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Old 06-12-2021, 10:00 AM   #1
budwich
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Default Revox A77 MKIV, No play speeds

I have had this 10 inch reel to reel since new. It lost its play speeds a while back and has been sitting patiently ever since. FF and FR both work. When play is selected, the tape guide mechanisms "arm" themselves to position but nothing happens after, no take up reel effort or drive reel movement. Likely candidate would be some form of classic "badcap" maybe, hoping. It was only ever lightly used and probably only fully functioned about 20 years ago.
Any hints / pointers on where to start looking, ie... board / area. Thanks. I have to track down my service manual somewhere in the house... :-)

Last edited by budwich; 06-12-2021 at 11:28 AM..
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Old 06-12-2021, 12:29 PM   #2
R_J
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Default Re: Revox A77 MKIV, No play speeds

In play mode, it is the Capstan that drives the tape, Is the capstan motor working? Check that the motor is not seized, and check C151 motor capacitor.

Last edited by R_J; 06-12-2021 at 12:37 PM..
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Old 06-12-2021, 02:15 PM   #3
budwich
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Default Re: Revox A77 MKIV, No play speeds

ok... thanks. Nothing is turning in the head area which I assume has the capstan / pitch roller "feed". How does that couple into unwind / takeup of the tape at the reel motors?
I haven't gotten inside yet. I did find my service manual... its quite "intense" with three language spread all over ever page. Will take a bit of navigation to locate things but your "hint" of the c151 will certainly help move me to "start" position in this. Thanks again.

edit: I just checked the capstan and was able to spin it with my fingers. Didn't feel stuck but had some "flywheel like inertia" which I kind of expect.

edit2: One other thing, the "end of tape" detector light is not lit so the reel motors operate even without tape... hopefully just a burnt out light bulb although it looks new with no burnt spots.

Last edited by budwich; 06-12-2021 at 02:37 PM..
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Old 06-12-2021, 05:32 PM   #4
R_J
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Default Re: Revox A77 MKIV, No play speeds

I don't think the end of tape light could be the issue, I would think without the lamp the unit should still play but would not shut off at the end of tape. I downloaded the Revox A77 service manual but I am not sure if it is the same as the A77 MKIV

Last edited by R_J; 06-12-2021 at 05:55 PM..
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Old 06-12-2021, 06:15 PM   #5
budwich
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Default Re: Revox A77 MKIV, No play speeds

I only have a hard copy of the SM (came from studer / revox).
I have been trying to read thru the schematic but I must say they are difficult as I don't understand the designation for the pin / connector to where to jump on a subsequent page. You are right about about the bulb, it should not in itself cause an issue with play back BUT, I pulled the bulb and measure it, it is fine at about 100ohms. Based on this, that likely indicates that the 21V power "rail" is not functioning... WHICH might be causing problems elsewhere, including control / speed selection.
I have to figure out how to get the case off, the thing is heavy and the SM says to lay it on its face.... not sure that's a great idea. Attached is a picture of the thing, just for reference as an A77 can be a bunch of versions which are not easily marked as far as I can see.
Attached Images
File Type: jpg revox a77 plus.jpg (1.21 MB, 4 views)
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Old 06-12-2021, 06:29 PM   #6
R_J
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Default Re: Revox A77 MKIV, No play speeds

There is a fuse, F102, (630ma) for the +21v, it is on the tape drive control board. The Capstan speed control circuit is supplied by +21v. So you are likely on the right track.
Attached Files
File Type: pdf a77.pdf (15.32 MB, 8 views)

Last edited by R_J; 06-12-2021 at 06:40 PM..
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Old 06-12-2021, 06:39 PM   #7
budwich
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Default Re: Revox A77 MKIV, No play speeds

I got the case off. I checked the two fuses that I see on the power board. They meter good. I have to figure out / rig something up to circumvent the cabinent interlock system to allow powering up with the case off. It uses what appear to be a couple of metal prongs to push on something inside a female receiver. The insides look ugly in terms of access while powered to measure points and / or removal of boards.... at least at first look.

edit: just to clarify from the first post, the reel motors are turning in play when there is no tape mounted but the capstan does not turn so there is some form of take up / unwind happening.

Last edited by budwich; 06-12-2021 at 07:25 PM..
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Old 06-13-2021, 05:59 AM   #8
budwich
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Default Re: Revox A77 MKIV, No play speeds

Use some screws in the "shorting plug" to override the case "interlock". Confirmed that I DON"T have 21v dc on yellow red out of the power board.... only about 1v. Retested the fuses in and out of the holders, they are good (low resistance). Have to check thru other points on the card to see what's up. Operational access is somewhat limited. I haven't looked at how to remove the card yet but it does not look like fun.

edit: transformer output is 24v AC.

Last edited by budwich; 06-13-2021 at 06:12 AM..
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Old 06-13-2021, 09:54 AM   #9
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Default Re: Revox A77 MKIV, No play speeds

There is an issue with the readings in and around Q102 (sdt9201 NPN). The emitter shows -30.4v and base -29.9v while the collector is 0 (expected as it is tied to ground). Schematic shows a test voltage of -6v at the emitter... not good. checked q102 in circuit diode test appears to be good. Perhaps the capacitor c101 (1000 uf 35v) is an issue... likely based on age and the fact that the unit exhibited the problem from little to no use which is unlikely to take out the transistor.
I haven't figure out how to take the board out without a lot of disassembly as there is a few screws in places that have limited access.
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Old 06-13-2021, 10:05 AM   #10
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Default Re: Revox A77 MKIV, No play speeds

have you a link to the service manual ? i only have mk1 mk2 ones
its ok i have it now .
maybe the 1k resistor thats from emitter collector is bad ? or the cap as you said .

Last edited by petehall347; 06-13-2021 at 10:13 AM..
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Old 06-13-2021, 10:24 AM   #11
R_J
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Default Re: Revox A77 MKIV, No play speeds

Quote:
Originally Posted by budwich View Post
There is an issue with the readings in and around Q102 (sdt9201 NPN). The emitter shows -30.4v and base -29.9v while the collector is 0 (expected as it is tied to ground). Schematic shows a test voltage of -6v at the emitter... not good. checked q102 in circuit diode test appears to be good. Perhaps the capacitor c101 (1000 uf 35v) is an issue... likely based on age and the fact that the unit exhibited the problem from little to no use which is unlikely to take out the transistor.
I haven't figure out how to take the board out without a lot of disassembly as there is a few screws in places that have limited access.
It looks like Q102 is not being turned on to lower the voltage to -6v. Check the resistors in that area, like R107 across Q102, R103, & zener ZP12. I would also work the pot P106 back & forth in case it the problem.
The cap C101 & C103 are likely not causing this problem,
Does the service manual I attached seem to match yours? (power supply page 48)

Last edited by R_J; 06-13-2021 at 10:33 AM..
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Old 06-13-2021, 11:16 AM   #12
budwich
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Default Re: Revox A77 MKIV, No play speeds

sorry, I didn't originally see the attachment. Yes it is the same as my hard copy and pictured unit. Thanks.

I have been trying to get at the board more readily. I have to move / remove the brake actuating solenoid as it is blocking access to one of the mount screws of the board.

With my limited understanding of things, not understanding how the voltage gets to be so negative (some sort of "chopper", I guess) although if I do the "simple math" on the difference, you get about "27v" on the schematic (21 and 6) while currently I see about "31v" (1 and 30). Of course, that's my "simple". :-)
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Old 06-13-2021, 11:52 AM   #13
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Default Re: Revox A77 MKIV, No play speeds

do you have the +27v ?
edit ....i guess you have but worth checking .
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Old 06-13-2021, 12:13 PM   #14
budwich
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Default Re: Revox A77 MKIV, No play speeds

yes I check the 27v source, its fine... I was just playing with math numbers for no reason. sorry for the "dribble".
I checked the resistors as suggested, they measure as spec'd although r107 was "moving" as it must have some "charge path" somewhere causing the meter to "drift" towards the value as something charges.

I also checked the 12v "test point" (top of r103, clamped by the zener), it wasn't good. Was giving me just under 1v. Perhaps that points to a dead ZP12 (d104)?
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Old 06-13-2021, 12:23 PM   #15
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Default Re: Revox A77 MKIV, No play speeds

The regulator circuit controls the negative side and not the positive side like a "normal" circuit would. R107's reading could be effected by a charged C101. make sure it's not open.
The -6v voltage is with respect to ground (0v) since the circuit is not working and q102 is effectively open, you measure -30v on its emitter, as Q102 gets turned on, the emitter voltage will drop to -6v and this will show as +21v on C101 (+) with respect to ground.

It might make it easier to understand the circuit if you think of the [+21v] line as ground, [-6v] line as -30v input and this would regulate a -21v output at [0v].
So I would remove R107 and check it out of circuit to make sure it is not open.

Last edited by R_J; 06-13-2021 at 12:53 PM..
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Old 06-13-2021, 12:53 PM   #16
budwich
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Default Re: Revox A77 MKIV, No play speeds

pulled one leg of r107 to isolate it. It test 1k. Its seems strange that the zener is not hold 12v as it should. Physically, it doesn't look damaged.
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Old 06-13-2021, 01:00 PM   #17
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Default Re: Revox A77 MKIV, No play speeds

Quote:
Originally Posted by budwich View Post
pulled one leg of r107 to isolate it. It test 1k. Its seems strange that the zener is not hold 12v as it should. Physically, it doesn't look damaged.
Diodes can short and still LOOK good. check that the diodes are not shorted
Quote:
Q102 (sdt9201 NPN). The emitter shows -30.4v and base -29.9v
This does not seem right, I can see -30v on the emitter, but if the transistor is good it should not show -30 on the base, It could be that the transistor is in effect open so it will not conduct to provide a path to ground for the regulator circuit.

Last edited by R_J; 06-13-2021 at 01:08 PM..
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Old 06-13-2021, 01:29 PM   #18
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Default Re: Revox A77 MKIV, No play speeds

I did check the transistor (in circuit... its heat sink mounted and would a bit to get it out), it seemed to diode check correctly for an npn transistor. The voltage difference does seem to suggest some form of "diode junction operation" of ~.5v or so.

The zener diode seems to pass some checks but I can't check the "12v ceiling".
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Old 06-13-2021, 03:03 PM   #19
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Default Re: Revox A77 MKIV, No play speeds

you should test both those diodes . a couple of 9v batteries and current limiter resistor will do to test the zener .
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Old 06-13-2021, 05:17 PM   #20
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Default Re: Revox A77 MKIV, No play speeds

I have a large Pioneer. Its reel motors work independently of the capstan drive, like yours. After a time in storage, I had to replace the [capstan]motor-to-captstan-idler drive belt. It had 'frozen' to the flywheel. Worked like a charm after that!! I always thought of the electronics as being bulletproof, like the heads. Maybe a cap went bad, but check the mechanicals first. One caution; when you get around to cleaning the pinch rollers, be VERY CAREFUL of the cleaning fluid you use. Good old isopropyl may- in the long run- dry out the rollers (!!), but it won't eat them up like some marketed fluids.
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