tape head sudden magnetization, help please - diyAudio
Go Back   Home > Forums > Source & Line > Analogue Source

Analogue Source Turntables, Tonearms, Cartridges, Phono Stages, Tuners, Tape Recorders, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 13th June 2006, 07:05 PM   #1
diyAudio Member
 
Join Date: Dec 2003
Location: Buenos Aires, Argentina
Default tape head sudden magnetization, help please

I have an old NAD tape deck, -NAD6050c, circa 1985- that I have revamped to the best of my knowledge -new head, new rollers and belts, etc-
When I record some tapes everything is ok for a while -very
good playback, good channel matching, etc-, but at some instance, recordings became muffled and unbalanced.
I have tried some demagnetizing and it works for a while, then the problem starts again.

Apparently, the magnetization is severe, because it managed to erase the top end of formerly recorded tapes -I have checked the azimuth and is ok; I have it ĻstandardizedĻ with a STL test tape, so I believe that azimuth is out of question-...

The problem arises most of the time when recording in high bias and with Dolby C; in other settings the issue is less severe, but still exists.
Somewhere I read about the possibility of ageing caps -and some dc offset- reaching the tape head and magnetizing it.
I have the manual of the deck, and I believe that I could be able to identify some caps related to tape head dc blocking if neccesary -I`m not a pro, but I have managed relatively well with another issues-; but I really donīt know still.
Does anybody have any theory about what the problem is?
Could it be related to some internal bias adjustment routine that the manual mention, but that I havenīt done because I canīt understand still what the hell they`re trying to say in that paragraph? -apparently, a bad traduction job-
The deck sounds really well when working on spec, and I wish to keep it, but this issue is driving me nuts...

Thank you very much for your patience!
  Reply With Quote
Old 14th June 2006, 07:35 AM   #2
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Quote:
Could it be related to some internal bias adjustment routine
that the manual mention, but that I havenīt done because I canīt
understand still what the hell they`re trying to say in that paragraph?
Hi,

Reproduce that section verbatim here and we'll try and make sense of it.

/sreten.
  Reply With Quote
Old 14th June 2006, 09:53 AM   #3
damianm is offline damianm  Argentina
diyAudio Member
 
Join Date: Jun 2006
Quote:
Reproduce that section verbatim here and we'll try and make sense of it.
Hi everyone, first post here.

These are the images I sent to Federico-

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Hopefully someone will make sense of them - we certainly haven't.

Take care
  Reply With Quote
Old 14th June 2006, 10:29 AM   #4
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

The above is about setting calibration levels for the recorder,
seems to require a calibration tape with a 400HZ 0dB tone.

It is not part of the bias adjustment directly, though if its wildly out.....

/sreten.
  Reply With Quote
Old 14th June 2006, 12:44 PM   #5
damianm is offline damianm  Argentina
diyAudio Member
 
Join Date: Jun 2006
Quote:
The above is about setting calibration levels for the recorder,
seems to require a calibration tape with a 400HZ 0dB tone.

It is not part of the bias adjustment directly, though if its wildly out.....
Hi Sreten. Federico and I have same-model decks, and we swap tapes often.

My deck does have some other problem, but it manifests itself on playback - recording seems to work fine.

The problem with the 400 Hz/0 dB tone is that we are unsure which level Federico's test tape is recorded at (an STL something-or-other test tape); I was wondering whether making him a tape myself with a 400 Hz tone at 0 dB might help? Azimuth is not perfectly matched between the two decks yet.
  Reply With Quote
Old 14th June 2006, 12:58 PM   #6
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi damianm,
If you are partially erasing tapes on playback, one of two things are happening.

Firstly, you may have some DC leakage current through the blocking caps to the playback amplifier. Seen that a few times on other models.

Secondly, the bias oscillator may be partially running. Probe with a 'scope to find that answer.

To troubleshoot I would see if the oscillator is running, if not then change those caps.

You didn't mention tape path. If the capstan bearing wears the tape will be drawn towards the back of the transport. You may see some slight crinkling on the outside edge of the tape. Always check the capstan bearing for wear. If your head shows any wear path at all, do not attempt to correct the azimuth. The tape will not travel across the head properly if you do. You will lose tape to head contact in the gap area.

-Chris
  Reply With Quote
Old 14th June 2006, 01:29 PM   #7
dnsey is offline dnsey  United Kingdom
diyAudio Member
 
Join Date: Mar 2004
Location: Shropshire, England
Do you always set the transport to 'stop' before switching the deck off?
Many designs arrange for the bias to decrease gradually, thus performing an 'auto-degauss' function, when set to stop. Switching of whilst running can cause residual magnetisation of the heads.
Another thought - try temporarily disconnecting the erase head, and see if there's a difference. If the bias oscillator is running, this will significantly reduce the problem - there may still be some recording bias, but it will be small compared with the erase signal.
You are storing your tapes correctly - away from magnetic fields and high temperatures?
  Reply With Quote
Old 14th June 2006, 04:01 PM   #8
diyAudio Member
 
Join Date: Dec 2003
Location: Buenos Aires, Argentina
Default nad6050c

Hi all! Thank you very much for your response.

First, Sreten:
I will post the bias adjustment method ASAP; pity that the print is in my house and I`m at work.
As an advance, and for what I remember, the procedure appears to be a little awkward, at least to me: the same pair of variable resistors is used to trim the bias in the three different positions of the tape selector... It escapes me how it`s possible to adjust one tape type without disadjusting the other two, but that`s how it work.

Second, Anatech:
Sadly, I have no oscilloscope, but maybe some skilled technician in BA could do the testing for me to check for some oscillator trouble...
Do you believe that the mentioned internal bias adjustment is somewhat related to oscillator malfunction?
I will try to find the caps and replace them as an extra. I think that if they are elcos maybe they`re leaky or out of spec...
Tape path: The tape is running smoothly, with no excessive tension; the record/play head is new -amir666, an equivalent replacement of the original-, and when I cleaned and carefully relubricated the whole transport, I detected no play in the capstan bearing -in fact, I was marveled at the close tolerances that the machine exhibits-; when cleaned and relubricated -plus a pair of new rollers, one for the capstan and one for the hub; and new belts-, the transport performs as new in all of it`s functions, including autostops in all runnning modes, pause etc.

Third, dnsey:
Erase head disconnection is a good tip, I will try it.
For what I remember, Iīve never powered off the deck without stopping the transport firstly. But itīs good advice.
Tapes are stored away from strong magnetic fields, at room temperature -some 20 deg C-

Last -but not least-, Damiān:
Good to see you here, amigo!
We`ll get those bloody NADs running someday, don`t dismay! I can`t stand the idea of seeing them as landfill in some remote location.
Maybe as living room oldtech adornment -as the old Akai reel to reel- maybe, but not in the trash...
And how good the old cassette sounds in those decks when everything is working properly! I can`t believe it!

Saludos, and thanks

Analog rules, but how hard is...
  Reply With Quote
Old 14th June 2006, 05:24 PM   #9
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi federico moreno,
The adjustments usually cover normal CrO2 and Metal tape types. One should have two adjustments (one for each channel), the others shift the overall level for the other two types. These may be variable capacitors or resistors.

The adjustment should not have any effect in play at all. The master bias oscillator should be off unless the unit is put into record mode. That's why it's an easy first check for your technician.

Good to hear the mechanism is in good shape. The wear on the capstan bearing will be in the direction of the pinch roller pressure. You can't feel any play unless the thrust bearing is removed (at the back of the flywheel, normally about 0.5mm clearance).

Good luck!

-Chris
  Reply With Quote
Old 14th June 2006, 05:47 PM   #10
diyAudio Member
 
Join Date: Dec 2003
Location: Buenos Aires, Argentina
Default nad6050c

Hi, Chris.
Quote:
One should have two adjustments
Now that you mention it, I believe that the print states only two adjustments. But I think that I can`t get that print till tonight...
Now, in the PCB there are only two variable resistors -labelled vr107/vr106, if I remember well-
The manual states that the procedure is -more or less- the following:
a) deck in rec mode
b)front selector switched in the position of the type of tape to be calibrated
c)probes of tester ĻacrossĻ resistor NN -I don`t remember the number, but is a resistor in series with the head-, and, while rotating vr106 or vr107...
d)... the target value must be NNN mV.

Sorry for my lack of understanding of those issues, but I have two specific questions:
a) What is the meaning of ĻacrossĻof the resistor? Simply one probe in one leg and the other probe in the other leg of the resistor; or black probe in gnd and red probe in the resistor leg that points toward the head?
b)Target value: isn`t expressed in type of voltage, only states nnn milivolts. Is AC or DC?

Thank you very much in advance
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Magnetic tape head voltage output tomcuk Analogue Source 11 30th March 2009 03:20 AM
Nakamichi dr-2 3 head tape deck tyler477 Swap Meet 0 29th November 2008 03:54 AM
tape head amplifier Davide82 Parts 7 11th November 2008 01:59 PM
WTB: Tape head demagnetizer in Australia rabbitz Swap Meet 3 16th January 2008 10:28 AM
AKAI GXC-39D Tape Deck head advice gmphadte Solid State 19 24th October 2007 02:24 PM


New To Site? Need Help?

All times are GMT. The time now is 08:32 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2