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-   -   JVC L-A110 turntable speed problem? (http://www.diyaudio.com/forums/analogue-source/225763-jvc-l-a110-turntable-speed-problem.html)

soul music 15th December 2012 06:01 PM

JVC L-A110 turntable speed problem?
 
Hi, I have a JVC L-A110 belt driven dc servo turntable which was working fine until one day it sped up whilst playing a record. Now it plays 33's too fast at about 35 rpm and 45's too slow at 43 rpm with the trim pots adjusted at both extremes to try and acheive to correct speed. I've replaced all the electrolytic capacitors, sprayed contact cleaner on the trim pots and even replaced the IC controlling the motor!? I've checked the one transistor. What gives? Next stop the wheelie bin :confused:. I know this TT is nothing special but it did sound pretty good before this problem occured and I hate to throw anything away and i'm really frustrated this has beaten me - I mean there is not much in there and I have already replaced half of the componants. BTW I tried attaching some pics but they would not upload (maybe because i'm a newbie here?). Any Ideas?

soul music 15th December 2012 09:39 PM

3 Attachment(s)
Here's some pics.

hitachi_nut 18th December 2012 03:44 PM

Did you check the regulated output coming from the transistor beside R801 under load? The transistor looks like it is used as pass element in conjunction with D802 as reference forming a series regulator circuit. It's possible that it's not regulating anymore (shorted or not enough gain) given how tiny that transistor TO-92 package is relative to the load.

The regulated output voltage = ( zener voltage - Vbe drop which is typically 0.6V).

soul music 18th December 2012 09:21 PM

Quote:

Originally Posted by hitachi_nut (Post 3290079)
Did you check the regulated output coming from the transistor beside R801 under load? The transistor looks like it is used as pass element in conjunction with D802 as reference forming a series regulator circuit. It's possible that it's not regulating anymore (shorted or not enough gain) given how tiny that transistor TO-92 package is relative to the load.

The regulated output voltage = ( zener voltage - Vbe drop which is typically 0.6V).

I did a simple test using the diode test on my multi-meter to make sure the transistor was conducting in the right directions only and I think the voltage drop was about 0.7V so I assume the transistor is OK. I do have a transistor tester on my multi-meter but I don't know what readings I should get from it. I'm fairly new to electronics and so far I have just tried replacing 'suspect' parts. Maybe I could just replace that transistor anyway?

volken 19th December 2012 08:23 AM

How much ,,noise,, makes the motor when you take off the belt , mostly the bearings or collector from these motors are worn !
regards
Volken

Mooly 19th December 2012 08:29 AM

It looks like a simple regulator from what I can see that just senses back EMF from the motor for speed/load regulation.

I see a couple of small compressed disc ceramic caps which are notorious for going leaky so check them on the highest ohms range or preferably by substitution.

As a last suspect it could be the motor (electrically noisy commutator) but it wouldn't be my first suspect.

soul music 19th December 2012 10:24 PM

Fixed it !!!???
 
I have fixed the speed problem but i'm not sure how I did it??? I was just trying to sort out the motor which was loose on its mounting by packing it out with some extra washers and in the process I accidently stripped the thread in one of the screw holes so I twisted the motor around a little and used the other unused holes instead and just for good measure I also squirted some servisol down the shaft. I plugged in the deck to check that it was'nt vibrating now I had secured the motor properly and was amazed the TT was turning really slow on 33 (it was going too fast before)??!! So I adjusted the trim pots back to where they were before the problems started and lo and behold the speed was more or less spot on??!! Also when the speed problem occured I was having to 'bump start' the TT by rotating it a little by hand to start it spinning, now it is starting as it should when I move the arm over??!! Iv'e no idea what is going on but i'm guessing it must be the motor and as with all these 'miracle' fixes I don't think that it's going to work for very long. Any thoughts?

hitachi_nut 20th December 2012 07:18 AM

Diode test with a DVM is not a good indication of transistor functionality. You already have it in circuit. Measure voltages relative to circuit ground will give you meaningful information. Doing it this way is a learning experience and will likely prevent useless and time-wasting parts replacement in the future.

Based on your latest post, I vote for the motor, they don't last forever especially brushed DC motors with internal speed regulator. I've replaced them in the past.


Quote:

Originally Posted by soul music (Post 3290488)
I did a simple test using the diode test on my multi-meter to make sure the transistor was conducting in the right directions only and I think the voltage drop was about 0.7V so I assume the transistor is OK. I do have a transistor tester on my multi-meter but I don't know what readings I should get from it. I'm fairly new to electronics and so far I have just tried replacing 'suspect' parts. Maybe I could just replace that transistor anyway?


soul music 20th December 2012 01:29 PM

Quote:

Originally Posted by hitachi_nut (Post 3292288)
Diode test with a DVM is not a good indication of transistor functionality. You already have it in circuit. Measure voltages relative to circuit ground will give you meaningful information. Doing it this way is a learning experience and will likely prevent useless and time-wasting parts replacement in the future.

Based on your latest post, I vote for the motor, they don't last forever especially brushed DC motors with internal speed regulator. I've replaced them in the past.

Thanks, I did remove the transistor from the circuit to test it. I quickly learned that testing individual componants in circuit is not the best. For example I tested the diode in circuit and it seemed to be faulty, I took it out to replace it but found when I tested it out of circuit that it was fine (so I put it back). I guess the capacitors in the circuit cause false readings. Anyway as you pointed out I learned quite a lot from this (including soldering/desoldering). Regarding the motor I guess it is near the end of it's life, is it possible to get new ones? Does it have to be the same make and model? or can I substitute it with another similar motor?and where would be the best place to get one? (I live in the UK).

Mooly 20th December 2012 07:25 PM

Great that you've fixed it :)

A couple of thoughts. Isn't the motor on rubber grommets to stop mechanical noise transmitting to the deck ? It probably should be "wobbly".

It seems like the lube has done the trick. Just put a drop (one drop) of fine oil on the bearing... use a little not a lot :)

The motor would have to be identical type for the regulator to work correctly. These regulators keep the motor speed constant as the load on the motor changes and they achive this by means of "back EMF" sensing.


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