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

[fireman]

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.

[Enzo]

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.

[olddawg]

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.

[fireman]

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)

[fireman]

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

[fireman]

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

[Enzo]

Just my terrible recollection, but I;d expect tens of volts.

[Jazz P Bass]

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

[fireman]

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

[fireman]

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.

[g1]

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.

[olddawg]

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.

[fireman]

G1 Thanks for helping with uploading the manual.

[fireman]

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

[Enzo]

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.