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Thread: Fostex early A8 8 track reel to reel will not erase or record

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    Fostex early A8 8 track reel to reel will not erase or record

    Hi , does anybody remember reel to reel tape machines? I have a fostex A8 from the early eighties that has stopped eraseing and only just records, I checked the heads, the erase head has no shorts and has continuity of around 12 ohms on each track, the record/reproduce head is also free of shorts and has continuity of around 110 ohms on each track.

    I scoped the erase head with the tracks armed for record and got a slightly distorted sine wave bias signal of around 8 v peak to peak at about 86khz ( manual specs 100 kHz ) calculated from oscilloscope , I don't have a frequency counter and the scope hasn't been calibrated while I've had it.

    The manual I have is a bit hit and miss, some of the circuit boards are different and the schematics are for the A8LR model which records on all 8 tracks at once, mine only records on 4 at once.

    If any body could help out with expected bias and oscillator levels this would be great, the test points in the manual do not exist on the record/ repro circuit cards that are fitted to this machine.


    The machine had not been used for a while but did record then suddenly stopped erasing, I though this may have been operator error as it did start to erase again but the stopped again and will not erase now.
    The machine still plays previously recorded material and has not had the alignment altered.

    Thanks in advance for any help.
    Last edited by fireman; 01-15-2018 at 01:25 PM.

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    Lifetime Member Enzo's Avatar
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    I don't have the manual handy, it is in storage, but that bias sounds a lot smaller than I would expect. 86kHz or 100kHz shouldn;t matter. Yes, one might be more optimal, but either would make it record or erase. Your problem is gross, not a nuanced frequency variation.

    4/8 channel difference shouldn't matter. I'd be surprised if the bias oscillator and drive are different between the two.

    If it comes and goes, I would suspect an R/P switch contact or a solder joint somewhere.
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    Old Timer olddawg's Avatar
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    Listen to Enzo. You should have a massive sign wave on the erase head when it’s engaged. The erase head should also have 8 tracks so you don’t erase all 8 channels at a time. It would not record if the bias oscillator wasn’t working. There’s a lot of switching to allow you to “punch in”. If it has a bunch of physical switches, break out the Caig Deoxit. If the switching is electronic find the correct schematic and use your scope. I remember Fostexs being a bit of a rat’s nest. Make sure your heads aren’t trenched or dirty. Do all 8 channels have the same problem? I’ve had to polish heads with the wooden part of a Qtip to get them clear. Also demagnetize the whole tape path.

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    Hi Enzo, thanks for the reply,

    I will try to post the parts of the manual/ schematics that do match the machine, although I struggled to edit the misspelt thread title)
    The system control board and the connector board are a match, the rec/repro boards are different, but because multiple tracks are not erasing I don't think the problem is on the channel pcbs.

    The connector pcb has the oscillator unit on it, this is a small Crome box 2cm x3cm it has connections for 24v, ground, output and what seems to be a trigger (on/off)
    Last edited by fireman; 01-15-2018 at 10:24 PM.

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    I tried to upload a photo of the wave form but get told I don't have permission.?

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    Old dawg, the erase head does have 8 separate tracks, I have made sure that they are clean, but do not have a dematizer.

    Recordings are now low quality and at a very low level compared to the previous recording.

    All tracks have the same problem, this makes me think it's either the oscillator or the logic control, signals make it to the meters the switches that select tracks to be recorded bring on the indicator leds and I can hear the relays ( one per channel) making and breaking, I even tested the switching of one with a battery.

    When you say a massive signal do you mean tens of volts or maybe hundreds.? Ball park figure?

    Thanks

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    Lifetime Member Enzo's Avatar
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    Just my terrible recollection, but I;d expect tens of volts.
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by fireman View Post
    I tried to upload a photo of the wave form but get told I don't have permission.?
    fostek.jpg

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    Thanks Enzo, I'm going to try and isolate the oscillator unit and see if the signal level increases to make sure that another component isn't grounding it out

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    Thanks jazz p bass, I hope I'm better with a soldering iron than I am with a computer!

    This is the waveform on the output of the oscillator unit when recording, the vertical is 1v/div , the horizontal is 5 micro secs/div.
    Attached Thumbnails Attached Thumbnails img_0845.jpg  
    Last edited by fireman; 01-16-2018 at 12:17 AM.

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    About the only thing I remember from these is we replaced a lot of relays. And the intermittent nature of the fault could result from an iffy relay. But it would have to be one that is common to all the tracks when record is engaged.
    I'll post the manual that you mentioned being incorrect, it may still be helpful to someone.
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    Old Timer olddawg's Avatar
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    I meant massive compared to the signal level. Your record head signal should be modulated on top of the bias signal and the erase head should be just the bias. Usually it's upwards of 30vac if I remember correctly. It actually is different for different types of tape. In multrack tape bias adjustments are critical so you don't erase or degrade adjacent tracks. If your recording is bad on new tape I would strongly suspect a weak bias. If you are going to use analogue tape you need a demagnetizer.

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    G1 Thanks for helping with uploading the manual.

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    Just to recap so far.

    I think that the heads are ok, no shorts on either erase or rec/ repro, all tracks have continuity, if the rec / repro plays it should record. ?

    The problem occurs on all tracks so is likely to be a 'common' parts of the system.

    The machine worked ok a couple of weeks ago and has not had any alignment changes or different tape used.

    The tape is new, not N.O.S quantegy grand master and is the right way round..

    The level from the oscillator is below what you guys would consider normal.

    The signal from the oscillator, I'm not sure if the wave should be a square wave when generated but it's a fairly good sine wave by the time it gets to the heads.

    Could the scope be pulling the signal down? I'm using the probe on X1 I think the input impedance is 1Mohm, the scope is 20 Mhz but is probably the same vintage as the tape machine it's not been calibrated but agrees with my Megger when measuring dc voltages, the Megger can't read the bias properly possibly the frequency is too high or it's not true rms.

    Sorry to ramble on but I don't want to be doing something stupid when you are all offering help and advice, I'm an electrician but I'm a bit out of my depth with things like expected bias levels and logic controllers.

    Thanks

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    Lifetime Member Enzo's Avatar
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    No the scope is not loading anything.

    The type of tape would not cause it to not erase at all. Tape type if wrong might cause a little loss of high end response in recordings.

    I think several of us want you to explore the low bias level.
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    I have put 3 photos in my album but cannot get them to apear in this reply!

    one is the schematic that I have drawn out again, the others are the scope shots of the oscillator output before and after removing the two capacitors c20 and c26, the after pic has the scope on 2 v/div so the voltage has risen to around 12v and the waveform is more sine shaped than before.

    Whilst removing the caps I did notice that they where both connected in reverse to both the pcb layout in the manual and the silk screen on the pcb, I am thinking that this would make no difference to a 100 kHz signal but will replace them as per the silkscreen.

    I also noticed that the resistor r54 which triggers the osc unit is 1 ohm on the parts list for the pcb but the actual resistor is 47 ohm looks to be original with factory looking paint and bend on leg, I'm thinking of changing this to a small pot to see if the value is critical. Not sure if this is a typo on manual or a mistake in production.?

    If somebody could move my pics here this post will make much more sense, thanks


    img_0845.jpgimg_0857.jpgimg_0851.jpg
    Last edited by fireman; 01-17-2018 at 09:35 PM. Reason: add photos

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    g1
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    When posting, click on 'go advanced'
    Then scroll down and click 'manage attachments'
    A separate box should open called 'manage attachments'
    Drag the images from the folder down into the area labelled 'attachments'
    Once they appear in there with checkmarks, click on 'insert inline'
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    Thank you, I had managed to get to the go advanced bit, but I couldn't get the images to move across, I'm on my iPad, the screen scrolls when I try to drag anything, I'll have to get the laptop out and try again.

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    Old Timer olddawg's Avatar
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    Have you tried "tickling" the oscillator circuit with a can of freeze? lol

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    photos are now attached to previous post, hope it reads better now.

    olddawg, no I haven't tried cold spray, I don't have any here, my workshop also doubles up as a dinning table,

    though I have had some success in increasing bias level, after disconnecting the two caps on the output of the osc unit to test and getting a small increase measuring at the output terminal I replaced the caps as per silkscreen (polarity) I put a new 1uF in but didn't have a 33uF so had to use the original. I then removed the rec/repro boards and tested at the connector, the rec bias was off the screen of the scope even on 10v/div (screen is 10 wide by 8 high) , so over 80v?
    the osc (erase) signal was around 20v, soooooo

    thinking..
    1 should have used x10 on probe to get a proper reading of rec bias

    2 the caps can't have been blocking the signal, it would have been present on osc unit output before.

    3 if the signal was leaking to ground though L1 it would still be doing it.

    4 the signal looses amplitude when the rec/rep boards are installed, this would make sense as there are multiple paths through the heads to ground.

    I measured the dc resistance to ground from the erase head cold term and got 50 ohms but this will be passing a 100khz signal so should be impead or is this splitting hairs?

    I put the track cards back in and tried the machine still doesn't erase and still only just records

    I'm goinig to try replacing one track card at a time to see if one is loading down the signal, they are all connected in parallel.

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