Author Topic: [SOLVED] Rigol DG1022 attempted repair (and reverse engineering?)  (Read 4147 times)

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Offline giosif

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Hi all,

I've recently got a Rigol DG1022 ARB function generator off eBay, I'm trying to repair it and I am hoping that, with your help, I'll manage to restore it.
I hope you'll have the patience to go through the whole ride.

The problem reported when I bought it was no signal on channel 1 after putting some voltage into it (see original report of the issue here - https://www.eevblog.com/forum/testgear/rigol-dg1022-problem/msg451006/).
On investigation, I found that there actually is an output on channel 1, but much lower amplitude than what the device is reporting on the display.
More specifically, for an amplitude of under 2V p-p, the output is correct.
For an amplitude of 2V p-p or above (up to 20V), the signal is 10 times weaker than it should be.
Thankfully, channel 2 is working, so I can use that for reference to some degree.

Now, for the things I managed to work out (please use the attached picture and annotations on it for reference):
  • For each of the two channels, the generated signal is coming out of some ADC's, then goes through a first set of relays - marked in the picture with 4 for channel 1 and with D for channel 2.
  • Both of these relays are in the "off" position (i.e. no power to the coil and the signal just goes through the relay with no change) if the set signal amplitude is below 2V p-p.
       For any higher amplitude setting, they are "turned on" (i.e. the coil is powered), which makes the signal go through a series of resistors before continuing to the next stage of the circuit.
       And this is expected behaviour, from what I could figure out about how the whole thing works.
  • Next in the path of the signal towards the output connectors, we have another set of relays - marked in the picture with 3 for channel 1 and with C for channel 2.
       I do not know exactly what these relays are for and when they get turned on, as none of my test settings managed to power any of the two relays.
       Judging by the set of resistors connected to them, I suspect they are used when the signal needs to be further atenuated or the impedance is set higher.
      Question 1: Is anyone on the forum reading this who has the same function generator and who would be willing to test if and when these relays are powered on?
  • Next in the path of the signal, we have yet another set of relays - marked in the picture with 2 for channel 1 and with B for channel 2.
       Here, things start to be somewhat different between channel 1 and channel 2:
       - for channel 2, the relay is in the "off" position if the signal amplitude is set to below 2V p-p, and in the "on" position for a signal amplitude set at or above 2V p-p.
       - for channel 1, there is no change on the position of the relay, irrespective of the set amplitude of the signal.
       The above behaviour for channel 2 is as expected, as this now takes the signal (which, at the same time, is being attenuated at relay D mentioned at step 1 above) through what I am guessing is an amplifying stage to bring the signal to the desired >=2V p-p amplitude.
       The behaviour for channel 1, I am 99% sure, is not normal and is the reason why the output at the unit's connector for channel 1 is so low.
       What I expect should happen is for the relay to turn on and change the path of the signal through an amplifying stage, similar to the approach on channel 2.
      Question 2: Would anyone of the same device be kind enough to confirm this (i.e. relay is turned on for amplitude >=2V p-p)?
  • I did trace the device that is supposed to turn on relay 2 for channel 1 and it is a SN74AHC595 8-Bit Shift Register - marked with SR1 in the attached picture.
       So, my guess is that the shift register is faulty.
       At this stage, I tried to use channel 2 as reference and look to confirm SR1 is not working properly.
       However, as you can see marked in the picture, SR2 and relay B are connected slightly differently when compared to SR1 and relay 2 - i.e. relay B is connected to pin 3 of SR2, while relay 2 is connected to pin 4 of SR1.
       And, unfortunately, I don't know shift registers that well to be able to evaluate if those different pins would make a difference in results or not.
      Question 3: Is there an easy way to check if a shift register is faulty or not?
       I'm looking for something that does not involve desoldering the component, as I already have a replacement shift register and I might just as well perform a replacement and skip the checking altogether.
       Plus, I don't have an SMD re-working station and de- and re-soldering such a component would be some challenge for me.
  • Now, the interesting thing is that, if relay 2 for channel 1 would turn on as I expect, the signal would then go to an operational amplifier.
       You won't see that in the picture, since I've removed it as it was faulty.
       Unfortunately, Rigol was "kind" enough to scrape any markings from this op-amp, so I don't know what make and model it is.
       I searched the forum and could see I'm not the only one looking to find out that information.
       I also pinged Rigol asking them if they could provide some info on the op-amp or at least provide a (aslo scraped) replacement op-amp - still waiting for a reply.
       In the meantime and since I don't expect Rigol to be that forthcoming with the information/help I requested, I extracted as many details around the op-amp as I could, in the attempt to narrow down the search for the specific make and model.
       So, as you can see in the attached picture, we'd be looking at an op-amp with the following specs:
       - SOIC 8 pin package, with thermal pad at the bottom
       - single channel
       - dual supply: +15V and -15V
       - enable/disable (power on/shutdown) pin (i.e. pin 8 )
       - [my guess] 10x amplification when using a 33 Ohm resistor on the (inverting?) input and an 800 Ohm resistor on the (positive?) feedback loop
       Question 4: Anyone that can make suggestions on op-amps with such characteristics?
       I did some searches but wasn't able to find op-amps with all the above specs.
       Also, suggestions made on other forum threads on the topic don't entirely match the above specs either.
       BTW, on the enable/disable pin 8 I mention above, I confirmed that it is connected to the coil of relay 2 and to shift register SR1.
       This looks to confirm my guess that SR1 should turn on relay 2 for signal amplitudes at or above 2V p-p and, at the same time, enable the op-amp.
  • Finally, on its way out of the device, the signal goes through a last set of relays - marked in the picture with 1 for channel 1 and with A for channel 2.
       These are easy to guess - they are used for turning the respective channel output on and off.
       And both of these work as expected.

Anyone still reading this at this stage?  ;D
If yes, I thank you for your patience and would appreciate any answers/pointers/sugestions/wishes/etc or, indeed, even other quesions that would allow me to get this device fixed.

Thank you!

Regards,
George
« Last Edit: June 01, 2016, 11:57:27 pm by giosif »
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #1 on: May 29, 2016, 09:52:25 pm »
Just an update: today, I mustered enough courage to try de-solder the two shift registers SR1 and SR2 and replace them with new components.
I've decided this after seeing a video on xDevs, watching Tin de-solder some components, including SMD, with just a regular soldering iron.  :-+
I'm happy to report that, after some sweat, emotions, etc. I've managed to replace the shift registers with new ones and that did the trick: relay 2 on channel 1 and the pad associated with pin 8 of the (still missing) op-amp are now being turned on when the signal amplitude is set to 2V p-p or above.  :-+
So, questions 2 and 3 were answered.
Question 1 still stands, but I'm not too bothered about it right now, as the behaviour is the same for both channels 1 and 2 (and channel 2 is working properly).
The main outstanding item is the op-amp.
I am yet to make any significant progress on that.  :box:

Regards,
George
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #2 on: May 30, 2016, 10:14:02 am »
Another update: I've just got the response from Rigol (I asked them for schematics of the main board and/or info on the op-amp) and they kindly told me to bugger off.
Actually, they suggested I buy a replacement main board as the only way of fixing the function generator.  :clap:
Oh well, back to searching for an unofficial replacement op-amp...
 

Offline 1kostik1976

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #3 on: May 30, 2016, 03:07:34 pm »
Hi giosif
how you determined that the amplifier is faulty? very warm?
 

Offline TurboTom

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #4 on: May 30, 2016, 03:10:26 pm »
You can probably try a current feedback operational amplifier like the Linear LT1227CS8. Since it's got offset null inputs on terminals 1 and 5 you'll have to cut terminal 1 since it must not be connected to GND. Otherwise it should fit on the foorprint.

Good luck,

Thomas
 

Offline 1kostik1976

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #5 on: May 30, 2016, 04:05:22 pm »
THS3095
« Last Edit: May 30, 2016, 04:07:56 pm by 1kostik1976 »
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #6 on: May 30, 2016, 11:16:18 pm »
Hi giosif
how you determined that the amplifier is faulty? very warm?

Yeah, I dreamt that dream too (i.e. hoping the op-amp was ok), but it wasn't to be. :-)
I reached the conclusion that the op-amp was dead because one of its input pins was shorted with pin 8 (enable/shutdown pin).
Also that pin 8 and, by consequence, the positive terminal of relay 2 were being driven to -4.5V, which was definitely wrong.
And that wasn't because of the relay, since I desoldered it and the -4.5V was still there.
At this stage, I desoldered the op-amp and the negative voltage went away.
It's most likely that this negative voltage messed up the shift registers as well.

Regards,
George
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #7 on: May 30, 2016, 11:24:37 pm »
You can probably try a current feedback operational amplifier like the Linear LT1227CS8. Since it's got offset null inputs on terminals 1 and 5 you'll have to cut terminal 1 since it must not be connected to GND. Otherwise it should fit on the foorprint.

Good luck,

Thomas

Thanks, Thomas!
I'll give that a look.

Regards,
George
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #8 on: May 30, 2016, 11:36:23 pm »
THS3095

Nice. That looks like the most promising candidate.
Or do you actually know this is what Rigol are using for the DG1022?

Thanks,
George
 

Offline 1kostik1976

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #9 on: May 31, 2016, 03:30:40 am »
HI
I do not know. all the answers are in the photo that you posted. + 30 minutes of searching on digikey ......
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #10 on: May 31, 2016, 06:58:46 am »
HI
I do not know. all the answers are in the photo that you posted. + 30 minutes of searching on digikey ......

Ok. Thank you for your help and time spent on that!
I ordered the part and will let you know once I have the component in place.
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #11 on: June 01, 2016, 11:53:26 pm »
IT'S A...LIVE!!!  8)
Thanks to 1kostik1976's suggestion for the op-amp (i.e. THS3095), I now have a fully functional ARB function generator.  :-DMM
 

Offline giosif

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Re: Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #12 on: June 01, 2016, 11:56:13 pm »
I should also mention that I eventually figured out the answer to question 1 from my original post: relays 3 and C kick in when the signal needs to be atenuated into the tens of mV p-p area.
 

Offline 1kostik1976

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Re: [SOLVED] Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #13 on: June 02, 2016, 01:50:01 am »
HI
Congratulations.
 

Offline giosif

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Re: [SOLVED] Rigol DG1022 attempted repair (and reverse engineering?)
« Reply #14 on: June 02, 2016, 06:10:40 am »
 


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