looking for help faultfinding 2ch audio amp

Discussion in 'The Projects Forum' started by snh, Oct 13, 2012.

  1. snh

    Thread Starter New Member

    Oct 13, 2012
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    Hi,
    I have an Audio Innovations Alto integrated amp that I am trying to fault find.

    The left channel is good, the right channel not so good - there is audio there (from speaker) but very distorted. I managed to find some schematics and attempted the quiescent current setup. Left channel perfect but on the right I can setup the 50% supply voltage on right hand side of R22 but I get no voltage drop across it (when I should get about 30mV).

    I believe all transistors are 'on' that is there is about .6V across all B-E junctions, the zener is ok as are the 3 diodes near it.

    Alll caps have been replaced or swapped side to side.

    The VR2 pot is good.

    I am no electronics guru (just a bit of basic knowledge) - what would be my next logical step in fault finding?

    I know the fault is staring me in the face but I can't see it. One channel is good the other not, so short of swapping every component one by one (I have almost done that I think) what would you do?

    Thanks in advance
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    No voltage drop across a resistor must mean one of two things: either the resistor is a short, or there is no current flowing in it. If the bottom power transistor is blown open, there would be no current flow in R22.

    In these kind of cheap power amps, the power devices often fail.
     
  3. snh

    Thread Starter New Member

    Oct 13, 2012
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    Thanks for the reply bounty, couldn't agree more however, I have pulled those resistors out and checked them and even swapped them with the left channel. I have also changed out all transisitors and double checked the base emitter voltage on them (about 0.6V) .

    It's got me bamboozled, I can adjust the "half rail" voltage at R22 for about 36.5VDC with the pot VR4 but adjusting VR2 as no effect - the voltage drop across R22 is zero i.e. no current flow as you suggest.

    I don't understand how all the transistors appear to be 'on' but there is no current flow through that section? Where is it going?
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The 0.6V from base to emitter just proves the emitter-base junction is alive. The zero V drop on the R22 resistor tells you no current is flowing down the collector to the emitter. The base-emitter diode can work even if the collector is open.

    If you replaced the transistors, is it possible a trace is burned open? If those devices mount on a heatsink, is there a socket contact or some point that might be open?
     
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  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Since it's a complimentary output stage (and the top side seems to be working) it can hold the output to 1/2 rail. The bottom section sinks current, and that is not required to establish the output at half rail since there is no "sink load" on it.
     
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  6. snh

    Thread Starter New Member

    Oct 13, 2012
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    Ok I am going crazy here, I have been looking at this thing all day now and I cannot see for the life of me why there is no current flowing down that right hand side.

    I really appreciate your help bounty and I have checked everything you said and more. There is continuity everywhere that should be, I've lost count of the amount of times I have checked those transistors (using diode function of Fluke 87 DMM), and there are no broken tracks or open cct's anywhere (that I can see).

    TR13 & 5 are on heatsinks but I have checked the sockets and all is good.

    TR13 collector is good to VCC similarly emitter of TR5 is good to ground.

    Is there another way I can test this thing to see why they aren't conducting? I'm pulling my hair out here....

    Cheers
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Tough problem. One thing to try: print out a copy of the schematic. Turn power off. Use an Ohm meter to do readings across the traces and current paths in the lower circuits to verify continuity and correct Ohm readings where applicable. Use a highlighter to check off on the schematic. If you have continuity and correct Ohms, something must be breaking down under voltage.

    I would also use a volt meter to map all node voltages in the bottom circuitry (under power) starting at R31, VR4 and going down and across. That should tell you the problem. Adding R values to the schematic would make it easier.

    You might want to unsolder the collector lead on TR11 and stick a 1 Ohm resistor in series and see if there is any current flowing down. That is what feeds current to the bottom output stage.

    Check voltage across R36 and see if TR11 is getting fed any base current.
     
    Last edited: Oct 15, 2012
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    BTW: since the output is capacitively coupled... if you can set the output to 1/2 VIN, then the top power stage is pushing bias current down somewhere. If you are sure it's not going through R22, it has to be going through R29 and TR15/R36. A voltage map might show what's up.

    Has TR15 been checked out as good?
     
  9. snh

    Thread Starter New Member

    Oct 13, 2012
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    Thanks again Bounty, you wouldn't beleive how frustrating this has been, it almost got chucked in the lake today! haha Now relaxing with a glass of red but here's something I prepared earlier.....
    ALTO4_volts.jpg

    Tr15 has been checked and double checked and even replaced.

    I will try the 1ohm resistor trick on tr11 tomorrow.

    I'm not going to give up on this - I know it can be fixed....
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    have you measured both output resistors.also the darlington pair tr5/7.
     
    Last edited: Oct 15, 2012
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    VR4 sets the biasing of the lower stage and output centreing, and VR2 biases the upper stage.

    What is the problem you are getting now? The voltages you have written in don't look too bad. How do those voltages compare to the other (good) channel?
     
  12. snh

    Thread Starter New Member

    Oct 13, 2012
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    Thanks Dave - yes measured both R22/R26 out of circuit 0.4 ohm (supposed to be o.22 ohm), have even swapped with left channel - no difference.

    TR5/7 measure ok (as do all transistors).
     
  13. snh

    Thread Starter New Member

    Oct 13, 2012
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    Thanks RB, this is a learning curve for me so every little bit of circuit explanation helps to piece together how this thing works and where current flows etc.

    The problem as it stands now is:

    - no voltage drop across R22/r26 (cant get the 30mv to set up quiescent current using VR2 as per test schedule)
    - no audio on right channel (left ch is good)

    Today I am going to do some more comparative voltage/ohm readings between channels.
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Clearly TR5/7 are not pulling any collector current because you are getting a zero voltage drop across R22.

    IF the TR11 is pushing current into the base of TR7 (and all parts are connected as shown) their should be current flowing down TR5/TR7.

    I would try swapping out TR5/7/11 and remeasure. You said TR15 has been replaced so that only leaves those.

    You said:

    Measured how? I have seen bad transistors that will show a P-N junction on the B-E and C-B leads.

    When this type of design blows, the output devices generally get hit. I would swap the whole set and see.
     
    Last edited: Oct 15, 2012
  15. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Are you actually getting zero voltage across R26? That means there is no bias current in either the top or bottom power device's collectors. If so, the top set could be blown as well. It's possible you are getting the 1/2 supply voltage by the current flowing through the emitters of TR9/13? I can't tell for sure because there are no resistor values, but maybe there is no bias current flowing in either of the collector sets of the output drivers, it's just current from base to emitter.
     
    Last edited: Oct 15, 2012
  16. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Something occurred to me, you said:

    You keep saying the voltage drop across these resistors is zero. Is it possible your meter won't read low enough voltage range to get accurate value?

    If the bias idling current through the output power devices was 50 mA that would only generate a voltage of about 10 mV across the resistor. Takes a pretty good DMM to read that correctly.
     
  17. snh

    Thread Starter New Member

    Oct 13, 2012
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    Bit of an update, I ended up swapping out all transistors (again) either with new ones or side to side. When I changed the 2n3055 power trans side to side I noticed the voltage drop across output resistor R22 changed.

    While I could adjust for the 30mA on the left side in original config when I changed them over I could only get 8-10mA and something similar on the the right (dud) side.

    I thought I was on to something so I connected up a cd input and some speakers, expecting the fault to have changed sides. However left side still ran ok while the right had audio but was miniscule in volume and quite staticky.

    While redoing the current setup for left trying to get the 30mA as per the schedule (which I can't get by the way - only about 8-10mA max) I noticed that moving the volume pot caused the right to jump in and out.

    I investigated more and when I tweak the pot slightly by holding the shaft and applying a little down and left pressure I get clean audio and no hum!!

    I tried cleaning it with a bit of isopropyl alcohol and recrimping up the little lugs (they were loose) but it still crackles (and shunts the audio to ground?).

    I have a new one on order (its a 100k linear dual gang pot fyi), so hopefully tomorrow I will have this saga solved.

    I'll keep you posted, and Bounty you were spot on with your 10mA prediction over that resistor. the reason I got bogged down looking for 30 is that I did have 30 at one stage and could adjust either side of it. Still not sure what is going on there but will have a look when I get both channels back up. Phew!

    Thanks for your help - I would have given up long ago without it
     
  18. snh

    Thread Starter New Member

    Oct 13, 2012
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    There is peace and harmony again in the universe! haha :)

    New volume pot fitted and also found a dry joint on the input cap C15.

    We now have clear crisp music in the bedroom and does it pump with the Polk RT12's!! I can't get the volume knob more than 1/8th(ish), certainly no more than 1/4, before it becomes TOO loud. Sounds really good too, no hum/crackle etc.

    Thanks so much to everyone who helped, it's been a very frustrating experience (for me stumbling my way around the circuit) but also very rewarding.

    I'm now keen to dive into some other sort of electronic project....

    _DSF0008.jpg
     
  19. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Good to hear it's all working ok. :)

    Did you adjust for crossover distortion using a sinewave and 'scope? Or did you set the AB bias on those trimpots using a voltage procedure from the manual? I'm a bit worried the channel you were adjusting may no longer be setup right.
     
  20. snh

    Thread Starter New Member

    Oct 13, 2012
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    Thanks RB
    I don't have access to a sig gen or scope so I only did the setup as per the attachment on first page. I set up the half VCC voltage with VR3 and 4 (left and right). However I could no longer get the 30mv they talk about with VR1 and 2. I can only get 10mv max both sides and that is with the pot hard one way. I admit this is probably not ideal but the audio out of speaker sounds good.

    I'm all ears if you have a suggestion on how to better set this up?

    Thanks again
    p.s. I used to have 30mv on the left side but when I swapped over TR5 and TR13 it dropped back to 8-10mV.
     
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