Author Topic: HP 4274A LCR Meter - mini teardown  (Read 25639 times)

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Online Jay_Diddy_B

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HP 4274A LCR Meter - mini teardown
« on: March 02, 2014, 11:15:20 am »
Hi,

I have mentioned the HP 4274A LCR meter several times in my discussion of ESR Meters. I bought a second one today, so I thought I would share tips on buying this instrument and some pictures of the insides.

This LCR meter was manufactured in the 80s, by a joint venture between Hewlett Packard and Yogogawa. So you will see a little different style than HP equipment of the same vintage.

This is a high precision LCR meter 0.1% basic accuracy and 5 1/2 Digits of resolution. With option 001 or 002 you can measure capacitors with a dc bias voltage. The accuracy specifications are fairly complex. The accuracy becomes lower at high and very low impedances. I have attached a section from the Operators manual giving the accuracy.

The particular unit that I have photographed was manufactured in 1987.


Buying Guide

If you do a quick check on eBay you will see that most of the units are reporting Errors. This is quite normal  :D but can be used to your advantage to get a good price. The unit I am showing here I bought for $250.00


If you plug a unit in with nothing connected, you will get a display like this:



For it to work properly, the Lcur must be connected to Lpot, Hcur connected to Hpot and all four of the BNC shield connected together.

Here is a picture of the same unit with my home made Kelvin leads attached:



Also the Bias selector switch on the back should be set to OFF:



Jay_Diddy_B


« Last Edit: March 02, 2014, 11:18:29 am by Jay_Diddy_B »
 
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Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #1 on: March 02, 2014, 11:23:53 am »
Hi,

So let us have a look inside:



You can see the power supply section and the card-cage assembly.

This is a shot of the card-cage. The colour-coded ejector tabs are classic HP:



This is the A1 Range resistor and Null Detector Assembly:



With shields removed you can see some read relays and some high quality analog circuitry:



Here is close-up of the relays:



Jay_Diddy_B

« Last Edit: March 02, 2014, 11:28:46 am by Jay_Diddy_B »
 
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Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #2 on: March 02, 2014, 11:35:06 am »
Hi,
More pictures of the 4274A.
This is the A3 Power Amplifier Assembly, the circular black components are potted transformers:



This board is called the A4 Process Amplifier Assembly:



All these boards contain the Yogogawa Hewlett-Packard Logo:



The microprocessor board contains a Lithium battery to save the front panel settings in NVRAM:



Jay_Diddy_B
« Last Edit: March 02, 2014, 11:47:57 am by Jay_Diddy_B »
 
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Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #3 on: March 02, 2014, 11:43:26 am »
Hi,
To get repeatable accurate readings, the HP4274A uses a kelvin test fixture. The Kelvin test fixture can cost as much as the LCR meter.
Here is the 16047A installed on the 427A measure the Capacitance and ESR of small electrolytic:



Here is a close-up of the test fixture:



The inserts in the test fixture can be changed for radial and axial components:



Jay_Diddy_B
 

Offline rstoer

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Re: HP 4274A LCR Meter - mini teardown
« Reply #4 on: March 02, 2014, 12:04:30 pm »
Good information. Thanks.
 

Offline Vgkid

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Re: HP 4274A LCR Meter - mini teardown
« Reply #5 on: March 02, 2014, 12:08:44 pm »
Thanks for the teardown, where are the range resistors?
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Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #6 on: March 02, 2014, 12:23:51 pm »
Hi,
The range resistors are on the A1 assembly. Here is a photograph:



There are 5 covering 10, 100, 1K, 10K and 100K with dc and ac trims.

The resistors are marked TKD, this the Yogogawa influence.

Jay_Diddy_B
 

Offline Vgkid

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Re: HP 4274A LCR Meter - mini teardown
« Reply #7 on: March 02, 2014, 01:36:05 pm »
Im suprised to see that the resistors have a trim ability, i  figured that it would have tbrown off the freq response.
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Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #8 on: March 02, 2014, 01:47:53 pm »
Hi,

Here is the schematic from the manual:



The trimmers have a +/- 1% adjustment range. The trimmer capacitor are marked 'phase'. This is to make them resistive.

The 4274A is a 100 Hz to 100 kHz LCR meter. I also have the 4275A which covers the range 10kHz to 10 MHz. It would be interesting to see how different this section is for 10 MHz operation.

Jay_Diddy_B

 

Offline Vgkid

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Re: HP 4274A LCR Meter - mini teardown
« Reply #9 on: March 02, 2014, 02:24:03 pm »
It would nice to see a comparison between the two models. Im surprised they didnt just use one precision thin film resistor for each range. Also those are massive units.
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Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #10 on: March 02, 2014, 02:44:15 pm »
Hi,

One of the features of the 4274A is that you can adjust the test signal amplitude. The maximum current is 100mA. So this probably why there are some big resistors in the Range resistors.

Jay_Diddy_B

 

Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #11 on: March 02, 2014, 03:00:47 pm »
There is an interesting article on the 4274A and 4275A in the HP Journal:

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1979-02.pdf

Jay_Diddy_B
 

Offline dc101

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Re: HP 4274A LCR Meter - mini teardown
« Reply #12 on: March 02, 2014, 04:13:29 pm »
Nice pics man, looks in great shape.  Do you know the reasoning behind the potted transformers?  I've never seen them before.  Is it similar to how they have transformers in oil?
 

Offline Vgkid

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Re: HP 4274A LCR Meter - mini teardown
« Reply #13 on: March 02, 2014, 04:23:07 pm »
Going by the app note they should be for dc blocking in the signal path.
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Offline casinada

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Re: HP 4274A LCR Meter - mini teardown
« Reply #14 on: March 02, 2014, 06:50:22 pm »
Looks that those PCBs are pre silk screen days. All the labeling is etched on the PCB just like the circuit traces.I wonder who was the first to implement silk screens on the PCBs :)
 

Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #15 on: March 02, 2014, 10:18:28 pm »
Looks that those PCBs are pre silk screen days. All the labeling is etched on the PCB just like the circuit traces.I wonder who was the first to implement silk screens on the PCBs :)

This unit was described in the HP Journal in 1979. The schematics are definitely hand drawn. There is a high probability that these boards artworks were generated without a CAD system. I see some evidence of the boards being taped.

Before CAD systems were used, the process of designing a board:

1) The artwork was drawn with red and blue pencils. By tradition red was used for the solder side and blue was used for the component side.

2) Pre-cut paper donuts were used for the pads. Bishop Graphics was one of the main suppliers of these pads. The pads were placed on a sheet of Mylar.

3) A second sheet of Mylar was then placed on top. Red and Blue tapes were then run between the pads.

4) This artwork was normally generated twice actual size.

5) The artwork was then reduced to 1:1 with a process camera. Coloured filters were used to separate the top and bottom layers.

6) The boards were then fabricated from the films.

Generating a silkscreen layer was done, but it was time consuming.

Jay_Diddy_B

 

Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #16 on: March 03, 2014, 03:01:33 am »
It would nice to see a comparison between the two models. Im surprised they didnt just use one precision thin film resistor for each range. Also those are massive units.

Vgkid,

Here are a few pictures from the 4275A, the 10 kHz to 10 MHz model.

Front Panel
Here is a picture of the 4275a measuring the capacitance and ESR of a small electrolytic capacitor:



The 4275A agrees with the Jay_Diddy_B ESR meter adapter  :-DMM



So the 4275A must be good  :-+

Overview

This shows a similar construction to the 4274A. This unit has option 001, 0-35V dc bias source:



A1 Reference resistor assembly

With the shields in place:



Without the shields:



This is a close-up of the reference resistors. The resistors are similar to the resistors in the 4274A. plastic tubing has been used space the resistors of the ground plane. There is also a more complex circuit for ac trims:



Jay_Diddy_B




« Last Edit: March 03, 2014, 03:07:33 am by Jay_Diddy_B »
 

Offline Vgkid

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Re: HP 4274A LCR Meter - mini teardown
« Reply #17 on: March 03, 2014, 03:11:33 am »
Thanks for  the added pics.
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Offline dc101

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Re: HP 4274A LCR Meter - mini teardown
« Reply #18 on: March 03, 2014, 05:33:48 am »
Going by the app note they should be for dc blocking in the signal path.

Sorry, I should have been more clear.  I was curious what the potting was for on the transformers?  Vibration?  Heat?  They look pretty cool, I thought they were massive caps at first.
 

Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #19 on: March 03, 2014, 07:09:48 am »
Sorry, I should have been more clear.  I was curious what the potting was for on the transformers?  Vibration?  Heat?  They look pretty cool, I thought they were massive caps at first.

I don't much about the inside of the transformer.

It would make sense if they were toroidial cores. If they are toriodial transformers the plastic cup can be used to keep all the leads aligned.

The potting compound would seal the transformer from moisture.

It would prevent the transformers from being microphonic. This would prevent vibrations, especially from the fan, interfering with the measurements.

I am sure it is not for power dissipation, these are low power signal transformers. 

Jay_Diddy_B
 

Online BravoV

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Re: HP 4274A LCR Meter - mini teardown
« Reply #20 on: March 03, 2014, 01:41:46 pm »
The Kelvin test fixture can cost as much as the LCR meter.

Few years ago I was offered this exact fixture for about $150 nib, but I was not interested since I thought I need to have the LCR meter 1st which is hard to get. I guess I could make few pennies by reselling it at fleebay if I bought it ?

Offline dc101

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Re: HP 4274A LCR Meter - mini teardown
« Reply #21 on: March 03, 2014, 01:46:08 pm »
The Kelvin test fixture can cost as much as the LCR meter.

Few years ago I was offered this exact fixture for about $150 nib, but I was not interested since I thought I need to have the LCR meter 1st which is hard to get. I guess I could make few pennies by reselling it at fleebay if I bought it ?

Maybe, $150 seems to be what they're selling for these days.  There's a couple people offering a buy it now price for more than $300, so I suppose it depends on how long you can wait.  Jay's is in really good shape so something it that condition might go for slightly more.
 

Online Jay_Diddy_B

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Re: HP 4274A LCR Meter - mini teardown
« Reply #22 on: March 05, 2014, 12:53:58 pm »
The Kelvin test fixture can cost as much as the LCR meter.

This was an approximation. There are some fixtures on eBay for $150.00. These are only include one of the three sets of inserts. You have to be lucky or spend a lot of money to get one with all the inserts.

The BNC connectors on the front of the HP LCR meters are spaced 22mm apart. This spacing has been adopted by other manufacturers. You can also by other test fixtures like this one for around $60.00.



The point I was making, an LCR meter with the fixture is worth considerably more than an LCR meter without a fixture.

You can find many examples of Kelvin leads like these on eBay:



These have to be modified to work with the HP 4274A and HP 4275A. The shields have to be connected together at the clip end of the cable:



Jay_Diddy_B
« Last Edit: March 05, 2014, 12:55:38 pm by Jay_Diddy_B »
 

Offline CaptnYellowShirt

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Re: HP 4274A LCR Meter - mini teardown
« Reply #23 on: March 05, 2014, 06:24:10 pm »
Awesome teardown!

I was under the impression that the HP meters used the "Four-terminal pair" (4tp) DUT configuration. Its similar to the shielded four-terminal method, and both leads will function when hooked up. The difference with the pair method is current is carried on the shields of the Hcur and Lcur terminals, while the Hpot and Lpot terminal shields are just that, shields.

Check me on this though...
 

Offline TinkerGDS

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Re: HP 4274A LCR Meter - mini teardown
« Reply #24 on: January 02, 2017, 05:08:04 am »
I found this thread very interesting.  I just picked up a 4275A in very clean condition.  When I first turned it on I initially got the battery error.  I suspect the A9 battery is dead.  I want to replace it, but the service manual has no information on what type of battery this is.  Also, the service manual schematic shows the battery as 2.8v, but the battery on the board is 3.9v.  I see in your A9 board pic, your battery is also an ElectroChem.  Is your batter also the BCX-72-AA (3B64)?  I removed the battery and found out it is a Li-Ion battery.  Has anyone replaced these with a compatible type?
« Last Edit: January 02, 2017, 05:45:26 am by TinkerGDS »
 


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