Denon DP-72L Speed Problem

[PCMHiFi]

Hello - I have a Denon DP-72L with an intermittent speed problem. When it acts up, it is almost always during the first few minutes of playback. Best way I can describe it is that it seems like it is at least momentarily trying to slow down the platter, then bring it back up to speed. Sometimes the episode is so brief that the "Lock" light doesn't even flash. Other times, it lasts for about 5-6 seconds, by which time I've JUMPED out of my chair to stop it. When that happens, I try starting it again and it may speed up/slow down momentarily a couple more times, then seems fine for the rest of the listening session which is typically about 30 minutes or so.

I've had it since 2000 and until about a month ago, these episodes numbered less than 10. But within the past month, both the frequency and severity of its episodes has increased quite a bit. I can't think of anything I've changed around it that would be an obvious culprit.

I originally thought perhaps static electricity was annoying it but am not sure. When I go to touch the power button or the arm, I first touch a metal rack next to it to discharge any static electricity from myself. I tried hanging an unused dryer sheet on the wall behind where the 72 sits - about 10" away from the back of the plinth. That didn't help either.

So I'm wondering if this is really static electricity or something else. Any thoughts? Thanks.

[Alec124c41]

The first thing to try is cleaning all the controls and switches. A spray can of contact cleaner, such as Caig DeoxIT D5, with the straw nozzle, directed to the inner workings of the controls, with a short burst, and working the controls, should clean the layer of oxide that makes them go wierd.
You will have to go in through the bottom.

Cheers,
Alec

[EdAInWestOC]

I have a Denon DP-62L which is almost identical from an electrical perspective. The first question I have is: Have you had that table recapped?

The Denon DP-72L is over 30 years old now and the electrolytic capacitors are past their prime. You can find a service manual for the DP-72L in the library section of this forum. It is complete except for one part. The parts list in the Denon service manual omits a 10uF/16V non-polarized electrolytic capacitor (C72). Other than that the parts list is complete. The capacitor in question has to do with the servo control tonearm and not the speed control circuitry.

WARNING: If you do not have the table recapped you will be risking losing something non-replaceable. The Denon DP-72L is microprocessor controlled. The microprocessor in question is a Denon proprietary part and if it burns out, due to out of spec caps, you will not be able to get a replacement part unless you get lucky and get one from a parted out table.

I had a problem similiar to yours with my DP-62L recently and it was solved by recapping the turntable. The table required the replacement of all electrolytic capacitors.

The DP-62L and 72L turntables have a sophiscated speed controlled circuit and are significantly more involved than many turntables from the same era. If you are handy with a soldering iron and have some experiencing doing this type of work, working on the Denon circuit boards is pretty easy work. The parts density isn't too high and the resulting soldering work is not too difficult.

The speed control of this table is sophiscated but fairly straight forward. It is dictated by a quartz crystal and the rotation of the platter is monitored by the feedback from an magnetic sensor under the edge of the platter. There are two controls that are adjusted to have the speed lock function correctly VR1 and VR2. These are PCB mounted potentiometers that require a dual trace oscilloscope to adjust a one waveform to occur relative to the position of another waveform. The control is not an adjustment of the fine control of the speed itself. It is a phase lock loop adjustment.

If your table is starting to have intermittent speed issues I would look at getting the table recapped as soon as possible. Its a simple go/no-go situation. The adjustments mentioned above may have an issue like many potentiometers where the wiper is not making good contact on the resistive surface. This can cause issues but the problem is usually a failure issue and not intermittent.

You can try opening the bottom cover, noting the physical position of VR1 and VR2 (one is for 33RPM the other is for 45RPM) and working the pots back and forth. Return them back to where they started. Replace the bottom cover before you do any further testing.

The power transistors that drive the spindle motor are optically coupled in the DP-62/72L and the bottom cover must be in place for it to operate normally.

If you have any further questions feel free to ask. I completed my table within the past two months.

Ed

[PCMHiFi]

Thanks Alec. I might pick some of that up and give it a try...sounds like a good idea anyway as periodic maintenance.

Regards,

Pete

[PCMHiFi]

Thanks so much for your advice, and for all of the great information. With my poor soldering skills, I've been thinking for a while about sending it out to have the increasingly brittle patch/ground cables replaced with some top-quality jacks, along with bulb replacements, etc. With your mention of the capacitors, sending it to a good restoration shop is making more and more sense. I can't imagine that even twice what I'd spend on a top-notch restoration for the 72 would buy a "replacement" table I like even half as much. I plan to read up on repair/restoration shops over the weekend, though would appreciate any thoughts you might have there.

Meanwhile, I might get some of the contact cleaner and try Alec's recommendation along with your tip on the VR1 & VR2 pots just to see if I can calm it down. Until then, given what you mention about the microprocessor potentially being ruined by out-of-spec caps, I've stopped using it for now.

Regards,

Pete

[EdAInWestOC]

Thanks so much for your advice, and for all of the great information. With my poor soldering skills, I've been thinking for a while about sending it out to have the increasingly brittle patch/ground cables replaced with some top-quality jacks, along with bulb replacements, etc. With your mention of the capacitors, sending it to a good restoration shop is making more and more sense. I can't imagine that even twice what I'd spend on a top-notch restoration for the 72 would buy a "replacement" table I like even half as much. I plan to read up on repair/restoration shops over the weekend, though would appreciate any thoughts you might have there.

Meanwhile, I might get some of the contact cleaner and try Alec's recommendation along with your tip on the VR1 & VR2 pots just to see if I can calm it down. Until then, given what you mention about the microprocessor potentially being ruined by out-of-spec caps, I've stopped using it for now.

Regards,

Pete

Pete,
It is wise to stop using it for the time being. If I were you I would seek a repair shop who would be willing to recap your table.

I have used a repair facility in NY: http://www.stereorepair.net/. The hassle is that you have to pack up your gear but they seem to be professional and very detailed about their work. You might consider getting in touch with them and see if they would recap your table for a reasonable amount.

They would need to replace all electrolytics. That's a total of 42 electrolytic capacitors. None of these is the expensive large capacitance type. The largest capacitor is a 2200uF/25V electrolytic. These are all standard aluminum general purpose radial caps and if I remember correctly the total for all of the replacement capacitors was less than $40. Of course your cost would be mostly the labor to replace the parts and do the required adjustments.

I would also get quotes from local repair shops closer to your home. If you have a place close by that does this type of work it is much better than shipping the gear out and having to deal with the problems that can occur during shipment. That plus the extra bonus of starting a relationship with someone you can trust for future repair work.

Not to mention that the DP-72L is not a light item. It would probably cost around $50 to ship it via FedEx Ground.

If you are anything like me you love your table and want to keep it running for a lifetime. I am lucky to have an electronics background and some experience with a soldering iron. I would still like to find a local guy to take my gear to.

Be careful if you attemp to clean the switches in the DP-72L. The switches that you press are sealed type swiches that are on small circuit boards that are fixed to the bottom of a small plastic box structure. The switch that you press has a plastic shaft that contacts the momentary switch mentioned above. It looks like a small plastic cube mounted on the circuit board. I doubt that there is any way to access anything in the switch and if you spray anything in the area of the switch you could be complicating things.

My DP-62L has been running for 30 years and the switches seem fine. Every once in a while I have to press one of them more than once but I think thats just the expression of time. The speed switches themselves would probably not have anything to do with the symptom you first described. Both the 33 and 45RPM switch are directly connected to pins on the speed control microprocessor and they control which clock pulse it uses internally to lock onto the speed desired.

A momentary switch problem should not cause a speed problem. They are momentary type contact switches to begin with and the circuit does not depend on any current flowing through the switch for anything. The processor simply detects the pressing of the particular switch and internally switches to the proper clock pulses. If you press the 33 or 45RPM switch repeatedly it will not cause the processor to do anything, It will simply stay at the speed it is already set at.

Ed

[Whitneyville]

I re-capped a Technics PPL board last year for a "freebie" for train club buddy. Very similar symptoms. I figured it out that when I passed my hand (un-grounded) over the speed PC board the speed varied, that it HAD to be a capacitance effect. How so? The board is shielded with aluminum foil? Gotta be an electrolytic cap. $4 and a few hours later, it's solid as a rock on speed. Caps and pots are easily replaceable, but 30+ year-old IC's are rare as new B-17G de-icing boots. I would un-plug it and shut it down for service to prevent harm to this nice piece. The actual parts are cheap, it's mearly the "bench-time" to pull and replace the parts and a few minutes of 'scope work, but they're "due", and you don't want to "let the magic smoke out" of the IC's.

[johnyk]

i have a denon dp51f. just got it back from the shop. bad transistors caused the motor to go.i had the same problem, speed would all of the sudden slow then pick up. sometimes come to a stop.eventually wouldnt work at all.my advice. dont play it any longer and take it in for service,you may be able to get away with only replacing bad transistors before you blow the motor like i did.you may be able to save yourself some money? regards

[PCMHiFi]

Thanks so much for your notes Ed, Whitneyville, and Johnyk.

I haven't used it since Ed's original post, and will definitely heed all the warnings and not use it until it's fixed.

Sofar I've found a place in town with an A+ BBB rating that advertises they'll repair pretty much vintage everything. But turntables, (even record players!), are conspicuously missing from their list of specialties. I found another place I'd have to ship (or drive) to, that expressly says they can restore just about any Denon turntable; no BBB record to include complaints. I plan to call each place when I get the time (& they're open). But sofar with only the above information, I've been trying to talk myself into the possibility of taking 2 16-hour trips to get it to and from the Denon TT specialist safely.

While it would be nice if it didn't cost a bundle, to me the 72 deserves a full-blown overhaul with top-notch parts and labor. I was reminded of this when I was what-iffing complete failure of it. Plenty of nice TTs out there but I haven't found one at any price where if I imagine myself purchasing it, I wouldn't still miss the 72. Plus I'm guessing a full-blown resto on the 72 wouldn't buy much of a new TT anyway.

I'll post as things progress and I learn more.

Meanwhile thanks again for all of your insight and advice!

Regards,

Pete

[PCMHiFi]

I've been deliberating between shipping the 72 and a 9-hour (1-way) drive, and decided to ship it. Assuming it is brought back to life, will drive to pick it up to help ensure nothing jolts any of the adjustments on the way back.

So in the absence of the original box & styrofoam, I have been researching means of packing and related supplies. I am hoping to have it ready in about 3 weeks.

It's a marvelous paperweight in the meantime!

[PCMHiFi]

After about 10+ hours of packing, I wound up sending the main unit in a 29x26x26 box, and the accessories in a separate box. Now waiting to hear that everything got there OK.

[Roblog]

Pete,

How did your project turn out? Do you have a recommendation.

[PCMHiFi]

Pete,

How did your project turn out? Do you have a recommendation.

Wow - time really got away from me...sorry.

After about 6 months, the repair shop said the proprietary speed-control chip that cannot be replaced was toast. They had been trying to find a "donor" table but to no avail. So, the DP-72L, (affectionately called "Rosie" in our household), is gone. Shipping the entire non-working unit back to me was cost-prohibitive, so I had them send me back some of the useable parts, the last of which sold on eBay a couple of weeks ago.

I bought the 72 in 2001. Not too long after that, it started having occasional speed fluctuations. Problem got substantially worse circa April 2011. The BIG mistake I made was in assuming it was caused by interference from static electricity, and continuing to use it. At the time I had no idea that bad electrolytic capacitors could be the culprit. So my recommendation for anyone who has one of these or similar tables is don't wait any longer. Even if there aren't any speed problems now, have the electrolytic capacitors replaced before they fry the irreplaceable proprietary speed control chip.

I "replaced" Rosie with a Marantz/Clearaudio TT-15S1, (named "Klaus"), which I'll cover in another post. I like the TT-15, but would have gladly kept the 72 for the rest of my life.

[Roblog]

Thanks for the follow-up, I bid on some of Rosie's parts on eBay. The rubber mat was mint.
Did you use stereorepair.net in NY? I'm about to send them my 62L, which is just starting to have speed problems.

Rob