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NAD 3020A Amplifier Low Volume

Discussion in 'Troubleshooting and Repair' started by maikelson, Dec 15, 2020.

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  1. maikelson

    maikelson

    4
    1
    Dec 15, 2020
    Hi, I've dug out a NAD 3020A Amp from my parents gathering dust corner and taken on the so far enjoyable learning process of attempting to diagnose and repair its issue(s).

    It powers on, all inputs and outputs seem functional. But the volume - at maximum - is only a whisper. Left and Right sound balanced. And the 5 power lights (indicating 1-35W) don't illuminate - I presume this could simply be the globes or a deeper issue.

    I replaced 4 main caps (2200uf 35v) because they had some visible corrosion/leaking on top. This resolved an idle hum which is reportedly common in this amp. The volume may now be marginally better but is still a whisper.

    As a next step I removed and tested 4 main transistors (2N3055 and MJ2955). They seem to be open in the appropriate directions and showing ~300ohm resistance (30x10) as per pictured multimeter.


    I see some brown glue around some caps which I've read may become corrosive. Otherwise all appear to be intact, albeit aged and potentially dried. I figured I'd seek advice and direction before tinkering with any of these.


    To my eye the circuitry seems without abnormality. Except perhaps some orange/brown residue at some of the soldering points - see left hand side as pictured - these align with 4 fuses (intact).

    20201215_122705.jpg

    20201214_090658.jpg

    20201214_090718.jpg

    [​IMG]

    20201215_121030.jpg

    20201214_090557.jpg

    Any assistance is much appreciated.
     
    Last edited by a moderator: Dec 15, 2020
  2. davenn

    davenn Moderator

    13,883
    1,965
    Sep 5, 2009
    Hi there,
    welcome to EP :)

    That's not from the cap's but considering the apparent age of the unit, replacing them isnt a bad idea :)
    In fact, I would be suspicious of all electrolytic cap's


    Taking out one of the glued caps and looking underneath it would con\firm if there has been any corrosion :)
    probably not ( but maybe)
    As said, replacing them all would be good, they have all probably dried out inside to some extent


    That's just a bit of solder flux :)

    cheers
    Dave
     
  3. Bluejets

    Bluejets

    5,037
    1,052
    Oct 5, 2014
    That old meter may or may not be capable of providing enough bias to test transistors.
    Usually when they have a x1 range but none on this antique.
    Could verify by testing say an 1N4004 diode or similar.

    What are you using as an input..?
    What sort of voltage levels are you getting on the rectifier output and on the adjacent caps ..?
    Rectifier is the rectangular part centre of where you removed the output transistors.photo #5
    Caps for the dc output are the 4 adjacent I'd imagine.
     
    Last edited: Dec 15, 2020
  4. maikelson

    maikelson

    4
    1
    Dec 15, 2020
    Thanks davenn, very helpful. I notice in the service manual parts list CER, MY, and ELEC Caps. Assuming these correlate to Ceramic, Mylar, and Electrolytic. Assuming age related deterioration would your recommendation be to replace only the Electrolytic Caps?

    Bluejets, thanks for the illuminating comment regarding my old meter. I'm struggling to find good instructions on the tests you've mentioned. Could you please explain how I would do so?
     
  5. Bluejets

    Bluejets

    5,037
    1,052
    Oct 5, 2014
    Not knocking your meter, I use a similar analogue meter myself sometimes.
    The meter must be capable of providing enough bias to turn on the junction of a semiconductor.
    It is why you will see modern digital meters with a "diode test" range as their ohm scale just will not work.
    Normally, analogue meters such as you have there would have a 1 x range which would be ok for testing semi's.
    When switched to the higher ranges they will not normally provide the bias drive.
    It may well be your meter will do ok as there is no 1 x range which is why I suggested testing on a fairly standard diode such as an 1N4004.
    This will also be a good test for future reference as well as sometimes, meters such as yours, have the positive and negative switched differently on ohm test.
     
  6. Martaine2005

    Martaine2005

    3,564
    969
    May 12, 2015
    What’s that three legged component next to C637?. Just above the transformer in your last photo.
    It looks rather malformed.
    As previously said, ALL the electrolytics should be replaced as a matter of course.
    During the 80s, many companies used glue for components which did indeed prove to turn conductive. This should also be carefully removed/scraped away. Then try the amp.
    Use a good contact cleaner on all switches and pots too.

    Martin
     
  7. maikelson

    maikelson

    4
    1
    Dec 15, 2020
    It's a 2SD669. I think the light casting a shadow gives the illusion of it being malformed. Unless you mean the degree to which it leans?

    Thanks for clarifying replacing the electrolytics. I'll do that and a clean as directed.
     
  8. Martaine2005

    Martaine2005

    3,564
    969
    May 12, 2015
    No, not the leaning, it looks burnt/melted in the photo.
    But obviously light plays tricks in photos.

    Martin
     
  9. davenn

    davenn Moderator

    13,883
    1,965
    Sep 5, 2009
    Yes :)
    Ceramic and mylar will last a lifetime and more
    Electro's are the most unreliable component on the planet ;)
     
    Martaine2005 likes this.
  10. Bluejets

    Bluejets

    5,037
    1,052
    Oct 5, 2014
    No voltage readings yet..??
     
  11. maikelson

    maikelson

    4
    1
    Dec 15, 2020
    Just finished replacing all bar 4 elec caps, awaiting delivery of the these final few. Warmed it up to test and it's working and sounding glorious. I found some oxidation on the legs of some of the caps I removed so perhaps that and/or their age was the issue.

    Thank you all for your help.
     
    Martaine2005 likes this.
  12. Martaine2005

    Martaine2005

    3,564
    969
    May 12, 2015
    The oxidation was probably due to age but electrolytic caps dry out over time and can become ‘resistors’ or ‘short circuits’ causing all sorts of problems.
    Glad it’s working now. Don’t forget to clean all switches and potentiometers too.

    Martin
     
  13. Technomaniac

    Technomaniac

    93
    12
    Oct 31, 2020
    A lot of amps of this vintage have two pairs of RCA sockets with wire links joining them. The links can be removed if you want to insert an external graphic equaliser in the middle of the amp circuit. Sometimes when you acquire an amp the links are missing, which can confuse, as the amp doesn't work or has low gain. But also, if the links are present, they can be a source of poor performance, and should be polished up. They are sometimes chrome-plated and can oxidise.
    The old 200H multimeter you have is same as the first I bought myself when I was still at primary school. Nothing wrong with them, but lacking a low ohms scale. When I started my apprenticeship, I bought an AVO 8 Mk.III to use at work. I still have both of them, 50 years later. Just check that your amp has an earth in the mains cord.
     
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