Another JVC QL-F4 Direct-Drive Turntable Repair

This is the second JVC QL-F4 direct-drive turntable with speed control issues I’ve repaired recently. Come with me as I resolve the problem.

The JVC QL-F4 is a great direct-drive deck, perfect for someone stepping up from a belt-drive or lower-tier DD. They actually have stellar wow and flutter figures, really low for an affordable deck like this. Best of all, people don’t know about them so they are a cheap little secret 🙂

You can read about my previous QL-F4 repair here. There are various causes of control issues with this and other decks. In these cases,  the old ‘corrosive glue’ problem I’ve written about before has a part to play, as do dead and or dying capacitors, and the occasional semiconductor fault.

Features, Courtesy Vinyl Engine

The QL-F4 has the best of both turntable worlds: platter rotational accuracy and full automatic operational ease. The combined use of a coreless motor, a direct-drive mechanism and the renowned JVC Quartz technology is the secret behind its performance and its low wow/flutter of 0.025% WRMS.

The QL-F4 features a Quartz servo system which incorporates a precision Quartz oscillator, super servo frequency generator and responsive servo circuit. Its major highlight is the core-less direct-drive motor; this doesn’t “cog” and thus assures absolutely smooth and accurate revolution, and leads to low wow/flutter.

The turntable offers the convenience of fully automatic operation. Among its unusually versatile functions are:

  • Auto-repeat offering from 1 to 6 plays at the twist of a rotary control, with the ‘R’ setting offering infinite replays
  • Auto-lead in which raises the tonearm, moves it over the edge of the record and gently lowers it to the lead-in groove to start playing.
  • Auto-return which sets the tonearm automatically to its rest when a play is completed and then shuts the power off. As a bonus, the moment you move the tonearm manually, the QL-F4 can be operated as if originally designed for manual operation.

Other features include new gimbal support, one-row strobe, oil-damped cueing, anti-skate control and a low-resonance cabinet.

The Problem

This deck presented well, but it wouldn’t run. It would start up and then, frustratingly, slow to a halt. There were a few other smaller set-up type issues, but if it doesn’t work, nothing else really matters.

Cool looking, fully automatic deck
First a clean with a special foaming cleanser
Now I can inspect the deck and work out what we have

Closer inspection revealed problems I’ve seen many times before. The polychloroprene adhesive used in manufacture to glue capacitors to boards is a corrosive nightmare.

the corrosive glue had attacked the legs of this 2SD321, but it was salvageable with a little cleaning. The transistor had dry solder joints though, so I remade those…
Dry joints visible here
The worst of the problems lay here, in the power supply section of the mainboard. The polychloroprene glue has corroded several components to dust.

The Repair

In this deck, the glue ate through two diodes completely. I replaced those and the five main power supply capacitors, after first removing as much of the glue as possible.

Here you can see the two affected diodes and gluey mess. Both diodes are corroded through, the one above the now removed capacitor is clearly in two pieces! The deck won’t work like this.
The only solution is to aggressively scrape away the polychloroprene glue residue and clean the board with acetone.
I then replaced all the affected components, in this case, five capacitors and two diodes. Three of the smaller capacitors were not directly affected, but I replaced these because they are old and its a very easy job to get to them with the deck apart like this.

Adjustment

With the repairs completed, the final job is to adjust the power supply voltage and quartz lock, phase-locked-loop circuit. This is often very awkward with direct-drive decks like this one because you have to simultaneously probe several test points whilst the deck is running! Invariably, I set up a jig like the one in the images below.

Ah, the pain of adjusting direct-drive turntables!
You have to lift them to gain access to space and boards underneath. I now use a slightly better jig than this, but you get the idea.
In this case, both 33 and 45 rpm test points are probed whilst the deck is running. I’m using my gorgeous Tektronix TDS3012C for this job.
This is what we need to see, two waveforms with this frequency and phase relationship, cleanly locked together.

Final Setup

The last set of things to do in this case was to correct the errors caused previously by someone at West Coast Hi-Fi. They managed to not only install the wrong type of cartridge for this tonearm but also connect every single headshell wire to the wrong place!

I kid you not, each one of the four headshell wires was connected to the wrong cartridge pin. This is the risk with getting anything done by mainstream hi-fi retailers – they are generally clueless for the most part, especially when it comes to vinyl.

So, West Coast Hi-Fi installed a Rega Bias, far too short to work correctly with this arm. The resultant vertical tracking angle is all wrong, so I replaced this with an Ortofon 2M Red, a much better choice for this deck.

If the work is done carefully and with the right equipment, this is the result, beautifully stable and solidly quartz-locked platter rotation.
The Rega Bias, set-up correctly for this arm, and testing playing a record.
But the owner, at my suggestion, went for a cartridge much better suited to this arm, an Ortofon 2M Red.
Perfect fit, and much better VTA now with this arrangement. Good to go.

11 thoughts on “Another JVC QL-F4 Direct-Drive Turntable Repair”

  1. Hi Volker,

    I have one of these that was given to me and I had the exact same problems. However, now that I’ve replaced the diodes and capacitors, the LED strobe does not light up and the turntable turns about 5% too slow. Do you know what could be causing this? I’d really appreciate it if you would reply.

    1. Hi Blake, thanks for your question, Volker is my customer, I repaired his turntable for him and run this website and the specialist hi-fi electronics repair business associated with it. There are many potential causes of issues like this on your deck and others. Rather than guessing, we (or someone else with the diagnostic capability) needs to test the machine and find out what is causing the problem. Do you live anywhere near Perth? If so, I’d be happy to look at her for you. If not, a specialist repairer in your area would be a good next step. Let me know if you have any other questions. And of course you’re welcome, it’s my pleasure, happy to assist…

  2. Totally understand! I will look it up on the service manual. While I have your attention, just curious on what you thought about this JVC direct drive mechanism? I am working on a turntable project and plan to use this as the motor. Is it any good compared to other direct drives like the SL-1200 Or Marantz motors?

    BTW the motor assembly easily comes off if you pull on the edges. The spindle comes off the bearing. It needs a little pressure.

    1. Yeah I quite liked it. Not much compares to the SL-1200 or KD-600/650 motors, but for a more consumer grade deck, it’s a nice unit for sure! Yes check the service data, it will have a part number, you can look it up and get the specs. I can’t recall the details but get back to me if you get stuck.

  3. I am doing the same to the turntable. Can you please list the diode specs that you used? I have the same issue with those 2 diodes.

    1. Hi Naveed, thanks for writing, I’m sorry but I can’t remember, I’d need to look at the service data and go from there. I work on a lot of gear so trying to remember what parts I’ve used on a particular unit from months ago is similar to trying to remember what you ate for dinner on that same day! I hope that makes sense?!

Feel free to leave a comment and share your thoughts!