Friday, January 6, 2017

Fixing Denon PMA-980R Input / Volume Selector "Jumpiness" (English)


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What to do when Denon PMA-980R "goes mad"


If it wasn't for Revox and their excellent B200 series, it is likely that a PMA-980R would still be my main amp for audio entertainment. While my first "real" amplifier was a Denon PMA-360, which was best for its price, it always lacked remote control for me, like any other component in the rack.
When PMA-980R hit the market, it was right on the spot. Denon had proven good for me for some years then, I was certain this was a good match. Besides the sound quality, a lot of other attributes were just right: it is made of solid heavy metal (near 10kg), features a quality toroid transformer, plenty of relays, a Record Selector, a Source Direct switch, and overall a very good feel about it. Thick aluminium front plate, huge heatsinks, and only the most inevitable amount of plastic.
In my opinion, the amplifier is able to compete with newer devices, thanks to the excellent Sanken transistors used in the power amplification section. No compromises were made in the volume adjustment section which consists of a motor-driven quality potentiometer for remote availability which also features a LED position indicator that can be seen easily from meters away. This was the best way to combine quality and usability. Digital attenuation was too expensive for the PMA-980R margin, and going cheaper in digital is always lossy.
Instead of mechanical switches and heavy wiring, Denon engineers employ processor-controlled relays for managing the signal path and speaker outputs. Thanks to this, everything can be controlled remotely without penalties in signal quality.
Both knobs (Speakers and Input) do not have end positions but can be turned infinitely. They have "notch" positions however which convery a very upworthy feeling. Turning a knob won't redirect any signals right away, instead, the knob action causes the processor to calculate what to do, and adjust the relays accordingly. In theory, this is much more sustainable than approaches where the signal runs through a highly complex multi-way switch, or an entire cascade of switches. Eventually, any switch in the path can be a failure point, and using sealed relays (and thereby extremely short signal paths, too) reduces these failure points to the absolute minimum. Contacts in the open air will corrode sooner or later, causing noise, interruptions and undesirable crackling and peaks, up to a complete dropout of one channel or even both.
Signal paths were also kept short for the Record Selector whose multi-way switch is positioned near the Cinch terminals and driven by a Bowden cable to make it accessible to the front. Yes, no relays here, rather classic and corrosion-prone, but another set of relays whould have made the entire amplifier considerably more costly. It's not the current subject but might be in the future, we will see...

Construction Analysis

Knobs without an end stop nowadays are a certain indication of a rotary encoder (see  my Panasonic Jog/Shuttle experiments). Well, not quite here. The PMA-980R is actually using a resistor ladder as revealed in the service manual. Basically it is a large voltage divider with 12 taps. Paths are shown here in orange for loudspeaker selector and green for input selector:


It is immediately clear that a lot more positions are possible than there are inputs or speaker groups: while there are six actual inputs, and four loudspeaker configurations, each selector has twelce positions, each resulting in a different voltage that results from the amount of resistors (10kOhms, 20kOhms etc. up to  120kOhms).
My guess is that none of these taps is actually bound to a certain input or speaker group. If they were, it would not be possible to use the amplifier via remote controller and on the device itself consistently. Probably the approach is rather to have the processor constantly measure the voltage, and a change in the voltage is interpreted as a change on the knob in clockwise or counterclockwise direction.
Inputs are TAPE2, TAPE1, PHONO, CD, TUNER und AUX. From AUX the amplifier will roll over (clockwise) to TAPE2, and from TAPE2 counterclockwise back to AUX. However, the remote controller can switch to an arbitrary input anytime. If that happened, and the knob is used, then it will still safely address the next or previous input relative to the one currently selected. So the resistances and resulting voltages must be merely an indication of the turn direction of the respective knob.
Supposing the CD input is at 20kOhm, then a resistance of 30kOhm would switch to TUNER, and 10kOhm would go to PHONO.  But actually it's not the fixed values that count, rather it's the relative change compared to the previous value. If the voltage is lower, it is typically the result of the switch being moved into one direction whereas a higher voltage means the switch moves in the other direction.
The resistor ladder is (in my opinion) a little overdimensioned. To safely recognize the turn direction, three resistors would have been sufficient. It is not clear why 12 were chosen. Anyway, the switch will sooner or later reach its last position, i.e. the 12th tap on the resistor ladder, which results in 120kOhms resistance and the respective voltage. The next position is back to the 1st tap which is just 10kOhms. So the processor needs to be able to identify this as not -110kOhms but rather see it like +10kOhms. It is the same rule for the other direction where +110kOhms actually means -10kOhms. All of this magic is probably happening inside the processor.

At powerup, the most recently used configuration concerning speaker groups and inputs is probably restored from a buffered memory, and the selection is then associated with the current resistance value of the respective knob. The other voltages are all relative then.

Downside of "Mechanical" Components in This Design

Back to the actual issue. Corroding surfaces were reduced as much as possible, but inevitably the selector knobs will be subject to corrosion, too. While no audio signals pass these selectors, the corrosion will not affect audio at all. However, selector corrosion in the PMA-980R and similar designs may have you end up with one or more of these symptoms:
  • knob not reacting at all
  • the LEDs around a knob are flashing one by one or in random order, and the relays keep clicking. This may be intermittent at first and become more and more permanent
  • the LEDs do not follow the turn direction
  • the input or speaker selector acts randomly without any obvious trigger
All this is usually just due to corrosion or dust accumulation or both and can be repaired with just a little time, and some tools. However, there are other factors that might cause a similar behavior. For instance, if the processor supply voltage has too much ripple (usually due to dried-out electrolytic caps), it may also start acting weird. But let's concentrate on the much more likely for now.

Repair Approach

To get down to the switches in question, we need to do this:
  • remove upper housing (3 PH2 screws on each side and 2 PH2 screws at the upper back edge)
  • the Speaker, Input, and Volume knobs (be careful with the latter because its illumination requires a small cable to go to the front side of the knob. Do not pull too hard or you might rip the cable)
  • remove front plate (5 PH2 screws on the bottom and 2 PH2 screws on the top side)
  • remove some cable connections between the front plate and the rest of the amplifier. Unfortunately, this requires to clip several cable ties that we need to replace later
  • two hexagonal nuts (size 11) that connect the Speaker and Input selectors to the front plate, and thus connect the processor PCB and the front plate (the PCB can be removed easily after this step)
  • two small connectors at the left and right side of the board need to be unplugged
  • a metal strip across each switch needs to be desoldered, then the switches, each with 13 pins in total
  • tear down the switches
  • clean all contact surfaces
  • add a thin layer of contact spray
  • assemble the switches and solder them back in
  • restore the metal strips
  • test
  • assemble the amplifier

What You Need

  • PH2 screw driver
  • a ratchet with a hexagonal bit size 11 to loosen the Speaker and Input selectors from the front plate
  • side cutter to clip the cable ties
  • to desolder: a vacuum desoldering station is highly recommended, otherwise a powerful soldering iron and solder wick
  • to solder: soldering station or a soldering iron with max. 20 Watts
  • a sturdy gripper to help disassemble the switches
  • optionally a flat screwdriver
  • for cleaning:
    • several pieces of soft cloth, as lint-free as possible
    • some earbudy (Q-Tips), at least three per switch are probably needed
    • Isopropylic alkohol 90% or better

Disassembly

Housing

Loosen six black PH2 screws, three on each side of the device
At the upper edge of the back side, loosen two smaller PH2 screws.
The top cover can now be removed by pulling it right upwards

Front Plate

Prior to this step it is advisable to remove the knob handles for Speaker, Input, and Volume. Keep in mind that the Volume knob is still connected with a small white cable.
All connections between the front plate and the other boards inside need to be loosened. On the way you will have to clip five or six cable ties. Be careful when pulling the connectors out of their sockets to avoid that any cable is torn. Be extra careful at the edge of the main board where the volume knob's LED connector (and others) resides. The board is flapping in the air here, and warping it too far might overstress is.

The front plate is screwed down with two PH2 screws at the outer left and right edge, and another five PH2 screws on the bottom side (best accessible if you place the amplifier upside down). The bottom screws are those which are next to the front plate.
After this you should be able to pull the front plate away from the device with not much resistance. Be careful about the cables though, and remember how they were routed. A photo taken now might serve you well later.

Separate the Processor Board From the Front Plate

The processor board should already be loose because all that fixed it to the front plate were the size 11 nuts that we removed previously. Note that the board is still connected to other components of the front plate. On the left, there is a 3-wire cable between CN3C on the processor board and the IR receiver which is on its own PCB near the power knob. On the other side, there is a 4-wire cable between CN4A and the Source Direct switch and LED which are on a separate PCB as well.
Let's have a first glance at the processor PCB now:


The largest elements are the switches we need to fix. Each is secured by a metal "bridge" that goes across the entire switch and is soldered on both sides. Unfortunately, these might be a pain to remove because the metal tabs were bent on the soldering side before they were soldered down. So you will have to fight a lot of solder, and also ensure that the tabs are straightened again to get them out of the way.
A side view of the PCB:


Desoldering the Switches

I found it easiest to get rid of the metal strips by "flooding" the solder pads that fix them with fresh solder. It ensures that, while heating it all up, the solder liquefies everywhere.
Setting the desoldering station to 325°C, everything was heated up to that point, and vacuum was used to remove all solder. Multiple attempts are not a shame here. Once the metal tab is free enough, bend it straight with pliers and push it through to the other side.
The switches are soldered down by 12 pins in a circular arrangement, and a 13th one for the middle contact, inside this circle. They all need to be desoldered. You won't have any trouble putting them back in later because the 13th pin acts as a kind of key and ensures that only one position is actually possible.
As the pins are rather big, and solder was generously applied, one of the bigger nozzles is required on the desoldering iron. You might try with solder wick or the manual "one-shot" handheld vacuum desoldering pumps.

Disassembling the Switches

The switches after this procedure:


Bottom side. The middle contact can be seen to the right of and a little below the center axle:


The switch consists of three parts: the brown base, a green plastic piece, and the metal frame holding the axle, and clamping everything together.
The metal frame is secured to the base by two metal ends which spread apart beyond the base. They need to be pushed together far enough so the metal ends pass through the base. As the metal is quite sturdy, some force is required here. However, be careful not to push too hard or you will make your life harder when it gets to spread them back into the secured position later.
You can push them together slowly with less than maximum force and simultaneous wiggling of the plier (maybe +/- 10 degrees around the axis of the metal end).


Once the ends are close enough so they can pass through the slot in the brown base, you can try to loosen the base. I am using a flat screwdriver here as a lever:


Eventually the top part is off:


Removing the green cap requires the four "anchor" tabs at the bottom side to be pushed together, and meanwhile pushing them through the hole in the base:


You may begin with two tabs on one side, once they are loose, insert something between the base and the green plastic piece so the tabs won't snap back. Then loosen the remaining tabs and push the plastic cap out.


We can now see the slider in the green cap which in operation moves across the middle contact of the base, and makes contact with one of the twelve metal pads around it. The slider tracks are clearly visible on the metal pads as "tracks" where a silverish surface is shining through. The gold look is actually some kind of grease plus corrosion plus dust.
After a first wipe, things start to look better:


I switched to iso alcohol now, and cotton earbuds. You will be surprised how much dirt is still coming out, even if everything looks clean already. I recommend wasting earbuds until no more dirt accumulates on them.


The counterpart should also be treated. Even if the naked eye might not see it, a lot of dirt can be found here as well. But be careful with these delicate metal springs, they bend easily and should not experience mechanical stress more than is absolutely unavoidable. It is enough to just clean the rounded surfaces that mate with the base contacts. Even if the rest appears dirty, better don't get there, it might ruin the spring properties.


Enthusiasts might even sand the contacts with very fine sand paper, or even just a piece of usual printer paper
Now for the contact spray. There are many theories about it, so I have my own as well. I think contact spray is bad if you just use it to flood devices and their switches from a distance, apparently sparing the trouble of going into disassembly or even desoldering. This might help for a time but as the dirt stays where it is, and is at best redistributed a little, the issues will keep coming back. Surfaces "treated" with the spray will accumulate dust even quicker because the oil component of the spray never evaporates. The end result is usually a mess.
But a thin layer of contact spray here might help in the long run, now that everything is clean and shiny as new, the spray might help conserve this status for a longer time. Besides, we have exposed metal contact surfaces which might corrode if not treated somehow.
So my recommendation is to put contact spray on a cotton bud and wipe that across all metal surfaces. Do not spray the surfaces directly so everything is swimming in the spray. It does not improve things, and might even soil other parts of the amplifier if the excess spray finds its way out of the switch. As it is not hermetically sealer, this will happen.
So here is one switch, cleaned up, treated with contact spreay, prepped for reassembly:

Switch Reassembly

Basically you do everything above backwards. Clip the green plastic cap back onto the base. Don't worry about the position yet.
The axle can go in only one way because it has a flattened contour that needs to be aligned to the slot in the green cap. Push the metal frame down and route the metal tabs through the appropriate base plate slots. Once everything is pushed together fast, you can spread out the metal tabs again so they secure the entire assembly. This is needed to ensure that the gap between the base plate and the green cap is as small as possible, to avoid anything getting in there.

Soldering the Switches Back

As said above, the switches have only one position that allows them to be put into the processor board correctly. Find the middle pin and align it to the respective hole in the board. It's no matter which of the switches you put in which position as they are identical.
I recommend that you solder only one pin at first, then check whether the switch is sitting in the correct end position. It is easier to correct anything askew right now.
After all 13 pins are soldered, the brave might run a first test by connecting all the cables but not reassembling the entire devide. You do not actually need the handle knobs for the switches in order to turn them. But be careful not to cause any kind of short circuit in this test. Just do this if you really have enough experience!
Ideally you should not observe any more jumping or other random behavior now. The switches should react promptly and as expected.
The metal tabs are the final step of mounting the switches. To make your life easier next time, just don't bend the tabs on the soldering side again, like Denon production did. It is not necessary at all. Just push the metal bridge in and put solder on it. It might help meanwhile to push against the metal bridge from the other side to get it in as far as possible. This gives you the maximum surface to solder against so it is more secure later. If things become too hot for your fingers, consider using the pliers for this.


Reassembling the Amp

Put the PCB back to the front plate. First ensure that the cables on the left and right edge are plugged back in. If you removed the 11-wire cable at the top like I did, plug it back in now because you won't be able to reach this spot once the board is back in place.

CAUTION: the LEDs around the switches are routed to the front through little pipes which have little tolerance. So each LED needs to match its respective pipe exactly. If it doesn't, the LED might bend sideways. You might notice this only some time later when you find that one or more LEDs don't seem to emit any light. They actually do, but in a direction you don't want. So keep an eye on the LEDs specifically when you put the PCB down.
Besides, I found the IR sensor board in my PMA-980R was extremely loose. Just two plastic anchor clips would "secure" it, and they performed miserably at that, which resulted in the IR board wandering around behind the front plate, and IR commands not doing anything. So please ensure that the IR board is in its home position while you put the front plate back on. A loose IR sensor PCB could cause short circuits, and the loss of remote control is not what we want either.

Furthermore, when you reassemble the front plate, be careful to route all cables the same way they were before you started the disassembly. If a wire connection is apparently too short, that might be because the wires looped somewehere behind the front plate. Don't just pull them to their expected length. If you are in this situation, better reseat the front plate and ensure no cables are caught behind it. This is easy now, and much harder later on.

On the bottom side, push the front plate in between the chassis and the bottom metal plate, sandwich-like:


If you feel too much resistance here, or the panel cannot reach the end position, there is either a wire squished somewhere between front panel and chassis, or one of the push buttons did not find its way through the front plate. Main suspects here are Loudness, Subsonic, and Source Direct. Make sure you can see the buttons, reseat the front plate again if needed.
Eventually the front should go on without much effort. In its final position, you should be able to see the threads beneath the seven screw holes.
If you removed the plastic side covers left and right of the front, put them in now as their screws cannot be accessed once the top cover is back in place.
Reseat all cable connections, also don't forget the ground connection between the front plate and the front heatsink. Tie down the cables approximately where they were tied before.

Another test is advisable now that short circuit risks are practically out of the way (unless you drop anything into the open device under operation). Please be aware that dangerous voltages and charges are built up in the amplifier, so keep your hands away from any place but the front
The cover slides back home just being put down straight from above. Reset the two PH2 backside screws first, then the six side screws.

That's it! Should be like new, enjoy it!

Some Design Criticism About the PMA-980R


Denon did almost everything right with the PMA-980R. However there are some things left to be desired. What I am missing might be due to the price margin though.
The jumpiness issue might have been avoided with a) less than 12 positions for the selector switches, or b) with optical encoders. As discussed above, the different resistance values just serve the purpose of determining the turn direction. For this, three different resistance values would be sufficient. I cannot think of why you need twelve, it's unnecessarily complex.
A purely optical encoder might suffer from dust accumulation but as there is no mechanical-electrical contact, it won't suffer nearly as bad. An open optical encoder system (like in early computer "ball" mice) could also be cleaned easily without a full disassembly of the component.
Either Denon's engineers didn't think this far, or it was too costly at the time. It's unlikely that planned obsolescence played any role back in the day but in 1992 engineers knew corrosion problems well as practically all hi-fi devices had open potentiometers and switches forever.
I don't want to think about the amount of amplifiers and similar devices that ended up in the landfill just because of such a minuscule cause, and owners could not figure out that it might be fixed rather simply.

Another issue may surface during rough handling of the amplifier. Certainly a little out of spec, but I had one victim of an accidental drop that was beyond fixing. The reason is that the mainboard in the PMA-980R is badly lacking mechanical support. As you can see from the next picture, the mainboard is pretty large, filling up the center and right space:


However, the board is bolted down in just four places, near the heatsinks! The only things that secure the right side of the board are the cinch connector terminal (which is not designed for this job) as well as the motor-driven potentiometer which is surrounded by some metal. It's likewise not designed for holding the board in place because actually the solder joints of the potentiometer are the place where this happens.
The right front edge of the board is not supported at all, i.e. if you pull a connector or push it in, the board flexes considerably. It could go as far as the board getting a crack in the most stressed place.
This is particularly sad because the PCB designer provided some holes in that corner, but the case designer did not put anything near there for a screw to hold.
The board offers enough space for additional screws but there is nothing beneath the board to screw against.
I think that at least three or four additional screws would have been really nice.
All is well until a certain degree of acceleration is exceeded, especially when the device is dropped flat from some height. The drop damage may face in a split through the entire board.
Because an even greater design mistake is this: the buffer capacitors (the large brown / shiny cans in the middle of the board) are neither protected from vibration (other manufacturers would surround them with a rubber or foam rubber support), nor are they mechanically decoupled by any means from the board. Their weight becomes a considerable problem combined with acceleration. On a drop, they will put all their weight down on the rather thin board, and a crack rips right through the middle of the board (horizontally, in the picture above). It might not be a visible crack but just a hair line, anyway it's not economically fixable.
If the place of the PCB where these caps are positioned would have even the slightest support, e.g. from a plastic frame beneath the board, or a metal frame above the caps that clamps them in, this could not happen, or would at least not end so badly. A PCB of better quality (epoxy instead of the "hard paper" type they appear to have used) would also have been a good choice.
The following picture illustrates that there is nothing but air beneath the board 😟:




Further Advice:

A camera is expressly recommended for documentation of cable routing, screw positions, erc. Better make one more photo than missing one later.
As a matter of course, be aware that working on the device in its powered-up state is dangerous and should be avoided. It is not only dangerous for yourself but also for the device under repair. So if you work on its internals, be sure to power it off and even better, pull the plug. Components such as capacitors can store a lot of energy for a long time even after the plug was pulled so if you are working on these components or places where their connections are exposed, be sure to discharge them first. The easiest and most effective way to discharge a high-voltage capacitor is to connect it directly to a 230V light bulb. This will quickly eat all energy stored. If there is too much energy, the light bulb may be blown out, but that's still better than you getting this kind of charge.
Some legal stuff because you never know: please bear in mind that I am writing this as a hobbyist, not a professional. I describe personal ideas here which is only one of many ways such a repair can be achieved. I cannot guarantee that following this guide will lead to a good result, and cannot be held liable for any personal, physical, or monetary damage anybody suffers by following this guide.
I am open to advice if anything described here is wrong or can be done better. Please let me know in the comments if you find there is anything left to be desired.
Thank you!

4 comments:

  1. Great article, thank you very much!
    Could you also give a hint how to add and connect pre-out outputs to this particular amplifier? Thanks.
    Marko

    ReplyDelete
  2. Thanks for the feedback.
    Why would you want a pre-out? Do you have a dedicated power amplifier and plan to use the PMA-980R only as the pre-amp? The power amp section in the PMA-980R is excellent, so why not use that? It would be a shame to use only the pre-amp section of it, and also doesn't make much sense regarding energy consumption.
    If the power amplifier inside the PMA-980R is not good enough for your purposes, then the pre-amp section probably isn't, too. So better consider a dedicated pre-amp in that case.
    Probably it is possible to pick up the selected input somewhere inside to branch off a pre-out but really, I wouldn't know what to do with it.
    In case I misunderstood and you actually want to use the PMA-980R as your power amp, but have a different device for input selection, I'd just use the CD input, connect the pre-amp to the CD jacks, and leave it at that.

    ReplyDelete
    Replies
    1. Hi Johannes, thank you for your answer - and I apologize for answering back after such a long time - but I meant it for subwoofer. Not that necessary, but eventually useful.
      Seems like I am about to repair this amp soon, the problem is obviously the one described in your blog. Thanks again for sharing your experience.
      Regards,
      Marko

      Delete
  3. Hi Johannes,

    Thanks for the article, very comprehensive. But if I may, I’d like to offer people googling this problem an easy alternative fix.

    I have found a way to eliminate the 'jumpiness' without even needing to open the lid on the amp. All you need is a white marker pen, and the remote. The result is you will never have it jump ever again; the downside is you have to stop using the selector knob and only use the remote. But if you can handle that, read on!.

    I have had my amp for 21 years, and it’s still going strong. I love how reliable it has been.
    I too, have had it apart a number of times over the past 5 or so years (since the jumpiness started) , cleaning the selector rotor contacts (both the speaker and function selector). Only to find the jumpiness seems to creep back and at random times.

    What I discovered was that sometimes the jumpiness was worse than other times, and began to wonder if it became dependant on where the knob was sitting at. I figured out that of the 12 positions of the selector switches, one of them shorts the selection input on main IC to GROUND. When it's at ground, the IC cannot change its state, you have no resistor ladder, no voltage divider, no more jumpiness!

    (On the IC I'm referring to inputs #24-Function and #25-SP (Speaker Select) - as per your schematic where you have highlighted these lines green and orange).

    I worked out which knob position sets the IC inputs to ground, marked it carefully and elegantly with some white paint. And now just use the remote to select the speakers or function - and have never had it play up since!

    I figured out that the jumpiness gets worse the higher up the resistor ladder you climb. From the 'ground' knob position, as you turn the knob anti-clockwise, it climbs the ladder, making the input more and more unstable. If you turn it one click clockwise from the 'ground' position you are at the top of the ladder and the jumpiness is at its worse.

    So, all you need to do is place the selector switches at the 'ground' position, mark it with white paint (or however you choose) because otherwise you’ll accidentally use your hand to move it around and not recall where the ground position is, and then only ever use the remote to change the speaker or function settings (a bit painful sometimes when you can’t find the remote, but worth it!).

    I'm not sure how to attach a photo, so I'll just explain where the 'magic ground position' - once you pull the speaker or function selector knob off you'll see the knob-post has a flat side to it, just make sure that's pointing at 4 o'clock (or 120 degrees).

    I hope to get another 21 years out of it now!
    Ben
    New Zealand

    ReplyDelete

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