Electronics > Metrology

Restoration : Wavetek 4920/4920M teardown, repair and usage cases, come join!

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Now my humble lab welcomes those rare green-glow Datron/Wavetek magical pieces. No, not another 8.5-digit Datron 1281...  :P

We talking AC-nuttery this time, time to open new pandora box for me. Yes, it's Wavetek 4920! And not just any, but military version of it, 4920M.  :box:
What's different from "regular" 4920? From what I found out , M-version have next differences:

* Both N connectors for input, instead of N for WB and banana for regular
* No low mV ranges option
* More scarce menu system and few differences in features

I hence summon all Datron/Wavetek 4920 and 4920M owners to join me in this ACV-nuttery journey begin.. I know you folks are out there, hiding gems behind DC gear!  :popcorn:

There was recent sale of 5pcs Wavetek/Datron 4920M. Instead of waiting and hoping that some voltnut from EEVBlog will snatch one and show us it's insides, I decided to close ebay shopping year 2016 for me with this last bit, which happened to have more recent Wavetek-only branded 4920M. Is it a bust for 592$ or a "winner-winner chicken dinner"? Well, in worst case it's just another box with LTZ1000 reference inside and nice glowing VFDs  >:D

After some waiting on GSP shipping and delivery (which is great btw, packing on GSP is top notch!) box just arrived today. Excited? Hell, yea.

Will it be another 220+ hours repair project, like rusty 3458A? Perhaps even more so, as I have not much experience in AC stuff, don't have any thermal converters, and only calibrated measurement piece of ACV/ACI I have is Keithley 2001 with Tek cal from 2014. My 3458A had never ran SCAL, nor 2002's completed AC calibration. Anyhow, stay tuned, will be fun!  :=\

Break the rule "take it apart" and actually power this unknown black cat to see if she's alive.

VFD is unusual nice and emerald green. I actually dig it, looks way better contrast than Keithley's/3458A. Big digits too! I think it's same exact front panel as used in other 1271/1281/4950 Datrons.

Last cal January 2015? Really? Too good to be true, I say.

And calibration sticker says 2009. Sticker on the rear tells to replace battery in 2012, so it's 4 years overdue or someone did not bother to update sticker?

Running internal self-test reveals only one error: 4053, WB calibration validity..

No other issues reported. Running WB Wlinearity self-cal (lucky me, it's the only calibration available which does not require fancy AC sources and gear).

After ~10 minutes, self-test pass without single error..

Is it like a fairy-tale and one of those repairs where nothing was broken?

Only one way to find out, connect ACV input and see what's out there.
* Set 5.66ACV 50Z on my trusty HP 3245A source, which gives pretty close to 2V with 50ohm load. Frequency for test is 100e3 Hz

2.122V? huh?  ???

What HP 3458A has to say about it?

1.9984V! But we never calibrated HP DMM for AC.

1.9962V from Keithley... Now i'm sorta believe those two folks more, not the Wavetek... Uh, oh..  :scared:

I ran manual step tests via Pathway section as well.

All steps pass with range stated in service manuals (Thanks to ManateeMafia for scans!), except one :
Test 4035 (PA22) : AC RMS Conversion - Linearity (10V)
Expected in manual result: 2.350 < x < 2.450 V
Unit reported : 0.242792
This one sounds somewhat important don't you think?

So I tested various input signals 1kHz from Time Electronics 9823 calibrator on various ranges.

2V on 10V range gives 2.007988V (K2001 reports 2.00053)
same 2V on 30V range gives 3.30198V. What?  :wtf:

Similar problem on 200mV range  :rant:

Few more steps:

Next step - you already know it  :-BROKE

Removed top cover. Ta-daaa...

Digital board and it's SMPS supply is located on rear top side, analog ADC circuitry board is located in front of it.
Wideband module fully enclosed in metal can box on the side of it.

Volt-nuts easily spot the magical parts - Datron main reference module with Linear LTZ1000 + Vishay foil network resistor in hermetic TO-package and traditional LT1013 in hermetic can as well.
It's same module used in Datron 1271/1281's. Lonely LM399 is used as auxilary reference to check drift and output levels of main one. AFAIK it's used only for self-test procedure.

More parts around.

Processor is good old friend, Motorola 68000.

Firmware lives here:

And battery bodged up by someone else already. Still reads 3.04V, so it's sorta ok. Will replace anyway...

And none of those calibration keylock trash, usual plain switch, easy! (maybe that's not native though?)

They are not joking, everything screwed down, harness tied and locked in place, overall very nice construction.

Analog board PSU is linear, using jellybean 78xx and 79xx regs + LM337. Pretty common. Capacitors are subject to be replaced ASAP :)

Digital board talks with analog world only via 5 optocouplers and FARNELL TESTED ribbon cable :)

Let's remove front panel cover.

Well it was like this, I swear. Nothing superglue can't fix though :) Also check those ties around connector, so FP keyboard interface cable would not pop out unexpectedly. Nice attention.

And - What's that screw doing there? It's just stuck to mylar. And I don't see any place where it could come from, it's too long!

Oh well..

Let's see some real stuff now.

WB module

Well, copper rigid coax, thank you very much. No that cable junk here.
AC 50 ohm load is nice too, nothing less than array of 3xx ohm Vishay S102C foil resistors. Make sure that load is stable!

Then we have some DIP packages around and output circuitry..

And surely, we have TVC cell under that metal can, nicely protected and soldered tight all around. I'm not brave enough for now to remove the shield for sake of photo :)

Now, assemble everything back in (wearing gloves at all times, yes) and let's see what waits for us on bottom side of the instrument.

And there are: dual transformers, very similar to 12x1 Datrons and fancy Fluke 8508A.

AC board with bunch of packages and MAT transistor arrays.

Plenty of free space too:

And famous "Paffner" time bomb. Will order new filter with caps too. I don't want fireworks!

Teflon material PCB! What's there? ACV main attenuator network and floating guarded preamp! Why Teflon? Because it provide very high insulation and good stable frequency responce.

Carefully remove the board to see the fancy voodoo...

Oho-ho.... Isn't that worth every penny paid? Look at those monster Vishay (AE back then) foil resistors!

They are just gorgeous..

Well. I can't write anymore, TBD :)

And here goes post with stuff I did not figured out yet  :-X


--- Quote ---VFD is unusual nice and emerald green. I actually dig it, looks way better contrast than Keithley's/3458A. Big digits too! I think it's same exact front panel as used in other 1271/1281/4950 Datrons.
--- End quote ---
Yes, that's true - Datron used same front paneƶ/display board for 4920, 1271 and 1281 and I guess for some other tools also.

--- Quote ---After ~10 minutes, self-test pass without single error..
--- End quote ---
Out of my experiance the tool need to warm up >2 hours before running the selftest - neded to get stable temperature.

As the selftest runs fine, the basic operation is ok, thus I assume some recap is needed.

I'm aware of usual precision gear warm-up times :) It was sitting ~3 hours, but no warming up would cause 1.3V difference of same input signal in two ranges  8)
Unless that's is "normal" for these AVMS, which I doubt!


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