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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 22nd Jan 2022, 3:01 pm   #1
OldKitMan64
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Default Testing before replacing Reel Motor A77 Mk3/4

After getting my Revox A77 case apart I have found the reason for a strange burning smell and lack of power on the right hand reel motor of my A77 Mk3/4.
I have attached a photo to show what I have found.
This looks like the failure.Click image for larger version

Name:	A77_Reel Motor_Burned_BrokenWindings_Small_2B.jpg
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Now - I have sourced a replacement motor from eBay (1.021.205) which looks fine but before I replace this I wanted to check voltages and characteristics of what I should see on the 4 supply pins (from the motor supply board) on the A77.
I think from the manual I downloaded these are D13 (Yellow), D14 (Green), D15 (Blue) , D16 Brown.
I have not found yet how to check the supplies before swapping over
Does anyone have any test procedure on this that will help me out.
I have access to a good AC/DC multimeter and somewhere in one of my cupboards I have an old Philips Oscllioscope but have not used it in years.
Any help and pointers on this would be much appreciated to ensure things are stable before putting in the new motor
What is puzzling why the reel motor would fail like this. Anyone else seen this happen ?
Thanks
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Old 22nd Jan 2022, 7:32 pm   #2
ms660
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

So far as I can make out from the schematic, with the reel motors switch in the on position, the tape threaded up so the end of tape sensor is defeated and the fast forward function selected then D13 & D14 should be connected through to the 105V connection on the secondary of the mains transformer, D15 should be connected through to the 0V connection and D16 should be connected through to the 0V connection via the motor capacitor, so with the conditions above and no motor connected the AC Voltage between D15 and D13 or D14 is likely to be a bit higher than 105 Volts AC with no motor connected.

With play selected D13 & D14 should be connected through to either the 55V or the 42V connection on the secondary of the transformer depending on the reel size setting, again that AC voltage could be a bit higher with no motor connected.

Lawrence.
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Old 23rd Jan 2022, 7:10 am   #3
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

Do the windings appear to be damaged? Hard to tell from the photo. Is it possible the motor run capacitor is faulty?
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Old 23rd Jan 2022, 9:40 am   #4
Michael Maurice
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

I've yet to see a motor on any of the Revox's burn out like that. As the power for the motors come from the mains transformer via the relays, its hard to see how a high voltage to the motor could occur. As Tim said, the motor run capacitor could be the likely cause.
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Old 23rd Jan 2022, 10:28 am   #5
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

Not related to the capstan motor but if you have not already checked (as these have a habit of going up in smoke), check the suppression capacitors plus their resistors as per Post 11 in your previous Thread.

https://www.vintage-radio.net/forum/...d.php?t=187595

David
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Old 23rd Jan 2022, 10:39 am   #6
vidjoman
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

It might be a trick of the light but that winding looks damaged compared to the one next to it.
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Old 23rd Jan 2022, 2:43 pm   #7
TIMTAPE
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

A clearer photo would make it easier to say with more certainty. It needs to be taken closer to reveal the state of the windings at the place where it looks as if they could be damaged.
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Old 23rd Jan 2022, 7:57 pm   #8
OldKitMan64
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

Thanks again to all.
As I bought this machine second hand years ago - it is difficult to know exact year of manufacture but it is certainly one of the later models as it has the IC based speed Capstan control
I'm also just working off a Service Manual that I could find on the web which is not a direct match for what I have !!
Apologies for the picture - I will try to compress the 2MB file I have sand send that as the 168KB photo is not much use.
To answer one question - yes the windings are damaged. This is a mystery as the chassis has never been out of the box and the machine has performed perfectly up to now.
The left reel motor was working ok before the problems I encountered with the right Reel motor.
C113, C114 and C115 and the 4.7 ohm resistors all look perfect (no cracking)
Now I'm not sure about C152, C153 C151. The latter 2 must be OK as the motors were OK.
Would a faulty C152 take out windings . HAs anyone experience of this?.
A kit of all three new capacitors is available on eBay - but not sure I need them. There is no physical damage.
I hope the zip file works for the higher res picture of the windings - it is the evaluation version !
Attached Files
File Type: zip A77_Reel Motor_Burned_BrokenWindings_1.zip (1.95 MB, 29 views)
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Old 24th Jan 2022, 12:02 pm   #9
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

As it's right at the top of the case is it possible some corrosive liquid has found its way onto the coil? As Michael says, burnt out reel motors are a rarity.
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Old 24th Jan 2022, 5:20 pm   #10
Michael Maurice
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

A faulty motor run capacitor could cause this sort of damage as the coils wont be running at the correct phase, but I still cant see it doing that sort of damage,
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Old 24th Jan 2022, 9:09 pm   #11
TIMTAPE
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

I cant open the Zip file at the moment but if there's mechanical damage to the winding maybe the A77 chassis had become jammed inside the case - it happens - and someone tried to prize the case off with a tool, unknowingly snagging the RH reel motor winding.

I once had a similarly damaged reel motor winding but the damage wasnt too severe and I was able to resolder the one or two broken wires together.
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Old 9th Feb 2022, 6:27 pm   #12
OldKitMan64
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Default Re: Testing before replacing Reel Motor A77 Mk3/4

So just to bring this thread to a happy ending I am pleased to confirm the A77 is now back to life and fully functioning - well 99%
My initial inspection of the snubber networks and the vulnerable Capacitors C113, C114 and C115 was not very thorough (blame the bad light and my eyes !).
In fact the 4.7Ohm resistor on the Right Reel motor had fried and C113 was badly cracked and damaged at the rear and leaking goo.
C113 had 13.8ohms DC resistance - so this was not good for the reel motor windings, hence burning and broken wires.
I've replaced all snubber network components, plus new Motor Caps.
The replacement motor seems fine
Only minor issue I have is braking on small diameter reels. Strangely 10inch reels appears ok. Have cleaned brake linings etc and adjusted tension (trial and error as no tension measurement available).
I mostly use the 10inch reels now so not a big issue.
Thanks to all for the posted comments. It's good to solve a problem like this and see an old bit of kit come back to life.'
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