Denon PMA-715R Repair

[DaveWarnock]

Hello folks. I bought a Denon PMA-715R stereo amplifier recently for very cheap, listed as having one channel quieter than the other.



After some investigation I am 99% certain that the problem is the volume pot. I am basing this on:

• Significant static/crackling noises when turning the pot, along with random volume fluctuations that settle when I stop changing the volume.
• A large number of people with similar issues to this being resolved by replacing the pot.
• The amp does otherwise work fine, and both channels do get louder/quieter as I change the volume, just one is always quieter than the other.

Cleaning the pot with contact cleaner didn't work, so probably replacement time. Ok, easy enough to do - except I can't seem to find a suitable replacement.

The service manual is linked below.
https://freeservicemanuals.info/en/servicemanuals/viewmanual/denon/PMA915R/PMA715R/PMA915RG/PMA715RG/
That link above seems to omit the 715R schematic, which I was able to find elsewhere at the following link.
https://www.eserviceinfo.com/downloadsm/163319/DENON_hfe%20denon%20pma-715r%20715rg%20schematic.html

The volume pot is on a little breakout board as shown below. Apologies I don't have a better photo of it, I thought I took one but I must be mistaken.



Anyway the service manual lists the part as Variable Resistor 30kohm, with the remarks "VR201 Main Vol" and and the part number 2110761004.
This component is number 15 on the exploded view parts list, with the break-out board being part 13-3. The wiring diagram labels the board 1U-2736-3.




I've done some digging and found out (in conjunction with the markings on the component itself) that this is a motorised variable potentiometer made by ALPS. I cannot find a viable/economically sensible equivalent replacement part.

I can find things like the ALPS "Blue Beauty" that seem like a sensible and cost-effective substitute, but they're not available in 30kohms, only 20k and 50k, from what I've been able to find.

Anyway, I was talking to a friend about it and they suggested that it's probably a standard voltage divider circuit, which has me thinking I probably CAN use something like a Blue Beauty, if I can find the other parts of the voltage divider circuit and also change them out.

I've managed to identify what I think is the key area in the schematic.



This is where I start to get in over my head, if I wasn't already. What I'm looking for is:

• Advice on how to proceed - if my theory is correct, or perhaps if I'm totally wrong about the pot.
• What parts I should change out and what type of parts I should be buying.

I do not care about using a motorised pot as I did not get this with a remote control. I know I could probably use a universal remote but I'm not worried about that at all. I just want it to work for the most part, and I'm looking for an economical solution.

Thanks in advance for your time!

[DaveWarnock]

I wanted to write a little update on this. I've been doing more research and I think I misunderstood potentiometers. What I think I got wrong is that the potentiometer IS the voltage divider, or at least it is in this circuit.

I removed the pot last night and got some more information off the part itself, which has allowed me to do some more sleuthing. Unfortunately my search was rather fruitless. Here's what I found.

• Googling the part number leads me directly back to my own post. Good SEO here!
• Google the other markings led me to a German audio forum, which my browser translated for me. In that post the OP has found the same part in another Hi-Fi and is trying to source a replacement. OP links to several long-dead AliExpress listings for replacements, and is looking for alternatives to AliExpress or failing that an alternative part. The thread concludes with the posters telling OP that this part is basically discontinued and there is no alternative. OP's only chance is to try and service the pot itself or to do significant work to get a modern pot to work.
• I've had my own search on AliExpress (something I'd already done but I tried again) and there is a single AliExpress seller with an exact matching picture of the part for £20. This seller has 1 star and 3 reviews saying they sent out a totally different part, so that's a dead end as far as I am concerned.

So my understanding of the situation is that I have 3 choices.

• Service the original pot.
• Replace with a modern pot.
• Accept this is too much work and scrap the amp.

I've ordered what I expect will be a knock-off Alps pot for £5 as my Plan B. Plan A is to service the original pot.

I tested the pot last night with my multimeter and found something odd. As I suspected, I get different readings from the two channels. However, one of the channels when turned up to MAX seems to go back to 0. The other channel gives me the expected log range from ~0-30k. The other seems to start at about ~1.5k, goes up to ~30k, then seemingly drops to 0. So I'm wondering if one of the sweeper arms has somehow become offset, and that's why I'm getting a volume difference.

I think there's a pretty high chance of destroying the pot here in attempting to service it, but it can't really get much worse, and there's always Plan B. So I'll report back when I've made progress. I feel obligated now, being the top Google search for the part number!

One final note, slightly off-topic, but I didn't get any replies to my original post. Obviously I'm not entitled to replies and I'm not trying to complain that I didn't get any help. It would just be good to know if I did (or didn't) do something in my original post that discouraged replies so I can address that in future. Thanks!

[macboy]

I would have replied to your first post but I didn't see it. Oh well. Your post was good, you stated what you did to track down the problem, included relevant photos, schematic snippets, etc.  Better than many people do.

You are right that the volume pot is a voltage divider. The actual value of the pot usually doesn't matter much. A 20 k or 50 k pot will work just as well to simply attenuate the signal.

In your case, this is not so simple. Your pot isn't just a voltage divider. It also connects to some sort of 'loudness' circuit. This circuit boosts bass and treble at low volume settings, to compensate for the human ear's lower sensitivity at low and high frequencies vs mid frequencies. A typical potentiometer has 3 connections: the upper and lower legs of the resistor track, and the wiper. Yours has a fourth connection, which is partway along the track. This connects to another part the circuit around SW201, which injects a signal at this terminal, and affects the output of the pot in some way which depends on the position above or below this midpoint.

You can replace the pot with a standard stereo pot, and it will work to control volume but you will lose the loudness function. You may also find another 4 terminal pot, but anything other than an exact replacement will likely alter the operation of that loudness function.

[wasedadoc]

Before going to the trouble and expense of replacing the pot, I would confirm that it really is the cause.  Solder Some lengths of wire beteen the pot and the PCB to swap the two sections of the pot.  If the low volume problem remains on the same channel the fault is elsewhere, not the pot.

[coromonadalix]

yeah  one thing you should have done is shorting the 2 center pot side in parallel, or find a way to be in mono  in the power amp section,

that way you will be sure to have the same input and check if it's the cause or not on the output side


seeing some comments of this and that is not a grantee that you have or may have the same problem(s)


you need to think before dismantling everything, and sometimes changing parts for nothing

you need to have some troubleshooting strategy...

[DaveWarnock]

So I finally got to disassembling the potentiometer and I do not believe it can be repaired. It looks like someone has applied a lot of force at the upper and lower limits of the rotation. A metal tab is present to stop the sweeper being rotated too far, and the corresponding metal tab on the sweeper has been badly deformed and has snapped the plastic. I don't think I fully understand why this messes up the resistance values, but it does seem like flipping this part over just swaps the broken channel, so I'm fairly confident this is the issue and there's little more I can do to repair this (unless I can somehow find a donor pot).

So Plan B, I guess, which is already in the mail.

You can replace the pot with a standard stereo pot, and it will work to control volume but you will lose the loudness function. You may also find another 4 terminal pot, but anything other than an exact replacement will likely alter the operation of that loudness function.

Thank you very much for the feedback. The replacement I have ordered does have the loudness function, and I appreciate you checking that. I'm pretty sure the replacement is a low quality part so I'm not expecting much but I guess I'll see.

one thing you should have done is shorting the 2 center pot side in parallel

I'm not quite sure what you mean - do you mean I should have tried to join the two signals into a mono signal and run it through one side of the pot, then the other?

you need to think before dismantling everything, and sometimes changing parts for nothing
you need to have some troubleshooting strategy...

My strategy on getting this amp was:

• Visual inspection before I do anything. This actually did reveal an issue as the connector to the output terminals was improperly soldered, so I fixed that. I left that out of my original post for conciseness.
• Manual test of the amp to verify the symptoms. This is when I noted crackling on changing volume, and that one channel seems to work fine, and that volume does go up and down even on the quieter channel.
• Search for service manual, which I found, but got me no-where.
• Search for the issue, which suggested the pot was most likely the problem.
• I looked for a direct replacement and discovered this was not viable from an economic standpoint.

Then I came here, and after I didn't get any initial replies I proceeded with a teardown to get a better look at the pot. I would be receptive to feedback on my strategy and what else I could have tried, but I don't really think my approach was unreasonable?

[TimFox]

If it were mine, I would replace it with a 20k unit and ignore the tap for the "loudness" circuit.
This was a popular setting years ago, trying to implement a correction for the Fletcher-Munson curves of perceived loudness vs. absolute sound level.
http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/eqloud.html
The loudness circuit essentially boosted bass response at low gain settings;  as in your amplifier, the loudness circuit was usually switched in when desired, connecting to that tap.
However, to actually do the Fletcher-Munson correction, you need to know the relationship between gain and actual absolute sound level, which is a function of everything else in the system.

[David_AVD]

You can buy dual gang pots with the loudness tap. Here's an example: https://www.aliexpress.com/item/1005001929890809.html

[DaveWarnock]

You can buy dual gang pots with the loudness tap.

If it were mine, I would replace it with a 20k unit and ignore the tap for the "loudness" circuit.

If I can address both these points at once, what I have bought is a an ebay "Alps" Blue Beauty 50kohm dual pot with loudness. It WILL have the following issues:

1. It's almost certainly a Chinese knock-off as I understand the market to be totally flooded with them. I believe a real "Blue Beauty" will cost at least £20.
2. Even as a knock-off, I expect it will have the same problem as the real deal - it's a totally different size so I won't be able to just solder it into the existing board, or stick the existing knob onto the shaft.

It also sounds like TimFox is trying to tell me that the loudness circuit might work, but its audio profile might be wrong if I'm swapping out for another type of pot? To be honest I'm willing to live without the loudness circuit if I need to.

So in summary, I'm not certain how much work will be involved in using this pot - it might be quite a lot, but it's Plan B for that reason.

That said there might be a Plan A-2. I didn't really think that getting a sweeper from a donor pot was a viable approach here, but after having a look on ebay I've found someone selling old-new stock of a pot that has a different rating (100kohm) and no loudness circuit. It's posted along with measurements of the pot itself, and it looks like it might be workable as a donor sweeper, or failing that a more appropriate replacement. I've attached some pictures from the listing.

At £4.50 I'm tempted to just give it a punt and see what happens, but if I can find a similar pot (with or without a loudness tap) with the right resistance, that might be a better bet than the knock-off BB even if I can't use the sweeper arm as a donor.

I admit this seems like a long shot. I'm still mulling it over. I don't want to throw good money after bad, but maybe I'm willing to roll the dice a little on the donor thing after all.

[TimFox]

The "loudness circuit" requires the tap on the pot to function, but I never used it.

[DaveWarnock]

Thanks TimFox. I'm honestly not that bothered about keeping the loudness function - a working amp without it is better than a broken amp!

I have just rolled the dice after getting out my calliper and measuring the original pot. Based on what was posted on eBay, it looks like the casings, pins, and thread on the one I'm looking at are exactly the same. It'll take just under a week to get here, but I will update once it has arrived.

Thanks everyone for your comments and help so far, I appreciate it!

[floobydust]

Also note a scratchy volume control can be caused by leaky/shorted electrolytic coupling capacitors, on the pot's input and output side. That would be C215, C216 and the two downstream. If the op-amp's offset has gone negative, you can also get some DC at their output which causes the (reverse-polarity) capacitor to fail. I have seen this where op-amps age or the factory design had them backwards. C207, C208 failing can also upset things, scratchy BAL control as well.
Check there should be no appreciable amount of DC at the pot's terminals.

If you find the original potentiometer's parent family model number, you can track down the wafers and replace them. The extra hardware for the motorized option is what makes them unobtainium. I think you're already doing this. It's a bit of surgery.

When checking for one channel being lower volume, I inject a 1kHz tone and use a multimeter on ACmV to compare channels and track down the problem.

[TimFox]

Also note a scratchy volume control can be caused by leaky/shorted electrolytic coupling capacitors, on the pot's input and output side. That would be C215, C216 and the two downstream. If the op-amp's offset has gone negative, you can also get some DC at their output which causes the (reverse-polarity) capacitor to fail. I have seen this where op-amps age or the factory design had them backwards. C207, C208 failing can also upset things, scratchy BAL control as well.
Check there should be no appreciable amount of DC at the pot's terminals.

If you find the original potentiometer's parent family model number, you can track down the wafers and replace them. The extra hardware for the motorized option is what makes them unobtainium. I think you're already doing this. It's a bit of surgery.

When checking for one channel being lower volume, I inject a 1kHz tone and use a multimeter on ACmV to compare channels and track down the problem.

That's the nice thing about stereo amplifiers:  you have two identical channels to compare against each other when troubleshooting.

[floobydust]

You can hear fractions of a dB (amplitude) difference as a shift in the stereo image to one side. But on the bench it's harder to hear or measure with a scope, and some potentiometers track up and down so a low channel becomes louder above some angle.
I'm having really good success taking the pot apart and cleaning off oxide on the wiper and carbon track, as well as using PPE cleaner like DeOxit. Then oil or grease the shaft.
The chinese made potentiometers are basically a disaster, even Alpha Taiwan RV-16 I got some 1MEG from Mouser and 90% of their resistance was in the first 15 degrees. Sigh. It seems to be the carbon film deposition is the worst quality.

[TimFox]

To be fair to the manufacturers, it is hard to build an accurate log-taper potentiometer with carbon films.
If precise tracking between channels is required, the best results are with switched-resistor networks (stepped attenuators), which are more expensive.
A compromise, with worse log taper but better match between two units, is to use a linear-taper pot with a load resistor to approximate a log taper.

[andy3055]

So I finally got to disassembling the potentiometer and I do not believe it can be repaired. It looks like someone has applied a lot of force at the upper and lower limits of the rotation. A metal tab is present to stop the sweeper being rotated too far, and the corresponding metal tab on the sweeper has been badly deformed and has snapped the plastic. I don't think I fully understand why this messes up the resistance values, but it does seem like flipping this part over just swaps the broken channel, so I'm fairly confident this is the issue and there's little more I can do to repair this (unless I can somehow find a donor pot).

So Plan B, I guess, which is already in the mail.

You can replace the pot with a standard stereo pot, and it will work to control volume but you will lose the loudness function. You may also find another 4 terminal pot, but anything other than an exact replacement will likely alter the operation of that loudness function.

Thank you very much for the feedback. The replacement I have ordered does have the loudness function, and I appreciate you checking that. I'm pretty sure the replacement is a low quality part so I'm not expecting much but I guess I'll see.

one thing you should have done is shorting the 2 center pot side in parallel

I'm not quite sure what you mean - do you mean I should have tried to join the two signals into a mono signal and run it through one side of the pot, then the other?

you need to think before dismantling everything, and sometimes changing parts for nothing
you need to have some troubleshooting strategy...

My strategy on getting this amp was:

• Visual inspection before I do anything. This actually did reveal an issue as the connector to the output terminals was improperly soldered, so I fixed that. I left that out of my original post for conciseness.
• Manual test of the amp to verify the symptoms. This is when I noted crackling on changing volume, and that one channel seems to work fine, and that volume does go up and down even on the quieter channel.
• Search for service manual, which I found, but got me no-where.
• Search for the issue, which suggested the pot was most likely the problem.
• I looked for a direct replacement and discovered this was not viable from an economic standpoint.

Then I came here, and after I didn't get any initial replies I proceeded with a teardown to get a better look at the pot. I would be receptive to feedback on my strategy and what else I could have tried, but I don't really think my approach was unreasonable?

Looking at those photos, Since the wipers and the tracks are in good shape, I think you will be able to use a glue gun and repair the broken side. What seems to happen is that the missing plastic under that wiper is making the wiper not contact properly. Shouldn't be that hard.

[DaveWarnock]

When checking for one channel being lower volume, I inject a 1kHz tone and use a multimeter on ACmV to compare channels and track down the problem.

Is there any chance you can expand on this, or link me to a tutorial/video on what you mean? I do have a multimeter and vaguely know how to use it, but I'm not sure what I'd be looking for here.

Also note a scratchy volume control can be caused by leaky/shorted electrolytic coupling capacitors, on the pot's input and output side. That would be C215, C216 and the two downstream. If the op-amp's offset has gone negative, you can also get some DC at their output which causes the (reverse-polarity) capacitor to fail. I have seen this where op-amps age or the factory design had them backwards. C207, C208 failing can also upset things, scratchy BAL control as well. Check there should be no appreciable amount of DC at the pot's terminals.

That's really interesting and gives me something specific I can test. I have an ESR meter (the cheap and cheerful MESR-100) so I can certainly test those caps before I put everything back together. I have plenty of caps here that I use for repairing video game consoles as well, so I might even have something I can drop in as a replacement should I find a fault.

I'm having really good success taking the pot apart and cleaning off oxide on the wiper and carbon track, as well as using PPE cleaner like DeOxit.

I have DeOxit but I wanted to do a google before I used it on the carbon traces, and the jury really seems to be out on that. The company claims deoxit D5 is totally safe to use on carbon traces but there's some weird history there. From what I could gather, they changed the formula at some point (quite a while back, I think) and the original formula would degrade carbon tracks over time, but the new formula won't. The audio forums I looked seemed to be convinced that DeOxit is snake oil when it comes to pots though, so at best it's going to do nothing, at worst it will break down the carbon over time. In the end I cleaned it with 99% isopropyl alcohol and gave it a very light rub with a clean pencil eraser. I think that should be it sufficiently clean.

The chinese made potentiometers are basically a disaster, even Alpha Taiwan RV-16 I got some 1MEG from Mouser and 90% of their resistance was in the first 15 degrees.

This seems to be a really common thread when talking about these pots, so I really hope I end up not having to use the one I've bought. Still, if it's garbage but fixes the problem, I can swap it out for a better component in future.

Looking at those photos, Since the wipers and the tracks are in good shape, I think you will be able to use a glue gun and repair the broken side. What seems to happen is that the missing plastic under that wiper is making the wiper not contact properly. Shouldn't be that hard.

I had considered that but I wasn't sure it would work - partly because the metal "stoppers" are also really badly bent out of shape, and I wasn't able to apply enough force to "unbend" them. That said it really couldn't get any more broken, so maybe that's Plan A-3 if Plan A-2 doesn't work out.

Thanks everyone for the feedback and suggestions, I feel like I have a much better handle on this than I did when I started!

[wasedadoc]

Also note a scratchy volume control can be caused by leaky/shorted electrolytic coupling capacitors, on the pot's input and output side. That would be C215, C216 and the two downstream. If the op-amp's offset has gone negative, you can also get some DC at their output which causes the (reverse-polarity) capacitor to fail. I have seen this where op-amps age or the factory design had them backwards. C207, C208 failing can also upset things, scratchy BAL control as well. Check there should be no appreciable amount of DC at the pot's terminals.

That's really interesting and gives me something specific I can test. I have an ESR meter (the cheap and cheerful MESR-100) so I can certainly test those caps before I put everything back together. I have plenty of caps here that I use for repairing video game consoles as well, so I might even have something I can drop in as a replacement should I find a fault.

Nothing to do with ESR.  Just leakage (elecrical not electrolyte ) which results in a dc voltage acroos the pot.  Measure the voltage across the pot and if more than a few millivolts, replace the capacitor

[DaveWarnock]

Well folks I am back with good news!

First off, when my part finally arrived I deconstructed it as much as possible (see pictures below) and it looks like the part will be a perfect fit. There's just one problem. I cannot for the life of me remove the sweeper from the rod. It looks like the end of the rod was pushed down with some kind of press, kind of like a pop rivet. I have tried drilling it out as much as I dared but I cannot seem to separate the sweeper and the rod. I considered cutting the other end of the rod off to see if I could remove it that way, but I can't see any way to do that without damaging the plastic.

So I was sitting mulling this over and thinking about what andy3055 said about just glueing it back together. Then when I was looking at it again, it all fell into place. When it's in place the bent metal is acting like a fulcrum, raising the damaged part high enough that the sweeper comes into contact with the carbon trace in two places. Glueing it back down might actually be sufficient to resolve this. So with very little to lose, I glued it down and left it overnight. The next day I re-assembled it and tested it with the multimeter and got a clean log curve on both sides. The only issue is that there is a very small distance in the resistance between the two tracks, but I'm led to believe that's not at all unusual for this type of pot - the difference being 29.5kohm and 30.2kohm.

So after that I re-assembled it fully, soldered it back into its board, and tested the amp. Zero problems. Zero scratchiness. As expected one channel did sound slightly louder than the other, but a nudge on the balance knob resolves that issue.

So now I have a perfectly working amp, or at least perfect for my needs. Thank you to everyone for your suggestions, but a particular shoutout to andy3055 because I had 100% considered the original part a write-off. I would not have tried to glue it at all if you hadn't put that idea back into my mind.

[andy3055]

Glad it worked out for you. Usually, it is easier to repair a part like this than modifying every thing around and fit a different control. Great job!

[wasedadoc]

The only issue is that there is a very small distance in the resistance between the two tracks, but I'm led to believe that's not at all unusual for this type of pot - the difference being 29.5kohm and 30.2kohm.

So after that I re-assembled it fully, soldered it back into its board, and tested the amp. Zero problems. Zero scratchiness. As expected one channel did sound slightly louder than the other, but a nudge on the balance knob resolves that issue.

A difference in the end to end track resistance between the two sides of a stereo volume control introduces next to no difference in the audio signal voltage coming off the moving contact.  Even if it did, the difference in your case is only 2.8%.  In dB terms that is 0.2 dB.  If you are really hearing one channel slightly louder than the other I highly doubt that the resistance difference you measured is the reason.

[DaveWarnock]

A difference in the end to end track resistance between the two sides of a stereo volume control introduces next to no difference in the audio signal voltage coming off the moving contact.  Even if it did, the difference in your case is only 2.8%.  In dB terms that is 0.2 dB.  If you are really hearing one channel slightly louder than the other I highly doubt that the resistance difference you measured is the reason.

Maybe it's entirely in my head then, I didn't do the most precise test. I put on some music and closed my eyes! Thank you 

[TimFox]

The only issue is that there is a very small distance in the resistance between the two tracks, but I'm led to believe that's not at all unusual for this type of pot - the difference being 29.5kohm and 30.2kohm.

So after that I re-assembled it fully, soldered it back into its board, and tested the amp. Zero problems. Zero scratchiness. As expected one channel did sound slightly louder than the other, but a nudge on the balance knob resolves that issue.

A difference in the end to end track resistance between the two sides of a stereo volume control introduces next to no difference in the audio signal voltage coming off the moving contact.  Even if it did, the difference in your case is only 2.8%.  In dB terms that is 0.2 dB.  If you are really hearing one channel slightly louder than the other I highly doubt that the resistance difference you measured is the reason.

Old rule of thumb on audibility of 1 dB change:  if you ask your kid to turn down the music or TV, he will turn it down by 1 dB (just audible difference).