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Turntable Speed issue
Turntable Speed issue
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Old 4th December 2017, 10:10 PM   #1
MauMX is offline MauMX
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Default Turntable Speed issue

good evening,
a dear friend of mine gifted me his grandfather's turntable (i cannot recognize the model). i changed the belt, i put some oil in order to lubricate it properly but i still have some problems: the turntable speed is too slow (not so much) and the pitch regulation has reached the limit.

i decided to remove the bottom and give a look inside in search of some trimmer, i found two of them (33 and 45) but they are not working. inspecting the pcb i noticed a scratch on the board that cut off the two variable resistors.
i really don't know how a FG servo works i found no datasheet for the IC neither for the motor so i decided to ask here for an help

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Last edited by MauMX; 5th December 2017 at 06:38 AM.
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Old 4th December 2017, 10:30 PM   #2
billshurv is offline billshurv  United Kingdom
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Turntable Speed issue
Any chance you can attach the pictures rather than hosting externally as I can't view them?
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Old 4th December 2017, 10:40 PM   #3
DRONE7 is offline DRONE7  New Zealand
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Quote:
Originally Posted by billshurv View Post
Any chance you can attach the pictures rather than hosting externally as I can't view them?
Try right clicking them and select 'open link in new window'

Last edited by DRONE7; 4th December 2017 at 10:42 PM.
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Old 5th December 2017, 01:39 AM   #4
PRR is offline PRR  United States
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Check the DC power supply. If that's not obvious, find the biggest electrolytic capacitor in the turntable, replace it, observe polarity.

A weak main cap will not hold full peak AC voltage, give low DC voltage, motor can't hit designed speed.
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Old 5th December 2017, 01:42 AM   #5
kevinkr is offline kevinkr  United States
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Turntable Speed issue
There are a couple on that board I would replace as well if you are comfortable with a solder iron or know someone who is.

Observe polarity before removal!
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Old 5th December 2017, 02:30 AM   #6
benb is offline benb  United States
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Quote:
Originally Posted by MauMX View Post
good evening,
a dear friend of mine gifted me his grandfather's turntable (i cannot recognize the model). i changed the belt, i put some oil in order to lubricate it properly but i still have some problems: the turntable speed is too slow (not so much) and the pitch regulation has reached the limit.

i decided to remove the bottom and give a look inside in search of some trimmer, i found two of them (33 and 45) but they are not working. inspecting the pcb i noticed a scratch on the board that cut off the two variable resistors.
i really don't know how a FG servo works i found no datasheet for the IC neither for the motor so i decided to ask here for an help
Offhand I don't think you need to know all these things - just seeing those traces cut in the last photo suggests to me that it would work if you jumpered them over and adjusted each trimmer so is runs at proper speed with the front-panel speed control in the middle of its travel.
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Old 5th December 2017, 06:53 AM   #7
MauMX is offline MauMX
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Originally Posted by benb View Post
Offhand I don't think you need to know all these things - just seeing those traces cut in the last photo suggests to me that it would work if you jumpered them over and adjusted each trimmer so is runs at proper speed with the front-panel speed control in the middle of its travel.
today i will try to fix the traces, then if it doesn't work i will replace some capacitor.
my question is: why are the traces broken? it's quite strange


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Originally Posted by billshurv View Post
Any chance you can attach the pictures rather than hosting externally as I can't view them?
now they are visible.
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Old 5th December 2017, 01:50 PM   #8
Eric is offline Eric  United States
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Turntable Speed issue
It appears that these traces were cut with the intention of bypassing the speed control adjustments. Hard to know why it was done (what problem was being solved), but a good start is just laying a cut off resistor leg along the trace and soldering it down on both ends. This should restore the function of the trim pots.

See what the behavior is like when the pots are back in the circuit. Troubleshoot from there, if necessary. If things are still not working, I second the vote to go after caps. After ~15 years, caps are a random element. The goods is they are cheap and plentiful!
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Last edited by Eric; 5th December 2017 at 02:03 PM.
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Old 5th December 2017, 07:41 PM   #9
PRR is offline PRR  United States
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Oil! That motor is 39 years old. The oil may have gone stiff. Does it spin freely?
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Old 20th December 2017, 07:39 AM   #10
MauMX is offline MauMX
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sorry, i had to study a lot the previous week for some engineering exams, now i'm back to life

Quote:
Originally Posted by Eric View Post
It appears that these traces were cut with the intention of bypassing the speed control adjustments. Hard to know why it was done (what problem was being solved), but a good start is just laying a cut off resistor leg along the trace and soldering it down on both ends. This should restore the function of the trim pots.

See what the behavior is like when the pots are back in the circuit. Troubleshoot from there, if necessary. If things are still not working, I second the vote to go after caps. After ~15 years, caps are a random element. The goods is they are cheap and plentiful!
i fixed the traces, but nothing is changed i mean it continues to be a little bit slow and moving the trimmers nothing change. i checked the trimmers with a ohmmeter and they are ok.


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Oil! That motor is 39 years old. The oil may have gone stiff. Does it spin freely?
it's the first thing i've done, it spin very freely.

anyway i fixed temporarily the problem applying some scotch (0,3mm) on the motor pulley and i reached the correct speed anyway now i have 2 ways:
-reduce the diameter of the bigger pulley
-fix the board

there is another way i think, what about build a regulator with arduino? it should be easy i think, the only problem is that i really don't know how the motor feedback works.
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