Revox A77 Stops Playing (Transport Fault)

[Raving Loony]

I am new to posting messages on a forum so please accept my apologies if this post is incorrect.

I have what I believe is an early Revox A77 low speed (Frankenstein version) as made with some parts from other models.
I purchased this poor beaten Revox from EBay a couple of years ago as unfortunately a nice newly serviced B77 was out of my price range. Over time I have learned a lot about the workings of the A77 and have managed to get it working and sounding great. I am not an electronics engineer but have a basic electrical knowledge.
My problem is when playing a 10½ inch reel it stops about 10 minutes before the end. Any attempt to press the play or wind buttons results in a loud buzzing noise which I guess is the relays. I have changed and cleaned the relays but this has no effect on the fault. After powering off the machine for a couple of minutes I am able to continue to play the rest of the tape. There is no end of tape bulb present at this time.
I struggle a bit with a visual inspection of components as I am partially colour blind but I do have a meter and an ability to solder if given reasonable instruction on what to check or change.
I have a great passion for my Revox and If anyone would be kind enough to share their thoughts or experiences on this problem I would be very grateful and so would my Revox.

[NorfolkDaveUK]

Ack ....just re-read your post ..I skipped though it ..sorry ..

I`m really not sure why it would do this other than a tape jam . It really doesnt make any sense that it`ll play most of the tape and then just stop . Firstly I would recap the power supply . I.e. change all the capacitors . It sounds like something is getting worn out . That would probably be my first approach

[Michael Maurice]

Don't recap until you've found the fault, you could make matters worse.

First, there are three 470nF capacitors on the transport board. If they are the originals change them. Use only polypropylene types rated for 275V~.

Then try playing a tape. if the fault continues, without switching the machine off, remove the tape and hit, play, rew or FF. Check what the two spooling motors are doing.

Please report back.

[Raving Loony]

Many thanks NorfolkDaveUK for your reply.
There is no tape jam and no other signs of any power problems. It's only the transport that stops.
Maybe the power supply is a good starting point although I would probably struggle with identifying what components to purchase and replace due to my limitations. I am happy to give it a go if I have a bit more info or pics. My knowledge of reading a diagram is non existent. If there is anywhere I can get more information on how to do this it would be very much appreciated. I feel confident to do this once armed with as much info as possible.
Many thanks.

Many thanks Michael Maurice for your reply.
Forgive my ignorance but first of all I would need to identify which board is the transport board. If the capacitors are readable I should be able to identify them and replace them.
Just to clarify the tape recorder never switches off (only the transport stops).
After 45 mins play when it switches off again I will try what you suggested and let you know what happens but can't change the components without more info.
Many thanks.

[Ted Kendall]

The transport board is that which carries the relays. The capacitors to which Michael refers look like rectangular boiled sweets, unless they have failed already, in which case they look a charred mess. It's not a question of whether they go, but when, and it's best to replace them all before you get the firework display. This doesn't actually cause much further harm, but the smell is something to hear...

[Michael Maurice]

If you remove the machine from its cabinet, looking at the rear of the machine, the transport board is to the right, marked in red. The three capacitors are marked with blue lines.

There is forth one which doesn't usually give much trouble, because the voltage across it is around 60V its on the capstan drive board and can be a pig to replace but if I'm overhauling these machines I replace it.

I use these capacitors as replacements.

[NorfolkDaveUK]

The suppression caps aren`t causing this . They only suppress sparking in the relays . I`ve got 4 x A77`s her at the min , all with fried supression caps and all working fine other than stinking and sparking like a good`n This is being caused by a power supply cap getting hot and either discharging or slowing right down or something I should think , I would let it fail and then have a feel around , I can almost guarantee you there will be a hot cap in there somewhere (almost i`ve been wrong many times .lol). Probably up on the power supply . If you look at Maurice`s pic . its above the capstan motor . Power off as soon as its quit on you and have a feel around , be careful , its probably going to be pretty hot if this is the issue when you find it .

[NorfolkDaveUK]

BTW I came to this conclusion as it working again after a while , and thats probably because the cap has cooled down enough to start functioning again. let us know if you find a hot one.

[Raving Loony]

Many thanks for the info and the link.
I'm going to try NorfolkDaveUK's post to see if a power supply cap is overheating and see where I go from there.

[Raving Loony]

Hi NorfolkDaveUK
I am going to test out your theory later today. Logic suggests an overheating problem of some kind.
I had a quick look at the board and was wondering if there is already signs of overheating. I have attached a pic for your opinion.
Many thanks for all your advice.

[Ted Kendall]

As I've said before, the Rifa caps are there for a purpose. The fact that the machine may continue to run after they fail is beside the point. The youngest of them fitted to these machines have now exceeded their design life and should be replaced as a matter of routine. Their correct functioning is a necessary but not sufficent condition for proper operation.

As to the fault itself, have we established what is meant by "stop"? Does the pinch roller drop out or does the capstan stop with the pinch still engaged?

[NorfolkDaveUK]

oh of course they have to be replaced i wasnt suggesting you shouldn't , or , dont need too . no you must replace those no matter what, you know me I replace all the caps as a matter of course they are 50 years old in these machines , i was just saying its definitely not whats causing this fault . after sleeping on it , i remembered i had one B77 with a very similar problem and it was the transformer that was getting red hot so the op might want to check that too , in my case i had to replace the transformer because it was internally short .

as for your question Ted I`m pretty sure its engaging stop and not allowing him to re apply play until "something" has cooled down.

[Raving Loony]

Many thanks Ted Kendall / NorfolkDaveUK.
Yes I agree on the replacement of the caps regardless of the fault I have.

When the fault occurs it has the same result as pressing the stop button. When pressing any of the transport buttons straight after the fault results in a loud buzzing noise which sounds like the relays. After powering off for a few minutes all functions work again.

[NorfolkDaveUK]

It sounds to me that something on the power supply board is failing , once what ever it is is getting hot . the voltage is dropping , once it gets to a certain low voltage the relays are no longer able to engage because they dont have enough voltage/current , this will be cause by a failing cap somewhere on the power supply board . As i say have a feel around for a hot one first of all . My GUESS would be it will be getting hot . If its not then its just not holding charge after a while and once it does that then it wont discharge enough current to allow the transport to engage . Its definitely a low voltage/current issue you just need to figure out which of the rails its on . Take your meter , measure all the voltages coming from the power supply . then let it fail and check them again , there is going to be a low one some where , dont forget to look for a hot cap though . you will find one I think

[Raving Loony]

Many thanks.
I will take it apart again today or tomorrow and let you know what I find. I have a basic meter but not sure what the voltages should be so I can only make a note of before and after. Does my pic above show any signs or overheating.

[NorfolkDaveUK]

Ok they should be the following starting from the bottom ......

yellow , red , = 21v

green , blue , yellow , red = 0v (use the black probe here to reference the others)

white , violet , violet , black = 27v

check those ones , use black probe on 0v (any of them) and the red on the one you are checking , dont use the chassis as a reference , you wont get the right results .

This diagnoses is pure speculation at this point so I could well be way off , but it sounds the most likely given the symptoms . Let us know what you find.

the cap laying horizontal look decidedly dodgy to me (C101 - 1000uF) . . but you will know for sure when you start probing around.

[Raving Loony]

Many thanks for your detailed info. I would have use chassis otherwise. Will have to talk the wife into identifying the colours for me. Just tried making a video of the fault and it played a whole tape without issues but I know it won't last.

Looking at my above image the colours don't appear to be as you listed. Is this something I should be attending to or just measure the voltages according to the colours you have stated.

[NorfolkDaveUK]

the order of the wires isnt importants as long as they are connected to the correct supplies ,....which they are bud.

green to yellow in the middle set are your 0v .

[Raving Loony]

Many thanks

[NorfolkDaveUK]

btw,,...i was starting from the bottom on the schematic not the board.......sorry for the confusion...