• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Revox A77 Not Recording Advice

A

algarcia

New Member
Hi all, I recently bought a Revox A77 MkIII 2-track version that seems to be in good condition- besides the fact that I cannot get it to record!

Fast forward, rewind, stop all work. I had a pre-recorded 2track tape that plays fine, so playback is good too. One of the VU meters is visibly broken; the needle is stuck halfway up the meter instead of laying at rest to the far left. The other meter briefly jumps when the power is applied, but who knows if that one works either.

Here are the steps I try to record to a blank tape:

  1. Receiver Tape Mon. OUT -> Revox Aux IN
  2. Receiver Tape Mon. IN -> Revox Aux OUT
  3. Turn Revox on
  4. Engage the Red Record buttons next
  5. Press the Record and Play At the same time.

The background VU meter red dots are now illuminated.

I can monitor the source directly and hear sound but the VU meter doesn’t move at all. I can turn the record amp gain all the way up and that doesn’t change the meter doing anything, but the music gets louder.

I also bypassed the receiver tape monitor entirely and connected a 3.5mm/RCA cable to the output of my phone at full volume.

As a final test I changed the Monitor switch to NAB to hear the tape directly and turned up the playback volume and record amps to the limit and pressed record. I could hear the input very very faintly, keep in mind that if I had been monitoring the input, this would be earsplitting volume. Upon playing the tape back at that same volume I was able to hear that it was in fact “recorded” and not just crosstalk.

I went ahead and swapped out another record relay board and the behavior is the same. While I didn’t fully suspect the record relay would fix the issue, I figured I might as well try it.

Right now my suspicions have me thinking that it could be bad recording amplifiers, which would allude to the fact that I don’t see the source on the VU meters, but would both go bad at the same time? Or could it be something like the VU meter board?The manual also suggests that No Record could be caused by a bad oscillator.

What are the chances that it’s a bad record head? Before I aimlessly try different things I wanted to reach out and seek some advice on where to narrow my efforts. Any suggestions?
 
Register to hide this ad
Do you have a scope?

So if the deck is set to source monitor, you get plenty of audio signal on the line outs?
 
Hey tapetech, thanks for your fast reply yes I do have an oscope.
That's right, when the switch is set to "input" I'm able to hear what's present on the Revox Aux IN at full volume. However the VU meter needles aren't moving.
 
With your scope, report how much bias voltage (peak to peak) you have on the record and erase heads in reocrd mode.. (if any) Also indicate if a pure sine wave or distorted.
 
Hi:
On the record ampl board there is a 100mf 6 volt electrolytic cap on the end of the board next to the connector. Replace that cap on both boards and then you should see the VU meters working. I have had two different A77's with the same prob. I replaced all the electrolytic caps and orange drop caps on the boards afterwards
Replace the pots too , the wipers come loose and do not make contact.
 
Hi tapetech, sorry for the late follow up, let me try and post the scope traces I found for CH 1 and 2 reading directly off the pins on the record head with respect to chassis ground.


for CH1 the amplitude read the correct 500mV CH2 was slightly smaller at around 440mvVpk, but the frequency of the Sine wave on both channels was 130k instead of the 120k listed in the manual. I’m not sure if the test values in the Revox service manuals are nominal/typical values, but 10kHz off seems like it could be a lot for a tuned oscillator. Overall the shape of the wave was clean. What frequency have you seen as the modulation frequency before and is there some tolerance +/- of what frequency it could be?

Next troubleshooting steps I’m planning is checking the DC bias points of the transistors in the record amp. Also thanks for the suggestion edlipman, I’ll look at those caps first. Seems like pulling the record board and powering it off a 21V supply and measuring the points would be the easiest way to do this. Any tips on easiest way to do that? Or do you leave it in the Revox chassis?
 
Hi all figured I'd bump this thread since I have some time during the quarantine to work on the reel.

Thank you edlipman for your suggestion on replacing the caps and pots. Sure enough after replacing the caps the VU meter works.

However, I still am unable to record with this tape machine. Since I've replaced the capacitors and transistors and checked the DC bias points everything in the record amp looks good.
Does this mean it's the oscillator or the recording head? I attached the scope traces above and the oscillator is at 130kHz instead of the 120kHz it lists in the spec seems significantly shifted in frequency, but I don't know if that is still within an expected range for what you see on these decks, does anyone know?
What are the chances that the recording head fails? Trying to figure out where to go on this next. Thanks
 
So in record mode do you have audio signal on the record amp output? Record a 10K tone at zero VU. Using the scope probe, you should see this tone on both sides of C511 and R516 in the record amp.
 
Hi tapetech, just wanted to close the loop on this issue. I really appreciate all of your help on this. After more troubleshooting, it seems that the tape itself was the problem. I had another prerecorded audio tape that I recorded over and everything is working as expected. I'm new to open reel recorders and didn't realize that the issue could be incompatible tape. The person I bought the Revox from gave me the "bad" tape as well so I figured it was compatible.

Moving forward it sounds like I need a lower bias tape. What typical tape should I buy for lower bias A77?
The older small tape splices I've successfully recorded on look to say Mylar on the package and have a lighter brown color to them (not sure if because of age). They don't have a PN though.
 
Compatible tapes include Maxell UD or UDXL, TDK Audra, and SA, and Scotch 206/207.
There are others, but the above are good reliable and easy to find formulations. Use the higher BIAS setting on your deck with them.
 
Aren't most of those tapes NOS at this point? Are there any modern tapes that have similar specs to those? Would RTM LPR35/SM911 or ATR MDS-36 be compatible? I guess I'm a little lost at this point of what spec on these tapes is what makes it compatible with the Revox A77?
 
Yes, new tape that is compatible is available. The LPR 35 and SM911 are fine. The ATR is fine too, I believe.

Used tape can be a risk, but the majority is just fine, and cheaper. I did see my mistake about the TDK tape-it’s TDK LN, not SA (which is a cassette format chrome tape)
 
May sound silly - but I have experienced tapes wound up with the backside forward.... (forward meaning the side on which the heads sit)
 
Thanks for the suggestion, Thorsten, but the shiny side of the tape was away from the heads and the dull side was making contact with it.
 
Well I just made an order of NOS Scotch 203 and 207, so we'll see how that goes. I guess I'm just at a loss right now for why I have a reel of tape that records fine and another reel where the recording is so quiet its almost inaudible. Maybe it could be a lot of things, but I'm a rookie at this so I just want to be sure that I know which tape will work and dont buy more wrong tape.
 
algarcia said:
Thanks for the suggestion, Thorsten, but the shiny side of the tape was away from the heads and the dull side was making contact with it.
Click to expand...
Well, there are tapes like e.g. Agfa Professional at which the backing layer is much more dull than the magnetic layer to improve the winding properties.
 
Just wanted to close the loop that the Scotch 203 worked and sounded great. However, I ordered a 10.5" reel of RTM LPR-35 which really stole the show. That tape sounds fantastic and I'm super excited to get a lot of use out of this deck!
Thanks for your suggestion Thorsten, how would you go about rewinding a tape that is upsidedown the correct way? I tried thinking of a couple combinations of flipping the reels but couldn't think of a way to do it. Would I need to put it and a takeup reel on some external spools and manually wind it?
 
Tape can be wound on a reel or hub either oxide in or oxide out. If the tape is wound with the oxide facing out, that is referred to as B-wind. If the tape is wound with the facing oxide in, that is referred to as A-wind. If you have a tape that is B-wind and want to convert it to A-wind, all you need to do is place the tape (supply) reel on your R2R and when threading the tape to the take-up reel, twist the tape 1/2 turn before you thread the tape onto that reel. You can then fast forward the tape onto the take-up reel. After doing this, you can thread the tape in the normal fashion (with no twist) back to the other reel and rewind the tape back onto it. After doing this, the tape on the reel will be in A-wind orientation.
 
Sure enough, the tape was wound backwards. I'm glad I went through and changed those electrolytics and old pots but it looks like the issue this whole time was recording on the wrong side of the tape.
 
You must log in or register to reply here.
Back
Top Bottom