EG&G (princeton applied reasearch) 5202 Lock-in amplifier repair

[_Wim_]

Hi,
For a new repair project, I am looking for the service manual of an Princeton Applied Research (EG&G)5202  Lock-in amplifier. It seems that for most models the service manual is easily available, but not for the 5202 for some reason. I have checked already the following sites (+ many hours of googling...):

http://www.ameteksi.com/support-center/signal-recovery/product-instruction-manuals
http://bama.edebris.com/manuals/
http://elektrotanya.com/?q=keres
http://www.opweb.de/en/index.php
http://exodus.poly.edu/~kurt/manuals/manuals/Other/
http://www.eserviceinfo.com/
http://www.ko4bb.com/getsimple/index.php?id=manuals
https://www.manualslib.com/

If anybody here has a copy somewhere (or knows where a I could find one), that would be much appreciated.

[_Wim_]

Some pictures of the work in progress on the bench:

[_Wim_]

The boards in detail...

[_Wim_]

Some more boards...

[_Wim_]

Symptons of the patient so far:

BAD:
- In phase signal is always incorrect
- Quadrature signals is sometimes correct, but then suddenly goes all over the place
- Vector mode (magnitude and phase display) immediately make the error light pop-up
- VU-meter In phase does not return to centre
- Nobs of all potmeters are missing

GOOD:
- Amplifier locks correctly to reference
- No overload at input amp
- Power supply voltages are ok

Edit: typos

[_Wim_]

This explains why VU-meter for "in phase" does not work properly...

[Kleinstein]

Sudden jumps on such an old instruments could come from oxidized contacts in the range switches. So give them quite a few operations - this often helps to clean them. Usually one can also separately look at the input amplifier output (so without demodulation).

The vector mode could be a bit difficult to fix (special old analog parts). There are different ways (PLL loop like to adjust the phase shift or doing analog RMS and phase calculation) to realize the vector mode. The fault could be be well linked to the non working in phase signal.

In these old instruments there was quite some explanation in the normal instructions. There should be also a similar, lower grade model, without the second channel (and vector mode). You might this way at least get some of the schematics.

[_Wim_]

Thanks for your reply. I have indeed downloaded comparable manuals from EG&G, and as you correctly state, there is alot of info available. I will give all the contacts a good work-out, that is a good tip.

At least one thing seems to be fixed: the VU meter. I openend it up with an exacto knive (it was glued close), and the little spring was a part of the needle, see pic(some sort of damping coil?) After that, the meter could again be adjusted to be exactly in the center :-)

[VintageNut]

Very interesting that you have an RF version. The vast majority of the older lock-in amps are low frequency. I have a model 128A which worked when I received it. The calibration procedure was available on the web. Everything checked out good. I used it for a project and then set it on a shelf where it resides over a year later.

[_Wim_]

I have a low frequency version also (5204). I bought them together non working for 50€, a very good deal I think. The 5202 I have already fixed (did not document that here), had a broken op-amp and a failed 7805 regulator. Also needed quite some adjustment, but this was very clearly descibed in the service manual I found here:

http://elektrotanya.com/princeton-applied-research_5204_lock-in-analyzer.pdf/download.html

That why I also would like to have the service manual for this one, fixing it is one thing, but the fine adjustments are difficult without any documentation...And as you can see from the pictures of the boards above, there is alot that could be adjusted...

[_Wim_]

Symptons of the patient so far:

BAD:
- In phase signal is always incorrect
- Quadrature signals is sometimes correct, but then suddenly goes all over the place
- Vector mode (magnitude and phase display) immediately make the error light pop-up
- VU-meter In phase does not return to centre
- Nobs of all potmeters are missing

Edit: typos

Ok, the in phase signal problems are purely related to the VU-meter. When I monitor the signal at the BNC-connector, all is ok (well, a bit off, but that is due to alignment probably). After a good work-out of the switches and some cleaning, the sudden erratic behaviour seems to be resolved for now.
time
Vector mode seems to work also (at least the phase indication, not completely sure about the magnitude yet), issue seems to be incorrect use on my side (time constant needed to be set at 10ms for vector mode to work, but this was not the case during my initial testing)

So it seems the issues are now more mechanical, finding a suitable VU-meter, and finding some nice knobs for 10-turn potmeters. Still hoping to find the service manual for the alignment...

[_Wim_]

The panel meter is also fixed!  I disassembled the panel meter completely, and found there were some "pins" in the back on with the little spring could be attached also. These little springs are used as a counter weight. By adjusting their position onto the holder, you can fine-tune the needle movement. Without the little spring that was loose, the meter went to full scale with only +-25µA. When I measured the meter of the quadrature, full scale should be at 100µA. After I reattached the little spring, this was about right. By filling a bit with all of the little weights, I was able to make -100µA and +100µA exactly full scale (both extremes), and allowing the meter to return to the centre when no current supplied.

After doing some testing with the unit, it seems to operate now completely (magnitude in vector mode is also ok), just still needs a little adjustment and some knows for the pots. So this means I have now a lock-in amp for both low freq and high freq, and only paid 50€ for both.

[Vgkid]

Nice job, I might have given up on that meter.

[_Wim_]

I did too, until I realized after an hour of googling it would not be easy to find a replacement one that would fit nicely...

[TerraHertz]

Very interesting that you have an RF version. The vast majority of the older lock-in amps are low frequency. I have a model 128A which worked when I received it. The calibration procedure was available on the web. Everything checked out good. I used it for a project and then set it on a shelf where it resides over a year later.

Ha. I too have a 128A. It came with something else, and so far I haven't found any use for it. Been sitting on a shelf for ages, untested. Not even on any 'todo' list.
What manuals & info did you find? I only found this:
   http://www.k7jrl.com/pub/manuals/brookdeal/128a/128a.pdf   7,397KB
   Manual, includes schematics

[VintageNut]

Good question. I am away from my office this week. I will post what I have when I return.

Something I am going to try is to feed the output of a SMU into the Lock-In and feed 60Hz from the wall into the reference input to see how much 60Hz noise can be detected. This came up recently in a conversation about a SMU application that is sensitive to AC noise riding on a DC current.

Very interesting that you have an RF version. The vast majority of the older lock-in amps are low frequency. I have a model 128A which worked when I received it. The calibration procedure was available on the web. Everything checked out good. I used it for a project and then set it on a shelf where it resides over a year later.

Ha. I too have a 128A. It came with something else, and so far I haven't found any use for it. Been sitting on a shelf for ages, untested. Not even on any 'todo' list.
What manuals & info did you find? I only found this:
   http://www.k7jrl.com/pub/manuals/brookdeal/128a/128a.pdf   7,397KB
   Manual, includes schematics

[_Wim_]

I agree there are not so usefull any more, because FFT and spectrum analysis is now reasonable cheap nowadays. I did not buy them for an actual need, but more becasue they looked nice and were cheap, and it seemed a fun repair project.

I will leave them on the bench for now and see if they get used. One thing I planning to test is compare if I can get a better S/N with the lock-in amp then using a good ADC and FFT on a pc

[Kleinstein]

A Lockin can still be useful for some applications, like optics experiments or for an AC bridge. Different from an ADC / FFT solution you get near instant results - though the one frequency you need only. With good software the ADC + software (though generally not FFT) can be as good or slightly better.

[VintageNut]

My understanding is that when the environment is noisy, the lock in can pull a signal out of the noise when you know the frequency of the signal to recover.

[_Wim_]

My understanding is that when the environment is noisy, the lock in can pull a signal out of the noise when you know the frequency of the signal to recover.

But so does the FTT is you take a high sample length. You can think of a lockin amplifier as a fourier analysis of a single (reference) frequency. I agree with Kleinstein that you get "near instant" results, were an FTT (with long sample length) can some time (1 or 2 seconds), which might be important in some applications.

A lock-in also means you do not need to boot up a pc, connect a pre-amp to the pc, start software, configure software... but just need to switch on a nice looking unit and turn some nobs to the correct position

[Dr. Frank]

Congratulation on these LI amplifiers.

These are still superior to any digital FFT techniques in several cases.

At first, LI usually come with very sensitive and low-noise pre-amplifiers, combined with (selectable) high dynamic range, over 120dB.
They usually feature true differential amplifiers.

All this enables these instruments, to directly measure signals in the nV range out of many orders of magnitude higher noise.

This capability of LI still is far beyond FFT, as latter systems are dynamically limited, and also are difficult to be synchronized with the base frequency of the signal to be measured, so you easily get any kind of frequency leakage effects, or picket fence effects, which requires filtering, which again creates unwanted phase-shifts of sine and cosine components.
LI are much more straight forward in this sense. Also, with LIs, you can implement clever chopper techniques (e.g. optical) for dc signals, so to measure also these in the nV range with very high SNR.

FFT can be used better, if the signal is clean from the start, and if you are interested in the higher harmonics of the test signal, and you have the possibility, to synchronize the DUT signal with the FFT analyzer. Using LI amplifiers, you would have to stack them, each one for every frequency component. 


Well, I have used and repaired a lot of P.A.R. LIs, especially many P.A.R. 124A.
Crucial components were the reed contacts, which switch the gain inside the ac amplifiers.

So I de-soldered these from their coils, and replaced them with modern reed contacts,. That way, even 25 year old LIs were like brand new, afterwards...
Wonderful electronics..

Frank

[_Wim_]

Congratulation on these LI amplifiers.
....

Well, I have used and repaired a lot of P.A.R. LIs, especially many P.A.R. 124A.
....

Thanks! I will for sure try to put them to good use.

Do you have by any chance the service manual for the 5202 unit?

[Kleinstein]

Modern lock in amplifiers use ADC and digital calculation and this way get really good results. But they also start with really good (e.g. 16 bit or better) ADCs to get a good dynamic range. So you could do a lot with a suitable 2 channel digitizer and software, bit a simple DSO might not help much.

FFT is usually just looking at frequency bands, but usually not synchronized to the signal. So you usually loose the phase advantage of looking only at the in phase signal and ignoring the out of phase noise. So the FFT is good if you don't have a reference.

The other point with a lock in compared to FFT is that the frequency does not have to be that stable - you can still get a very low noise bandwidth with signal that has quite some frequency drift. Simple FFT will not give that - but more complicated software for correlations could do that as well but here the FFT is only one step.

With suitable software, a lock in might not have a big advantage (if at all) - but it is a ready made instrument, including the often quite good quality amplifier.  So it is easy to use.

[Dr. Frank]

Thanks! I will for sure try to put them to good use.

Do you have by any chance the service manual for the 5202 unit?

Unfortunately, I only have the 5204 operating and service manual.

Frank

[_Wim_]

Modern lock in amplifiers use ADC and digital calculation and this way get really good results. But they also start with really good (e.g. 16 bit or better) ADCs to get a good dynamic range. So you could do a lot with a suitable 2 channel digitizer and software, bit a simple DSO might not help much.

FFT is usually just looking at frequency bands, but usually not synchronized to the signal. So you usually loose the phase advantage of looking only at the in phase signal and ignoring the out of phase noise. So the FFT is good if you don't have a reference.

The other point with a lock in compared to FFT is that the frequency does not have to be that stable - you can still get a very low noise bandwidth with signal that has quite some frequency drift. Simple FFT will not give that - but more complicated software for correlations could do that as well but here the FFT is only one step.

With suitable software, a lock in might not have a big advantage (if at all) - but it is a ready made instrument, including the often quite good quality amplifier.  So it is easy to use.

Thanks for this detailed explenation, I am becoming more and more glad  I bought those 2