Hello again everyone,
So, after real life getting in the way of my project, I am back playing with the RT-707.
This time after using some spacers 20 thousandths of an inch thick to raise the rear of the head deck plate, I decided to do some simple recording tests. I had already attempted to perform the Azimuth alignments but was unhappy with how uneven the forward playback head seemed to be. I had a great deal of trouble trying to get the Azimuth set, followed by the EQ calibration. I found that the EQ levels were no longer very flat at the top end, particularly seeing variations between left and right channel which had not been in evidence previously.
I tried three different tape types, recording a 1KHz tone at about -3dB on them and watching and listening for variations in levels. I found that the thin tapes, BASF LP35LH and Mulann LPR35 performed well, but the older Quantegy 456 was a little uneven its levels between left and right channel, with right channel being down on the left. Whilst the Mulann SM911 was really uneven in its levels, this time left channel was strong and right channel was noticeably weaker. This suggested to me that the 20 thou of spacer was actually too much, and so I dismantled it all again, this time making up spacers of some 10 thousandths of an inch.
Here is one of the thinner spacers on the top of the rear head plate mount.
So after reinstalling the head stack, I then went back and re-performed all the Azimuth alignment and recording EQ adjustments. This time the Azimuth setting was a lot better and more even, and the EQ was also much improved although I still noticed a little left/right channel difference at the very top end at 12.5KHz, 16KHz and 20KHhz with right channel being a little down on level over the left channel. But the level differences were very small in millivolt levels and lets face it, at those frequencies you are not going to hear it. Between 1KHz and 10KHz everything was nice and flat and even, with once again the observed boost in levels at the bottom end below 1KHz, just as before.
I then tried some recording tests with a 1KHz tone at -3dB on the same set of tapes. This time the BASF LP35LH, Mulann LPR35 and the Quantegy 456 all performed well with nice even levels and very little variation between left and right channel. I was pretty happy with this and felt that the 10 thou spacers gave a much better overall alignment that seemed to be verified by the Azimuth, EQ and simple recording tests.
So now it was time to move on to the Recording BIAS and remaining adjustments. Before doing this however I did re-perform the Dummy Coil and BIAS Trap adjustments, and verified that everything was still good there.
First thing I needed to do though was choose what type of tape to setup the bias for. I have no idea what the original Pioneer STD-502 tape was, but presumably it was some form of high bias tape. Originally I had wanted to use the Mulann SM911 as the 'high end' tape for setting up the bias, but in light of the issues I am still having with this tape tracking across my heads I have instead chosen to use Mulann LPR35, which is the same formulation as SM911 but on a thinner tape.
As any of you who have followed Smurfer77's thread will know, he discovered that the manual for the Recording BIAS adjustments seems to contain some errors, or typos perhaps, with regard to the function of the BIAS Adjustment VR's numbered VR303 and VR304. Smurfer discovered that the instructions for dealing with these VR's is back-to-front. He found that when the manual said to turn the VR's Clockwise to increase the BIAS, in actual fact you needed to turn Anti-Clockwise to achieve that outcome. Refer to these posts:
here
To ensure that I understood what was happening in this regard on my machine I set up my scope to actually check the function of the BIAS VR's.
Sure enough, turning the VR's Clockwise does actually DECREASE the bias level. Here you can see that my Left Channel (Yellow Trace) has dropped in level when I turned the VR fully Clockwise. This is opposite to what the manual says!
So I setup my little Signal Generator.
And here I have it outputting 1KHz at -10dB (approx 316mV)
Then you set the Line Input pots to give an output level of -7db (450mV)
And then set the VR303 and VR304 BIAS Adjusters to give the peak level on each channel whilst Recording and monitoring the Playback with the Millivolt Meter. My left channel peak BIAS was about 480mV with the right channel being about 490mV
I found the Milliolt Meter tricky to read when you are looking for changes of 0.2dB, which is only about 9mV (according to my calculations), so I used to DMM to help out. At 1KHz it is quite accurate.
Here it is showing the peak BIAS level on my left channel.
So after finding the peak level for each channel I set the overbias on the BIAS VR's by turning them Anti-Clockwise until I got values around 470mV for the left channel and 480mV for the right. These adjustments affect one another so you need to repeat them a few times.
I then setup to perform the Record/Play Frequency Response test. For this test you need to input a 1KHz signal into the machine, followed by adjusting the Line Input pots to give and output level of -7dB (450mV). The manual then says you need to reduce the input level by 20dB without touching the Line Input pots. At first I wondered just how I was going to do that, but closer inspection of my little Signal Generator showed that it had an attenuation switch that would drop the level by 20dB, perfect!
You can see the attenuation switch in this image.
You then proceed to record the 1KHz signal and take note of the level from the tape output. Then you input a 15KHz signal, still at the -20dB level, and compare the output level. Than manual says that the level deviation from 1KHz to 15KHz should be -3dB +1.5dB or -1.0dB. Now this is where I ran into trouble, at these sort of levels we are only talking about very small millivolt measurements. My 1KHz -20dB output level was about 36mV left channel, and about 40 mV right channel.
Here is the left channel level at 1KHz -20dB.
(You can see that the Millivolt Meter and the DMM agree pretty closely, even at these levels)
And here I have changed the scale on the Millivolt Meter.
When I measured the 15KHz -20db output level the Millivolt Meter looked very similar at this scale, but the DMM was clearly out of it's depth at 15KHz and could not be trusted. The DMM said 9mV, which clearly does not match the Millivolt Meter.
As you can see, the Millivolt Meter level looks very similar to the previous image.
And again with the Millivolt Meter scale changed.
But at this scale we can see that the 15KHz -20dB signal has actually increased!
At these sorts of levels, just what reading would -3dB be if the 1KHz level was 36mV? I thought that 1db was 44.7mV, and that calculation seemed to work well at the 316mV (-10dB) or 450mV (-7dB) levels. 450 - 316 = 134mV = 3dB so 1dB = 44.7mV. Even when doing the BIAS adjustment I calculated 0.2dB as 9mV, with 44.7mV as 1dB. But am I actually correct here?
But just what should I expect down at levels around 40mV, clearly it can't be a difference of 134mV (3dB), that's crazy, so I have somehow misunderstood just what to expect here.
Also, what should I do about the fact that the 15KHz -20dB signal is stronger on the tape than the 1KHz -20dB signal. The manual says it should be -3dB from the 1KHz -20dB signal, and if not to once again adjust the BIAS VR's. I tried altering the BIAS VR's as they say in the manual, but I was having to move them too far before anything would happen. I then had to go back and re-perform the BIAS adjustment at 1KHz back to the 0.2dB overbias.
So I'm a bit lost now, I have read and re-read Smurfers thread on this subject but am none the wiser. Just what should I expect to see for these readings. I can see that Smurfer just turned his VR's anti-clockwise to get a -1dB deviation, but I am not sure just what to do now.
I would appreciate any input from you guys.
Cheers.
EDIT: OK, so I have just used Google to lookup some info on Decibels. This quote caught my eye:
"Are decibels linear?
Decibels express a power ratio, not an amount. They tell how many times more (positive
dB) or less (negative
dB) but not how much in absolute terms.
Decibels are logarithmic, not
linear. For example, 20
dB is not twice the power ratio of 10
dB."
So obviously my 'calculations' used in this calibration exercise are probably way off. No doubt I have demonstrated just how much of a novice I am in falling for this, and not realising that dB is a ratio.
I wonder how one can calculate how much difference -0.2dB is from 480mV for example, or how much -3dB is from 36mV. No doubt there are calculators out there, I just need to find one.
. As GPS says, you may expect a bit of odd or varying behaviour as the tape settles into the new position. Let us know how it goes.
