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JVC QL-7 Turntable; unstable speed

MonoMania

MonoMania

Member
I recently bought a JVC QL-7 Direct Drive table online from a seller who described it as "fully tested and working". In reality it has numerous minor problems, but only one that is worrying me:

The speed wavers very slightly with every revolution, unlike other JVC QL turntables I've owned which have had perfect speed stability. The fluctuation is subtle, but visible on the strobe.

When I listen with a makeshift stethoscope (screwdriver handle in ear, tip on plinth) I can hear a buzzing sound once per rotation. With the platter off and motor running there is an obvious clicking noise ~7 times per revolution.

Spinning the platter or spindle by hand with the power turned off produces no apparent noise.

It's still early enough to return this table to the seller. Should I?

Could this clicking noise have an electrical cause? I haven't replaced the electrolytic caps yet, as if the problem is mechanical and not repairable then I may have to send it back from whence it came...

Any advice would be appreciated :)
 
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Unfortunate that no JVC or quartz-lock direct-drive experts have chimed in...

I cleaned the speed adjustment trim pots with deoxit, and attempted to recalibrate... To no avail.
Still getting a slight slow-down with each revolution.

Could be as simple as replacing the capacitors, but it's looking like I'll just have to return it to be on the safe side... Unless anyone can offer insight from similar past experiences?
 
Sound every revolution points to something mechanical. It's odd that you can't pick it up when spinning it by hand. In any case, I'd probably send it back or ask for a partial refund as you did not receive what was described.

I think @totem has some experience with the big JVCs.
 
Just to clarify: Neither the clicks nor the buzz end up coming through the speakers. Even the speed instability is at the very fringes of audibility, but I'm looking for perfection.

The 7 clicks per rotation with the platter off may even be normal[?]

The vibration/buzz every revolution with platter on however, I suspect is related to the speed wobbling once per revolution.

I just don't know enough about quartz lock DD tables to be sure.

I've had other JVC QL tables where the strobe doesn't move even slightly, so I know something is up... If it was a cheap garage sale find I'd be quick to swap out the capacitors, but I paid around $375 USD for it, and don't want to get stuck with a turntable that I can't use...

If anyone out there has the same (or better) model JVC for sale I might be interested.
This model would be perfect for my purposes, if it was working correctly...
 
Well, I ended up receiving a partial refund to keep the machine...
If I can't repair it, then at least I'll have spare parts for another QL-7.

This means three things:

1. I will be replacing the capacitors ASAP
2. Advice would still be appreciated if anyone has any
3. If anyone has another QL-7 for sale I'm still interested
 
You'll need an oscilloscope to properly diagnose the drive as well as to make sure it's locking properly when you're done working on it.

A recap might fix it if it's a bad cap, but before doing any part replacement do the following:

1. Read up on the disassembly procedure for your particular model. At a minimum get the service manual from vinyl engine.

1. Look for physical causes of the problem. If you can, get an automotive stethoscope and try to pinpoint where the sound is coming from.

2. Once you get it open, look for obvious flaws. B bad caps may be bulging or leaking. In this era of JVC tables, a common problem is corrosive glue that eats components. Look for transistor legs that look eaten away or diodes or resistors that are corroded away.

3. Diode test all the transistors. Based on your description of the symptoms, my gut says there's something funny in your speed pickup circuit or in one of the phases of the motor drive. If the problem is a transistor that's gone open or shorted out, check the SM to make sure it doesn't need to be hfe matched before you replace it. The QL-A2 SM mentioned matching certain transistors.

4. Check all the voltage readings where the SM has voltages printed. They likely won't be exact, but if anything is way off it's a sign something in that part of the circuit is whacked out.

If all of that doesn't turn anything up, the scope is going to be absolutely necessary to diagnose the thing. Being able to see the various electrical behaviors will give you way more insight than anything else you could do.

Cheers,
Nathan
 
@EngineerNate Thank you very much! This is exactly the kind of detailed advice I was hoping for!

I actually haven't yet done the obvious voltage tests yet... I'd packed the turntable up to return before the seller agreed to the partial refund, and I haven't unpacked it yet. When I had it disassembled I got no further than cleaning the speed trim pots with deoxit and trying to realign the speed by eye.

VinylEngine doesn't have the service manual, but I've ordered a paper copy online, should have it soon. In the meantime I've been studying the SM for the QL-A7 (automatic version, with same drive system) which seems to have a very similar if not identical circuit board. In fact, the QL-7 has blank spaces on its board for the extra parts shown in the QL-A7 manual.

I wasn't aware of the corrosive glue problem - I'll look for signs of that. No visible indication of bad capacitors, but it's a 40 year old table, so they're well past their intended lifespan. I've ordered all 16 electrolytic capacitors and will be swapping them in when they arrive.

How does one "diode test all the transistors"?

I've noticed that there are four variable resistors on the circuit board... One sets 33.3 speed, one sets 45 speed, one apparently balances the rotation somehow... Any idea what the fourth one would be? The SM doesn't seem to mention it at all.

I've been trying to find a quality second-hand oscilloscope for years, ever since my HP scope picture tube burned out. I may have to buy one online and have it shipped. Would one of those absurdly cheap digital hand-held units from China be sufficient for this specific task? Or will I need something like a Tektronix?

Funny that you should mention the QL-A2 - that model has been my trusty DD table for years, which this QL-7 was intended to replace. Since the A2 was so reliable I figured moving up the JVC product line would be sensible. I never had to do anything more to it than replace caps and clean switches, and its' speed was always accurate.

Thanks again for your insightful advice! I will follow it all once the SM and capacitors arrive.
 
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Since this is a Quartz table, you can't really adjust the speed by eye like you can with a standard servo table. On the A2 (also Quartz lock) you set the speed by aligning two different waveforms in a specific manner for each speed. The TT-71 is a more sophisticated model so it'd probably be a bit more involved.

I use an Owon VDS1022i for now since I don't have storage space for a "real" scope. Works just fine. :)
 
@EngineerNate

I've acquired the correct SM, and ordered the Owon VDS1022i which will arrive in a couple
weeks - thanks for recommending it!

I examined the physical condition of the board and saw no indication of corrosion on any of the component legs. I swapped out all of the electrolytic capacitors while I was in there.

While waiting for the oscilloscope to arrive I'll check all the voltage readings and test all of the transistors as per your advice - do I need to remove the transistors from the circuit in order to test them or will an in-circuit test be sufficient?

Thanks for all your help! I'll report back in a couple weeks when I have the scope.
 
You can diode test in circuit, but the results may not be conclusive. If they test okay (there are many tutorials on diode testing transistors) you can eliminate them as a problem most of the time, but if they test bad (shorted or open when they shouldn't be) you may need to remove them or at least lift a leg to get a conclusive test. Knowing the schematic here is important-some will have nothing connected to them that should cause them to test bad, others will have other paths around them in circuit that may screw up your test.
 
I could be wrong here, but I don't think that's the case. The ql-7 houses the tt-71. The ql-5 drive is something different, the boards look like reconfigured versions of the drive in the QL-A2 and ql-f4.

Fortunately, the ql-a7 is simply the ql-7 with some automatic features tacked on, and vinylengine DOES have that one. Whooo!

https://www.vinylengine.com/library/jvc/ql-a7.shtml

Cheers,
Nathan
 
All I'm saying is that the motor controller board for the QL-5 has the same basic components as the QL-A2 board I worked on, it's just a semicircle instead of rectangular.

There are definite differences in the TT-xx drive units. Surely you're not asserting that the tt-71, tt-81, and tt-101 are identical internally.
 
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Alright. Did some digging.

Here's the QL-A2 schematic:
YugEljd.png


Here's the QL-5:
E8NTAeQ.png


Things to notice:
1. Near identical topology.
2. Same primary controller IC.
3. Same opamps setup in feedback and oscillator circuits.

Here's the QL-A7 schematic:
EpvHSek.png


Differences:
1. Additional transitors in both the feedback and oscillator circuits that aren't present in the other two.
2. Different (single instead of dual) opamp in oscillator circuit.
3. Different primary controller IC.

Similarities:
1. Same basic control schema. Notice that the drive transitors are almost identical between all three.
2. Same feedback philosophy. Sine wave generator into an opamp into the feedback loop. It seems that JVC came up with a good way of doing things re:Quartz lock and used it for everything, with tweaks for higher performance on the higher end models.

The QL-5 motor itself is likely much improved over the ql-a2 and ql-f4 but in operation they work (electrically at least) identically. There's definitely more going on in the ql-7.

The tt-101 and 81 have additional circuitry, iirc the 101 has both a dual Quartz lock loop and a "push-pull" dual drive setup, whereas the 81 just has the additional lock loop. @JP probably knows the most about that.

Cheers,
Nathan
 
@MonoMania

Sorry for the thread derail, but the QL-A7 schematic on VE should get you going, just ignore anything that's related to the auto lift functionality. The drive unit is identical.

Did you ever find the source of the ticking noise? That would be my biggest concern to sort out. It'd be odd for the electrical system to be making any audible noise so there may be something damaged our out of place in the mechanical system. While you're waiting for your scope, I'd familiarize yourself with the SM and find your test points. You can also do all of your diode testing now.
 
@EngineerNate

The oscilloscope has arrived, it's a great little unit for the cost, thanks for recommending it!
I also now have the proper QL-7 manual in paper form.

I'm just getting started here, not necessarily going in the correct order of operations...

I went through the adjustment procedure outlined in the manual:
adjustment.jpg


The speed seems to be locked perfectly (as done by eye weeks ago), I didn't need to re-adjust:
speed.png


However, I wasn't able to get a sine wave out of the "balance adjustment" on TP-2, just a noisy waveform... Maybe I have the scope set wrong? Is this a clue as to where the problem is? I adjusted the pot to minimize the fluctuation, but at no point did it generate a clean sine wave:

lock.png


I haven't yet checked most of the voltages, or done the recommended diode test of the transistors... Will be labor-intensive and frustrating to go over the entire board, but that is my next objective.

I will also pick up an automotive stethoscope to try to locate the source of the noise.

I'll drop the schematics here, but yes the QL-7 is the same as the QL-A7, in fact the QL-7 has empty spaces on the board for the absent QL-A7 parts. I was specifically wanting a fully manual turntable so I really hope to get this unit working - it would be ideal for my purposes.

board_assy.jpg

schematic.jpg


I've also noticed that the neon light for the strobe is flickering... Is this easily replaceable?
Any chance it could be affecting the circuit, or creating the illusion of speed instability?
Somehow I don't think that is the actual problem...

Speaking of which, here is a video of the 'actual problem':
Notice the strobe wobbling very slightly with each revolution.

@EngineerNate , thanks for all your help! I'm just a humble hobbyist with no formal training, so your expertise is immensely appreciated!

P.S: I have this thread 'watched', but have not received any notifications since my last post... Sometimes I get emails telling me that someone has posted, but most of the time I don't... Does anyone know why this might happen?
 
For reference, a list of all 16 electrolytic capacitors in the JVC QL-7

63v 220uf (2) ea
50v 47uf (3) ea
100v 47uf (2) ea
50v 10uf (2) ea
50v 2.2uf (1) ea
50v 1uf (1) ea
50v 0.47uf (1) ea
16v 10uf (3) ea
25v 33uf (1) ea
 
Good work! I'm just a hobbyist as well. ;) Before you check off the lock adjustment, make sure your scope is not set to dual triggering. I don't have mine here at the moment so I can't take screenshots to show you, but when I adjusted the QL-A2 I initially had the triggering set to dual and it was making everything line up perfectly even when things were a little off. Ideally you want to trigger off of the reference signal and adjust the feedback signal match it as best as possible. When it's set to dual trigger the two signals you're seeing are not being displayed on the same time axis.

To check if the motor is working properly, connect in pairs to the collector of x812, x815, x816, and x819. All of those waveforms should be roughly sinusoidal and virtually identical but out of phase from one another. If they all look good, your problem is likely elsewhere. Be careful doing this, you'll likely need some mini-grabber leads to get in there without damaging anything. Alternatively you may be able to get a lead attached at the associated locations on the motor plug, pins 3, 8, 9, and 12.

Regarding tp-2, I wouldn't be surprised if the sinusoidal waveform in the manual is artistic license. Looking at the schematic I'm not seeing a good reason for it to be a perfect sine wave. @JP is probably the most knowledgeable guy here on these tables, maybe he can comment.

I'd also try to get a probe on the fg coil output to make sure everything is kosher there.

Cheers,
Nathan
 
You could well be right about the dual trigging.. First time I'd played with this scope.

I'll explore further and let you know what I discover.

Thanks again for your help!
 
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