Post by thread:[EE]: Microcassette mechanism interfacing

Hi, I've just salvaged a microcassette mechanism out of an old answering machine. Reverse engineering the motor segment was easy enough - run the motor in reverse and the mechanism switches into a fast-rewind mode, run it the other way and the mechanism switches into record/playback mode. I've also identified what appears to be an optoreflector on the controller board. It's situated under one of the tape drive pulleys, and has three leads. Two appear to be the LED anode and cathode, the third seems to be an output of some description. The sensor is SMD, in a clear case. I suspect all I need to do is feed it 20mA across the LED, then take the signal from the output pin, but I'm not sure if the output is open-emitter, open-collector or totem-pole (I suspect it'll be o/c). The big problem is interfacing the magnetic head to something. I don't have the foggiest clue what I need to do to record on the tape. I know there's supposed to be an AC bias signal fed to the record head and the erase head, but I can't find any concrete info on how to determine the frequency and amplitude of that signal. The same applies to playback circuitry - no info on output levels or required amplification. Does anyone have any schematics lying around that I could get copies of? Most of the stuff I've seen consists of one chip, the heads and a few passives. Something that's based on common ICs (standard opamps, transistors and passives) would be great. Thanks. -- Phil. | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB, spam_OUTphilpemTakeThisOuTphilpem.me.uk | ViewFinder, 10BaseT Ethernet, 2-slice, http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI ... If there's one thing I can't stand, it's intolerance.

{Quote hidden}>-----Original Message----- >From: .....piclist-bouncesKILLspam@spam@mit.edu [piclist-bouncesKILLspammit.edu] >Sent: 18 May 2005 09:54 >To: .....piclistKILLspam.....mit.edu >Subject: [EE]: Microcassette mechanism interfacing > > >Hi, > I've just salvaged a microcassette mechanism out of an old >answering machine. Reverse engineering the motor segment was >easy enough - run the motor in reverse and the mechanism >switches into a fast-rewind mode, run it the other way and the >mechanism switches into record/playback mode. > > I've also identified what appears to be an optoreflector on >the controller board. It's situated under one of the tape >drive pulleys, and has three leads. Two appear to be the LED >anode and cathode, the third seems to be an output of some >description. The sensor is SMD, in a clear case. I suspect all >I need to do is feed it 20mA across the LED, then take the >signal from the output pin, but I'm not sure if the output is >open-emitter, open-collector or totem-pole (I suspect it'll be o/c). > > The big problem is interfacing the magnetic head to >something. I don't have the foggiest clue what I need to do to >record on the tape. I know there's supposed to be an AC bias >signal fed to the record head and the erase head, but I can't >find any concrete info on how to determine the frequency and >amplitude of that signal. The same applies to playback >circuitry - no info on output levels or required amplification. > > Does anyone have any schematics lying around that I could >get copies of? Most of the stuff I've seen consists of one >chip, the heads and a few passives. Something that's based on >common ICs (standard opamps, transistors and passives) would be great. Simple preamp design at http://www.elektroindonesia.com/elektro/el0101a.html Can't help with the recording side. This is the kind of thing there used to be hundreds of DIY designs for in the popular electonic/hi-fi magazines many years ago but with tape well out of favour there is little interest these days. I guessing you aren't after "hi-fi" results from this? Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

In message <EraseME23075D38FE1C8144847DFAECA3565F2704E54E6Aspam_OUTTakeThisOuTpai-smx-01.europe.bkhm.net>> "Michael Rigby-Jones" <Michael.Rigby-Jonesspam_OUTbookham.com> wrote: > Simple preamp design at > http://www.elektroindonesia.com/elektro/el0101a.html That looks useful - thanks. > Can't help with the recording side. This is the kind of thing there > used to be hundreds of DIY designs for in the popular electonic/hi-fi > magazines many years ago but with tape well out of favour there is > little interest these days. I've found loads of references to magazines, but the magazines themselves seem to be long gone :( "Hobby Electronics", "Electronics Monthly" and ETI are three names that seem to get thrown around a lot... > I guessing you aren't after "hi-fi" results from this? Not even close. Something similar to one of those handheld tape recorders would be fine. I'm thinking about using it to store FSK encoded data, or maybe just speech. At the moment I just want to get some sound out of this thing :) Later. -- Phil. | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB, @spam@philpemKILLspamphilpem.me.uk | ViewFinder, 10BaseT Ethernet, 2-slice, http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI ... FANATIC: one enthusiastic about something you don't care about.

> > I guessing you aren't after "hi-fi" results from this? > > Not even close. Something similar to one of those handheld tape > recorders would be fine. I'm thinking about using it to store FSK > encoded data, or maybe just speech. > At the moment I just want to get some sound out of this thing :) The old Commodore Datasette used, IIRC (a long time since I had one apart), a 7406 or similar to drive the record head with logic. Playback was only a little more complex. With anything but the very poorest quality cassettes they were reliable, and the addition of an azimuth adjustment + signal strength LED meant you could "tune" them to any tape, eg commercial or another, mis-aligned Datasette recording

Some interesting usenet discussions on bias: http://groups.google.ca/group/rec.audio.pro/browse_thread/thread/17c77b8b1eda3520/b06338a999a09aa9 http://groups.google.ca/group/sci.electronics.basics/browse_thread/thread/ad231ea9e375433e/ This surprised me: Quote An historic side note: before there were all-in-one chips, cheaper tape recorders skimped on circuitry and eliminated the bias oscillator. Instead, they used DC bias techniques -- the same as you'd use to move a tube or transistor into its best operating range -- and ran the tape head in an always-positive or always-negative mode. This had the same effect on zero-crossing... but you can imagine what it did for s/n as the head became magnetized. Some early tube recorders (e.g., 1954 AC-operated Ampro) and transistor ones (mid-60's $30 Lafayette portable) had this arrangement. Dave Philip Pemberton wrote: > Not even close. Something similar to one of those handheld tape recorders > would be fine. I'm thinking about using it to store FSK encoded data, or > maybe just speech. > At the moment I just want to get some sound out of this thing :) > > Later.

In message <KILLspamqwf.scvfo.rKILLspamsgndj.aa.ij> Peter <RemoveMEplpTakeThisOuTactcom.co.il> wrote: > Tape playback preamp is in f.ex. LM387 datasheet. Recording is done > without ac bias in those things. Sometimes dc bias is used (resistor > from +Vcc feeds through head). The one thing I need to work out is the bias current. 5V Vcc, with a record/playback head impedance of 92.5mH. Same thing for the erase head - except that's 92.28mH (yeah, I know, big difference). > The opto is nearly certainly open > collector and needs a pullup (or pulldown!) between 10 and 100k. It is > used to sense tpe end. I figured it would be for something like that. Might be good for determining the rotational speed too. Speaking of which, I need to find out what speed these tapes are usually run at (in normal rec/play mode). Later. -- Phil. | Acorn Risc PC600 Mk3, SA202, 64MB, 6GB, spamBeGonephilpemspamBeGonephilpem.me.uk | ViewFinder, 10BaseT Ethernet, 2-slice, http://www.philpem.me.uk/ | 48xCD, ARCINv6c IDE, SCSI ... I've got 256K of RAM, so why can't I run Windows 3.1?

At 02:27 PM 5/18/2005, Philip Pemberton wrote: >In message <TakeThisOuTqwf.scvfo.rEraseMEspam_OUTsgndj.aa.ij> > Peter <RemoveMEplpTakeThisOuTactcom.co.il> wrote: > > > Tape playback preamp is in f.ex. LM387 datasheet. Recording is done > > without ac bias in those things. Sometimes dc bias is used (resistor > > from +Vcc feeds through head). > >The one thing I need to work out is the bias current. 5V Vcc, with a >record/playback head impedance of 92.5mH. Same thing for the erase head - >except that's 92.28mH (yeah, I know, big difference). If you go direct digital, you don't need to worry about that, and the record and play circuits are trivial.

--- Philip Pemberton <philpemEraseME.....dsl.pipex.com> wrote: > > The one thing I need to work out is the bias current. 5V Vcc, with a > record/playback head impedance of 92.5mH. Same thing for the erase > head - except that's 92.28mH (yeah, I know, big difference). > Well there are a few things to consider (ehh?). First are you performing FM or standard direct recording, and is your signal analogue or digital. If the latter there is a lot less to worry about. The bias is used to improve the low frequency response of the tape, at the cost of high frequency response. However if you are recording digitally you don't need it and the bandwidth of the tape is much higher due to recording in a narrow frequency range. > I figured it would be for something like that. Might be good for > determining the rotational speed too. Speaking of which, I need to > find out what speed these tapes are usually run at (in normal > rec/play mode). Sounds like you are looking at recording analog? 1.5" IPS for Cassette tape and 0.75 IPS for microcassette tape. The capstan is what you should use to determine tape speed (at least recording speed). Since this is actually fairly precise you can use it to determine tape position relatively well. Unfortunately fast forward and reverse would make that scheme untenable. You might be able to pulse the motor and measure the EMF generated by it when there is no power applied to acruately estimate the speed of the capstan. The worst problem with all tape mechanisms is the drive belts. I can't find replacements for anything anymore reguarding them unless I look at a restoration web site these days, Stephen R. Phillips was here Please be advised what was said may be absolutely wrong, and hereby this disclaimer follows. I reserve the right to be wrong and admit it in front of the entire world. __________________________________ Do you Yahoo!? Yahoo! Small Business - Try our new resources site! http://smallbusiness.yahoo.com/resources/

On Wed, 18 May 2005, Philip Pemberton wrote: > In message <EraseMEqwf.scvfo.rsgndj.aa.ij> > Peter <RemoveMEplpEraseMEEraseMEactcom.co.il> wrote: > >> Tape playback preamp is in f.ex. LM387 datasheet. Recording is done >> without ac bias in those things. Sometimes dc bias is used (resistor >> from +Vcc feeds through head). > > The one thing I need to work out is the bias current. 5V Vcc, with a > record/playback head impedance of 92.5mH. Same thing for the erase head - > except that's 92.28mH (yeah, I know, big difference). Something between 1 and 100kOhms should do it. The impedance is not relevant, shoot for dc current. The series resistor at the output of the recording amp can give a clue. Try a bias resistor between 10 and 0.5 times that resistor. >> The opto is nearly certainly open >> collector and needs a pullup (or pulldown!) between 10 and 100k. It is >> used to sense tpe end. > > I figured it would be for something like that. Might be good for determining > the rotational speed too. Speaking of which, I need to find out what speed > these tapes are usually run at (in normal rec/play mode). 1.2 or 2.4 cm/s . Imho forget about the solwer speed, the fast one is bad enough. In answering machines only 2.4 cm/s was used afaik. The motor usually has a built-in regulator, but only if not reversible. It used to acceptable to run it off straight dc with no regulation, the mechanical load being nearly constant. Peter

At 01:27 PM 5/18/2005, Philip Pemberton wrote: >I figured it would be for something like that. Might be good for determining >the rotational speed too. Speaking of which, I need to find out what speed >these tapes are usually run at (in normal rec/play mode). <catching up on email> Standard cassette tape runs at 1 and 7/8 inches per second (IPS). Some high-end cassette decks could run them at double speed (3.75 IPS). These are all based on the early tape standard speed of 30 IPS, 15 IPS, 7.5 IPS, etc. I've still got an old Phillips deck that runs from 15 IPS down to 15/16 IPS. Old broadcast logging machines run at that speed (15/16 IPS) as well. Many fond and some not so fond memories coming back - I used to look after all the tape decks (multiple racks full of Ampex 350 decks) at one of the local radio stations as well as the multi-track and 2 track mastering decks several of the recording studios here in town. Interesting to see what happens when the closed-loop servo controls go ballistic while shuttling full reels of 2" multi-track tape at full speed (not pretty). dwayne -- Dwayne Reid <RemoveMEdwaynerspam_OUTKILLspamplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 21 years of Engineering Innovation (1984 - 2005) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email.