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Nap 135 help: self-muting and fan adjustment

Discussion in 'd.i.y.' started by hifiaf, Jul 27, 2020.

  1. hifiaf

    hifiaf pfm Member

    I recently bought some CB Nap 135s, which after recapping are sounding very good. There are two quirks in operation, though.

    I should note here that I only recapped the amp and regulator boards, not the fan boards — and I didn’t replace the zener diodes on the amp boards. I did test that the fans worked by turning up the fan trim pot (they both came on) and then returning them to their pre-set positions. I do now have the caps in hand for recapping the fan boards, so if that’s all that’s needed, it’s easily done.

    First, one of the amps likes to suddenly mute its output. It doesn’t turn right off, because the idle hiss is still audible when it goes. It seems to do this only if I’ve been driving it relatively hard (about 9 o’clock on my 32.5) for about 20 minutes or more. If I then turn it off and back on, it plays happily once again. The case is not hot to the touch at any point. The fan never turns on. I thought maybe that the process that is supposed to trigger the fan is instead triggering this self muting.

    The other amp doesn’t self-mute but it’s a bit overzealous with its fan. My speakers are relatively efficient Yamaha NS-1000Ms, so I wasn’t expecting to hear the fans at all unless I really got things loud. The other day I left it turned on and noticed that the fan was running when I came back in the room — it was a hot day, but still... I see where the trim pot is for the fan — I figure I just need to turn it down a bit. But is there a standard way of setting this pot?

    Thanks for your help!
  2. Gervais Cote

    Gervais Cote Predator

    Are you able to take the temperature of the casing near the output transistors BEFORE the fan turns on ?
    My NAP 160 BD runs steadily at 43 Celsius with no problem at all which is quite warm when touching it with my fingers.
    I would personally set it to start the fan at 50 Celsius or if too nervous maybe 45.
  3. Gervais Cote

    Gervais Cote Predator

    Another thing I did is to add an open style of wooden rack I made myself to lift up about 50 mm my NAP 250.2 from the shelf’s surface. This allows more air to circulate all around the casing and really help to keep the amplifier cooler.
    Before adding this spacer, the temperature of my 250.2 was steady at 38-40 Celsius but now as I touch it, I simply don’t feel any heat at all, it’s not even warm.
  4. hifiaf

    hifiaf pfm Member

    Thanks, @Gervais Cote. My sense is that heat itself is not the issue at this point -- I don't feel any warmth from the case when the fan turns on on the one, or the channel mutes on the other. I do have them in a fairly well-ventilated spot.

    How would one take the temperate at the output tranistors to set the fan turn-on point? I guess open the case, blast the music, and get a thermometer into the vicinity (I have one of those cooking thermometers that might work)?
  5. Gervais Cote

    Gervais Cote Predator

    I have a temperature probe on my multimeter so rather easy for me.
    Also, the output transistors can be quite hot and still in their comfort zone so that’s why I verify the temperature on the casing or cooling fins most of the time.
    Another thing to verify are the heat sensors that active the fan and/or thermal relay to see if they are working properly although they rarely fail.
  6. hifiaf

    hifiaf pfm Member

    I found this on the Naim forum:
    Hard to believe the temperature could be as high at 70 degrees (based on the case being only warm), and I don't believe the mains supply is actually tripped since I can still hear the idle hiss... And it does jump right back into service when powered down and then powered back up.

    But checking the heat sensors sounds like a good idea. Do you know how to do that?

    Thanks again!
  7. Gervais Cote

    Gervais Cote Predator

    I would interchange the sensor from one amp to the other to see if the same problem occurs.
    Or, if you don’t have a lot of patience like me, buy 2 new sensors and install them.
    I would also change the thermal grease under the output transistors as it may have dried a bit thus not transferring the heat properly.
  8. hifiaf

    hifiaf pfm Member

    Great advice -- thank you! Do you happen to have a part number for the sensors?
  9. hifiaf

    hifiaf pfm Member

    I tried swapping channels just now, and the self-muting moved from left to right — so definitely the amp, not something else.

    I opened the case up right away and measured the temperatures (I didn’t realize my DVM does temperature!). Everything was 29-31 degrees C, including the transistors and every part of the heat sink. So not hot in there. This should eliminate thermal paste as a culprit.

    Also, as soon as the amp cut out, I powered it off — and as soon as I pressed the switch to the off position, music came back ON until the reservoir caps had emptied. Is that strange or just how the relay works? I was having some issues with my switches before cleaning/lubricating them with Fader Lube.
  10. hp1

    hp1 pfm Member

    The cutting out of the amp is caused by the regulator circuit being incorrectly set or a fault . the rule of thumb is to position the trip pots some where near the centre of there travel but to do it properly it is done with a dummy load and ammeter ,i think 10 amps per rail has been mentioned.
    If the fans are coming on for no reason check the caps on the board and if ok adjust the pot to suit.
  11. hifiaf

    hifiaf pfm Member

    Aha, thank you @hp1! I take it the two trim pots in question are the ones indicated with the red arrows in this photo?


    Those are the only two I didn't touch during my service of the 135s -- I used the others to set the voltage on the rails and the quiescent current.

    A dummy load sounds beyond my current capacities -- but setting the pots to the centre of their range seems doable as long as it's safe...
  12. hp1

    hp1 pfm Member

    The two halves of the over amperage sensing circuit are linked together so that if one rail trips the other does the same. from your description of the problem it sounds like one of the rails is under set so the whole thing switches off in a predictable manner, as I see you have them marked I would go through the 4 possible directions one at a time to find which pot needs tweaking and in which direction. only move them a few degrees at a time.
  13. Gervais Cote

    Gervais Cote Predator

    By looking at your pictures, the thermal paste seems to be still okay as it’s not brownish and there seems to be enough of it.
    Heat is definitely not a problem here according to your readings.
    As for the adjustments to be performed, I don’t have a good enough knowledge about it but many PFM members do so let’s wait for their comments.
  14. hifiaf

    hifiaf pfm Member

    The photos above are of the "good" amp -- the problematic one has the other type of trim pot, which is harder to mark. This photo (showing all the stuff I replaced during my recap) shows the actual boards and the initial position of the two over-amperage trim pots:


    Neil McBride's page says the following about the trim pots:

    Speaking of thermal paste -- it does look in bad in one spot, under the transistors linked to the fan board. Not sure if that might be relevant:


    If the over-amperage trim pots are set correctly -- what causes over-amperage? It wouldn't be playing too loud in my case. A faulty component somewhere in the reg board?
  15. colasblue

    colasblue pfm Member

    If the amp is tripping off but still clearly on (ie lights on) then its definitely the regulator tripping. The thermal trip turns all the lights off too and it comes back on on when it's ready.

    If its voltages are set correctly and the current trip isn't set ridiculously low then a thing to consider is that it might actually be doing its job properly!

    So check that you don't have an intermittent short in your speaker cable or crossover and also check that you haven't accidentally switched signal and ground on the amp's input which can also cause this sort of issue.

    It should be more or less completely impossible to trigger this sort of trip in correctly set up normal usage. Playing a bit loud certainly shouldn't do it.

    The thing that worries me in your description of the fault is that you say you can still hear the hiss when the amp is "off" in which case it isn't actually off at all, it's just lost input signal (but not ground) - which the amp is incapable of doing on its own. So perhaps the real problem is upstream of the power amp somewhere.
  16. Gervais Cote

    Gervais Cote Predator

    I would also verify the input connectors inside the 135. They are usually robust but on some other Naim amps some DIN connectors are used and the soldering sometimes breaks.

    I am a bit late with it but here an interesting link :
  17. hifiaf

    hifiaf pfm Member

    Thanks for the suggestions, @colasblue -- this is extremely helpful!

    Well, annoyingly, the lamp on that power switch has died (and it was brand new and only lasted about a week -- another possible symptom of the problem?), so it's hard for me to know. But I do indeed hear the idle hiss after it trips... And then when I turn the power switch "off" the music comes right back on until the reservoir caps drain, which seems weird.

    Voltages are definitely correct (thanks to your guide!). I did turn the current trip up just a little bit (one side was indeed very low) and I've now been listening uninterrupted for the last hour (previously, the time was a very predictable 25 minutes... so strange!), so perhaps it was just the current trip setting. But...

    I've tried switching my speaker cable and speakers and the problem follows the amp, not the speaker cable or speaker -- so I don't think it's the cable or crossover. But it could very well be the input. I installed new XLRs when I did the recap. I'm quite sure I've got signal and ground in the right order but will double check. Also probably worth checking all my solder connections, too...
  18. sq225917

    sq225917 Bit of this, bit of that

    Were the xlr pins labelled from the inside or outside? Remember, when you solder from the inside everything is flipped 180. ( which is why I built your paradise psu loom, as people often balls this up).
  19. hifiaf

    hifiaf pfm Member

    So handy that I take pics of everything! I seem to have got all this right. (Note that I disconnected one of the two XLRs to make one 135 a dedicated left-channel amp and the other a dedicated right-channel amp.) Also note that my XLRs are metal bodied and that I needed to file off the AES notches to get them to fit into the enclosure. Perhaps that could mess things up...?


  20. hifiaf

    hifiaf pfm Member

    Well, it held out longer, bit still tripped (after about an hour and a half of continuous, loud playing). I just swapped the lamp in from the other lamp, so we'll be able to see if the light goes out next time it trips...

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