I recently purchased a JVC TT-81. The speed stability is not good. wow & flutter measures about .3% using Kenwood FL140 meter. It appears to be primarily a wow problem as opposed to flutter (FL140 can do separate tests). This is consistent with he pattern on the table's strobe. The pattern is generally stable but does jump occasionally.
I replaced all electrolytic capacitors and used low leakage caps in the two positions that were originally low leakage caps. I followed the service manual procedure for adjusting the table. Still the problem occurs. I do notice that the waveforms in the Lock Adjustment procedure do tend to shift, similar to the pattern on the strobe. This leads me to believe that the underlying frequency generation might be the problem. However it rapidly gets beyond my skill to troubleshoot further.
I'd appreciate suggestions for further analysis/repair. Better still would be someone who would repair this table for me. I would certainly pay the going rate for repair work.
Many thanks.
I replaced all electrolytic capacitors and used low leakage caps in the two positions that were originally low leakage caps. I followed the service manual procedure for adjusting the table. Still the problem occurs. I do notice that the waveforms in the Lock Adjustment procedure do tend to shift, similar to the pattern on the strobe. This leads me to believe that the underlying frequency generation might be the problem. However it rapidly gets beyond my skill to troubleshoot further.
I'd appreciate suggestions for further analysis/repair. Better still would be someone who would repair this table for me. I would certainly pay the going rate for repair work.
Many thanks.
Your turntable has its own version of a direct drive servo crystal controlled controlled motor ,this uses an integrated frequency generator in the motor.
A disc with grooves is integrated with a doughnut shaped magnet attached to it .
What I see is that the pulses from it are directed to a stator coil which is attached to a doughnut shaped PCB ,its these pulses that control the motor .
I would check out if your deck has been dropped or knocked as this is pretty critical in controlling the motor , check the motor and the board for any sign of movement or unbalance in relation to each other .
I would also download the very good explanation and oscilloscope testing points to check out the various waveforms to narrow this down .
Its available on vinyl engine if you log in.
A disc with grooves is integrated with a doughnut shaped magnet attached to it .
What I see is that the pulses from it are directed to a stator coil which is attached to a doughnut shaped PCB ,its these pulses that control the motor .
I would check out if your deck has been dropped or knocked as this is pretty critical in controlling the motor , check the motor and the board for any sign of movement or unbalance in relation to each other .
I would also download the very good explanation and oscilloscope testing points to check out the various waveforms to narrow this down .
Its available on vinyl engine if you log in.
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Thanks for the information. The guy who shipped it to me did not install the transit screws which lock down the transformer. It must have bounced around because the main PCB was broken loose from it's mounting - the standoffs were broken away from the chassis. I could see no physical damage to the PCB, but who's to say what might be cracked. I have downloaded the service manual from VinylEngine, but it does not include the schematic. So I seem to be missing the key that links the waveforms at various point to test points on the schematic. I'll take another look.
The motor stator and the stator coil have a very intimate relationship they need to be exactly equally balanced to transmit data that means equal in distance all round from each other.
Does anyone have a schematic they can share? It doesn't appear to be available on vinylengine.com
Still not found a full blown schematic and most comments online going back a long time say the same --only block diagrams available but the scope tests point should help to narrow it down.
As I said the board holds the PRINTED coils --if that board is dislodged/broken etc then it requires precise alignment not just a "that will do " repair .
As I said the board holds the PRINTED coils --if that board is dislodged/broken etc then it requires precise alignment not just a "that will do " repair .
I did dismantle the motor. The index ring was broken loose from its attachment point. I re-installed temporarily but am still getting variation. The strobe is generally stable except once per revolution it moves left then right. So it will definitely alignment. How do i do that?
It must be exactly parallel the printed coils all the way round --no variations.
If there is little space between them then use old fashioned automobile feeler gauges or if there is a greater distance then use an accurate steel engineers ruler to check all round .
If there is room use an internal adjustable gauge that machine lathe users use.
Nowadays I think they have a digital display .
If there is little space between them then use old fashioned automobile feeler gauges or if there is a greater distance then use an accurate steel engineers ruler to check all round .
If there is room use an internal adjustable gauge that machine lathe users use.
Nowadays I think they have a digital display .
Excellent. I think i have it concentric but there is vertical runout. Gives me something to work on. Cheers
I think I have it set so that the index ring is exactly parallel to the printed coil and also concentric with it. However I still have the problem. The square wave from TP5 in the Lock procedure moves just like the pattern on the strobe - i.e. left a bit and then right and then stable. Does this about once per revolution. The sawtooth wave part of the lock procedure stays stable. I suppose this indicates a problem with the signal generator part of the motor. But could it be part of the circuit that converts the sine wave coming from the frequency generator on the motor into a square wave?
Progress. I worked to get the frequency generator parts as well aligned as possible. The strobe is now very stable. Short of some sort of factory alignment jig I think it's as good as it will get. I'll put it on the FL140 meter later today to see if it meets spec.
Okay , as we don't have a schematic we have to use the block diagram so each test point has to be checked starting back at the initial circuit to produce the signal that is converted to stablise the motor .
Test points that show on the scope as okay should be checked all along the chain till the faulty one is reached .
If the sawtooth waveform stays stable and the square wave which is timed by it moves then a fault exists in the TP 5 circuit the first vertical in the square wave on each square wave should line up in frequency with the rise on the sawtooth waveform .
Check the EXACT frequency of the sawtooth and see if the square waves are EXACTLY the same frequency ,it possible there is a timing error in the conversion in that part of the circuit to a square wave probably a faulty capacitor.
We posted at the same time ---so it was a case of alignment ? good its sorted now.
Test points that show on the scope as okay should be checked all along the chain till the faulty one is reached .
If the sawtooth waveform stays stable and the square wave which is timed by it moves then a fault exists in the TP 5 circuit the first vertical in the square wave on each square wave should line up in frequency with the rise on the sawtooth waveform .
Check the EXACT frequency of the sawtooth and see if the square waves are EXACTLY the same frequency ,it possible there is a timing error in the conversion in that part of the circuit to a square wave probably a faulty capacitor.
We posted at the same time ---so it was a case of alignment ? good its sorted now.
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What's the likelihood of two posts at the same time from people 5000 miles apart...cool.
I measured the wow & flutter with Kenwood FL140 and it's about .1%. Above spec but probably not audible? May be the best I can do without some sort of factory alignment jig.
I measured the wow & flutter with Kenwood FL140 and it's about .1%. Above spec but probably not audible? May be the best I can do without some sort of factory alignment jig.