Lapping is a Myth? | More diagnostics for cassette magnetic heads
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- Опубликовано: 2 июн 2021
- In my experience lapping - the physical wear down of the surface of a magnetic head that touches the tape - doesn't happen to cassette heads in the same way it does to the heads for higher speed, higher tension, open-reel formats. But they do fail - in this video I show how this failure manifests, as well as more tests you can using bits and pieces from your scrap pile to distinguish between faults with the magnetic heads, and other areas of the system.
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Very clever fabricating your own test equipment to repair portastudio tape heads. Love it. 👍
You are kind...I actually got the idea from a circuit bender's RUclips video but I can't find the link
Excellent presentation! I could and will listen for hours, since I just subscribed...
Great timing on this video I have eliminated everything electrical so I will be looking at the tape path next. In the normal run of things is it just vertical movement that will need adjustment in regards to the erase head.
Yeah I think so....I’m always cautious about giving a definite answer because although I’ve probably got more experience with this stuff than most folks who watch my videos I’m still trying to figure it out myself! You could also check the head assembly raises without obstruction, that was the issue with the marantz at the end of the video.
Tetrakan Supermonobloc thanks for the videos, whilst I am handy enough with a DMM and soldering iron there is no way I would have the ability to figure out all the steps to troubleshoot the problem.
Manual lapping by hand is not a proper approach because even 0.2 mm deviation makes significant difference. Particularly with 1/4" open reel pro decks like Revox/ Studer. A jig is highly recommended with a USB microscope to observe abrasions and angular depth
Wear on cassette heads does happen, mainly because (1) many home cassette decks use cheap, soft heads, and (2) because the pressure pad presses the tape into the face of the head. But quality ferrite heads do not wear much, it's true. I hope you're demagging those R/P heads after testing them for continuity.
For context I've never worked in a shop or had formal academic training with electronics, and I'm figuring out the cassette stuff by myself, pretty much because I couldn't find any resources about it when I started trying to repair portstudios about six years ago. Yeah I demagnetise heads as a matter of course once I've established all 4 segments are intact, though to be completely honest, ive never, ever noticed a positive difference in performance after doing so! Ive read hearsay about Phillips doing a study which concluded demagnetising was unecessary for cassette format players and recorders, though ive never seen the study itself, only people alluding to it in forums etc. So im curious, what would you anticipate happening if someone ran a continuity test on the coil in a head then failed to use a degassing wand afterwards?
@@Tetrakan Hey, we all started out with no experience or education, so don't let that bother you. Plainly, you're a lot further along than most folks who find this stuff interesting. I started out repairing audio gear in 1977, so I've been at it for a while.
Anyway, the current from an ohm meter or continuity checker is DC, and - may - be enough to magnetize the core of the head being checked. If it does magnetize the head, then each time you run a tape past it, it - may - shave a bit of the highs off the recording. It all depends on the amount of current coming from the tester, the coercivity and retentivity of both the head's core material and the tape you're playing, and how many times current is run through the head in the same polarity.
That said, the current from a DMM is likely to be much lower - and therefore less problematic - than the current from an analog VOM, or worse yet, a mechanic-style continuity tester with a light bulb and battery. These last can actually burn up the fine windings in a head. I check heads with a DMM's ohmmeter function all the time, with no problem, but then I always demag a tape deck after doing anything that brings tools near the tape path, mainly due to the fact that even tools that are nominally non-magnetized can still have a weak magnetic field around them.
As far as demagging cassette decks on a regular basis, there's probably no need to in the case of a two-head deck, because the bias (when recording) will demag the R/P head, and the erase current will demag the erase head. As long as nothing is introduced to the machine that can magnetize other parts of the transport (like the cheapo rotating-magnet "demagnetizers" that record shops used to sell in the '70's), all is well. In a three-head deck, though, the play head never sees bias current, and leaky coupling caps (if present) in the front end of the playback preamp can magnetize the play head. But this can only be truly fixed by replacing those leaky caps.
Keep up the good work!
I appreciate the detailed response. Yeah all the multitrack units I work on are 2 head units and I've only ever used my DMM for continuity test, so I guess that explains why I haven't heard a big difference after using the degaussing wand, but I hadn't realised that the current from VOMs was so much more significant than a digital multimeter - I'll be sure to mention that caveat if I'm discussing the topic on youtube in future, thanks again.
@@Tetrakan No problem. The current from a DMM is usually in the low microamp range or less (except on diode check, where it can be higher). With VOM's the current through the device under test has to be enough to move the meter movement directly, and so on a cheap 1,000 ohms-per-volt VOM the current can exceed a milliamp, and that's possibly enough to cause a problem.
@@ScottGrammer about the need for demag: I had a NAD 6040 deck with Dolby (B) / HX. If not demagnetizing for a while, the recordings included very low frequency thuds along high frequency signals. It was 90's when I looked at the rec signal with scope and saw the bias changing according to the high frequency content. Checking now, it's called adaptive biasing and the original Dolby HX was rejected by the industry because of its inherent flaws.
To the point: bias signal wasn't enough to keep play/rec head demagnetized. Now thinking if the HX caused magnetization... Anyway, the low thuds in the recording did not appear on freshly demagnetized head, so this has been my personal experience and justification for demagnetizing. To be fair, on any other tape players I can't say I've proved the benefits of demagnetizing. I've just assumed it's good practice.
Thank you for this video, for some reason videos where the magnetic head is taken apart are really hard to find. I'm working on a project that involves building my own tape-recording head, do you know of a video where they take apart the magnetic tape recording and reading head?
Very good video, some useful tips. Test equipment brought to you by Boards Of Canada.
Hey there, off topic but.. when you test the op amps in these tascam units, do you do it with the power on? Thanks for all the videos!
Hi, yeah. I avoid testing linear power supplies ( portastudios with an internal supply ) with the power on unless it's absolutely essential because there is some risk of electrical shock or damaging components if you short something with your probe, but most things after the power supply can be tested safely with the power on, and on the case of active components like opamps it's impossible to test them without power. My basic process is read the serial number on the chip and Google the data sheet so I can see which pins are the power rails and which are the audio inputs and outputs (plural because usually a chip will contain two discrete opamps) then it's 1) check the opamp is receiving power using a DMM 2) check ac audio is reaching the input and 3) check the audio is exiting the output at an appropriate volume and without distorting - either a oscilloscope or an audio probe can be used for steps two and three...I find it's s bit quicker and more intuitive to use an audio probe. If step one fails, troubleshoot the power system, if step 2 fails, your issue is earlier in the audio signal path. If step three fails, replace the opamp. If all three tests pass, the problem lies later in the audio signal path.
@@Tetrakan ok thank you!
Is it possible to repair a faulty erase head coil?
dude thats a badass marantz. is it nice? I rarely see them in portable mtr form. I have the regular pmd222 and its a sturdy machine.
Thanks for your videos, I'm restoring a porta one and you're a total guru. So the erase head on continuity measures 9 ohms on channels 2 and 4, and open circuit on channels 1 and 3, in fact these two channels don't erase properly. So I'm afraid the coils are open, there's no way to repair it right?
Thank you. Take those continuity tests at the base of the head in case it’s the connecting wires that are giving you an open circuit result, but if you’re still getting that result there’s nothing for it but to obtain a replacement erase head unfortunately. A healthy replacement head could be taken from any unit with that GEC generic transport in it though - at worst you might have to make small erase bias adjustments - so look out for parts donors on eBay and Craigslist, you could probably take a couple of bits from it and sell it on for most of the money to another repair hobbyist who needs other parts from it? Good luck!
@@Tetrakan Yes I checked directly under the head and reflowed the solder joints on the head pcb but still open. Since Ch 1 and 3 share the same part of the circuit, I'm guessing in the past something shorted and DC fried the coils, oddly I can't find no shorts now but I'm changing all electrolytics just to be safe. Thanks for your reply and advice on finding a replacement
@@Tetrakan also since you work on a lot of these, is it common to find open heads?
I’ve come across it on maybe a dozen heads over a couple of hundred units. 50/50 erase heads vs play record heads.
for this type of tests, i like to use the cassette to aux adapter you get with 5 bucks on internet :D the one i have, had a 4 track head which is perfect
hi at last someone done this as a yourtube video about time bob
Try telling that to the record/playback head on the Pioneer CT-S310 I picked up to refurb recently. I realise that it's 31 years old now, but there's a 0.2mm deep gash across the head from tape wear and it's well beyond the point where lapping could save it.
I've lost track of how many cassette heads I've had to change over the years due to excessive wear. All heads wear like this eventually in my experience. It's just a matter of time.
Ok. Your mileage may vary. This is specifically a multitrack cassette recorder channel - I've refirbished at least 300 MCRs and have yet to encountered heads that require lapping. Whether that's due to lower hours on these units or different materials vs. consumer stereo decks, I can't say.
Sir I have head with 4solder points and I have to connect aux with it please tell the connection 🤟🏻❤️
I'm sorry I don't understand. If you contact me via Facebook you could show me a photograph of what you mean?
@@Tetrakan thanks sir but I got the solution 👍
Спасибо! Мой Tascam 424 mk2, в гараже долго стоит, надо доставать, пока не заржавел....
ha ha ha i had one i got a better test meter with khz for a £12 i was using the red thing alot it stopped working
and i was talking to a ex bbc audio tec i know told me what to get
hi out there i have otari soundcraft 760 deck i will say the head are very hard there is no wear on any of my deck
i only got them because sometime started to play up on each deck and i can jazz up the deck to get them to work
the fostex m80 a8 do have soft heads i had to jazz up a fostex m80 as head was dead
the uk model as better to fix up i am not a pro just like you are i keep my self in the dark and not post over the net
the show of don't like it what i say is the real life of gear like you are doing you are ace un cover the real parts
Good information, thank you
Who on earth would lap a portastudio tape head!