Revox a77 - alignment problems?

[heavenorlasvegas]

Hi all,

Just mustered up enough scratch to grab myself a revox a77 from a local tech, and while the tech said he recently restored the unit, I’m having the following issue with it:

Channel 1 and channel 2 have very different volume output level and frequency response.

Channel 2 has more hf than channel 1, and channel 1 has a higher overall output but less hf than channel 2.

I have cleaned the tape path. The heads don’t look too worn. I’ve also adjusted the screws that hold each head in place. I get various results by adjusting the screws but Still no dice.

Any advice sos? Thanks again y’all.

Roger

[MarkOne]

Well, from memory, calibration of a tape deck involves a test tape and an oscilloscope. Someone with direct experience will, I’m sure be along to expound on the process, but I’m sure it’s quite involved with multiple steps.

[Nazard]

In the first instance I would talk to the tech you bought the A77 from. As a matter of interest, have you owned reel-to-reel machines before and which model A77 is it?

I have cleaned the tape path. The heads don’t look too worn. I’ve also adjusted the screws that hold each head in place. I get various results by adjusting the screws but Still no dice.

If the screws you have altered have a small amount of red paint on them, these are azimuth adjusting screws, (for both record and replay heads), and shouldn't be touched unless you are familiar with setting up a reel-to-reel, and have the equipment for adjusting azimuth, e.g. test tape, oscilloscope, etc..

I am sure it can be sorted out but it's probably better not to adjust anything else before you speak with your tech.

[Arpangel]

I wouldn’t touch any screws, or make any adjustments without the right tape, have you demagnetised the heads?
Have you given them a "good" clean?
Lining up a machine isn’t that hard, you just have to have the right alignment tape. I like doing this, it’s very therapeutic
If in doubt, get help.

[Sam Spoons]

Same advice here, if the tech has restored the machine it should not be exhibiting those issues. But make sure it's not something in your signal path first, just in the interests of avoiding embarrassment (we've all been there )

[Hugh Robjohns]

First, are you talking about tone/level differences when replaying a previously recorded tape, or recording onto and then replaying the tape?

Second, if you monitor the input through the machine, does it come out with the right levels/tone?

Tweaking head alignment screws randomly is guaranteed to mess things up, and don't rush to damagnetise anything. You can seriously mess the machine up if you get that wrong and self-mag wouldn't cause those symptoms anyway.

The obvious thing to do is get the machine back to the tech to check over. It nay be nothing sinister — it is possible for alignment pots to shift when subjected to vibration in a car, for example.

But aligning tape machines is non-trivial and requires expensive alignment tapes, test gear, knowledge and experience.

[Tim Gillett]

Especially these days few people know their way around such legacy gear. There are techs and there are techs. You might try a forum specializing in vintage tape technology such as Tapeheads or UK Vintage Radio Repair and Restoration...... The head faces on the A77 are very difficult to see let alone clean properly. It's a weakness on a mostly excellent machine.

[James Perrett]

Lots of good advice already. With the A77 a small mirror is useful when cleaning the tape path - Bib used to include one in their tape head cleaning kits. I'll bet that there is still some gunk left on the edge of the head or one of the guides.

What sort of tape did the tech set it up for? Hopefully it was one of the tapes that you can buy new because you don't want to be testing a machine with old tapes with unknown history.

What sort of tape are you using? If you are trying to use Ampex/Quantegy tape then stop right now. You'll be spending all your time cleaning the machine. The same goes for some types of 3M (Scotch) tape.

Sadly a couple of the expert Revox technicians have died recently so there aren't so many places that you can take it to but, having said that, there is plenty of service information available so that most competent technicians can work on them.

[James Perrett]

Just re-read your post and realised that you've messed around with the heads. Probably the best thing you can do now is to take the machine to someone who knows what they are doing, get them to remove the whole headblock and send it to Terry Summers at Summertone who can restore the heads and re-position the heads properly on the baseplate.

Don't go messing around with anything else - you'll probably break something else if you do.

[heavenorlasvegas]

Hi, thanks for the replies all.

I have not tried demanetizing the unit, i may give that a shot. As for cleaning the heads, I have taken the face plate off and have given them a good clean so I’m pretty sure that is not the issue.

As for the alignment, and maybe someone can shed some light on this for me because I’m a little bit confused.

When aligning a machine “properly”, A test tape and oscilliscope are used to determine what head screws to adjust. Is that correct?

can this not be done by ear? by running any signal through the machine and adjusting until there are no phase issues etc?

Not sure why an oscilloscope and test tape would be needed. I understand techs need to eat, but Seems a bit unnecessary, no?

[Hugh Robjohns]

I have not tried demanetizing the unit, i may give that a shot.

DO NOT ATTEMPT DEMAGNETISING.

If you use the wrong gear, or use it incorrectly, YOU WILL WRECK THE MACHINE and cause someone a lot more of a challenge sorting it out. None of the problems you described indicate a magnetization issue.

When aligning a machine “properly”, A test tape and oscilliscope are used to determine what head screws to adjust. Is that correct?

The height and zenith are normally set at the factory and NEVER normally adjusted again. Only if the heads are replaced are they adjusted —initially with a jig and/or by eye using a temporary material painted on the head to reveal the tape path across it. The azimuth is adjusted using a reputable test tape, scope and audio level meter, and should normally only need adjustment during regular maintenance (unless you're handling tapes from other machines routinely).

...can this not be done by ear? by running any signal through the machine and adjusting until there are no phase issues etc?

If it's almost correct then yes, it can be fine tuned by ear listening in mono and tweaking for max HF (or for deepest null of the stereo difference signal)... but if you tweak too far there are false peaks and its easy to get confused and end up in the wrong place. Correct use of a test tape and scope avoids that risk.

Not sure why an oscilloscope and test tape would be needed.

Read up on professional tape alignment and you'll understand...

[James Perrett]

Not sure why an oscilloscope and test tape would be needed. I understand techs need to eat, but Seems a bit unnecessary, no?

If you have Reaper and an audio interface then you have an oscilloscope suitable for azimuth alignment already (look in the JS effects). This won't work for motor adjustments though - you need a proper scope for that.

A test tape is essential because you need a known standard to align the machine to.

There is far more to alignment than "head screws". Do you have the service manual? If not, then download the appropriate version for your machine (there are 4 main versions) which might help but you need to be methodical and disciplined.

Don't touch a demagnetiser unless you really know what you are doing. You can really screw things up if you don't use it correctly. I have only ever encountered one seriously magnetised tape machine - and that's out of a hundred machines or more that I've owned or serviced.

[Arpangel]

I used to "line up" my machines, I had tapes, but it was a rough and ready good enough for rock-n-roll type of situation. I was told, and shown how to do it by a friend, but I always used to forget some of it, I muddled through, and left out bits and pieces. I was OK lining up the Dolby's, I/O levels, record playback calibration, but things like azimuth and anything more involved, forget it.
Bottom line they always sounded fine to me.

[James Perrett]

If Tony's friend is who I think it is then what he calls "muddled through" is probably a pretty thorough calibration by many people's standards. Certainly Dolby calibration on an A77 (it is a fairly rare optional extra) isn't for the faint hearted from what I remember.

[sonics]

All this talk of tape calibration just makes me so glad I don't have to do it any more. So much "wasted" time...

I would like to find a Tascam 4-track to recover some old material, though. But that's it!

[heavenorlasvegas]

UPDATE:

I was able to bring both channels to equal levels by raising the repro level for channel 2 on the amplifier card. this involved basically taking the whole machine apart. the front panel knobs are an absolute bear to get off and i still havn't figured out how to get them back on! More in a bit.

thank you guys so much for your input.

roger

[Arpangel]

If Tony's friend is who I think it is then what he calls "muddled through" is probably a pretty thorough calibration by many people's standards. Certainly Dolby calibration on an A77 (it is a fairly rare optional extra) isn't for the faint hearted from what I remember.

Ha! You’re right James!
I remembered at the time, but now, if you asked me, I’d have trouble!
I have his voice ringing in my ears "come on boy! you can do it" that applied to everything, from lining up a tape machine, to miking up a symphony orchestra.

[James Perrett]

UPDATE:

I was able to bring both channels to equal levels by raising the repro level for channel 2 on the amplifier card. this involved basically taking the whole machine apart. the front panel knobs are an absolute bear to get off and i still havn't figured out how to get them back on! More in a bit.

Sounds like you are making things far harder than they need to be. To adjust replay level you just take the back off, lay the machine down on its back and all the adjustments you should ever need to make will then be right in front of you. No need to take the knobs off or remove the front panel.

You really need to look at the service manual...

[Hugh Robjohns]

... this involved basically taking the whole machine apart...

No, it really doesn't!

You are the perfect example of the saying, A little knowledge is a dangerous thing!

As James says, you just need to remove the case — and bypass the safety interlock in the mains connection. Details in the service manual.

The full service manual is readily available on the web.

I'm all for people getting their hands dirty to expand their knowledge and understanding, but please do yourself (and all of us) the courtesy of reading the service manual first.... before you destroy the machine (like you've destroyed other project gear in the past), or kill yourself.

[Tim Gillett]

UPDATE:

I was able to bring both channels to equal levels by raising the repro level for channel 2 on the amplifier card. this involved basically taking the whole machine apart. the front panel knobs are an absolute bear to get off and i still havn't figured out how to get them back on! More in a bit.

thank you guys so much for your input.

roger

If channel 1 repro trim pot is at say 2 o'clock then when adjusted correctly channel 2 will normally be very close to 2 o'clock as well. Repro gains rarely if ever need adjustment. Don't change the repro gain to compensate for a record gain problem. Repro is meant to be the reference by which we set record. As Hugh and James said follow the Service Manual.

[Arpangel]

Altering levels like this could make things worse, as Tim says, repro level is a reference, you could be making things worse by adjusting this not knowing fully what the record level is set at, this is where test equipment comes in.
If unsure, take it to a specialist, these are great machines, and it would be a shame to mess it up.

[Mike Stranks]

I'm reminded of something I saw affixed to a VDU (it was a long time ago!) which was depressing for me as a (then) IT trainer:

"When in doubt press buttons indiscriminately"

The A77 was/is a very good piece of kit. Your well-meaning, but uninformed, fiddling about runs the real risk of turning yours into a shambolic mess.

[Arpangel]

I'm reminded of something I saw affixed to a VDU (it was a long time ago!) which was depressing for me as a (then) IT trainer:

"When in doubt press buttons indiscriminately"

The A77 was/is a very good piece of kit. Your well-meaning, but uninformed, fiddling about runs the real risk of turning yours into a shambolic mess.

Put the screwdriver down, and step five paces back (you distract him while I call the RSPCVTR)

The Royal Society for the Prevention of Cruelty to Vintage Tape Recorders

[ken long]

You need a test tape, a baseline. Wouldn't be messing with any settings until you have one.

[Brian M Rose]

RTFM Indeed!
Slightly different, but when I had to line up cameras, I would lock the door and disconnect the phone. It was a bit like a computer game, 'You have been killed by a dragon. Play another game?'
Except this could easily be, 'You have reset to Factory Setting. Do you want to return the camera to Japan?' Yes, it cold be that bad. I too have a Curated Revox A77. It does need re aligning but I don't have a Test Tape (although I do a scope etc) So the screwdriver is kept well away!