JVC TT-101 Won't Stop

The Stop button on my recently purchased QL-10 doesn't work. It worked at first after the TT warmed up, but then quit completely. I since have replaced all 37 electrolytic caps on 3 boards, but it made no difference. The power voltages are correct. The button itself is fine and shorts pins 6 and 9 of the P8 connector on the main board, but the motor doesn't stop. What could be wrong? Any particular transistor or adjustment? Please help! -Alex
Have you downloaded the Sevice Manual from Vinyl Engine?
It is comprehensive....and has a trouble-shooting guide.
Safesphere, You seem to be a fairly competent trouble-shooter, just based on what you've already done. When I first saw your post, I thought right away you must have a problem with the switch. I am told they are prone to malfunction mechanically. But if you're sure the main power switch is OK, you might look at any of the other switches on the panel. They may be wired such that any one of them can cause the failure mode you are experiencing. Think of it this way… at least your TT101 runs. Are there any other faults in its function? Because you seem to know how the switch is wired, I assume you already have downloaded the Service Manual, as Halcro correctly suggests.
Yes I have the service manual, thank you. All other switches work fine. The only other potential issue with functioning is that, when the TT starts, it first overspins and then comes down to 33.33. On startup the first measurement after zero is higher than 33.33. It is inconsistent. Usually it is between 34 and 45, one time I saw over 70. Even when it is 34 I still see the platter spinning faster, but slowing down before the digits appear. The next number is usually 33.33 or slightly higher, the following one is always 33.33. This effect is less noticeable at 45. I also hear some motor noise on startup only that is clearly related to overspinning. I have access to another copy of TT-101 that does not show this behavior. I can try measuring transistor voltages, although it is physically challenging with the platter mounted. I don't have an oscilloscope other than through the PC sound card. Where can I get a good one at low cost? Thanks!
Here is an excellent oscilloscope for little $


Best of luck

Just note that likely you need a set of probes for the suggested oscilloscope.
Unfortunately, the TT101 is very complex. Mine has nearly driven me nuts until I stopped fretting about it (because I realize that I also own 4 other great turntables that perform flawlessly, and it is of little consequence to me if my TT101 is a pain in the ass). Any of the hundreds of solder joints can have gone bad over the 30 or more years since they were built. Re-soldering suspect joints is a place to start. Another occult source of problems is those beige-colored multi-pin connectors. inside those connectors one may have a cracked wire or bad solder joint that is not accessible for inspection. I am about to ditch one of them and just directly solder the wires to the pins in hopes of curing a very elusive intermittent issue I have with mine. I was able to determine that tugging on the harness that goes to that connector can induce the failure mode I am trying to cure.
I have measured the voltages on some transistors at different speeds. I can see that pushing the Stop button triggers the following sequence of transistors (per the circuit diagram on page 38 or 39 of the service manual): 629 through 626 and 624, but not 625. The voltages on 625 are off, the same at any speed (12V on C or E and 11.3V on B), and don't depend on VR646. The voltage on the other side of R650 is 0.6V. This makes me think that X625 is gone and needs to be replaced, but I'm not an expert and would appreciate if someone could please confirm my conclusion. Thanks! -Alex
If you are not already seeking help elsewhere,you may find a teck hanging out on Audiokarma or diyaudio.
Safesphere, Wish I could help, but you've already gone further than I in parsing out that circuit. Early on in the repair of my unit, I gave it to a very competent pro who found no issues with the ICs or the discrete transistors, after he replaced all the electrolytic capacitors. I own a 'scope but have never applied it to repairing the TT101, because the problem is very intermittent and elusive in the first place.
Safesphere, I am bumping your thread. I did some re-wiring on my TT101 over the weekend, also found a bad electrolytic capacitor on one of the PCBs that are mounted under the top cover. (This was the only original lytic that we failed to replace on the first go-around, and it's the only lytic to be found under the top cover.) In the process of "improving" the wiring integrity, my other goal, I screwed up something. I need to ask you about the order of the wires in one of the connectors on that board. I hard-soldered them to the male prongs, because the connector was very loose. Now I fear I may have gotten the order incorrect. I will post the parts number for that connector, and maybe you can check yours for a reference. (The wiring is color-coded, as you know.) I turned what was once a rare intermittent problem into a consistent problem. Aaarrrggghhh!!!
Safesphere and anyone else with a dismantled TT101,
The connector in question is nearest to IC810 on the Crystal PC Board Assembly, TDC-7B. This PCB is located under the top circular piece that surrounds the platter. TDC-7B is depicted on page 31 of the owner's manual. The order of the four wires going into this connector, as I have it, is: brown, (space), orange, yellow, green. Please check, if you can do it without much trouble.

Upon second look, I think I've got it correct, but I am grasping at straws, trying to figure out what I did that could have worsened my problem. I cannot find anything else that could be causing my TT101 to upchuck. Thanks.
I find it useful to take lots of digital photos before I work on equipment. Quick and easy and can save a lot of time.
I did just that. Unfortunately I had unplugged this particular connector before taking a photo of that part of the PCB. Anyway, I hope Safesphere or some other TT101 owner responds, but if not, I do have the service manual which with some effort can be used to confirm the color codes of the wires. Actually, I am nearly certain I made no mistake, just grasping at straws here. Very weird.