On 3/19/2017 1:07 PM, Michael Thompson via cctech wrote: >> >> Date: Sat, 18 Mar 2017 21:46:21 -0500 >> From: Jay Jaeger <cu...@charter.net> >> Subject: Re: LINCtape/DECtape Head Alignment >> >> Curious: How are you measuring the signal from the head? Do you have >> an honest to gosh differential probe, or are you using some other >> technique? (If you have a differential probe, then the TU56 manual >> indicates that you should see 10mv-12mv (the addition of the two paired >> heads together), so as a first guess I am guessing you are looking at >> the coils one at a time. >> > > We used the procedure in the TU56 maintenance manual, and used two G888 > modules to make the equivalent of the G500 described in the manual. The > G888 modules really cleanup the high frequency noise mixed with the head > signals. >
Actually, I finally found a way to measure the heads and do it repeatably. Fortunately, my scope can go down to 500uv/division (5mv/division with the 10x probes). First, updating to the latest software on my DS2072A helped - but just a little. Getting the settings just right - not all of them immediately intuitive - helped more. (BTW, if you do not have a DSO, they are WONDERFUL. If you haven't seen one of these things in action, go to the EEVBLOG on YouTube My normal analog Tek scope now has a tear in its eye. Poor thing - it served me well. I got my scope during a time when Rigol had a promotional offering, so I got the decode features and a few other things that one might normally pay extra for. I got my from Saelig and they offered a discount to match that offered by other vendors who join the EEVBlog *forum*). https://www.youtube.com/watch?v=_TSr9nFN1GU Here is what I learned over the past couple of days: 1. I set the bandwidth limit on the input to 20MHz. So this limits the bandwidth before it even gets to the Math processing and reduces the sample rate. Maybe this gives the 'scope more time to do other processing? Anyway, this made a VAST difference. I can still see the waveform without doing this, but it is attenuated and more quantized. 2. At times I did not have the 10x multiplier on the input channels set to match the probes, which meant my signal readouts were 10x off. This was another important mistake that lead to incorrect readings. (D'oh) 3. I then used the "Math" function on the scope and select a low bandwidth filter, and set it to 50Khz (more or less matching the TU56 manual's 60Khz identification of the Tek module they used). This gets rid of almost all of the noise (I live less than 2 miles from a bunch of TV and radio towers!) This way, I reliably get decent signal readings off of the heads. In fact, in this configuration, I could even set the Math to A-B and sometimes see the wave as the manual offers intended, though it was chock full of noise (unfortunately and understandably, the scope can't do both low filter and A-B at the same time.) (It might even be that I could use the scope to capture the data, and then post process it, but what I have now is just fine. It was helpful that I had reasonable confidence in the head to begin with. JRJ
