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Proton 520 (similar to NAD)

Discussion in 'classic' started by Radfordman, Apr 26, 2012.

  1. Radfordman

    Radfordman pfm Member

    My recently acquired Proton 520 integrated amp sounds very good. It is similar to NAD amps of the same vintage, I believe that NAD’s were actually made by Proton.

    I have a problem, one output channel as it has a DC offset of around 300mV, and the other channel is fine, close to zero. I have tried adjusting the offset adjustment pot for the 300mV channel, but can’t get in any lower.

    I would like to fix the problem eventually, anyone any ideas?

    But I am considering putting some large value electrolytic caps back to back (to make a bi-polar capacitor), in series with the loudspeaker outputs, so as to protect the speakers.

    I have got plenty of suitable capacitors, for example 6,800uf 80v
  2. colasblue

    colasblue pfm Member

    This fault is probably caused by a high value open circuit resistor in the active rail regulation circuit that feeds the front end of the power amp circuit.
  3. Radfordman

    Radfordman pfm Member

    Thank you, most helpful. I have the circuit diagram and it looks like there are independent power supplies for each power amp channel.

    I will post a link to the diagram, and perhaps you would indicate exactly where I should look.
  4. Radfordman

    Radfordman pfm Member

  5. colasblue

    colasblue pfm Member

    Can't open that - sorry needs a USERID & password or something

    I've found a version of the manual and I'd be looking at R801 through 810 and very carefully at 804.
  6. Radfordman

    Radfordman pfm Member

  7. colasblue

    colasblue pfm Member

    Oops take that back, I'm looking at the wrong thing!

    The circuit seems very similar to a NAD 7150 I had more or less the same problem with and its probably got the same root cause.

    VR's 5&6 should alter the offset but I'm guessing one of them has no effect.

    I found a fault in one of R607, 609 or 617 (can't remember which). Obviously it could be the even numbered equivalents in the other channel depending on which one is iffy.

    Your problem seems so similar its probably a really good bet that its a fault in that general area.
  8. Radfordman

    Radfordman pfm Member

    Thanks, yes, turning the offset pot for the afflicted channel, does not have much effect. I will have a go at checking the resistors that you mention.

    It may not be for a few days, will do it in daylight too, being an older person my eyesight is not so good.

    This amp still sounds very good, perhaps better than the NAD 3130 that I also have, and that is pretty good.

    If the 7150 circuit is similar, perhaps I should consider one of those.

    I hope to report back soon.
  9. colasblue

    colasblue pfm Member

    No don't !

    I found the 7150 (presumably the same as the 3150 amp wise) had an annoying background hum which couldn't be cured. Others have had similar experience.

    If you get over that it actually does sound very good indeed but it's a bit of a spoiler.
  10. Radfordman

    Radfordman pfm Member

    Thanks, then perhaps I won't bother with those.

    The Proton 520 is very quiet, in fact I'm running it into Tannoy HPD 315's which are fairly efficient and would show that up. Incidentally, both my NAD 3130's are quiet too.

    I wonder which, if any, of the NAD badged amps are close in design to the Proton 520.
  11. Michael J

    Michael J pfm Member

    Thank you for that still available download, which I have just found via a Goggle search after having picked up one of these heavy little beasties for £5 at a junk sale. The right hand channel in mine appears to be dead through headphones, but I haven't yet tried splitting the pre-out from main-in to see whether it's just the power stage that is faulty. It might be no more than a blown internal fuse, I'm about to pull the case lid off to see. :)
  12. Radfordman

    Radfordman pfm Member

  13. Michael J

    Michael J pfm Member

    There is only one fuse that I can see inside it - 800mA 250V and, as the thing powers up, that is not the fault.
  14. Michael J

    Michael J pfm Member

    Well, that was interesting... it's not the power stage that's blown. All its line inputs are the same, no right-hand side at all, but it at least drives headphones nicely when it is fed a stereo signal from something else to main-in. :)
  15. Radfordman

    Radfordman pfm Member

  16. Michael J

    Michael J pfm Member

    I haven't tried it with speakers yet because I can't be arsed - yet! I'll get around tuit when I can be arsed to unclip the wires leading to the big old Baker 12" whizzer-coned jobbies from the back of the 3020 that is just here (reaches out and affectionately taps the ugly duckling that cost £2) and clip 'em into the back of the Proton just there (gently kicks it with a toe), along with rejigging the pre-out/main-in arrangement again (seeing as I have reset everything to its standard bewildering complexity of spaghetti stretched between multiple amplifiers and too many sources scattered liberally along one side of the room).

    I don't think it's just a dirty switch as all the line inputs (DAD, Video, and Tuner on the rotary selector as well as Tape on a separate push button) have the same fault. It needs the lid off again and a proper looking at now that I know that it is not a dead power stage. This stereo system could do with being tidied up a little. In the meantime, I think it's now time for some ZZ Top through too many loudspeakers at once. Sod the Proms. :)
  17. Michael J

    Michael J pfm Member

    The fault is not the rotary input selector. After perusing the wiring diagram (which shows a purely passive input set of sockets, pcb traces, a multi-way switch, and a 1k resistor immediately before Tape Rec out), I have two CD players connected to it, thence Tape Rec out to a line input on the 3020, and working fine as a fancy external switch box. It does mean the source is driving through 1k, but that is not an issue when the receiving end is several tens of k. The Proton is not even plugged in to the mains. :)

    If the source has a variable output level then it would be possible to bypass the active preamp circuitry entirely by using any of the three non-tape line-inputs through rec-out direct to main-in. Which reminds me (he says, looking at the mahoosive box right in front of him), my Emerson tuner does indeed have such a control, appropriately enough labelled OUTPUT LEVEL, conveniently situated on its front panel.

    I am a tad suspicious that the delay circuit (to prevent switch-on thumps etc.) might not be behaving correctly. A failed FET in one channel immediately before Pre-Out would explain things...
  18. Michael J

    Michael J pfm Member

    The fault might be partly the rotary input selector... but is not only that. Connecting a tt to the phono input has revealed that aforesaid switchgear is a piece of carp and lent extra support to the thesis that no power is being supplied to any part of the preamp electronics Right channel which remains stubbornly silent. Left channel can be made to work unreliably but seems best when "lodged" just before the correct click position. It might make a decent stage for a mono cartridge.

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