then the IEEE-488 address. After that it should reflect values from 0 - 9 based on what you have dialed on each decoder position. I understand you're having trouble with the 5 Volt supply, have you corrected this yet?JLMForgot to mention, when making this 16.681K junction measurement be sure to reference your negative meter lead to the junction of capacitors C416 and C417. This is the +150V and -150V supply common. WARNING !!! There is nearly 300 Volts potential in this area if across the two caps 416 & 417 .. so be really careful here. This supply is the main function of the output stages driven by the differential amplifier created by Q301 and Q302. Please take note this can be a dangerous unit to service if you're not familiar with this circuitry. Best of Luck!

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 09, 2015, 09:01:18 pm
content: Quote from: jlmoon on April 09, 2015, 04:22:41 pmThanks for the tips JLM. I had already measured the +-150V rails at the rectifier without knowing beforehand what the voltages were supposed to be.As far as I can tell, that's the only functional rail at the moment. The 5V problem is still not fixed - I am concerned that the transformer might be damaged.

author: jlmoon
author_rank: Unknown
timestamp: April 09, 2015, 09:41:35 pm
content: Quote from: nidlaX on April 09, 2015, 09:01:18 pmNidlaX,I seriously doubt the windings are damaged, the windings used for the 5V generation are quite large in size making it pretty tough to burn up or short those, most of the time when severe loading is present on the secondaries of a transformer, the primary windings are usually the ones to melt or short down and start blowing fuses.The first thing I might suggest is to isolate that 5V supply down to just the bridge & filter caps and regulator. If you do this be sure to remove and cap the windings that make the +- 150V so as not to bang the diff amp on the rails and blow up the output buffers due to improper DAC drive from the linear circuits. This will save you lots of grief. Actually I would recommend you isolate the HV supply until you get a good scaled output from the reference ladder and the DAC output op amp. Once you get the 5V working and all the control systems then you can move forward into the power stuff that drives the output amp. Does this make sense to you?I have repaired and calibrated a few of these including JWallings 521.I think I can point you in the right direction if you give me some readings in various places. Let me know.JLM

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 09, 2015, 09:50:54 pm
content: Quote from: jlmoon on April 09, 2015, 09:41:35 pmAgreed, but the physical evidence is present that melting has occurred on at least one of the windings, unless it's just a manufacturing defect.I'll take your advice and isolate the HV supply. Should I simply disconnect the transformer taps to the board and set them aside?

author: jlmoon
author_rank: Unknown
timestamp: April 10, 2015, 12:06:22 am
content: Absolutely Yes. Cap them up so not to spoil your day.

author: dacman
author_rank: Frequent Contributor
timestamp: April 10, 2015, 12:12:45 am
content: 2.6 Vdc is not enough to energize the relays. This would result in no output and no feedback.What is the line voltage switch set to?What is the voltage at (what would be) the input to the LM309K if the 5V connector is disconnected? How much current can it source? What is the voltage across the HPCL-3700 IC pins 1 & 4?Do you have a 5V power supply that can source about 1.5 Adc that can be used to power the 5V circuit?

author: jlmoon
author_rank: Unknown
timestamp: April 10, 2015, 03:41:32 am
content: Quote from: dacman on April 10, 2015, 12:12:45 amNor would 2.6 volts be enough to properly run any of that digital logic. Would be handy to remove output lead of that 309K and feed with a 5 V current regulated supply to determine if there is some excess loading going on, which would explain the heating of that bridge rectifier pin on pcboard and limiting the output on that 309K. Is that regulator hot to the touch?

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 10, 2015, 08:14:13 am
content: Quote from: jlmoon on April 10, 2015, 03:41:32 amWhen I installed the new rectifier, I was getting 0V from the rectifier output. That is currently the issue that I'm focusing on.I thought about powering the regulator externally, but I was worried about damaging something.

author: jlmoon
author_rank: Unknown
timestamp: April 10, 2015, 04:27:41 pm
content: If you have 0 V I would isolate supply and if output is normal then look for a shorted tantlum cap or component downstream.

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 10, 2015, 11:26:18 pm
content: I desoldered C420 and it seems to be ok. However, I still measure ~0 Ohms across transformer taps 10 and 11.

author: dacman
author_rank: Frequent Contributor
timestamp: April 11, 2015, 01:45:27 am
content: Do you mean that there is no output and the fuse is still good?Are the +/- 15 VDC supplies OK (at two each 7815 and 7915 regulators).In the third set of pictures there is a white connector with a green circle imposed on it. If this connector is pulled, the 309K would be removed from the circuit. The pins are standard 0.093” pins.The 5V supply transformer windings should have low Ohms due to high current output.

author: nidlaX
author_rank: Frequent Contributor
timestamp: April 11, 2015, 06:11:44 am
content: Quote from: dacman on April 11, 2015, 01:45:27 amI wish I knew more about the specifications of the transformer. Suppose I have disconnected the rectifier D402 for the +11V DC output to the +5V LM309: should I still measure an AC voltage across taps 11 and 12?The mains fuse is fine, I have not measured the +/- 15V supplies yet. I will test the LM309K by itself.

author: dacman
author_rank: Frequent Contributor
timestamp: April 11, 2015, 02:42:55 pm
content: Taps 11 and 12 or 10 and 11? (I have access to a 522 manual at my day job, but I don't access eevblog at work. The manual would need to be available on-line.)There should be AC voltage across this open transformer winding; I would guess 8 VRMS or more. There should be AC voltage across it, even if the rectifier is installed.If that unit has a separate Line Voltage Selector switch (115V/230V) I would want to know that it was good. I would Ohm it out, or at least exercise it a few times to try and knock off any possible oxidation.Jlmoon is giving good advice. I wouldn't want 150V feeding back into the power amplifier. I also would not want the power transistors going into saturation, which could happen whether or not the relays are energized.

author: nidlaX
author_rank: Frequent Contributor
timestamp: June 27, 2016, 12:53:27 pm
content: I've revisited this repair over the past few days. Today, I fixed the 5V power supply problem with a new bridge rectifier. I drilled through the carbonized area of the PCB and soldered a bodge wire directly from the positive leg of the rectifier to the corresponding capacitor. The unit powers and goes through the self test.Now, I have some weird problems with the output. If I set negative polarity, I can get output on all the ranges for all possible values most of the time. (Sometimes I get an overload when I am increasing the voltage) If I set positive polarity, I can only get output up to about 0.5 V on the 100 V range or about 0.2 V on the 10 V range. Anything else would trigger an overload, and I would continue to get overload until I back the set value down to near 0. At first, I thought this could be a front panel switch issue, so I sprayed each switch out with deoxit and actuated them many times. This does not seem to solve the root problem, however. I get similar results if I attempt to program the output over GPIB, but for some reason I can only reach 0.2 V on the 100 V range when setting remotely as opposed to the 0.5 V I can obtain when setting locally.Are there any suggestions for how I should proceed to troubleshoot this?

author: nidlaX
author_rank: Frequent Contributor
timestamp: July 12, 2016, 03:55:45 am
content: Some more progress on this: the output at Z303 appears to become increasingly unstable at positive voltages. I see a half-sinewave like ripple when I increased output to >= 0.5 V on the 100 V range. FB and INP appear stable relative to SG. I swapped Z303 to little effect - the instability still appears eventually as I increase the voltage. The output will hold at any negative value, but overload will trip if I try to increase the voltage, so it seems that there is an issue any time the non-inverting input is greater than the inverting input.Time to keep digging.

author: dacman
author_rank: Frequent Contributor
timestamp: July 12, 2016, 11:00:11 pm
content: The output of the DAC (Z101) should be the same with the polarity switch set to either + or -. Polarity gets inverted by FETs Q311 through Q314 (between Z101 and Z303, two FETs turn on and the other two turn off, inverting the input to Z303).Check the output transistors (Q306, Q308) and drivers (Q305, Q307), R311 and R315 (3.3 Ohm resistors) and the other output stage resistors, and the MJE350 transistors (Q301 to Q304). Also, check the diodes (D305 to D309) and make sure the +/- 150 Vdc rails are ok.

author: nidlaX
author_rank: Frequent Contributor
timestamp: July 13, 2016, 01:19:56 am
content: Quote from: dacman on July 12, 2016, 11:00:11 pmYes, the DAC output and polarity switching were both checked early on.Quote from: dacman on July 12, 2016, 11:00:11 pmThanks, I'm continuing to go through the output amplifier stage.

author: dacman
author_rank: Frequent Contributor
timestamp: July 13, 2016, 02:02:57 am
content: There will be a little ripple on the output of Z303, even if it is working well. The ripple on the +/- 150 Vdc power supplies is higher than the output noise specification, and the way this is reduced is that Z303 follows the ripple (which is on top of the feedback) and takes most of it out. FB will look stable but Z303-6 will not.

author: nidlaX
author_rank: Frequent Contributor
timestamp: July 13, 2016, 03:12:35 pm
content: Quote from: dacman on July 13, 2016, 02:02:57 amHow much should I expect to be normal? Should it be on the order of the ripple of +/- 150 V vs power_gnd?

author: nidlaX
author_rank: Frequent Contributor
timestamp: July 16, 2016, 01:11:40 am
content: I don't know why I keep triggering myself by not checking the simple things first.When probing Q301 through Q304 today, I noticed that the symptoms were "improving" depending on which transistor I was hooked onto. Turning the unit around revealed solder joints that were basically falling off to light pressure on the transistor legs. Given how bad the soldering job is throughout my unit, I should've known to triple check all the suspicious looking joints. 5 minutes of reflow later and I have stable output on all ranges again.I could say repair completed, but I know deep down that I should reflow just about every single joint on the unit because they all look like shit. Thanks EDC or whoever worked on this unit last...

author: Samogon
author_rank: Frequent Contributor
timestamp: September 27, 2016, 10:02:53 pm
content: Quote from: jlmoon on April 09, 2015, 04:22:41 pmGood pointjlmoon!I have 520a with "overload" turning on output +/-23V. On the junction between R334 and R335 measures few volts.Also noticed that at power supply +/- 150 volts experience voltage drops to +/- 130 volts depending on what polarity output set on control panel.I assume something in Output Amplifier has failed and drags down PSU voltage.Still searching for it. If you have ideas i would appreciate your input.

author: dacman
author_rank: Frequent Contributor
timestamp: September 28, 2016, 12:28:07 am
content: Check the output transistors and associated components (drivers, resistors, and diodes). The MJE350s (and associated circuitry) can also fail. I'd be curious to know what Series the unit is (on the back panel). There is a resistor on the other side of the rail from the MJE350s. It was originally a 13 kOhm 2 Watt carbon (it could short and take out the MJE350s). It was revised to a 10 kOhm 5 Watt WW. The two 22 kOhm 2 Watt bleeders in the power supply were revised to 47 kOhm bleeders (they can short and blow fuses).

author: Samogon
author_rank: Frequent Contributor
timestamp: September 28, 2016, 04:54:50 am
content: Serial is 14022I did check power transistors they are easily detached, drivers need to desolder to check, will do tomorrow.Resistor is 13k and two others 47k.There is charring on MJE350 and resistors also charing on +/- 150 rectifier output.

author: jondaddio
author_rank: Newbie
timestamp: September 09, 2018, 07:10:49 pm
content: Hi Jwalling,Any chance I could get a copy of your scanned schematic and manual for the Khron-Hite 522 DC Voltage Calibrator?TIA,Jon

author: dacman
author_rank: Frequent Contributor
timestamp: September 10, 2018, 01:44:42 am
content: Jwalling did state they were uploaded to ko4bb and they are there:http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/EDC

author: Jwalling
author_rank: Unknown
timestamp: September 10, 2018, 09:33:20 am
content: Quote from: jondaddio on September 09, 2018, 07:10:49 pmAs I said earlier in this thread, I uploaded it to KO4BB back in 2015.http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=06_Misc_Test_Equipment/EDC

author: jondaddio
author_rank: Newbie
timestamp: September 10, 2018, 03:14:35 pm
content: Thanks Jwalling and dacman !! I didn't see the link earlier.