Hi everyone.
I finished the basic preparation for converting my b77 from 4 tracks to 2 tracks.
The reproduce pcb was more or less similar, bought a used 2 track oscillator pcb and carefully made the needed changes for the transformation on the recording pcb ( quite harder than the reproduce one since there are some differences to the 4 track one).
I found some quite good prices for all 3 (play, rec, erase) 2 track heads (used ones) and i placed them on the headblock. Soldering and re wiring was a breeze. Used clear tape for initial allignment.
Monitor amp check (first step of the manual calibration process) went fine and it works perfectly well.
Then i started with the calibration of the playback head and i came across the first issue. I’m using a basic calibration tape that was very good for successful calibration of the 4 track playback head.
But now with the 2 track head when the switch is set to stereo there is no signal coming in the right channel. The mono section of the switch works perfectly well.
Tried to change the head cables (green where yellow goes) and then i only have left. So it’s not an issue of a faulty head or bad pcb connections, but rather something else either with the initial head adjustment (height or zenith - never done it before and there is no real guide on how to do it properly) or the calibration tape is not appropriate for 2 tracks.
Any advice would be much appreciated. I think i may need some more down thevroad. But this step,as you may know, is really important.
Thanks in advance
I finished the basic preparation for converting my b77 from 4 tracks to 2 tracks.
The reproduce pcb was more or less similar, bought a used 2 track oscillator pcb and carefully made the needed changes for the transformation on the recording pcb ( quite harder than the reproduce one since there are some differences to the 4 track one).
I found some quite good prices for all 3 (play, rec, erase) 2 track heads (used ones) and i placed them on the headblock. Soldering and re wiring was a breeze. Used clear tape for initial allignment.
Monitor amp check (first step of the manual calibration process) went fine and it works perfectly well.
Then i started with the calibration of the playback head and i came across the first issue. I’m using a basic calibration tape that was very good for successful calibration of the 4 track playback head.
But now with the 2 track head when the switch is set to stereo there is no signal coming in the right channel. The mono section of the switch works perfectly well.
Tried to change the head cables (green where yellow goes) and then i only have left. So it’s not an issue of a faulty head or bad pcb connections, but rather something else either with the initial head adjustment (height or zenith - never done it before and there is no real guide on how to do it properly) or the calibration tape is not appropriate for 2 tracks.
Any advice would be much appreciated. I think i may need some more down thevroad. But this step,as you may know, is really important.
Thanks in advance
You have the head height wrong. L and R of a 4 track tape should occupy half of each channel of a 2 track head. Sounds like you have R in the space between.
Thank you very much EJP for that. Could you please advice (or send a guide link) on how to do it properly?
I have no problem removing the head block or do it in place if it can be done.
I have no problem removing the head block or do it in place if it can be done.
I'm with ejp. If the replacement heads are used there will be some wear 1/4" wide where the tape needs be. Also, it must lie flat, no bends or ripple, and the other heads, not yet aligned, can distort the tape path. You've taken on an interesting challenge, but it absolutely can be done with only a calibration tape and a lot of patience. So, first thing is tape path, then other planes playback alignment, then record.
All good fortune,
Chris
Should add: Trust the tape guides for tape path references. Might need some little mirror on a stick to do this while you're still right side up.
All good fortune,
Chris
Should add: Trust the tape guides for tape path references. Might need some little mirror on a stick to do this while you're still right side up.
Thank you very much Chris Hornbeck for the positive input.
Could somebody please explain to me if i have to adjust the other two screws on the head “board”-have never touch them and don’t know what they adjust (not the azimuth one, but the larger next to it and the one at the bottom) or should i take the whole headblock out and adjust the rear screw?
In my eye with clear tape the tilt (that’s what i adjusted by tightening the rear screw) of the heads seem good-parallel to the tape path and parallel to one another (maybe not perfect - can’t tell with just magnifying glasses and a hand ruler). Can’t really tell what exactly goes on inside the head “shell”
Could somebody please explain to me if i have to adjust the other two screws on the head “board”-have never touch them and don’t know what they adjust (not the azimuth one, but the larger next to it and the one at the bottom) or should i take the whole headblock out and adjust the rear screw?
In my eye with clear tape the tilt (that’s what i adjusted by tightening the rear screw) of the heads seem good-parallel to the tape path and parallel to one another (maybe not perfect - can’t tell with just magnifying glasses and a hand ruler). Can’t really tell what exactly goes on inside the head “shell”
The thing is that i took the headblock out, re alligned everything, put it back on, tried the 2 upper screws adjustments (they just raise or lower the signal voltage - more or less coarse azymuth adjustment), but in my case it’s not fine or coarse adjustment since there is no signal coming out of one side.
Please re-read if you can my original post. As i said both green and yellow leads (set of 3) on either set of 3 pins on the mainboard work fine as right or left channel on their own. But combined together in STEREO only one channel plays (signal output).
Please re-read if you can my original post. As i said both green and yellow leads (set of 3) on either set of 3 pins on the mainboard work fine as right or left channel on their own. But combined together in STEREO only one channel plays (signal output).
Ignore the mu-metal shield around each head; they're not trustworthy for adjustment. Only the actual tape on the actual head matters. Tape heads typically have three adjustment screws, but these don't individually adjust in the three planes we're interested in - they interact, and work in their own personal planes. I gave away my B77 decades ago so can't reference the wire colors. Probably safe to assume that you got them right.
All good fortune,
Chris
All good fortune,
Chris
Back in my days I used to use some kind of mild felt-tip pen to "colour" the heads, then "play" a length of disposable tape. The ink then wears off the head and you can see exactly where the tape touched the head. Throw out any tape you used.
Afterwards clean the heads, capstan and tape guides thoroughly (of course). (isopropyl alcohol)
In general (but not your issue) demagnetize often, also when you think you don't have to, do it anyway. Also demagnetize the tape guides. At all times make sure any calibration tape is stored away securely before you even think about the demagnetizer. And lock away the demagnetizer before you reach for the calibration tape.
When using a 2-track calibration tape to calibrate a 4 track head keep in mind the "crosstalk" is heaviest in the low frequencies, this will give a false low frequency boost. I'm not sure how this will work the other way around, I think you should be OK using a 4 track tape to adjust a 2-track head.
Afterwards clean the heads, capstan and tape guides thoroughly (of course). (isopropyl alcohol)
In general (but not your issue) demagnetize often, also when you think you don't have to, do it anyway. Also demagnetize the tape guides. At all times make sure any calibration tape is stored away securely before you even think about the demagnetizer. And lock away the demagnetizer before you reach for the calibration tape.
When using a 2-track calibration tape to calibrate a 4 track head keep in mind the "crosstalk" is heaviest in the low frequencies, this will give a false low frequency boost. I'm not sure how this will work the other way around, I think you should be OK using a 4 track tape to adjust a 2-track head.
Thanks for the advice. I will try it. The degmatnize part i really do.
What really makes me worry is the non signal. Not low or bad. But dead. Like hard phase problems.
What really makes me worry is the non signal. Not low or bad. But dead. Like hard phase problems.
Perhaps this forum is more specialized: https://www.tapeheads.net/
I don't remember exactly but it's possible the 2 tracks don't align exactly with the 4 tracks which does mean one track will be off while the other is good. Then again many calibration tapes are full track.
I never really worked much with consumer type recorders so when I was dealing with 4-track machines it would be 1/2 inch tapes.
I don't remember exactly but it's possible the 2 tracks don't align exactly with the 4 tracks which does mean one track will be off while the other is good. Then again many calibration tapes are full track.
I never really worked much with consumer type recorders so when I was dealing with 4-track machines it would be 1/2 inch tapes.
I do prefer this forum here. There are forums i don’t feel good to start threads. Just a personal thing.
I will keep trying and maybe someone with good knowledge may step in.
I will keep trying and maybe someone with good knowledge may step in.
A calibration tape is a full-width recording, not 2-track or 4-track. So the issue must be with the playback head cabling.
Thank you icsaszar. It was a cabling problem. The playback is sorted. Now i’m facing the same problem with the recording head.
No recording on the right channel. Tried to replace the relays on the oscillatir board. No change.
I have to admit that i modified heavily the recording pcb (for fast speed) and it feels like it’s either the recording pcb or the recording head interaction with the playback head.
The pcb doesn’t seem like it has an obvious fault, and with naked eye and by moving the recording head by hand the recording head seems slightly lower than the playback one.
But could such a difference cause a dead channel?
Maybe i have to resolder a bit the wiring or clean the oscillator pin header.
No recording on the right channel. Tried to replace the relays on the oscillatir board. No change.
I have to admit that i modified heavily the recording pcb (for fast speed) and it feels like it’s either the recording pcb or the recording head interaction with the playback head.
The pcb doesn’t seem like it has an obvious fault, and with naked eye and by moving the recording head by hand the recording head seems slightly lower than the playback one.
But could such a difference cause a dead channel?
Maybe i have to resolder a bit the wiring or clean the oscillator pin header.
Why not use a scope to see if you have signal on the recording head? At least where you think you ought to have it?
A scope is really nice in adjusting tape heads anyway. A cheap USB scope or so should work fine (Lab-Nation or such).
A scope is really nice in adjusting tape heads anyway. A cheap USB scope or so should work fine (Lab-Nation or such).
I have one. A trustworthy old analog one. I was worrying about excessive magnetic field with the probe. But yes i will do it. One question.
Can the pair of leads be connected on the probe like + and “0”? Or i risk a short?
Can the pair of leads be connected on the probe like + and “0”? Or i risk a short?
Checked with oscilloscope. Equal strange frequency on both + pins of the recording head.
I say strange because even if i feed the in with a 1k signal the frequency is closer to 20k.
Maybe something with the oscillator.
The important thing is that the head takes signal so now the problem is either on the oscillator board or the recording pcb.
I say strange because even if i feed the in with a 1k signal the frequency is closer to 20k.
Maybe something with the oscillator.
The important thing is that the head takes signal so now the problem is either on the oscillator board or the recording pcb.
Had never seen the bias waveform before (in my ears it’s like a noise around the signal that i have to reduce to minimum whenever i’m setting it).
So apparently it seems that i have biased signal on both channels of the head, so now i have to check the in and the out of the recording board.
If it’s healthy on the in and wrong on the out then the problem is on the right channel,of that board. If it’s wrong on the in then it’s on the right of the oscillator pcb.
i’m studying the schematic now to find where to put the oscilloscope probe.very tight space .
So apparently it seems that i have biased signal on both channels of the head, so now i have to check the in and the out of the recording board.
If it’s healthy on the in and wrong on the out then the problem is on the right channel,of that board. If it’s wrong on the in then it’s on the right of the oscillator pcb.
i’m studying the schematic now to find where to put the oscilloscope probe.very tight space .
Nope don't adjust to minimum, actually the deal is:
First line up the head correctly using the calibration tape. Then if need be (and possible) adjust the frequency response.
Then swap the calibration tape for a blank tape and adjust the bias. I don't remember exactly what to adjust for, I think it was indeed best high frequency response and then a little more until that drops off again (2-3 dB). Then adjust the recording frequency response for best flat response. You feed into the recorder while recording and measure off the playback.
Revox should have all those settings because it's a consumer-Studer and the Studers had adjustments for that.
Note that the bias level is for THAT type of tape so if you have different brands/types of tape you need to find some middle ground. I think it varies the most for Long Play and Triple Play tape and such as those tapes are thinner so have a thinner magnetic layer. Of course it also varies if you use chrome or ferro tape (like cassette decks that have switches for the different cassette types).
If you set bias too low you have more distortion especially in the silent bits, if you set it too high you have poor high frequency.
First line up the head correctly using the calibration tape. Then if need be (and possible) adjust the frequency response.
Then swap the calibration tape for a blank tape and adjust the bias. I don't remember exactly what to adjust for, I think it was indeed best high frequency response and then a little more until that drops off again (2-3 dB). Then adjust the recording frequency response for best flat response. You feed into the recorder while recording and measure off the playback.
Revox should have all those settings because it's a consumer-Studer and the Studers had adjustments for that.
Note that the bias level is for THAT type of tape so if you have different brands/types of tape you need to find some middle ground. I think it varies the most for Long Play and Triple Play tape and such as those tapes are thinner so have a thinner magnetic layer. Of course it also varies if you use chrome or ferro tape (like cassette decks that have switches for the different cassette types).
If you set bias too low you have more distortion especially in the silent bits, if you set it too high you have poor high frequency.