Toa EPM 104 impedance meter

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi All,
Does anyone have the calibration instructions for an impedance meter made by TOA? The model is EPM104, the ZM104 may be similar. Mine is reading high at as you go up the scale. I tested it against two resistor standards. It may be in spec, but I'd like to optimize it rather than make a correction graph.
Schematic would be cool. I could trace it but I'm lazy.

-Chris
 
Chris,

I believe I have one of the meters in my service van. Does it have X's 1,10,100, and 1000 buttons and a cal button on it? The Toa is a very primitive inaccurate meter. It will only measure at 1K.

I tried about 15 years ago to get Toa to send me out some information on the meter and they wouldn't do so. I ended up calibrating the meter using a Sencore impedance meter with loads verified by the Sencore. There is a pot for every range. Using plain resistors doesn't work well. Use a transformer with a speaker attached (plain 70volt speaker of commercial variety). I will call Toa today and see if they will send me some cal information on it.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Joe,
Thanks, you're very kind! Is a schematic available?

I did attempt to calibrate the meter myself using fixed resistors and did manage to improve it.
The Toa is a very primitive inaccurate meter.
The big problem is that the readings vary with battery voltage! :mad: It is a handy little beast and allows easier tracing and troubleshooting.

When you are looking at an amp rack (or four), it allows for very quick troubleshooting. It's accurate enough for that - just!

-Chris
 

Audi O

Member
2011-11-16 10:23 pm
OK, so I know that this thread was started over 6 years ago & is probably no longer of interest to the original posters!......(but...)

I found this thread whilst searching for an answer to the same question, "how to calibrate my (similar) ZM-104". I could see that there are 5 preset potentiometers inside the meter which most likely are there in order to facilitate calibration, but I was reluctant to start adjusting them without some reassurance from another source. In particular, I needed to know which 3 of the 5 pots were used to adjust the 3 switchable ranges.

I got my hopes up towards the end of this thread, only to find that, disappointingly, no-one posted back with any info - so my trawl through the vast depths of the internet for an answer continued!

After wading through numerous sites which either just sold the meter, or reproduced the irrelevant but rather useful application sheet written by Syn-Aud-Con & found on the TOA website;

http://www.toaelectronics.com/downloads/reviews/TOA_SAC_ZM-104.pdf

I eventually struck gold & found the zm-104 service manual, which contains a circuit diagram &, in section 4, the much sought after calibration instructions.

In case anyone else, like me, comes upon this thread looking for the calibration info, here is where I found it. The manual is free, & the download link is towards the bottom of the page. Enjoy!
 

Audi O

Member
2011-11-16 10:23 pm
It doesn't download for me.

Strange. It works fine for me, & I also notice that it has been downloaded a couple of times since I last posted here, so I assume that others have had no trouble. I could have posted the direct download link here, but the webmaster may not like it, so I just linked to their webpage.

Just to be sure you are doing it right....

Right-click on the text "To download the file, please, click here !". Then click "Save Target/Link As".

Any more problems let me know, & I'll sort out something else.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Audi O,
Thank you for that. I really like the accompanying article you ran across.

I ended up calibrating my unit via a trial and error method. I got it much closer than it was, but I'm never comfortable unless I can use the factory procedure. One thing I did notice is that the calibration is affected by the battery voltage. I confirmed this using a variable power supply instead of batteries. What this means is that the EPM 104 is an approximate reading device, so don't assume any great accuracy from it. It does a fantastic job of tracing PA speaker lines and showing improperly loaded amplifiers. That is far more common than you want to know about (overloaded amps).

I stopped looking after I had mine set up, so thank you very much for posting the manual.

Hi O.B.F.,
Downloads fine. The manual is at TOA ZM 104 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

-Chris
 

Audi O

Member
2011-11-16 10:23 pm
You're welcome, anatech.

Thanks for the tip on variation due to battery voltage. I'll be interested to see (probably after some testing) whether the 3 ranges become equally "inaccurate" as the voltage varies. That is, whether the 3 ranges still all give the same (inaccurate) reading, or whether 1 range becomes more inaccurate than another. It seems that a similar inaccuracy between all the ranges would be easier to remember & make allowances for, as the batteries become weaker.

I also have an Adastra digital audio impedance meter. I find this easier to use because it has additional features like a large illuminated display, & a reading-lock etc. It's main limitation for me has been the lack of sufficient audio output power, needed when I use it as a quick audio source at the head-end, to test the location & correct working of speakers connected to a large installation. They are hard to hear, particularly in a noisy environment (construction site or pub, for example).

The TOA ZM-104 was recommended to me as a much better meter, so I got this meter second-hand. The only downside to this cheap purchase was that I immediately noticed that 1 of the 3 ranges was giving a completely different reading to the other 2 - hence the need for calibration!
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Audi O,
I have a few calibration resistors I keep in the box. This allows me to check the rough cal. in the field as the batteries die down.

I also dealt with very noisy environments, like waste treatment plants. For situations like that, I use a 60 watt amp driven by the meter (after I check the line impedance). One of the Bogen TPU series are excellent for that. They tend to be light and compact. The features these amps have can be pretty handy too. You can hang one on the backboard quickly thanks to the flanges.

To be honest with you, I'd rather stick with the various Bogen products and be done with it. The problems and call backs with some other brands aren't worth it for me. If a client wants to use scrap, used or bargain amps, I'll get the wiring in, phone interfaces and what ever else, test and certify the cabling and let some other poor guy deal with crappy amps. Music shops tend to be the worst, so I allow them to under cut me, install fan cooled amps (rather than convection) and wait a couple years to install the right stuff after the client as figured out the truth. :devilr:

Life is too short for games.

The most important resistor to keep is a 100 ohm unit. 5% is fine as the meter isn't going to be much better anyway. If you can, attach these to the inside of the case so you can clip on and measure without worrying about dropping the darn things. Probably three values will let you know where you are. That's it, easy.

-Chris
 

Audi O

Member
2011-11-16 10:23 pm
Good advice, thanks.

Now I'll be keeping a few resistors in with the meter - though I'm guessing the wire-wound variety might over-complicate the issue because of their inherent inductance!

If there's already a working & connected amp on site, & I just need an audio source for doing the testing, more & more often these days I just use my mobile phone as that source. It's always with me anyway, so it just means one less piece of equipment I have to carry with me.

In fact, not only can it provide audio by playing mp3's, but with the right apps installed, it can now often replace several pieces of equipment. The latest app I'm currently experimenting with on my Android smartphone can provide a complete audio toolkit (albeit not as good quality as dedicated pro equipment) with functions including;

Signal generator (sine, square wave, white noise etc) spectrum analyzer, speaker polarity checker, SPL & RT60 meter, chart recording etc.

The only thing I need to remember to do first is to mute certain other functions while I'm doing some of the tests, otherwise, for example, everyone on site may get to hear my ringtone at full volume through the speaker system if I get an unexpected incoming call!
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Gaetan,
No, you couldn't use the ZM-104 to tune bass reflex speakers. As Burnedfingers said, you need a decent low frequency oscillator, high output would be a bonus. You also need a jig with about 1K0 in series with the speaker system. Take the signal from across the speaker, and another right off the generator and input that in XY mode into your oscilloscope. You use this to detect for phase information. The input from across the speaker will give you an impedance curve. You would hunt for the two maximum values and the minimum value. Add some math and shake wildly for some idea of the current tuning. A sealed box or free woofer will give you one maximum.

Of course, if you ran into an HP 3580A spectrum analyzer, you could sweep the box and look at the impedance curve. You could also use a sweep oscillator and use the ramp output to drive the horizontal of a digital storage scope to essentially get the same type of picture (an envelope instead of a trace). I've done this in every way I mentioned. THe most useful is the 'scope and oscillator with a meter to put a number to the peaks and valleys. Use a 10 R resistor to generate a calibrated sweep. This is where using an amplifier bus you more headroom and easier cal ratios (like 1:1 for example). I made a box that had the cal resistor, leads, switches and jacks to connect all the equipment in one neat package. I did a lot of work with that stuff.

-Chris