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Revox A77 MK1 Problem

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Vasil Penev

New Member
Hey. A friend of mine gifted me the Revox. We've made recordings with it at his place and it sounds great, I probably haven't heard a more sweet sounding machine, the heads are as good as new.

Ive been always bugging him to play this machine and I noticed one time, power was on, light was on, but it started and stopped just like that. After we tried a few times, after a few resets it went back in action.

Now, as it usually happenes, I got the machine, i noticed speed slowing down sometimes while playing. Then on the next day it started stopping. I cleaned the tape path so many times, lots of gunk was there, especially on the pinch roller, it is gone now, and the problem seemed to dissapear but I was wrong.

Apparently it was an electrical problem all along, I guess a transistor. Then capstan stopped turning at all. It should be on at all times.

Please tell me a simple transistor or condensator change will fix the problem and I haven't burnt the motor trying to start it. I have a proper technician at hand, hes not free atm, and Im terrified I f*cked the motor up. I'm not an engineer I didnt realise you shouldnt turn it on at all when it has such a problem.

This machine sounds so sweet and is built like a tank I am so sad -.-
 
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Hello and welcome here.
You describe several symptoms. I assume the only problem is that the capstan motor stops, correct?

If so, try the following:
Turn the unit on and check if the motor starts. If it starts and stops after 1-2 seconds, then very likely the problem is the capstan motor speed adjustment trimmer. The wiper may have become loose.
If it doesn't start at all, then it's probably the power transistor on the motor control board - or something around it - that is at fault. It should be easy for a technician to find the problem.

BTW, it's very difficult to destroy the motors. If they don't get hot, they are probably OK.

One word of caution. There are some old capacitors in these decks - read the forums for details. If, when you power up the deck, you hear a loud bang and smell a fishy smoke, don't get terrified, it's the RIFA capactor in the back that has blown.
 
Yes, SaSi has given good advice, from the 100's of threads I have read, many citing the RIFA caps as ticking time bombs. Have your technician replace all of them, if you want a reasonably trouble free deck. Most of the time, they fail open, meaning you simply have a broken path in a circuit. On occasion, a cap can fail short-meaning it simply acts like a piece of wire, sending along it's current to the next component in the path, which may kill a transistor or an IC, and some of these devices may not have direct replacements by now.

And yes, I have also read that the motors are robust AC units, and likely have survived.

The A77 is a very good machine-it's worth spending some money to have it restored to safe and reliable operating condition.
 
SaSi said:
Hello and welcome here.
You describe several symptoms. I assume the only problem is that the capstan motor stops, correct?

If so, try the following:
Turn the unit on and check if the motor starts. If it starts and stops after 1-2 seconds, then very likely the problem is the capstan motor speed adjustment trimmer. The wiper may have become loose.
If it doesn't start at all, then it's probably the power transistor on the motor control board - or something around it - that is at fault. It should be easy for a technician to find the problem.

BTW, it's very difficult to destroy the motors. If they don't get hot, they are probably OK.

One word of caution. There are some old capacitors in these decks - read the forums for details. If, when you power up the deck, you hear a loud bang and smell a fishy smoke, don't get terrified, it's the RIFA capactor in the back that has blown.
Click to expand...

I used to do that, although used to doesn't describe it properly, it used to turn on prperly then slow down, then come back to life, but it all happened in a matter of a day. I guess it must be the transistor. Some other people on here have said that if the tacho head went bad it gave other symptoms. I hope I didnt ruin the engine and I have a connection to a very skillful oldschool technichian.... Its just that there is pain in my heart... I really hope I didnt blow the motor.... I know that transistors have a shelf life, if only that, and that we're speaking about high voltage ones, esp the power transistor I heard runs at 80 volts, again I have medical education, sorry if Im wrong

I have no knowledge of capacitors, apart from the obvious when it doesn't look right, I haven't sensed a smell of a burnt one, nor do I hear a noise when I turrn it on or off. When I asked my father, who's lived where it was at, he said, big capacitors go down with a bang, and he said transistors have a certain amount of time to live, then they are gone...

I know a friend who lost alot of money because replacement transistors weren't made anymore and he lost clients because the ones in stock had a shelf life that has passed. I dont feel well... I hope my friend fixes this machine because it deserves to work. It sounds so good.

I
 
oldvinyldude said:
Yes, SaSi has given good advice, from the 100's of threads I have read, many citing the RIFA caps as ticking time bombs. Have your technician replace all of them, if you want a reasonably trouble free deck. Most of the time, they fail open, meaning you simply have a broken path in a circuit. On occasion, a cap can fail short-meaning it simply acts like a piece of wire, sending along it's current to the next component in the path, which may kill a transistor or an IC, and some of these devices may not have direct replacements by now.

And yes, I have also read that the motors are robust AC units, and likely have survived.

The A77 is a very good machine-it's worth spending some money to have it restored to safe and reliable operating condition.
Click to expand...
ok I read some now I understand, I'll tell him what I read here, maybe a bad cap burnt a capacitor
 
Vasil Penev said:
is this irony, sorry, but this guy is a captain first rank and has been dealing with radiotechnics all his life, several friends rely on him to fix their machines
Click to expand...
Just trying to be funny-an adaptation of an old Doctor's adage: "Take two of these and call me in the morning"

I think your tech should be able to fix the deck, and you will feel better, and the worry about the motors will be alleviated.

Glad to hear you are set on getting it fixed-I am confident you will be happy when it returns to you.
 
Have the Rifa caps been changed out of this deck? If not, they need to be, it's the first thing to do to an A77; they've probably failed or are failing. There's three or four of them, it's an easy swap, the board doesn't even need to be removed if you're careful.
If you change these out the deck may be ok unless they took something else with them when they shorted.
 
I got it back, it was a faulty transistor, capstan is working now and speed is alright.

But now I notice something Ive noticed before. On CH1 - left channel - when recording, the levels sometimes drop I can see it on the meters and hear it on playback. I tried a few methods of recording from a record, and the best sound I got was through the headphone jack of my amp, although I get no monitor this way (if I take the sound from the output in the amp (it says tape I and has an input and output) it does something with the sound, boosts the highs and it is too loud. So until I get a standalone phono it sounds best from the headphone jack (ofc you have to be careful with the volume control)

So what happens it the highs especially sound like a butterfly is flapping it's wings ontop of the sound if that makes any sense. Then it goes normal for a few seconds, then it does it again or drops off for a second or two (like almost no highs and low volume), then comes back again, the higher the recording level is, the worse it is, and maybe even starts affecting the right channel as well but Im not sure. This flapping effect is most noticeable when I record through phono or line in but from digitalised rips from records.

From pure digital test recordings, it sounds much better, but it is still there, I guess the noise in those recordings from analog makes it more prominent.

When I switch the input channels while recording the problem remains in the left one. When I switch the output channels upon playback, problem moves to the other channel, but it is in the recording no doubt.

Ive cleaned the tape path many times, the heads look pristine (and sound like it). The delete head was the dirtiest, there was alot of light brown gunk in a spot, when it all came off I think there is something like a tiny scratch on it. The other heads look nice, but are really hard to clean because of their casings. I still did what I could, the whole cleaning process didnt affect the problem at all, I know how a dirty or damaged head sounds like - muffled - but this doesnt sound like that

Even recording at 5 and volume knob on amp so low that meters dont even reach the red, the problem still appears, if you bump the levels it becomes worse, I record some tracks from digital to check again, because its much more apparent when you rec from a source with noise (you hear the noise flapping if that makes any sense). I guess thats another electrical issue :(
 
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You need to re-examine the heads again. You HAVE TO use a dental mirror to examine the heads for tape oxide deposits. And use a magnifier with strong light. I would bet there is still some oxide around the top track which is the left channel. And also know that sometimes IPA is not strong enough to remove all deposits. You need to use a stronger solvent if that happens. Be aware that if you have a defective tape, it can clog up the head in as little as 5 minutes, so even if you correctly cleaned the heads, they CAN go back to being dirty very quickly. You need to find out which of you tapes are defect and which are ok. If you keep using defect tapes, your problems will never go away.

When testing a deck, it is always best to use new tape like SM911, LPR90, ATR Master, MDS36.
 
Hmm cleaned the heads again and again the left channel just does what it pleases, becomes louder than the right, then becomes quieter, then muffled, I think its really an electrical problem it doesnt sound like dirty heads, but Ill clean more
 
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