How to visualize the signal of a tape recorder head on an osccilloscope - Page 1
Forum section: electronics / projects

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: December 23, 2022, 11:52:31 pm
content: Dear All,I would like to visualize the signal of a magnetic tape reorder, as taken directly at the magnetic head.Is it possible ? What is the level of the signal there.I have tested directly with the oscilloscope, but could not see anything but noise.What would be a simple schematic to be able to visualize this signal ?Thanks for any suggestion.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 24, 2022, 01:50:32 am
content: I would say try to get it after the head amplifier if its possible, maybe try at x10 or x100 probe, the impedance could kill a signal ... or try to build some head amplifier to help

author: tooki
author_rank: Super Contributor
timestamp: December 24, 2022, 11:50:29 am
content: According to this head catalog, the voltage at 1kHz is anywhere from 0.04 μV to 0.65 μVhttp://www.jrfmagnetics.com/Nortronics_pro/nortronics_silver/Nortronics_silver_catalog.pdfSee also:http://www.jrfmagnetics.com/and click the Magnetic Heads link. (Stupid frames-based site makes it hard to link to a specific page.)By the time you’ve used a 10:1 probe, you’re looking at voltages far, far too small to see on an oscilloscope. A typical oscilloscope can go down to 1mV/div. So if your head outputs, say, 0.3 μV, then it’s divided to 0.03 μV. Supposing a typical scope’s 10 vertical divisions and 8 bit ADC, that’s 25 steps (LSBs) per division. 1 μV/25 LSB= 0.04 μV/LSB. So your signal is less than one single LSB of the ADC. Effectively, nothing your oscilloscope can see.You need to amplify your signal (a lot).As for your original question: you want to visualize the signal from the head… well it’ll just be what’s on the tape. It’s an analog audio waveform. You can see the same thing by playing back the same audio from a PC or any other audio source. Or open a digital file of it in Audacity.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 24, 2022, 12:26:11 pm
content: Quote from: JacquesBBB on December 23, 2022, 11:52:31 pmWhat kind of magnetic tape do you want to probe, and with what kind of magnetic heads do you use?For the audio tapes and audio cassettes, the signal is in the uV range, way too small for direct probing. At most you can increase the playback speed, that would give higher voltage but still too small for direct probing.However, if you want to read the magnetic record on bus tickets, credit cards, etc. those are usually using harder magnetics, with much stronger fields. That is possible to probe directly with the oscilloscope, and if you swipe fast the signal can get as big as 10-100mV, directly from a cassette tape magnetic head, no amplifier.I know that because it happens that only a couple of days ago I've clean my wallet from expired papers and subway tickets. Here the subway tickets have a magnetic stripe, and they are printed on a thin cardboard (about 0.5mm thick paper the size of a credit card). It happened that on the same workbench I've seen two items neaqrby, the expired subway ticket with its magnetic stripe, and a bag with spare cassette heads that were already on the bench. That gave the idea to try to see the pulses on the subway ticket. I've cut away the magnetic stripe and swipe it fast over a mono cassette head, out of curiosity. Got very strong and clean pulses of about 10-50mVpp.LATER EDIT-----------------Still have them on the bench, so I've repeated the experiment, took some screenshots and made a new topic about it:https://www.eevblog.com/forum/projects/fun-with-flags-stripes-bare-hands-reading-a-magnetic-card-subway-ticket/

author: wasedadoc
author_rank: Super Contributor
timestamp: December 24, 2022, 01:56:38 pm
content: Analogue audio tape: the signal at the LINE OUT socket is just a massively amplified version of the signal from the head and with some relatively simple frequency shaping.Analogue video tape: typically (eg VHS, S-VHS, Betamax, Video-8, Hi-8) the video signal from the head(s) is a relatively constant amplitude carrier of several MHz which is frequency modulated with wide deviation by the luminance component of the video.

author: tooki
author_rank: Super Contributor
timestamp: December 24, 2022, 02:14:48 pm
content: Quote from: wasedadoc on December 24, 2022, 01:56:38 pmFor VHS/S-VHS: That’s how the hi-fi audio works. But the standard mono audio track is a simple linear track of baseband audio. (Same for the obscure linear stereo version.)Betamax’s audio situation is basically the same as VHS. The only difference is that I don’t know whether linear stereo was ever popular in Betamax.Video-8 always uses FM audio, Hi8 also supports digital audio.

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: December 24, 2022, 02:22:19 pm
content: Thanks to all for your very informative answer.I understand that the level of the head is much too small fro my scope. I will need to design a small amplifier to visualize the signalat the head of the tape recorder ( here it is a Denon UDR-F10) cassette recorder.Just a few precision :My problem is not to read the tape, but to find the fault of my tape recorder. Before tracing the signal through the board, I wanted already to be sure af its existence and amplitude at the head.Then I found that it was interesting by itself to try to get the signal directly at the head of the tape recorder. I have various opamp here. Do you think an instrumentation opamp as an INA128 would be a good idea ?

author: RoGeorge
author_rank: Super Contributor
timestamp: December 24, 2022, 02:49:16 pm
content: Playback heads are very reliable, never seen an interrupted one. At most they wear out mechanically. With time, the magnetic tape will dig into metal/glass of the heads. If so you should see the dent/dug channel with a magnifier. Even so, the heads will still give about the same signal, just with less than normal high pitch frequencies (muffled audio).If you touch the head pins with a finger you should hear huge audio hum from mains. Find a schematic and start by checking all the DC voltages are OK. Also, cassette players used to have mechanical switches that may cover in oxides over time. Make sure the defect is not from faulty contacts. Then you can start debugging by schematic.Apart from amplification, for audio tapes you need a bandpass filter with a specific curve response, called RIAA cassette tape corrections. These are slightly different depending on the cassette tape magnetic materials. The player can read some wholes poked in the back of a cassette tape and thus identify the correct correction curbe to apply for each type of tape. Noise is also a big problem, since the signal from heads is very small.TL;DR- it is not possible to visualize the audio from the heads by directly probing with an oscilloscope- you already have the best heads amplifier in the cassette player which you want to repair anyways, so no need to make another preamplifier

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: December 25, 2022, 05:29:01 pm
content: Quote from: RoGeorge on December 24, 2022, 02:49:16 pmI understand, but I checked the signal after. this preamp, and it does not look right. I would like to compare with what I can get with a simple preamp after the tape head. I am thinking of using an instrumentation amplifier as the INA128.

author: floobydust
author_rank: Super Contributor
timestamp: December 25, 2022, 05:40:06 pm
content: Another approach is to put a tape head degausser near the head, that will generate fairly large AC mV sine-wave.Rarely I have seen it damage a head preamp IC. Treat the head like 1/2 a transformer. Most tape head degaussers are a solenoid-style inductor.Tape heads usually fail open-circuit due to the very fine magnet wire. This is not common though, you can measure and compare L/R ohms with both channels. Note this magnetizes the head from the multimeter DC current.Making a test head preamp is difficult due to the very high gain required. Lot's of noise pickup and oscillations.

author: wasedadoc
author_rank: Super Contributor
timestamp: December 25, 2022, 05:42:42 pm
content: Quote from: JacquesBBB on December 25, 2022, 05:29:01 pmDoes it sound right? If not, in what way? Both left and right or only one? Do cassettes recorded on another machine also not sound right? Do cassettes recorded on this Denon sound right when played on other machines?

author: wasedadoc
author_rank: Super Contributor
timestamp: December 25, 2022, 05:45:54 pm
content: Quote from: floobydust on December 25, 2022, 05:40:06 pmDo not measure head resistance unless you have the means to demagnetise immediately afterwards.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 25, 2022, 05:48:28 pm
content: Quote from: JacquesBBB on December 25, 2022, 05:29:01 pmWhat do you mean by "it does not look right"? In what way you find it wrong, and how do you expected it to be instead. Post a picture or an oscilloscope screen capture. How do you test it? What signal do you use? Does it sing at all?- comparing won't help- INA128 is not a good choice, there are dedicated ICs for magnetic heads, like e.g. LM381Ahttps://www.waynekirkwood.com/images/pdf/National_AN-64_LM381.pdfRead the datasheet, will give you some explanation, schematic and will answer your title question, too, (heads signal typical 800uV at 1kHz). Keep in mind that the level is proportional to the frequency, that's the main reason you need those RIAA corrections, and you can not use a fixed amplification opamp.- however, if you prefer to make your own preamplifier, that's a nice projectgo ahead and search for schematics for inspirationhttps://duckduckgo.com/?q=schematic+cassette+tape+preamplifier&t=ffab&iar=images&iax=images&ia=images

author: JacquesBBB
author_rank: Frequent Contributor
timestamp: December 25, 2022, 11:12:45 pm
content: Quote from: wasedadoc on December 25, 2022, 05:45:54 pmThanks, but too late. I already checked the resistance of both channels. I found 150 ohms on one side and 152 ohms on the other side.At least I know that the head is OK.

author: floobydust
author_rank: Super Contributor
timestamp: December 25, 2022, 11:56:17 pm
content: For auto-reverse cassette players, I have seen the (head) switches or flex cable go open-circuit. So the head was fine but the connection from it to the preamp was bad.I would say look at your preamp circuit next.