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Author Topic: possible mod for the HM 2140 Heathkit Meter  (Read 2844 times)
N8NSN
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« on: July 10, 2009, 11:01:01 AM »

A few years back I acquired an HM-2141 for my VHF gear.  I like the way they look and the way they work.  Good design.  I like the 2141 with the peak reading capability.

I would like to acquire an HM 2140 for my HF gear.

  The only draw back for the HM 2140 (HF) is that there are only two power settings of 2000 and 200 watts...

I run a great deal of QRP CW from an old HW-5400.  It is my main QRP rig for now.

I would like to know if anyone has found a mod to convert the 200 watt reading in the meter to a 20 watt reading through the circuit.  And perhaps convert the 2000 watt to 200 watts.  Or perhaps leave the 200 watt selection alone and drop the 2KW section to 20 watts.

Am I correct in thinking this would require a resistor change in the coupling unit or perhaps on the circuit board internal to the meter chassis?

*** Any one who has any ideas on this or has performed this modification to the HM 2140 would be greatly appreciated for some response.  

I'd like to have the matching set of the meters...  2140's go for relatively inexpensive prices...

Thanks,

Jim N8NSN (former kc8byf)
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N0XE
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« Reply #1 on: July 10, 2009, 12:20:37 PM »

It may not be super accurate, but close enough,  not sure on your 2140, but on the HM 102 wattmeter you can set the meter in the SWR position, then input a known and accurate 10 watt source to the meter, turn the SWR forward control until it reads 100 on the 200 watt scale, (this represents 10 watts)   You now have a range that can be used as a Qrp wattmeter, and will show a higher reading up the scale and much more readable then using the normal wattmeter side which the scale leaves a lot to be disired for low power use, yes your in the SWR mode. I have done this with my HW 8 and my HM 102 and seems to be pretty close. I compared it to my OHR WM-1 Qrp meter and it was surprisingly close.  When I applied my known 2 watts out from my HW 8, sure enough doing it this way showed an almost perfect reading of 20  on the 200 watt scale and that would  mean two watts and  a reading of 5 watts would show 50 and so forth. This by the way is described  in the HM 102 manual for using the HM 102 for Qrp use, I see no reason why this same technique would not work with the HM 2140 as well. Yes there is probably a way to actually modify it , but why if this will work?, give it a try, 73 Jim N0XE
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N8NSN
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« Reply #2 on: July 10, 2009, 01:38:43 PM »

Thanks Jim,

That is a very nice feature for the 102.

The 2140 has separate SWR/Reflected metering from the Forward/Watts out meter.  (2 individual meters)  

The 2140 has 8 separate functions

SWR has

SET / FWD   and  SWR / REF

PWR has

PEP / AVG   and  LOW / HIGH

and of course the selectivity variable resistor for the SWR / REF meter set.

The SWR meter set has no affect on the Power meter at all.

Good info on the 102 though, thanks

Jim N8NSN
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N8NSN
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« Reply #3 on: July 11, 2009, 07:11:12 AM »

Are there any ideas specifically for the HM 2140?

I have looked all over the internet for the schematic on the 2140...  Maybe if I can look over the schematic in detail I can understand how this particular part of the circuit works.  Then perhaps, find a solution to modifying the meter for QRP levels.

Nowhere on line is a DL'able PDF manual and schematic to work through this...  Any help would be appreciated.

73,

Jim N8NSN (former kc8byf)
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N8NSN
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« Reply #4 on: July 11, 2009, 07:39:37 AM »

I have managed to finally find the schematics for the sensor unit and the meter circuits at

http://www.tech-systems-labs.com/

Would anyone care to do some online exchange of brain storming?  I need some help to understand this circuit and exactly where to get into it to transfer the 200 watt scale to a 20 watt scale...

I am certain that if we manage to come up with a good mod for this that posting it on the QRP reflectors will have a lot of positive results.  The HM 2140 and HM 2141 are really nice meters.  The 2141 already has a 30 watt setting that is very accurate for the VHF QRP work here.  The prices people are asking for QRP capable meters on line are simply ridiculous.  Especially so for the people who attempt to limit their monetary input into the radio lifestyle and like to mod things to boot.  Not to mention that the HM 2140 and 41 are just cool looking units.

Any QRPers out there?

Any QRPers want to work on a mod for a HM 2140 in QRP mode ops?
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K4DPK
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« Reply #5 on: July 11, 2009, 10:50:45 PM »

I think those manuals have been removed.  When I downloaded it, the folder was empty.

But....

Typically, a wattmeter has a pickup medium (often a current transformer) a diode for rectification to low-level DC, and a series resistance to the meter.  This series resistance is the multiplier in what is essentially a DC voltmeter circuit.

You can make a 20 watt meter from a 200 watt meter just by reducing the value of the multiplier resistor, providing the current transformer is sensitive enough.

The multiplier is often a fixed resistance in series with a trimpot (variable resistance), and it may be you only need to reduce the value of the fixed resistor and then re-calibrate with the trimpot.

73s and GL

Phil C. Sr.
k4dpk
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N8NSN
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« Reply #6 on: July 12, 2009, 08:21:08 AM »

Hi Phil,

Great stuff, Thanks :-)

I have a good response from a post I placed at QRZ dot com in the Q&A section, as well.

Strange the link I pasted for the schematics did not work there...

I have both of the schematics saved in a zip folder for the coupling unit and the meter circuit as well.  If you need a copy of them I would be more than glad to send them to you.  I can likely switch their format to a j-peg and send them if you wish...

I am new at this type of mod.  I have never modified a meter in any capacity.  Would the reference resistors be in the coupling unit or in the meter circuit board, internal to the meter chassis?  I have over looked the schematics briefly and am in the learning process of how these circuits perform their work.

73,

jim N8NSN (former kc8byf)
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N8NSN
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« Reply #7 on: July 12, 2009, 08:32:12 AM »

here is a link directly to the "schematics page" from TSL...

http://www.tech-systems-labs.com/heath_schematic.htm

I scrolled down to the HM 2140A and there is where I found the schematics for the sensor circuit and switch circuit.

If my thinking is correct I am lead to believe this mod would be done in the sensor unit?

73,

Jim N8NSN (former kc8byf)
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KA5N
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« Reply #8 on: July 12, 2009, 09:13:35 AM »

This is just a simple Ohms law problem.  Select the range you want to change and divide by the value of the range you want to change to and select resistors of that ratio.  In fact it would be simple to put in a switch to select ranges.  
About the only caveats is that the diodes in the circuit have about .3 volt forward drop, so you can't go to too low a range.  Probably be just fine at 20 watt range.  Also you will have to calibrate the new range against a calibrated wattmeter (you could also use a calibrated oscilloscope for calibration).
Allen
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N8NSN
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« Reply #9 on: July 12, 2009, 10:25:26 AM »

Thus far it would seem we are getting exactly where we want to be.  A HUGE thanks to everyone for all of this help.  I love learning.  Also this is a topic in the QRZ dot com Q&A forum.  I am following both threads and saving all the replies as I would like to make an article of this mod after it all works and make photos of the procedures as well.  Crediting all replies in the article will be done as well, if each person is OK with that.

I have a further query on this topic for everyone...

Looking at the Sensor Schematic I am seeing:

LOW REF CAL followed up by X-101 @ 2200 ohms

HIGH REF CAL followed up by X-102 @ 22K ohms

HIGH FWD CAL followed up by X-103 @ 100K ohms

LOW FWD CAL followed up by X-104 @ 4700 ohms

So far I have ascertained that this mod will
be done in the sensor unit?

Am I correct in thinking that I will need to modify both the
FWD and REF resistors for both meters to read accurately?  If so:

I am thinking that if I install a DPDT switch
in the sensor unit to facilitate 2 each resistor
values for both the FWD and REF section of the
LOW end of the circuit; that this may be the
correct mod.  Huh

After the correct resistor values are found I am wondering
if recalibrating the variable resistors of
R-104 for LOW REF CAL and R-107 for LOW FWD CAL
on the QRP settings, if this is going to make the
standard 200W settings inaccurate...

If this would make the standard 200W settings no
longer calibrated; my thinking is that a permanent mod
to QRP would be more feasible.  Then, in the event of
possibly selling the meter either sell it as modified
for QRP or switch it all back to factory.

Am I on the right track with the mod being performed
in the sensor circuit?

Any thoughts?


Here is the circuit schematics link
just scroll down to HM-2140A:

http://www.tech-systems-labs.com/heath_schematic.htm

Again, Many many thanks, & 73,

Jim N8NSN (former kc8byf)
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KA5N
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« Reply #10 on: July 12, 2009, 11:02:09 AM »

Replace BOTH the fixed resistor AND the variable resistor in any case.  Example: if you desire changing a 200 watt range to a 20 watt range then replace the fixed resistor with one that is 1/10 the value of the original do likewise with the variable. Then calibration for one range doesn't affect the other range and you can swap ranges with a DPDT switch.
Allen
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N8NSN
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« Reply #11 on: July 12, 2009, 05:47:55 PM »

Hi Allen,

Hope no one is getting aggravated with all my questions...

What I am gathering is that I will need to make a small circuit board with two variable resistors and two fixed resistors on it.  One set of a fixed and a variable for the FWD calibrations and the other set of a fixed and variable for the REF calibrations.  Then cut the traces before the original FWD and REF component feeds, run wire from the in bound side of the cuts to the centers of the DPDT, Run one side off the DPDT to the new circuit and then off the new circuit to the out bound end of my trace cut on the opposite side of the original variables and fixed(s) trace cut  and then the other side of the DPDT to the original circuit?  So, I would need to cut traces on both sides of the original circuit and make in essence the capability to use the original circuit and a secondary route or bypass and return loop to and from the new QRP calibration circuit... Hard to read but easy to see in a schematic representational drawing in my mind...  However, it seems I may need two DPDT switches to open and close the out bound side of the original circuit... Or even better a 4PDT switch. *** Am I heading in the correct direction?

Man!  This is fun stuff!
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N8NSN
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« Reply #12 on: July 12, 2009, 06:46:19 PM »

LOW REF CAL followed up by X-101 @ 2200 ohms

HIGH REF CAL followed up by X-102 @ 22K ohms

Obvious, there is a multiple of 10X here ^^^^^
on the fixed resistors

The variable resistors for the REF High and Low are a multiple of 10X @ 10K and 100K


HIGH FWD CAL followed up by X-103 @ 100K ohms

LOW FWD CAL followed up by X-104 @ 4700 ohms

However, this is not a 10X multiple ^^^^^
closer to 21.28X for the FWD meter

As well the variable resistors in the High and Low FWD circuit are off set at 20K Low and 100K High.  A multiple of only 5X

Seems like the 10X theory in the REF circuit is good as a bowl of beenie weenies at a camp fire. But, on the low to high ratios in the FWD meter reading circuit I am coming up with mixed ratios of 21.28X fixed resistor value (high to low) and 5X for the variable resistor values (high to low)... Should I stay with these ratios or just drop everything in the QRP mod circuit to -10X of what is factory in both variable and fixed for the FWD circuit?


If staying at the -10X principal, for the QRP circuit I am thinking a 470 fixed and a 2000 ohm variable for the FWD reading meter...

Then, a 220 fixed and a 1000 ohm variable for the REF meter in QRP mode.

Does this appear to be correct?

Then... I have a question, not for the modification process, but rather just to "know the answer".

I have noticed that there are fixed capacitors in parallel with the variables for the HIGH REF and FWD calibration variables, but no parallel caps on the low end FWD and REF variables...  That's a plus in my mind to not have to mess with the parallel caps in the low side modification.  I do wonder, just for educational purposes; why the parallel caps on the HIGH side variables...?  Any reasoning I may learn to understand here?

Allen, and all others in this thread, have you fellas looked at the schematics for this sensor (coupling) circuit?

We're on a roll !

Jim N8NSN (former kc8byf)
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KA5N
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« Reply #13 on: July 13, 2009, 05:31:25 AM »

One more time.  The farward resistors are higher than the reverse because the reverse voltage is lower than the forward voltage.  VSWR is a ratio between the farward and reverse voltages.  The power you wish to indicate is set up by resistors.  The ratio of powers (between existing and what you desire) is the ratio of the resistors required and has nothing to do with the ratio of the farward and reverse sets of resistors!!!
You only read forward power not reverse power.  Yes I have looked at the schematic and like I said initially this is a simple Ohm's law problem.
P=E^2/R  amd I=E/R.  Since this wattmeter has a peak reading response, the amplifier that follows the front end takes a current and presents it as a power reading or an SWR reading.
Allen
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N8NSN
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« Reply #14 on: July 14, 2009, 08:34:58 AM »

Hi Everyone,

I am understanding this circuit much better now :-)  I appreciate all the input.  I want to thank you all.  A special thanks is in line for VK1OD, Owen.  The articles you sent me are exactly what I wanted to read.  What great information you have offered on the subject of linearity and/or the lack there of in a multi level FWD RF POWER circuit.


I am looking forward to taking on the project and feel much more confident to do so.  The article to eventually be put together will take some time. I want to ask, formally, if everyone who has offered input to the original questions is OK with me referencing their input within the article.  Is this OK?  I have saved all information offered along with any links to other educational sources to do this.

I have to find a decent deal on a set of HM 2140 or HM 2140A meters now.  This may take time, as well.

73,

jim N8NSN (Dayton, Ohio)
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