Hello All, I am preparing to calibrate a 36 year old 8 channel reel to reel. It involves measuring a.c. voltages (audio sine waves at .300 volts). The service manual states that you must use a VOM with a resistance of 50k ohms per volt...since my VOM is no longer in operating condition, I am forced to use a DMM, which has many times the ohm/volt resistance. My question is...How will I use a DMM, and still maintain accuracy while compensating for the different load of the DMM? It is a critical item, because it effects playback, and record bias levels. I remember seeing, somewhere on the internet, a table showing what value of resistor needed to be paralleled with the test leads to compensate for the high internal resistance of the DMM. I think it concerned calibrating an old tube tester, but I can't find the resistance table. It seems that I have not been able to properly set up the machine using the DMM. Two failed attempts so far. Any suggestions?

There were some 50k ohm/volt VOM's, but that was on DC ranges only and I'm not aware of any VOM's that had that high of input impedance for AC. It's most likely at the time the tape recorder was made, they were looking for it to be tested with a meter made during that time period that would put the least load on the circuit as possible. What you are doing with the DMM should fill the bill, and 300 mV AC in the realm of digital meters should not be anything challenging to measure, provided the DMM you are using can accurately measure the AC at the frequencies being put in to it, which in the audio range it should if its anything decent. The failed setup is more likely due to there being something wrong within the recorder causing the discrepancy rather than how you are measuring the AC signal with a DMM. An oscilloscope would be the preferred tool of choice when measuring AC on items where frequency might be a variable with measurements.

-Mark-

Could it be 5KOhm per volt instead of 50?
RRM

Hey Mark, Thanks for your reply. I do have an oscilloscope...just ordered a user/service manual for it..RCA WO-535A..I know it's an old, earlier solid state scope, but it should probably do the job ok. My DMM is a cheap Radio Shack model. I don't have the budget for a Fluke, or a Tek O'scope, so I'm stuck with what I've got for now. The Teac R to R is in very good shape for it's age. The heads and tape path have been reconditioned and laser aligned. The fact that I have twice attempted to bias and calibrate it for one tape brand...(EMTEC SM911, which is a new replacement for the long obsolete Ampex 456)...sounds like MUD!, yet it sounds fantastic with a different tape type...(AGFA pem469)..also another long gone, and different spec'd tape, points to calibration error rather than defects in the electronics. I also want to thank the Retired Radio Guy for his response...I have checked both the user and service manual..the required meter is 50kohms/volt. Another item I don't own (but should) is a frequency counter. Any suggestions on a budget model frequency counter and DMM that will be accurate? Thanks again for your help.

You can use your DMM with a shunt resistor (connected to the meter lead tips by alligator clip, or across the meter lead connectors where they plug into the meter, whichever works easiest).

Sounds like you'd be on a 1 volt scale for what you're doing, if you had a multimeter, so for 50k ohms/volt, that would translate to 50k of resistance across the DMM leads to simulate a VOM. Go a little higher if you have to make a combo of resistors to get there, to compensate for the DMM's internal resistance slightly lowering the parallel combo of the 50k and the meter's internal.

And on those types of calibrations (audio), you really need to match the manufacturer's recommendations on impedance to get the right results. And by the way, don't use an inductive type of resistor, e.g. wirewound.

I'm not trying to be a wiseguy or anything, but was curious so I did a little experiment for fun. Using my HP 3325A I used a 500 Hz sine wave as a reference frequency with output set to 150 mV and measured it on the Fluke 8505A. It read .30382 VAC as it should not using a 50 ohm load. I then put a 50k ohm shunt across the leads, and the reading was .30342 VAC. I can't see a difference of 400 uV making any significant performance difference with the tape recorder, and that amount of change isn't anything you would likely even be able to see or interpret with an analog AC meter.

I repeated the test with my audio spectrum analyzer where I could change and examine the frequency quickly and measure the signal amplitude. I tried 40 Hz, 500 Hz, 1kHz and 10 kHz with the same results- the 50k shunt being in place across the leads made no significant amplitude measurement difference compared to when the measurements were taken without it in place.

-Mark-

But the tape recorder signal will probably be of a different source impedance than the signal generator. That is what determines how the level will change with different loads applied.

Vince

I couldn't agree more. Theory has value, but unless you know the particular audio component design in question well enough to make that call, follow directions.

Vince stole my words. Make it plus 1.

One issue about old tape machines using transistors that popped up back then
is DC ground potentials, (millivolts) that happened when relays grounded out heads
(erase and record) during playback. This feeble DC current screwed everything up because
every pass of the tape on play showed changes in S/N and weakening of highs. The damn
heads were becoming magnetized. Maybe this is why you cant set up and get balance .
Current is so high on a solid state circuit board, that DC drop happens on ground traces.

Ok....here are the specs. of the Teac reel to reel. Input -10db (.3v) impedance greater than 20K ohms. Output -10db (.3v) impedance greater than 10K ohms. The signal generator I used..Eico Model 377. Output..100 mw into 1000 ohm (internal) load. Output stage is a 6K6 cathode follower. Hope this clarifies things a little.

Looking at the recorder's specs for the impedances that we now know, I'll stand behind my replies. I did my experiment unterminated because I simply had no way of knowing what the recorder's source impedances were. I think they were looking for the measurement to be taken with a meter of greater impedance than the source impedance to be sure the meter used did not load the circuit- really that simple. In this case the difference in measurements between a 50k ohm/volt impedance AC analog VOM (if one even existed, about the highest I have seen is 25k/volt) and a typical DMM of today which has between 1-10 megohms of input impedance on AC is nearly indiscernible at audio frequencies. I beleive the problem being encountered here with the recorder is a variable other than the method of measurement being used. That the type of tape being used had the biggest bearing on the performance, I would center my attention to what the difference is there. I'm not an audio tape expert, but Radiotechnician might be heading down the right path.

-Mark-