Another JVC QL-F4 Direct-Drive Turntable Repair

So this is the second JVC QL-F4 I’ve repaired with speed control issues. These relate to the old ‘corrosive glue’ problem I’ve written about before.

You can read about my previous QL-F4 repair here. 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 🙂

Features and blurb, courtesy of the 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 core-less motor, a direct-drive mechanism and the renowned JVC Quartz technology is the secret behind it’s 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, 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. It wouldn’t run though. 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 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 main board. 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. Both are corroded through, the one above the now removed capacitor is clearly in two pieces!
The only solution is to aggressively scrape away the glue residue and then clean 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 now.

Adjustment

With the repairs complete, 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 the space and boards underneath
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. This genius not only installed the wrong type of cartridge for this tonearm, but also connected every single headshell wire to the wrong point! 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 literally 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.
 

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

  1. 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?!

  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.

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