Help repairing Denon PMA-250SE

[Figlet108]

So on recommendation from istari_knight on some other thread I bought a Denon PMA-250SE from ebay to put together a good value system for my folks.

Unfortunately it turns out that the volume pot is faulty and I need to replace it. Question is, what part is it and where can I source one?

First here's a pointless photo of the amp's innards, just because it's fun to look at things I don't understand:
https://lh3.googleusercontent.com/-_9ajw5adsM0/U2QHzV_hiQI/AAAAAAAACU0/AwFNyYXKhGY/w1034-h689-no/Denon+Amp_resize.jpg

Here's the pot:
https://lh3.googleusercontent.com/-ubxDb31DDeU/U2QIPzrhwwI/AAAAAAAACVQ/bG64FLC7rEw/w1034-h689-no/2014-05-02+21.17.59_resize.jpg

It's marked as a 926G 100KX2
https://lh6.googleusercontent.com/-NpM8WO3lqo4/U2QHzZMwOGI/AAAAAAAACUw/vakb7H-6r4c/w558-h689-no/Denon+Pot_resize.jpg

thanks folks :)

[istari_knight]

Why did you buy a faulty one ? :lol:

The stock one is a 100K dual log "D" shaft with loudness tap... Not the most common [typically!]

You could get one of these: http://www.ebay.co.uk/itm/100K-ohm-Log-Logarithmic-Stereo-Dual-Gang-Mixer-Volume-Splined-Potentiometer-Pot-/231167319180?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item35d2a55c8c or possibly something nicer like an RK27 and fit it on flying leads ? Either way finding one exactly the same as original would be expensive, time consuming and pretty pointless IMO. That cheapie one I linked to has a splined shaft but as the volume knob is plastic it should be easy enough to "make it fit."

Whats wrong with the original anyway ? Track damage or just noisy ?

[Effem]

http://www.maplin.co.uk/p/omeg-100k-andohm-log-dual-gang-potentiometer-fx12n

Because the shaft is made of plastic it can easily be sawn to length

[istari_knight]

Also, if you want to upgrade performance into another dimension above the stock unit swap that opamp for an LM4562 ;)

[Figlet108]

Why did you buy a faulty one ? :lol:
I agree it's an obvious question :)
The fault is that when the volume is turned right down to zero, there is still loud music coming from the right channel. The seller had it in storage for ages and when he tested it just before listing on ebay didn't test it on zero volume so to him it sounded ok. Fair enough, I'm willing to take that in good faith.



The stock one is a 100K dual log "D" shaft with loudness tap... Not the most common [typically!]
What's a loudness tap? And if I got one of the 3 pin options you guys are suggesting how do I wire it up?



You could get one of these: http://www.ebay.co.uk/itm/100K-ohm-Log-Logarithmic-Stereo-Dual-Gang-Mixer-Volume-Splined-Potentiometer-Pot-/231167319180?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item35d2a55c8c or possibly something nicer like an RK27 and fit it on flying leads ? Either way finding one exactly the same as original would be expensive, time consuming and pretty pointless IMO. That cheapie one I linked to has a splined shaft but as the volume knob is plastic it should be easy enough to "make it fit."
Thanks. I'll just go for the simplest and cheapest as frankly in the setup my folks will use it in (and with their hearing aids!) any upgrades won't be appreciated.

[Figlet108]

http://www.maplin.co.uk/p/omeg-100k-andohm-log-dual-gang-potentiometer-fx12n

Because the shaft is made of plastic it can easily be sawn to length

Thanks Frank. That's a good 'instant gratification' option that I could go and get tomorrow to just get the job done.

[Figlet108]

Also, if you want to upgrade performance into another dimension above the stock unit swap that opamp for an LM4562 ;)

James, as a matter of fact I have a whole bunch of those LM4562 opamps lying around from when I was rolling opamps on the ASUS Xonar Essence sound card in the bygone days of the TFS. So I will definitely do the swap. Thanks for the tip.

[istari_knight]

Fair do's, I've had a few like that in different amps... the end stop gets damaged & the wiper just keeps going.

leave the loudness taps open [ie. pretend they don't exist] they boost low frequencies at the pots first bit of travel... Unless used at very low volume you wont notice any difference, even then it'll be minimal. The two pins furthest right are the loudness taps in your picture above. You may find the Maplins pot won't fit as pin spacing is different but if using fly leads it won't matter.

I liked the improvement from opamp rolling but as with everything YMMV :D

[Figlet108]

Right, I've ordered the ebay one as you're right, the Maplin one wont fit and I just want to solder back onto the PCB.
Thanks for the loudness tap explanation - very interesting, and how to wire it up.

Wow, my questions answered and problem sorted in double quick time - this is an example of what's great about forums and this one in particular :)

[istari_knight]

Right, I've ordered the ebay one as you're right, the Maplin one wont fit and I just want to solder back onto the PCB.
Thanks for the loudness tap explanation - very interesting, and how to wire it up.

Wow, my questions answered and problem sorted in double quick time - this is an example of what's great about forums and this one in particular :)

We aim to please :)

[Figlet108]

So, just a quick update to say that the new pot arrived and I just fitted it.
Problem solved and amp working perfectly :cool:

Thanks for your help James.

I'll swap out that opamp and plumb it into the main system and have a proper listen now it's working... (after the tedious job of reassembling it back into it's case etc...)

[istari_knight]

Nice one - Another thing saved from landfill :thumbsup:

Dont know if the opamp will give you any grief... Mine was alright but if you do get oscillation just stick some 10nF-100nF caps across the +/- rails [pins 4 & 8] to ground.

[Figlet108]

James, just a quick follow up.

Looks like the 2 tone controls don't work since I replaced the pot (they did work before). Do you think it's to do with the unconnected loudness pins, or more likely I've broken something?

Thanks.

[istari_knight]

Sorry, only just spotted this. Shouldn't have, they are unrelated [relatively] :hmm: What about balance ?

Did you change either opamp ?

[Figlet108]

No, the balance works fine

No I didn't change the opamp yet - all I changed was the pot.

Maybe I've damaged one of the board tracks when (de)soldering without noticing?

I've dismantled the original pot and cleaned it and was thinking of trying it back again just to satisfy my curiosity...

[istari_knight]

Yes that's got to be worth a try. I think I've got the schematics on an external HDD... I'll have a look for you.

[Figlet108]

I forgot to post the obligatory photo:
https://lh5.googleusercontent.com/-zSMipE1T7xo/U26Oi52fEqI/AAAAAAAACVs/rRAOge3dmgA/w856-h554-no/Pot+Internals_resize.jpg

I'll swap them out once the Euro-voting is over...

[istari_knight]

Yep, that's a loudness tap.

Unfortunately I don't have the schematics for this model, only the 250mk1 & 350SE both of which are very different to yours. I cant see how the tone controls could be affected by the loudness tap... Maybe if they were in the NFB loop ? Possibly, linking the empty solder pads [that previously held the loudness taps] to the end of the pots carbon track would restore functionality but its nigh on impossible to say without a circuit diagram.

If it works with the old pot back in place then we can assume that's the case !

[Figlet108]

So, turns out that when I put the original pot back the tone controls do work again, so there must be something going on there with the loudness tap connection. Unfortunately the cleaning did noting to fix the pot.

With the new pot and connecting the loudness pads as you suggested didn't fix the tone controls...

Very frustrating...

I found this on ebay (http://www.ebay.com/itm/NOBLE-100K-DUAL-LINEAR-POT-WITH-LOUDNESS-TAPPING-/400419069005), but it costs half of what I paid for the whole amp...

[istari_knight]

Thats a shame... Although that's the gamble you take on a faulty item [not that you knew it was !]

If the amps otherwise in good nick I would suck it up & buy that pot from HK, if its a minger then maybe just try & live without tone control ? Not much help I'm afraid but without the circ diagram its guesswork in the dark :( FWIW this is the first amp I've known where removing the loudness taps has caused such a problem.

[Firebottle]

Here is something to check Jason, you may have come across this as well James:

If the original pot has a metal support bracket or part metal body with pcb lugs, check to see if these are used as links across to other tracks on the pcb.
This will be the 0V or ground tracks, probably not the case but worth a check.

I've never come across a loudness tap affecting tone controls either :scratch:

:cool: Alan

[Figlet108]

Hi Alan, thanks for that suggestion. I had a look but couldn't see anything obvious.

However, Les the seller has very kindly agreed to pay for the £15 pot from ebay, which is very kind and very much appreciated.

Cheers Les :cool:

[istari_knight]

Top man Les :clap:

[Figlet108]

Les is the one on guitar :guitar:

GLtwfGKd2V0

[Figlet108]

Right, just to reach a satisfactory closure with this thread, today I received the replacement pot with loudness tap from ebay that I mentioned above.
I soldered it in place and would you believe it everything completely works :)

So against all the odds, the tone controls do seem to be connected to the loudness tap somehow.

Anyway, after this educational epic, I finally have a working cheap-but-good amp I can send to my folks. Under the circumstances I'm going to pass on replacing the opamp and just get this in the post to Greece and be done with.

It sounds excellent by the way - who'd have thought for such a cheap old amp...