Tascam 38 Input Cal (setting meter to "0")

[explodingPSYCH]

I'm attempting to follow the manual's instructions for calibration but one issue I'm having is being able to set the VU meter's to "0" with a .3v input (and output) signal.

I've checked the voltage of my incoming test tone and ensured it's at 0.3v. I then connected the oscope to the output of the tascam and fed it the tascam the 0.3v test tone. Then I set the input trim until it read 0.3v peak to peak on my oscope.

However, with this input trim, the meter trim pot (R141) does not have enough range to bring the needle past about -3. (I currently set the meters to read -10)

Am I missing something?

EDIT: I did some additional adjustments plugging the unit into one of the two mixers it will be plugged into while taking measurements. One is directly to my RME M32 AD pro converters and the other is my Midas Venice F32 both of which have a 10km impedance (instead of the 50k mentioned in the manual). With the RME plugged in, I was able to increase the input trim more and get a little bit closer to zero.

Not sure how to address this issue. Should I adjust the input trim to a different value to make up for the impedance difference?

 

[SkywaveTDR]

The very first thing you need to use is a proper dB meter. .3V is not the correct input and if you do the calculations using the A=20Log V formula a -10 dBV signal is 316mV. I never use voltages except where told to check for a certain value like in cassette decks that say a test point should be 245mV. Otherwise I always use dB figures on a leader LMV181A. All that other equipment does not belong in the calibration area as it like a I phone is not a correct piece to do this work with. This is why things can get more confusing.

There are some occasions and I have just finished a Tascam 32 today where the pot (Trimmers) were open so a trimmer I took out said 202 which should have an end to end value of 2000 ohms. Well these all measured open and so I put Bourns Cermet 5K trimmers in and they allow better range and better adjustment as they are ceramic substrate. There are a few occasion where I put different values of trimmers in as I know what I am doing and usually just move them up one notch so to speak. A lot depends on what parts you have on hand and I have a lot of cermet trimmers here- probably at least 1000 of them. Horizontal and vertical with wiper forward is the kind used in the thirty series. They make trimmers with center terminal back as well. So put in the right signal and terminate into 47K resistor provided the meter you have is 1M ohm input resistance, then it might make it otherwise it is time to put some cermet trimmers in.

 

[explodingPSYCH]

Sorted this out. I don't have access to a decibel meter yet, but I'm going to look into an affordable one....

There was a couple of issues. First was not loading the 50k. I put a 47k and 3.3k resistor on the hot of my lead to the oscope. Was still way off though.

The issue is that I needed to convert the -10dbV into peak to peak voltage. There are online calculators to do this, but it's roughly .89V peak to peak.

Once that was sorted everything made sense. I will likely come back and fine tune with a decibel meter when I get one, but until then, I'm hoping this will be close enough.

 

[SkywaveTDR]

OK, That is why my entire career I have used or bought the right equipment to do the job. I can remember when many years ago I bought the Leader LMV181A new and was like $795 plus tax.
On E bay as shops closed down they would sell used one for as low as $100. I bough some then too and also have a LMV185 which is a two channel unit- these are backup unit on two benches as I do now have two Audio Precision One plus Portables here. The first one cost me $5029 in 1997. The second one was $1100 last year. LFM39A is also a well sought after device for speed setting and wow and flutter measurements- This is a Leader wow and flutter meter. I have obtained some for as low as $39 but some were bought for parts. They are not so easy to get anymore as the supply of shop shutting down has dried up. There are some out there at $300 from used equipment sources. I don't use scopes for measurement most the time just phase alignment and then troubleshooting. I have digital DPO types scope on bench 2 and a quad trace CRT unit but also a Quad 2247 Tektronix scope on bench 1. Now days a Rigol scope that is digital and a storage scope can be obtained for less than $300 in the US. It is not the same as a Tek scope but with a 5 year warranty and being they all are made in China anyway why not?

 

[SkywaveTDR]

You would be surprised that I get questions all the time where people are using abstract methods to measure a signal. Why make the job hard on yourself as the job can be complicated the way that it is before adding the interface or DVM meter complications. I don't think the load is what the problem is but on 1 M ohm input impedance meters I put a 47K resistor across the input as the manual tells you to. The minimum load is said to be 10K while maximum load is 2K but most stereo equipment is set up with 47IK input resistance. So that is what I use- the level difference should not be that much different as the source impedance is usually pretty low around 100 ohms.