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Old 11th Nov 2021, 11:39 pm   #1
Michael Maurice
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Default Revox A77 MK4 capstan problems

This machine came to me with no capstan rotation at all after the owner decided to do a recap and changed a few other parts as well.

This board is based on two IC's and is very similar to that of a B77.

I've managed to get the motor going and its stable at the slow speed but at the fast speed 7½, it is all over the place.

Measuring the pick up frequency, its correct at 3¾ giving 800Hz but at 7½ it goes up to over 1600Hz, then goes down to about 1300Hz, then slowly back up but never settling on the correct speed.

Apparently it did this before the owner decided to carry out his 'service'

Any ideas?
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Old 13th Nov 2021, 1:04 am   #2
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Default Re: Revox A77 MK4 capstan problems

Am wondering if it is related to the tacho pick up head. I have read before that is it more critical on 7.5 ips speed.

The manual refers to air gap between the pick up head and rotor in section 6.2.1 adjustment, although does not really explain how to adjust.

David
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Old 13th Nov 2021, 1:49 am   #3
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Default Re: Revox A77 MK4 capstan problems

The plastic moulding/bracket of the tacho head is prone to cracking up. Plenty of A77s need new tacho heads and there are people looking for them. Mine is Araldited.

David
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Old 13th Nov 2021, 9:15 am   #4
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Default Re: Revox A77 MK4 capstan problems

A reversed polarity or wrong value electro or other component he replaced on the PCB?
7.5ips switch faulty or misadjusted?
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Old 13th Nov 2021, 10:07 am   #5
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Default Re: Revox A77 MK4 capstan problems

A reversed capacitor is unlikely if it did the same before his ministrations, but then can you be sure that it's just the same?

David
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Old 15th Nov 2021, 11:41 pm   #6
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Default Re: Revox A77 MK4 capstan problems

I'm well and truly stuck, I've actually ordered a new board for it, in the meantime I'd like to try and get this one working but its quite difficult to work on.

A question: Should I have an output from pin 3 or the 555 timer chip no matter what happens at the input pin 2?

A diagram of the Mk4 board is in the attached file.
Attached Files
File Type: pdf A77 capstan control board.pdf (129.9 KB, 117 views)
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Old 16th Nov 2021, 11:35 am   #7
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Default Re: Revox A77 MK4 capstan problems

Looks to be a monostable so no waveform at the input would mean no waveform at the output so far as I know.

Lawrence.
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Old 16th Nov 2021, 12:19 pm   #8
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Default Re: Revox A77 MK4 capstan problems

I have always found this confusing, with a stationary capstan motor presumably there is no tacho signal to the board, so how does the NE 555 get triggered to produce an output to drive the motor, i.e. chicken and egg situation. Maybe on power up the NE 555 initially becomes free running.

David
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Old 16th Nov 2021, 1:13 pm   #9
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Default Re: Revox A77 MK4 capstan problems

I was under the impression that the 555 ran free uncontrolled until a tacho signal was received and then the motor slowed down to it's set speed which is why the motor is heard to start to run and then slow down.
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Old 16th Nov 2021, 1:22 pm   #10
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Default Re: Revox A77 MK4 capstan problems

Quote:
Originally Posted by Analogue man View Post
I was under the impression that the 555 ran free uncontrolled until a tacho signal was received and then the motor slowed down to it's set speed which is why the motor is heard to start to run and then slow down.
I think that's just the turn-on settling transient of the control loop Revox chose. Earlier A77 with the LC tuned discriminator before the 555 was ever thought of do this too. It's a reasonable response, the turn-on surge prevents trouble with bearing stiction from stationary.

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Old 16th Nov 2021, 1:29 pm   #11
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Default Re: Revox A77 MK4 capstan problems

Some 555 data here including some application circuits:

https://www.ti.com/lit/ds/symlink/ne555.pdf

Lawrence.
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Old 16th Nov 2021, 2:48 pm   #12
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Default Re: Revox A77 MK4 capstan problems

The symptom suggests to me the basic speed regn cct is working but the feedback loop is unstable, not tuned properly, so it can't reach a quiescent state. Maybe a cap out of spec.
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Old 16th Nov 2021, 3:04 pm   #13
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Default Re: Revox A77 MK4 capstan problems

As 555s are being discussed, I assume everyone knows about the 555 design bug? It can result in unexpected behaviour in some circuits, but I don't think this one is susceptible.

On an output pin transition, the plain bipolar 555 output stage will momentarily try to short the supply pin to ground, and emit a group of fast pulses on the output pin. ALL makes are at risk, as the artwork was either shared or copied perfectly, bug and all. The burst of pulses plays hob with digital circuits, but a pulse averaging discrim should be OK. But you do need good decoupling.

CMOS versions of 555 are well-behaved.

David
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Old 16th Nov 2021, 6:51 pm   #14
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Default Re: Revox A77 MK4 capstan problems

Check the wavefrom at E against the Revox data. I'd say the issue is possibly around that diode matrix area assuming HT etc is correct.

John
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Old 24th Nov 2021, 9:45 am   #15
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Default Re: Revox A77 MK4 capstan problems

I ended up buying a board and after changing a couple of caps and the preset, it worked.....sort of.

However

When you pressed play the capstan stopped, when you pressed FF, the capstan reversed!

I found out what it was, can you guess?

There are more problems which when I've had some answers to the above, I will go into.
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Old 26th Nov 2021, 11:23 pm   #16
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Default Re: Revox A77 MK4 capstan problems

I'm surprised nobody even tried to guess, well here's the answer:

The customer replaced all the capacitors in the machine from a kit he obtained from Germany. This included a motor run capacitors and it was here that the problem lay. There are two black wires one goes to the capstan motor, the other to the FF motor, the customer crossed the wires.

Now I've got that running, does anyone know what causes severe lack of torque in one of these motors?
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Old 26th Nov 2021, 11:27 pm   #17
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Default Re: Revox A77 MK4 capstan problems

Lack of current or too low phase shift capacitor. Are the bearings free?

Capstan or spool?

If capstan, short the AC terminals of the bridge. This will put full power/speed on the motor. Enough torue now? Check the voltage from the transformer is OK

David
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Old 27th Nov 2021, 12:46 am   #18
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Default Re: Revox A77 MK4 capstan problems

Its the take up reel motor, the one affected by the error in wiring. The phase shift capacitor is new, the bearings are free
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Old 27th Nov 2021, 2:32 am   #19
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Default Re: Revox A77 MK4 capstan problems

Quote:
Originally Posted by Michael Maurice View Post
I'm surprised nobody even tried to guess, well here's the answer:

The customer replaced all the capacitors in the machine from a kit he obtained from Germany. This included a motor run capacitors and it was here that the problem lay. There are two black wires one goes to the capstan motor, the other to the FF motor, the customer crossed the wires.
On my MKIV a brown and a black wire are joined at the capstan motor run capacitor terminal. Had he swapped this black wire with the black wire on the FF motor capacitor terminal?
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Old 23rd Jan 2022, 10:50 am   #20
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Default Re: Revox A77 MK4 capstan problems

I finally got this machine working. The reason for the poor take up spool rotation was that the black and white wires on the capstan board had been reversed!

I now had a fully mechanically functioning machine, but now it was the electronics turn. There was a huge difference in output between CH1 and CH2 which was eventually traced to a cracked print around one of the capacitors that the owner had changed.

After that it was plain sailing through the alignment before it was ready for the customer to collect.

As an aside I dont know where he got his kit from but I didn't think much of the quality of the parts, also why get the owner to change all the semiconductors? I'm sure it was this that caused the problems with the capstan board.
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