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Heathkit TA-17 Amplifier

Discussion in 'Troubleshooting and Repair' started by Ron Cianciaruso, Apr 14, 2016.

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  1. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    Friends,
    I am trying to repair an old HealthKit TA-17 Amp and having a problem that I cannot solve. I have changed every Cap, both non polarized and Electrolytic. I have changed every resistor and transistor and have the following problem. On the attached schematic I am getting 80V DC on the Emitter, Collector and Base on Q13, Q14 and Q15. This is causing the amp to destroy components as you can imagine. I have also replaced the diodes correctly leaving me with a brand new amp, but this problem still exists. I am new to schematic reading but from what I see 80V comes in on [f] on the Power Amp Section and should hit the 1000ohm and 330ohm and reduce ( I think)... any ideas? I really appreciate any help to point me in the right direction.

    Regards
    Ron
    http://harmony.demont.net/documents/schematics/amps/heathkit_TA_17.gif
     
  2. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    Q12, R313 (330) and R314 (1000) effectively form a voltage divider for the base of Q13. You should see approximately 37.2 volts on the collector of Q12.
     
  3. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    Thanks Bart.

    Incidentally, I just discovered that Q12 and Q13 where bad... probably from a previous component failure. I replaced Q12, Q13, Q14 with freshly tested components and I now have proper voltages. Onwards with the troubleshoot. Will keep you posted on next step here.

    Many thanks - Ron
     
  4. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    Hi Ron.
    Glad to hear you are making progress. If you're new to this amp, you'll find it to be very versatile and fun to use. Do you have the original Heathkit speakers for it? The combo is wickedly loud!
    Bart
     
  5. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    I wish I had the speaker cabinet for this, but do not. I've not gotten it working to the point to try it out, but as soon as I do, I will be playing some WHO songs with my bass on it to road test it :)
    Ron

    P.S. - I was fortunate enough to get the complete original build guide for this amp, so I have great reference material including test points and voltage readings. Good stuff
     
  6. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    The WHO was and is a great group. One of my favorites. I too have the manual. It has saved me more than once when I had an issue or question with the amp. I bought my amp in 1968 or '69. It's failed only twice over the years. Both times it was shorted output transistors. Both times it was "operator error". I would rate this amp "excellent" in reliability. I'll be recapping it this summer to make sure it sees another 40+ years or reliable service!
     
  7. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    I am still in the thick of it... She powers on now and holds, but no audio output. I suspect Q10/Q11 may have been fried as well, so will change those out again this weekend. It can also be Q1, Q2 NS Q3 in the pre amp section, but I am less sure of that since the Bass input produces no sounds... my money is on q10/q11. Will let you know. BTW - I am a huge WHO fan and know Pete and Rog personally... we exchange emails.... I got to play onstage with Roger last year in Foxwoods Casino CT... I played Bass with him and sang harmonies with him to Wont Get Fooled Again.... talk about a dream....I am seeing them both end of May out in LA for a charity event... very small.. will get to hang with them and Eddie Vedder - Life is good.
     
    Last edited: Apr 15, 2016
  8. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    Bart - I am yet again stuck. I have all of my voltages correct and I have absolutely no sound coming out of the amp. Nothing, not a crackle or anything. I've checked Q10 /Q11 in the power driver section and they look normal. The amp powers on and looks otherwise healthy but no sound.. any idea where to look ?

    Thanks
    Ron
     
  9. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    Hi Ron,
    Do you have access to an audio generator and oscilloscope? IMPORTANT: Do you get an audio "thump" from the speaker when you first turn on the amp? If all three channels are dead, the problem must be in the POWER AMPLIFIER section. First, verify 80 volts at "F" and 25 volts at "XX". Now carefully verify B-C-E voltages on Q10 through Q16. Is C303 (Q11-C) original? If so, it may be open and not allowing the audio signal to pass. Let me know what you find out and we'll continue from here.
    Bart
     
  10. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    Bart
    I do NOT get the thump on power up and I do not have a scope/AG at this point. For voltages, I have 79v and 31v at those points. The schematic says there is a 20% variance, so not too worried about the 31v vs. 25. C303 has previously been replaced. I will go thru Q10 - 16 again and check them all and report back. I did find an error of my ways on this... there are a pair of 22ohm resistors on the board in R312 and R319 that I had replaced with a 2.2.ohm. This likely caused Q12 to suffer. I don't see any potential damage that could have been caused by R319. I have since checked every resistor to ensure no more goof ups and I am certain those are all good... I know things are better as my initial symptom was C307, R317 and R315 all went up in smoke separately. Those problems are gone for now..... At any rate off I go to investigate...

    many thanks
    Ron
     
  11. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    Here are the readings.. something is still amiss

    C303 - +side to ground = 31.6v ( Its a 15v cap.. I am surprised it did not pop)
    Q10 - B=.052 E=0 C=31.5 ( Not Normal )
    Q11 - B= .004 E=0 C=.063 ( Not normal)
    Q12 - B= .72 E=1.3 C=34.1 (Looks relatively normal)
    Q13 - B=35.9 E= 36.2 C= 79 (Looks relatively normal)
    Q14 - B=35.2 E=34.1 C= .30 (Looks relatively normal)
    Q15 - B=37 E=37 C=79 (Looks relatively normal)
    Q16 B=.21 E=0 C= 35.6 (Looks relatively normal)

    I really truly appreciate your help on this.
    Ron
     
  12. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    2.2 ohms at R312 may have destroyed Q12 and D601, D602. Q12-13-14-15-16 are DC coupled. Keep your fingers crossed they all survived! No speaker "thump" indicates C604 (4000/50V) is not charging or is open. Check for 38 volts on the + terminal of C604.
     
  13. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    Just got your voltage measurements. I'll look at them shortly.

    PS: Thank me AFTER we get your amp fixed! LOL!!
     
  14. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016

    I am getting 38.2 on the + side of C604
     
  15. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    Q10-C is directly coupled to Q11-B. The voltages must be equal. Use your ohm meter on the X1 scale and measure the resistance between Q10-C and Q11-B. It should read a fraction of an ohm. Could possibly be a cold solder joint or a break in the trace.
     
  16. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    38.2 is good. If you press your ear against the speaker with the amp on, do you hear any noise/hum at all?
     
  17. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    ok, I removed Q10, Q11 , the 82K resistor, the 3.3k resistor and the 10/15v Cap, and reinstalled them paying attention to clean solder joints. Now I do get some noise from the speaker like a hum. Plugged in a guitar to the Guitar and Bass input jack and nothing yet.... but there is an improvement :)

    resistance between Q10C/Q11B is 0.7 as well.
     
  18. Bart Rydzynski

    Bart Rydzynski

    39
    0
    Apr 14, 2016
    Are the Q10 and Q11 voltages now correct? Audible turn-on "thump" from the speaker?
     
  19. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    No thump on power up. But I do get HUM

    Q10 readings :
    B .61 - Good
    E 0 - Good
    C 4.4 Good


    Q11 readings

    B 4.4 - Good
    E 3.6 Good
    C 15.4 ( Marginal Good - SB 11) Given {xx} Voltage is 31 , I would expect this.

    I plugged an audio cable in the guitar/bass jack and touched it.. nothing
     
  20. Ron Cianciaruso

    Ron Cianciaruso

    54
    0
    Apr 14, 2016
    A small update. The MIC input is working. plugged in a cable and touched it.. lots of noise...
    the Bass and Guitar are not....
     
    Last edited: Apr 17, 2016
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