steerpike2
Super Member
No, not normal. A 'reverse' surge makes me thik even more it's a fault in the motor drive electronics. A fault in the servo system could make it surge, but would only make it do it in the 'normal' direction.
Revox B 795
- Thread starter Robie
- Start date
No, not normal. A 'reverse' surge makes me thik even more it's a fault in the motor drive electronics. A fault in the servo system could make it surge, but would only make it do it in the 'normal' direction.
Just to be clear, the turn on surge goes in both CW and CCW directions. This is beginning to make sense based on the following:
The required hand assist at start up is necessary because the platter only receives motor assist at certain rotational positions. The hand assist rotates the platter to a position where the motor engages and it can then spin up to speed. However, at speed, the platter is receiving intermittent motor assist throughout each full rotation and this is the cause of the wow/warble. Strange as the PL speed light is now constant at speed and there is no intermittent blink as before (progress thus far).
Furthermore, with the power switch on and the arm in its rest position, when the platter stops spinning, it rocks back and forth before stopping. I just rotated the platter by hand (with the power switch still on) and there are 6 positions in each complete platter rotation where the platter will stop. At each of these 6 positions the platter will rock back and forth slightly before stopping in the middle. With the power switch off, the platter will stop anywhere and spin freely so it seems that the bearing is fine.
If the platter stops in the middle of one of the 6 positions, there is no platter surge/movement when the power is turned on, but the platter will not spin up to without hand assist.
The required hand assist at start up is necessary because the platter only receives motor assist at certain rotational positions. The hand assist rotates the platter to a position where the motor engages and it can then spin up to speed. However, at speed, the platter is receiving intermittent motor assist throughout each full rotation and this is the cause of the wow/warble. Strange as the PL speed light is now constant at speed and there is no intermittent blink as before (progress thus far).
Furthermore, with the power switch on and the arm in its rest position, when the platter stops spinning, it rocks back and forth before stopping. I just rotated the platter by hand (with the power switch still on) and there are 6 positions in each complete platter rotation where the platter will stop. At each of these 6 positions the platter will rock back and forth slightly before stopping in the middle. With the power switch off, the platter will stop anywhere and spin freely so it seems that the bearing is fine.
If the platter stops in the middle of one of the 6 positions, there is no platter surge/movement when the power is turned on, but the platter will not spin up to without hand assist.
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steerpike2
Super Member
Absolutely and definiitely a dead phase. I.e., one of the components in the schematic section I posted a pic of earlier, or in the motor itself.
If you're lucky though, just a dry joint.
If you measure the two voltages across the two motor phases with your meter, you can narrow it down to a motor winding fault, or an electrial component fault.
steerpike2, thanks for the diagnosis.
As I am really just an electronics newbie here (and that's even a stretch), can you please describe in a little more detail where to measure?
For the Motor PCB, I suspect it may be somewhere on the jack exiting the Motor PCB but I don't have a clue on where to measure the motor.
As I am really just an electronics newbie here (and that's even a stretch), can you please describe in a little more detail where to measure?
For the Motor PCB, I suspect it may be somewhere on the jack exiting the Motor PCB but I don't have a clue on where to measure the motor.
I'll be checking for diode (all four of them) continuity, the four output transistors will be checked for operation and the motor windings resistance checked.
Bob: don't change R73 & R74 - the re-adjustment procedure is a bear. I suspect the transistors, or the connection between the motor driver board and the motor.
Bob: don't change R73 & R74 - the re-adjustment procedure is a bear. I suspect the transistors, or the connection between the motor driver board and the motor.
steerpike2
Super Member
First & easiest, with all power OFF, unplug the motor from the drive PCB, and measure resistance on the motor wires between (1) yellow and green and (2) yellow and black.
The two resistance values ought to be very close to each other. (within 10% at least). Probably a few tens of ohms, or in the low hundreds. If they differ sinificantly, the motor has an open circuit or short circuit winding. Short circuit is a big fix, but Open circuit is probably just a broken solder joint where the motor coils attach to the pcb inside the motor itself. (see not at the bottom re: opening the motor)
If the motor is OK (both resistances above are very close), replug the motor. Now you need to do measuremts with the motor running, which is a physically tricky arrangement. Nice meter leads with clips essential.
And the platter must be on - its inertia is an integral part of correct motor operation.
For voltage measurements, the earth wire on the power supply board (black) is the ground reference (negative meter probe).
Measure the 2 AC voltages on the black amd the green wires of the motor. *If the motor is good* (checked above) you should find *these two voltages differ significantly*. These will fluctuate, and be quite tricky to read accurately (a moving coil meter might be easier to use here if you have access to one). The peak voltage reading is a good enough estimate.
If the motor resistances checked good, yet you find these two voltages also show up as equal, then chances are your meter isn't really up to the job.
(SECTION A)
Confirm: So far, the motor checks good, and the two drive voltages don't match.
On IC10, measure the AC voltage on pin 1 and on pin 7. Again, both readings should be the same, to wihin a few percent.
If voltages are the same, go to (SECTION B)
If the voltages differ significantly, proceed to measuring on the motor plug (drive pcb socket where motor wiring plugs into it).
These voltages are likely to be small, hopefully not too low for your meter.
Compare voltages on grey & blue wires. Should be the same.
Compare voltages on white & violet wires. Should be the same.
If the above pairs of voltages are significantly different, the Hall sensor is damaged.
If the above pairs of voltages are the same, more or less, then IC10 is damaged
(SECTION B)
If you are here, the pairs of voltages measured above must have been equal, or very close.
Measure the voltages on the (-) pins of C15 and C25. Again they should be the same. If NOT the same, one or more of D7, D8, D11, D12 are faulty. They are cheap enough to replace all four. (I think you replaced C15 and C25 already)
IF the (-) pin capacitor voltages are equal, the fault lies in one of the 4 power transistors Q4..Q7.
IF you need to open the motor itself, come back here first! There are two set-point screws on the motor that adjust the phase balance (revox calls it the wobble component). If you knock these out of alignment, that is a MAJOR fix.
Good advice, well laid out plan of attack...
Those power transistors are about 50c each (that's my bet) and the diodes about 7c. At mouser - numbers available upon request. I also have reasonable substitutes in the bins here... IC 20 (10) is RC4559 and is about $1.
Bob, when measuring on the IC's etc - insulate all but the very tip of the probe so when you slip (we ALL slip) you don't short circuit to anything else.
Oh, and those "wobble screws" are closely related to R73 & R74 settings, thus leaving R73 & R74 alone is a very good idea.
on the schematic that calls the resistors R73 & R74, it's IC 20
while the schematic that calls them R57 & R58 it's IC 10...
Those power transistors are about 50c each (that's my bet) and the diodes about 7c. At mouser - numbers available upon request. I also have reasonable substitutes in the bins here... IC 20 (10) is RC4559 and is about $1.
Bob, when measuring on the IC's etc - insulate all but the very tip of the probe so when you slip (we ALL slip) you don't short circuit to anything else.
Oh, and those "wobble screws" are closely related to R73 & R74 settings, thus leaving R73 & R74 alone is a very good idea.
on the schematic that calls the resistors R73 & R74, it's IC 20
while the schematic that calls them R57 & R58 it's IC 10...
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I'm this far (after a dropped retaining disk and 2 hour search).
Resistances on the motor pins were spot on.
Found the earth/ground wire on the power supply board and checked voltages with the black DVM lead on this wire. The other end of this ground/earth wire is connected to an amp's earth/ground connection.
AC voltage on the black wire on my cheap Sears Craftsman (model 82141) DVM is variable when (checked at the motor power plug) between .1V and .3V. It is more often .3V than the lesser values.
Voltage on the green wire (also checked at the power plug) is a constant .1. So I presume this is a sufficient difference to proceed to the next step. Regardless, I will presume so and pursue both paths outlined as the next steps.
steerpike2
Super Member
One of the reasons I like ReVox so much: everything - aside from an occasional microcontroller - is standard run-of-the-mill industrial parts. Rarely any custom components that are impossible to find.
This is good news in that the motor itself is OK. Still the possibility of a flakey Hall Ic, but we will hope not!
Robie is over here right now.
The additional troubleshooting steps he attempted resulted in a no start motor when it came in here this afternoon. It just sits there.
The motor phases check ok, 28 ohms or so, the drive transistors and diodes check ok, with continuity or appropriate resistance between components, starting at the 3.31k hall input resistors through the power drive transistors through to the socket.
When it is powered up, and rotated by hand, as it spins down at 33 & 45 the tach circuit turns on the speed led indicators as it passes through the appropriate velocity on it's spin down. NO resistance is felt between attempting to spin it with power off or on.
When spinning, there is 6v being fed to the hall's ORANGE wire WHEN the appropriate velocity is reached and below that velocity to zero.
Above that velocity, the 6v to the halls disappears.
I have yet to read & try the tests in the 12-06 11:39 post, but I will be after posting this.
The additional troubleshooting steps he attempted resulted in a no start motor when it came in here this afternoon. It just sits there.
The motor phases check ok, 28 ohms or so, the drive transistors and diodes check ok, with continuity or appropriate resistance between components, starting at the 3.31k hall input resistors through the power drive transistors through to the socket.
When it is powered up, and rotated by hand, as it spins down at 33 & 45 the tach circuit turns on the speed led indicators as it passes through the appropriate velocity on it's spin down. NO resistance is felt between attempting to spin it with power off or on.
When spinning, there is 6v being fed to the hall's ORANGE wire WHEN the appropriate velocity is reached and below that velocity to zero.
Above that velocity, the 6v to the halls disappears.
I have yet to read & try the tests in the 12-06 11:39 post, but I will be after posting this.
steerpike2
Super Member
That is normal and correct. The PLL-lock detection circuit + LED operates at all times, even if the motor isn't switched on.
That also is correct. The orange wire is essentially the motor speed control voltage. Above correct speed, it thinks the motor is running too fast, so cuts power to the motor. Below correct speed it needs to accelerate the platter, so applies full voltage to the motor.
It sounds like both Hall elements are now dead; and before probably one was.
(That's not as much of a disaster as it may sound since they need to be matched devices, so if one had to be replaced it would be best to do them as a pair.) My manual does not show part numbers for the Hall devices though - I think ReVox preferred to supply the motor as a complete, correctly aligned unit.
I had been suspicious of the IC 10 rc4559 dual op amp, so I subbed in LM358's that I had for first one with no apparent effect, then all 4. Also to no effect
I was getting a sine wave out of pin 1 of the IC 10.1, whose frequency reflected the rotational speed of the manually rotated turntable. I was probing pin 7 for the other side (awkward without an extension cable for the tonearm cable) when I shorted to pin 8 +12v. Bob noticed a momentary "surge" of the platter that was not repeated.
That gave me hope that the hall sensors may not be zapped, which I intend to explore further.
Once I get back to this point, is there a way to fake out the motor board and get it to run without the tonearm cable, or even the whole tonearm board deactivated? It has to be a logic signal, but is it steady state? I can HEAR the microswitch actuating in the tonearm as it moves into place, but it feeds logic circuits.
I will be examining for that, but if simple question nets the answer rather than involved studying.....
I don't have a tonearm extension cable, the limiting factor toward better access of the motor drive board.
Once on the motor drive board, what are the outputs of the hall sensors in your experience?
Edit: motor driver transistors survived again, diodes & caps ok. New LM358 put in on general purposes..
looks like arm signal asserted high, incoming on grey wire through power supply, pulled low / allow run by just not connecting power to arm driver board. ALL IC's now socketed....
.
I was getting a sine wave out of pin 1 of the IC 10.1, whose frequency reflected the rotational speed of the manually rotated turntable. I was probing pin 7 for the other side (awkward without an extension cable for the tonearm cable) when I shorted to pin 8 +12v. Bob noticed a momentary "surge" of the platter that was not repeated.
That gave me hope that the hall sensors may not be zapped, which I intend to explore further.
Once I get back to this point, is there a way to fake out the motor board and get it to run without the tonearm cable, or even the whole tonearm board deactivated? It has to be a logic signal, but is it steady state? I can HEAR the microswitch actuating in the tonearm as it moves into place, but it feeds logic circuits.
I will be examining for that, but if simple question nets the answer rather than involved studying.....
I don't have a tonearm extension cable, the limiting factor toward better access of the motor drive board.
Once on the motor drive board, what are the outputs of the hall sensors in your experience?
Edit: motor driver transistors survived again, diodes & caps ok. New LM358 put in on general purposes..
looks like arm signal asserted high, incoming on grey wire through power supply, pulled low / allow run by just not connecting power to arm driver board. ALL IC's now socketed....
.
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steerpike2
Super Member
Maybe not. Pulling IC10 pin 7 to +12V WILL give a strong pulse of torque from the motor - because it will energize one phase coil full-on. This won't produce continuous rotation though, because the pulse isn't correctly timed.
That's the same phase that was dead before (no drive on the green motor wire)
Any idea what might have happened to the other phase that was working OK before?
Remove the arm-electronics-board's connection to the power supply - thats the plug on the power suply board closest to the EDGE of the PSU board, the motor should run continuously.The motor control board gets +5v on the grey wire from the arm electronics to STOP the motor.
I haven't measured, or got any quoted figures, but a quick look at the gain-setting resistor values suggests that the inputs to the op-amp (IC10, pins 2,3, 5,6) would be *around* 25mV.
<EDIT> That will the AC component when rotating; there is also a steady-state DC offset, but I have no idea what that is. (if I HAD to guess, I'd say around 2V)
Exactly so - didn't see you'd found this out before I wrote it above.
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steerpike2
Super Member
In a private message from:
Maybe you can be convinced to hang on to it though - they are great turntables, and a pity to give up on it totally for a relatively minor component; save it for a repair-in-future.
If the distances weren't so impossible, I would buy this one from you as it is!
I can't find the exact type number or brand for the original Hall devices, but the Honeywell one I do have info on looks like it could be a replacement: If not a drop-in, then with some minor resistor mods to get it working.
at ZA-RS
http://za.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0304267
At R234.26 (about $30) they aren't dirt cheap; but other suppliers are likely to be cheaper - ZA-RS has such an exceptional range of components so I look there first.
The alignment - in the manual - is described as quite involved. But that was in 1978! More sophisticated measuring tools are now commonplace (PC-based signal analysis) so I think Hall replacement isn't a big issue really, just requiring logic and attention to detail.
Robie said:
Maybe you can be convinced to hang on to it though - they are great turntables, and a pity to give up on it totally for a relatively minor component; save it for a repair-in-future.
If the distances weren't so impossible, I would buy this one from you as it is!
I can't find the exact type number or brand for the original Hall devices, but the Honeywell one I do have info on looks like it could be a replacement: If not a drop-in, then with some minor resistor mods to get it working.
at ZA-RS
http://za.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0304267
At R234.26 (about $30) they aren't dirt cheap; but other suppliers are likely to be cheaper - ZA-RS has such an exceptional range of components so I look there first.
The alignment - in the manual - is described as quite involved. But that was in 1978! More sophisticated measuring tools are now commonplace (PC-based signal analysis) so I think Hall replacement isn't a big issue really, just requiring logic and attention to detail.
SaSi
Seriously Illogical
The service manual shows several test points with the expected waveforms to verify correct operation and adjustment. Have you checked these in case they pinpoint the problem source?
Have you also checked that there's no faulty link between the motor control PCB and the motor itself? It appears that the servo circuit is fully functional but power isn't getting to the coils. Is power actually coming out of the driver transistors (pins 1 & 6 on the plug?) - is that signal actually getting to the coils?
I don't want to sound like a smarta** but the circuit appears to be pretty robust (and two of them survived my not so great surgical skills) so I am suspecting simple things that might have been missed.
Perhaps it's time to bring out my semi-faulty 795 and work along.
Have you also checked that there's no faulty link between the motor control PCB and the motor itself? It appears that the servo circuit is fully functional but power isn't getting to the coils. Is power actually coming out of the driver transistors (pins 1 & 6 on the plug?) - is that signal actually getting to the coils?
I don't want to sound like a smarta** but the circuit appears to be pretty robust (and two of them survived my not so great surgical skills) so I am suspecting simple things that might have been missed.
Perhaps it's time to bring out my semi-faulty 795 and work along.
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steerpike2
Super Member
I was thinking that too, Sasi. The evidence points to the Hall ICs, but I'm not aware of them being particularly susceptible components to damage.
One thing that just occurred to me now... the plugs used on the various wiring harnesses are crimped onto the individual wires - there is the possibility of corrosion there - maybe worth checking the actual cables and the multipin plugs.
Thanks again, steerpike2 and SaSi.
I just got off the phone with Jack Clark of JM Techical in Nashville, the only Studer/Revox Factory Authorized repair facility. I was calling to get a part # for the Hall sensors. He said that the Hall sensors were not available but that complete motors were. Approximate cost of, gulp, $400 to $500.
He asked what the symptoms were and suggested it had to be in the motor circuit, most likely bad ICs. He says he's never seen a motor nor a Hall sensor fail. Very nice guy, no affiliation. So I am feeling a bit more optimistic than previously.
My plan is to replace all the ICs now that Mark has "socketed" the board. A big thanks to Mark as well!
I just got off the phone with Jack Clark of JM Techical in Nashville, the only Studer/Revox Factory Authorized repair facility. I was calling to get a part # for the Hall sensors. He said that the Hall sensors were not available but that complete motors were. Approximate cost of, gulp, $400 to $500.
He asked what the symptoms were and suggested it had to be in the motor circuit, most likely bad ICs. He says he's never seen a motor nor a Hall sensor fail. Very nice guy, no affiliation. So I am feeling a bit more optimistic than previously.
My plan is to replace all the ICs now that Mark has "socketed" the board. A big thanks to Mark as well!
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