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

[TiN]

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

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. 
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! 

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 

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.. 

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? 



Similar problem on 200mV range 



Few more steps:





Next step - you already know it 

[TiN]

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

[TiN]

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

[pelule]

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.

Yes, that's true - Datron used same front paneö/display board for 4920, 1271 and 1281 and I guess for some other tools also.

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

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.

[TiN]

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 
Unless that's is "normal" for these AVMS, which I doubt!

[quarks]

Bookmark
Looking foward to see the inside

[ManateeMafia]

I will get out my 4920 (non M version) for some comparison tests.

They should be nearly identical in most respects barring any options. I just need to get a suitable source set up 

Of note: my unit runs a POST at power on including ADC linearity. No warmup time required but the limits are probably pretty wide.

[Vgkid]

Went to see about reading a manual for this, and TiN's site is throwing an error :/ .

[TiN]

Of note: my unit runs a POST at power on including ADC linearity.

Same here, it does short version of selftest once powered.

There is plenty space to add some extra PCBs into chassis if desired too No noisy fans too, which also means no dust in unit. 
Ordering capacitors and Delta mains filter tomorrow at DigiKey. 

TiN's site is throwing an error :/ .

What error?

[TiN]

Uploaded pics in second post. Beauty! If anyone have more information about magic stuff there - do tell! 

[doktor pyta]

Great stuff
Do You have part of the schematic showing what is inside the metal can ?
Some day I will show pictures of my vintage thermal transfer standard...

[dacman]

A Cal Validity error means it's past its due date.  It's not a hardware nor measurement error.  My guess as to why the readings are off is that someone adjusted it that did not have the proper standards.  One clue is that the calibration label dates do not match the dates in memory.

[lowimpedance]

Nice bit of kit, in good shape inside but whats up with the outside !............. never ceases to amaze me the treatment such precision gear get. Well it is the 'M' version  .
Which also goes some way to explaining the 'spare' screw, just in case another gets lost you know  .
Anyway its still a winner. 

[ManateeMafia]

Simple test of 2v @1kHZ  on 10V range 2.000270 VAC

same signal on 30V range = Error Ur

It seems the fw will not measure below a certain range input value (10%?) but yours is off enough that you don't get that message.

[zlymex]

Enjoying the show, even for reasons of very nice and big photos, as always.

[dacman]

A footnote for the specifications for the unit is that the specifications are valid from 30% to 110% of range.  It has been my experience that it will under range at about 10% of range, although, 0.99 Vac to < 15.0 Vac is supposed to be a valid input for the lower calibration point on the 30 Vac range with 10 Vac being nominal.

It appears that the Ur point is 9% of range (2.7 on the 30 Vac range, and 0.9 on the 10 Vac range).

[Vgkid]

TiN's site is throwing an error :/ .

What error?

It was throwing a 404 error earlier, but it is working now.
That attenuator board is rather dreamy. Interesting to see the hermetic network in a sea of hermetic resistors.
Interestingly it appears that the meter is using the rather ubiquotous squaring + exponential amplifier in the process of rms conversion. But the more interesting side does not involve this...

[zlymex]

I got three questions:
- Are those big fat hermetic resistors also from Alpha Electronics just like the smaller ones?
- Was Pickering Electronics(marked on the black relay) related to John R. Pickering(of Datron) in any way?
- Are all those plastic foil resistors from Vishay?

[TiN]

1. I think so, BOM lists them as Alpha as well.
3. Did not check every single one of them, but ones I looked at are Vishay S102C's. 

Bit more reversing on TVC in WB module. It's manufactured by US company Best Technology which is still seem to be around, and have insulated UHF-type hermetic thermal converter used here. One used in 4920M is listed as insulated 5.0mA type, likely S.7 type, as TVC DC output amplifier is configured to provide signal gain of 867 (1 mV output from TVC will be amplified into 867 mV) by LT1028 ultra-low noise amplifier. Gain is set by fixed 8.66 k and 10  R203/R204 resistors.
Higher voltages, like 10mV would cause opamp to saturate due to it's +11VDC and -19VDC power supply. Unless my math is messed up somewhere.

[acbern]

At a theoretical 10mV output the TVC would already be destroyed. They are very sensitive, I cooked one just by a short switch spike of my Datron 4808, although I was aware of the potential issue. So should you do any tests, be very carefull. Also, not sure the S7 would be a correct replacement, more usual are 200ohms/V types, although of course this may be totally different here.

[TiN]

Here are my DUMP? constants output on 10V range:

4920M log
DUMP? Range 10.0 V, 0 = 001,+0.3000000E+01,+0.3000000E+01
DUMP? Range 10.0 V, 1 = 002,+1.0000000E+01,+1.0000000E+01
DUMP? Range 10.0 V, 2 = 003,+0.0000000E-40,+0.0000000E-40
DUMP? Range 10.0 V, 3 = 004,+0.5000068E+05,+0.8776189E-01
DUMP? Range 10.0 V, 4 = 005,+0.2000027E+06,+0.8704789E-01
DUMP? Range 10.0 V, 5 = 006,+0.5000067E+06,+0.8533230E-01
DUMP? Range 10.0 V, 6 = 007,+1.0000137E+06,+0.8916546E-01
DUMP? Range 10.0 V, 7 = 008,+0.0000000E+00,+0.0000000E+00
DUMP? Range 10.0 V, 8 = 009,+1.0000001E+00,+1.0818163E+00
DUMP? Range 10.0 V, 9 = 010,+0.9765597E+00,+0.0000048E+00

And here are good ones from calibrated 4920, read by fellow ManateeMafia.

Good 4920 log
DUMP? Range 10.0 V, 0 = 001,+0.3000000E+00,+0.4175232E+00
DUMP? Range 10.0 V, 1 = 002,+1.0000000E+00,+1.3915722E+00
DUMP? Range 10.0 V, 2 = 003,+0.9999817E+05,-0.2553497E-02
DUMP? Range 10.0 V, 3 = 004,+1.0000093E+04,-0.8196756E-04
DUMP? Range 10.0 V, 4 = 005,+0.2000019E+05,-0.3578179E-03
DUMP? Range 10.0 V, 5 = 006,+0.5000044E+05,-0.2259891E-02
DUMP? Range 10.0 V, 6 = 007,+1.0000088E+05,-0.8410558E-02
DUMP? Range 10.0 V, 7 = 008,-0.0001296E+00,+0.0002198E+00
DUMP? Range 10.0 V, 8 = 009,+1.0012110E+00,+1.1019141E+00
DUMP? Range 10.0 V, 9 = 010,+0.9765555E+00,+0.0000049E+00

Those three first values on the right shows me that calibration is lost 100%, as they are defaulted now.
Quick set-cal to Keithley 2001's reading allowed 4920M to show correct value, and updated constants as:

4920M log after pre-calibraiton
DUMP? Range 10.0 V, 0 = 001,+0.2997650E+01,+0.3263413E+01
DUMP? Range 10.0 V, 1 = 002,+1.0000350E+01,+1.0880032E+01
DUMP? Range 10.0 V, 2 = 003,+0.0000000E-40,+0.0000000E-40
DUMP? Range 10.0 V, 3 = 004,+0.5000068E+05,+0.8776189E-01
DUMP? Range 10.0 V, 4 = 005,+0.2000027E+06,+0.8704789E-01
DUMP? Range 10.0 V, 5 = 006,+0.5000067E+06,+0.8533230E-01
DUMP? Range 10.0 V, 6 = 007,+1.0000137E+06,+0.8916546E-01
DUMP? Range 10.0 V, 7 = 008,+0.0000000E+00,+0.0000000E+00
DUMP? Range 10.0 V, 8 = 009,+1.0000001E+00,+1.0818163E+00
DUMP? Range 10.0 V, 9 = 010,+0.9765597E+00,+0.0000048E+00

So, no need to worry about saving calibration NVRAM on this box. 
Seems like cal data was reset/lost, but functionally it have no problems (remain to be confirmed, it could have excessive noise/drift/whatever problems, not diagnosed by self-test).
Now we face bigger question - how to calibrate this unit..

[acbern]

I also bought one of these. Mine worked, said CAL invaild and it also seemed reading incorrectly. Battery was ok, replaced it anyhow (without loosing any data). Then I started to calibrate ist, using a Datron AC calibrator, just for the sake of checking. What was strange is that when doing the CAL sequence where you need to apply specific frequencies at each voltage step, it does calibrate, and is fine at these frequencies, but when I apply e.g. 100kHz instead of 200 (where it was spot calibrated), it is off by quite a bid (several percent). That is wrong.
So my recommendation is you do a CAL with an AC source based on K2002 or 3458A (preferred; SYNC mode) and check what happens when you test the frequency range. Maybe there are other issues, just as the ones I see. If not, it may be worthe sending it to CAL (if you cannot do it yourself). I use a Ballantine 1605 for this (if I ever get there).

[TiN]

Same seller? I see same thing. But that's expected, as flatness and other special cal constants are lost too. Neither of my 2002s or 3458 had AC calibration, so most trusted is K2001 which had full cal almost 3 years ago. I can calibrate HP 3245A (sources up to 20Vpk-pk 1Mhz or 200Vpk-pk 100kHz) but it's nowhere near required accuracy.

So in the end I'll likely poke local cal labs if they take this 4920M for full cal+adjustment, as I don't see much of other cost-effective choices here.

[pelule]

Have tried to upload the doc to KO4BB, but failed.
Thus I uploaded them in my Dropbox:
 - Datron 4920M_service_manual_Vol-1 & 2
 - Datron 4920 AVMS Users Handbook
 - Datron 4920 Design, Application and Performance
 - Datron 4920 Calibration

If someone have interest, here is the link (will be valid until Sunday).
https://www.dropbox.com/sh/8y662mm9ckdzzat/AACGJvTBm1Nxm1hD_TcC-vrha?dl=0

[lowimpedance]

Have tried to upload the doc to KO4BB, but failed.
Thus I uploaded them in my Dropbox:
 - Datron 4920M_service_manual_Vol-1 & 2
 - Datron 4920 AVMS Users Handbook
 - Datron 4920 Design, Application and Performance
 - Datron 4920 Calibration

If someone have interest, here is the link (will be valid until Sunday).
https://www.dropbox.com/sh/8y662mm9ckdzzat/AACGJvTBm1Nxm1hD_TcC-vrha?dl=0

The 4920M vol 1 and 2 are on KO4BB but not the others. Maybe have another go with those, anyway thanks for sharing.