Amp repair question

Discussion in 'Electronics & Electrics' started by Clacker, Jul 4, 2019.

  1. Clacker

    Clacker Member

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    I have scored a NAD amplifier that immediatley shut off with smoke that I'm trying to repair.

    I've managed to narrow it down to the left side power amp board and from what I've been reading, a failed power transistor that has shorted is a possible casue.

    Because I don't know the history of the unit, I was hoping I could maybe disable the channel that has failed and run the unit for a period of time before repairing to see if everything else is OK.

    Obviously if it's Front Left that has failed its a problem and, assuming its a power transistor, could I snip the legs to remove from cicuit of one/both transistors for that channel to test?

    Thanks
     
  2. Technics

    Technics Member

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    It's reasonably likely that if output transistors have blown then the driver stage before them will have also been damaged.

    The hardest part of fault finding in solid state Class AB amps is working out what is going on when the feedback loop is broken. Basically everything will be abnormal until everything works. Depending on the bias arrangement for the output transistors it may also need to be adjusted once they are replaced to ensure the quiescent current is correct.

    That all said, schematics for a lot of NAD amps are available online so I'd start there. They tend to be fairly simple designs so it should be relatively easy to repair. What model number is the unit? That might help in terms of being able recommend a way to disable that channel.
     
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  3. OP
    OP
    Clacker

    Clacker Member

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    I have found the service manual online today (first hit on google nad t765 sm) and the model is T765.

    Thanks for the reponse!

    Edit: The serial number is T765G02XXX. The second revision says T00951 and above but the last characters are definitely G02XXX
     
    Last edited: Jul 4, 2019
  4. Technics

    Technics Member

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    I didn't realise it was a HT receiver. You could start by disconnecting the J5804A connector to disable the main supply to the left board entirely and see if the amp will power up. You could then try disconnecting Q5517 and Q5518 and see how you go. You may run into problems because the amp has protection for DC on the output and over-current.
     
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  5. OP
    OP
    Clacker

    Clacker Member

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    My bad, as opposed to a stereo amp or something else like car audio?

    Disconnecting J5804A is what got the amp to stop turning off. I could then get sound out of the FR speaker.

    I read that testing in circuit is not always appropriate for transistors but do you think I can detect a short with a DMM just in case it's one of the surround channels?

    I assume FL is suspected due to being primary sound?

    Thanks
     
  6. Technics

    Technics Member

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    With the amp off I'd put the DMM into a low resistance range and one probe on to V+ (pin 1) of J5804A and the other on to each of the amps speaker output. Then move the probe to V- (pin 2) of J5804A and the other to each amps speaker output. The faulty channel should have a near short from pin 1 and/or pin 2 of of J5804A to the output (unless an emitter resistor has also failed).

    Note that when I say speaker output, you can't use the actual speaker terminals because they will be disconnected by the protection relays. The easiest access point is probably pin 2 of P5501, P5601 and P5701 which are the Bias test points for FL, SL and SBL respectively.
     
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  7. OP
    OP
    Clacker

    Clacker Member

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    I'm away for a few days so can't take it apart today but I managed to add coat hanger wire to my probes to reach the terminals in-situ. Definitely FL circuit is the problem.

    Q5017 is shorted on all pins, both ways.
    Q5018 is 777ohms on C+ to B but shorted on C to E.

    I tried another set of transistors for SL and SR (one each left and right board) and they matched within a few ohms and no shorts.

    So reckon if I cut those FL transistors pins that'll be enough to disable circuit?

    As all the circuits look the same, think I could swap wires from the pre-out connector and pipe FL signal through another output?

    Cheers
     
    Last edited: Jul 5, 2019
  8. Technics

    Technics Member

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    It should be. They appear to be blown so you don't have much to lose.

    It would be possible but you may have issues with the output relays not being enabled for the correct channel. You might have to re-route the relay control signals from J5801A appropriately as well as the input signals.

    Just be sure you don't have any shorts on the speakers connections (or a bad/shorted speaker) for the next time. It's usually why the outputs blow up.
     
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  9. aokman

    aokman Member

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    Whatever you do, make sure it is on a current limited supply when you power it up. I have saved countless amps with a current limited light bulb inline with the power, also helps for confirming faults.
     
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  10. Technics

    Technics Member

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    Yes this is a very good idea. I have such a set-up and a few different wattages of incandescent bulb. The idea is that the bulb will glow brightly while the bulk caps charge up when it is first switched on and then dim right down if the amp is idle. If it stays bright then you have a short still. Given the original post it sounds like the damage is unfortunately done in this case. If the output transistors are replaced it would be advisable to use a dim bulb test setup the first time the amp is powered up.

    I'm reluctant to recommend it to others because wiring up such a device is probably illegal (at least in some, if not all states of Australia). Come to think of it. OP should throw the amp out, order a new one from Gerry Harvey, immediately wrap themselves in cotton wool and lock all the doors and windows. It's the only way.
     
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  11. aokman

    aokman Member

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    Certainly already toasted but I have lost count of the amount of times I think an amp is fixed, hook it up, then find I missed some little bastard in another stage waiting to take the whole amp out again :lol:

    https://antiqueradio.org/dimbulb.htm

    I would normally warn about safety but if you cannot make a dim bulb tester easily, you have no place messing with an amp and it’s likely to bite you. I understand everyone has to learn sometime though but do it with low voltages.
     
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  12. OP
    OP
    Clacker

    Clacker Member

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    Haha, already have the parts to make one, two GPOs wired in series using the incandescent as a current limiter:thumbup:
     
    Last edited: Jul 6, 2019
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  13. aokman

    aokman Member

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    Good job :) it is something you will keep forever so go for it. Only reason I mention caution is I have had many close calls over the years and its been more luck than anything else.

    Everyone has their best lessons through trial and error, the problem is that 1970s and 1980s products take no prisoners and bite hard, it was a different time where OH&S was not even a design consideration :)

    Once you blow up an output stage or short a large capacitor bank, it is burned into your brain literally :lol: but I am one of those people who learns through experience and not a textbook.
     
    Last edited: Jul 6, 2019
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  14. OP
    OP
    Clacker

    Clacker Member

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    Is
    Been lucky to learn from others; from a poor kid in the 90s surrounded by “snow” after he installed an electrolytic cap backwards to a new starter that destroyed a UPS on his first day :)
     
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  15. aokman

    aokman Member

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    They are the best lessons that you NEVER forget. :) Pity school wasn’t so easy :)
     
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  16. OP
    OP
    Clacker

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    OK, back home now and had a chance to play with this for an update.

    I took the heatsink/fan assembly out to get access to the power transistors legs and found the following:

    Fan 1 (which is actually second from back) was not working. Probably not surprisingly, the left channel board circuit above this was Front Left which may explain the failure.

    Both 5W 0.22 ohm resistors attached to emitter on the two transistors had failed open circuit.

    I cut all 3 legs of both FL tranisistors, cleaned and put back together and unit is now working!

    Was pleasantly surprised reading the manual over the weekend that this amp has an "enhanced stereo" mode where all channels on each side play the stereo content for maximum loudness :D.. my current Sony (from 2001) would only do a virtual surround mode which I was always disappointed with.

    So my plan is: hookup 4 spare speakers I have to surround and back surround (can't turn off fronts but hopefully that doesn't matter), let it run low volume for a few hours then give it a squirt (keeping in mind reduced fan cooling). Do that for a few days.

    If all good, I'll take power output module out again, try and diagnose using other circuits as a guide and order parts as required.

    Thanks very much Technics for your time on this and your contribution aokman. :thumbup:
     
  17. Technics

    Technics Member

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    That sounds like good progress.

    Having the emitter resistors fail is quite common when the outputs blow. Sometimes they are metal plate, low inductance types so check what was used originally.

    Given at least one output has a short from collector to base it would be prudent to check the transistors in the prior stage as well if you do attempt to repair the channel.

    You will also need to check/adjust the bias with new output transistors. Often amp manufacturers will use transistors that have been binned for particular gains and the replacements may differ. I didn't see the spec for the bias current (as determined by the voltage measured across the bias test points. Usually in the 10's of millivolts). The best bet would be to measure the existing channels and adjust the repaired channel to match.
     
  18. OP
    OP
    Clacker

    Clacker Member

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    Yeah pretty stoked as hopefully the rest of the amp is OK and this is a great centerpiece for me to get back into HT after many years (kids)!

    I did see the service manual had bias setup details but as you said, using existing values has worked out well for me so will use that to check prior stage and bias values of the other channels.

    If the front channels (which can't be turned off) don't have speaker loads on them, do they get used/hot?

    Will be giving it a work out tonight :D
     
  19. Technics

    Technics Member

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    I have a nice Pioneer unit, B&W speakers and a one year old. I know why you see so many of those speakers on the second hand market with the tweeter domes pressed in. Little fingers seem to be strongly attracted to the magnetic fields.

    Yes it is there. I just missed it the first time. The manual also lists the emitters as wire wound types so nothing special required there.

    At idle and with no load they still consume the idle/quiescent current which is dissipated by the emitter resistors and the class A gain stages will consume a small amount of power all the time. It's not a whole lot though.
     
    Last edited: Jul 13, 2019 at 6:15 PM

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