Revox A77 Mk-4 Fast Forward/Rewind problem

[ofery]

Hi,

When I fast-forward or fast-rewind the reel, I hear a fast playback from the speakers. It seems that the tape is touching the head while he is fast-forward or fast-rewind.
Is that normal ?
I can see that the clutch separates the tape from the head, but maybe not enough ?

Regards /Ofer

 

[jan_stevns]

Hi,

When I fast-forward or fast-rewind the reel, I hear a fast playback from the speakers. It seems that the tape is touching the head while he is fast-forward or fast-rewind.
Is that normal ?
I can see that the clutch separates the tape from the head, but maybe not enough ?

Regards /Ofer

Usually this is caused by a bend tapelifter -

There are no adjustable parts. Under the pinch roller arm, you can see two small flanges (blue) - If they are bend, or the pinch roller arm aren't returning properly, the tape isn't retracted enough.
On most Revox'es the tape are running app. 1 millimetre from the heads - and since the playback amplifier isn't muted, you might hear faint sounds while winding.

 

[ofery]

Hi,

I checked the tape's gap from the heads & its ~1 mm from the right head & more from the other two.
Is that means I'm OK & its a normal faint sounds ?

Regards /Ofer

 

[tapetech]

On the switch circuit board (right-hand side), there are two leaf-switches (S5) that mute the audio in fast wind mode. Cleaning the contacts will get rid of your audio output in fast wind mode.

 

[ofery]

Should I open the tape's cover for that ?
If yes then I might take it to my Revox's lab - I'm not a technician ...

Another question please:
My fast rewind is very slow when using 26cm reels, the fast forward is much faster. Any idea why ?

Regards /Ofer

 

[LAB3]

Should I open the tape's cover for that ?
If yes then I might take it to my Revox's lab - I'm not a technician ...

Another question please:
My fast rewind is very slow when using 26cm reels, the fast forward is much faster. Any idea why ?

Regards /Ofer

There are sevral reasons ....motor run cap was leaking oil on that motor and my Mk IV fast foward/rewind was slowing down. I would not keep doing this and have it checked.

Ofer

My Mk IV has been restored... Thanks Tinman.... it has always done this
if volume is up some... I was told the great recording heads are still picking up the recording and my tapelifter is not bent and adjusted correctly. I move the selector from tape to input so my meters are not pegged out and I don't hear the super fast music tones . I was more worried about the meters being maxed years ago when Dad gave me the Mk IV he purchased new back then.
I used a Q-tip whlie rewinding and eased the tape down farther (several MM)
and i still heard the tones. Move the input selector... this seems to be normal for a Mk IV.

Louis

 

[tapetech]

Move the input selector... this seems to be normal for a Mk IV.

Louis

It's not normal. Did you read post #4? There is a muting switch which kills the audio in fast wind mode. There is no need to "move the input selector" if the mute switch is functioning properly.

 

[LAB3]

It's not normal. Did you read post #4? There is a muting switch which kills the audio in fast wind mode. There is no need to "move the input selector" if the mute switch is functioning properly.

Thanks. No I did not read it...duh.. Grand Son was crying. The Mk IV has been in the family from day one. I was 28 when Pop got it new in 1978 and it has always done this. Marc (Tinman) had it for several months and did a complete tear down...replaced over 100 "caps" etc. I asked about this and was told it was normal......... I will call Marc(Tinman) tomorrow and ask about this. Odd it did this when it was new:scratch2: is the Mk IV Dolby model different... muting switch???

 

[Tinman]

I'd have to look at the schematics, but MY A77 doesn't HAVE a muting switch.

Hmmm...

 

[tapetech]

The muting switch is connected to the pinch roller arm. In the arm down position, the play amp output is muted. Go see the switch for yourself on your unit. (You don't need to dissemble it). The linkages for the switch are out of adjustment or the switch contacts are oxidized (or both problems exist). Page 21 of the A77 service manual describes the muting switch operation. It's shown on the schematic as well (S5).

All the versions of A77 I've repaired have had play amp mute switches. I guess it's possible some don't have it, but I've never seen it.

-

 

[LAB3]

The muting switch is connected to the pinch roller arm. In the arm down position, the play amp output is muted. Go see the switch for yourself on your unit. (You don't need to dissemble it). The linkages for the switch are out of adjustment or the switch contacts are oxidized (or both problems exist). Page 21 of the A77 service manual describes the muting switch operation. It's shown on the schematic as well (S5).

All the versions of A77 I've repaired have had play amp mute switches. I guess it's possible some don't have it, but I've never seen it.

-

Thank You for the information. Tomorrow when the Grand Kids are back home I will look for my Service Manual and see if my Mk IV has this switch. 61 and still going to school on these machines.

 

[Tinman]

I just looked... nope, no muting switch. Mine does NOT have one. Wonder if it was modified?
I found it on the manual, but there are only empty spots on the mother board where it's supposed to be.

Louis, If yours DOES.... Then I just plain overlooked it while focusing on the other boards. I apologize if so.
Let me know, I'm curious since my own deck most certainly does not have it. I checked twice.

Learned something new, the schematic (not so clearly) shows it. I had to really look for it to find it.
Well..... it's no excuse regardless, but Louis's machine was such a challenge that I probably would not have
noticed that even IF I knew about it.

 

[tapetech]

The mute switch is activated by lever #54 as seen in the above exploded parts diagram. The switch contact pads are printed on the switch PCB.

It doesn't make much sense why someone would remove the switch from an A77. It serves a usefull function.

 

[Tinman]

The mute switch is activated by lever #54 as seen in the above exploded parts diagram. The switch contact pads are printed on the switch PCB.

It doesn't make much sense why someone would remove the switch from an A77. It serves a usefull function.

You'd think....

I think I overlooked it on Louis's deck because I plain didn't expect it to be there. This IS awkward.... I try very hard NOT to make mistakes, but I guess I still do. Could I have missed it when I took the head carrier out to replace the heads?

 

[LAB3]

The mute switch is activated by lever #54 as seen in the above exploded parts diagram. The switch contact pads are printed on the switch PCB.

It doesn't make much sense why someone would remove the switch from an A77. It serves a usefull function.

I have misplaced the box with the Service Manual..... can you see this mute switch or the wires without removing any covers?? I have the head cover off the door opened and I see the two tabs on part 54 and they look perfect.

 

[Tinman]

Yes you can. With the cover off and the flap down, look under the pivot for the pinch roller arm. You should see a copper leaf with 2 contacts touching 2 copper pads on the circuit board. (No wonder I didn't notice that) Mine is missing that contact leaf, but the 2 pads are there. When you move the pinch roller up and down you should see a lever move the leaf contacts up an down on the 2 pads. Try a drop of deoxit, since these are copper, not gold.

Marc

 

[tapetech]

I have misplaced the box with the Service Manual..... can you see this mute switch or the wires without removing any covers?? I have the head cover off the door opened and I see the two tabs on part 54 and they look perfect.

Yes, you can see the switch without removing any covers. When in play mode, the lowest part of arm 54 pushes a vinyl covered metal lever (the mute switch) . In the stop or fast wind mode this switch lever is not contacting arm 54.

 

[LAB3]

Yes, you can see the switch without removing any covers. When in play mode, the lowest part of arm 54 pushes a vinyl covered metal lever (the mute switch) . In the stop or fast wind mode this switch lever is not contacting arm 54.

I see it ....it has a white cover on the tip of the lever. In the stop position the 54 tab is about 3/16 of a inch from it. When in the play position it pushes the switch lever to the left and useing the wood end of coton swab I can see it is not going any farther to the left. Looks like the front has to come off to acess the mute switch.

 

[LAB3]

Yes you can. With the cover off and the flap down, look under the pivot for the pinch roller arm. You should see a copper leaf with 2 contacts touching 2 copper pads on the circuit board. (No wonder I didn't notice that) Mine is missing that contact leaf, but the 2 pads are there. When you move the pinch roller up and down you should see a lever move the leaf contacts up an down on the 2 pads. Try a drop of deoxit, since these are copper, not gold.

Marc

I don't see the two cooper pads on circuit board. I need to remove the Revox from the cabinet and place it on the table where I don't have to lay down with a flash light.

 

[tapetech]

The switch contacts (4 contact points in all) will be very corroded/oxidized, so some cleaning with very fine sandpaper may be needed. Follow up with a little DeOxit.

It's not a very reliable switch design, BTW. Almost all of the A77s I have repaired over the years have had defective mute switches.