Need expert help on Transistor amp circuit toubleshooting Fisher 250-tx

Discussion in 'Technical Repair' started by jimkarl, Jul 13, 2016.

  1. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    Have heavy distortion and weak output on right channel. Have narrowed problem down to the Driver (amp) board. I do not have an oscilloscope, just an hand held audio tester that I use to debug with (along with volt, cap,esr, transistor meters) Schematic and Voltages of the bad channel (right) are shown at the bottom. Right channel parts are represented by everything in the ().

    Before some can explain to me what is wrong, I want to understand what is supposed to be right. For example, On the left (good channel) Clear audio comes in Base pin 2 of Q801. I hear good clear audio out the emitter pair, and well as the other base pin 6. In contrast, the left channel has no audio out the emitters pairs and distorted audio out both other base pin as well as the collectors.

    So starting with the good left channel, I am confused right off the bat why the audio is coming out the emitters

    **My question 1 - I thought the audio path was to go out the collectors? But it seems in this case the audio is coming out the emitters, then onto the base of Q805 & Q807. Then out the collectors? and onto the base of Q809 & Q811? Then out the collectors again and to the speaker? Or do I have that wrong? I want to clearly understand that first and what the expected audio path is from input on that driver board to output

    NEXT - For the bad right channel, Voltages are severely messed up. There's clearly not enough voltage Collector Q804/Bases of Q806Q808, and the final stage Q810/Q812 look like the base/emitters might be shorted. Both were easy to disconnect from the board and test (simple pulloff tabs). Q810 tested fine. Q812 was not registering well. Turns out the little fusable link tied to it's emitter is dead. I will have to replace that and I have the part. Doing a diode check on that transistor, it shows a lower voltage .47, whereas all the others are above .5. Further, on a transistor tester, it is shown to be leaky as well. So it's needs to be replaced. It's a custom fisher part TR01058-5 no public datasheet I can find and google say NTE182 is the only substitution. The 5 watt associated R838 .39 ohm resistor is also registering high (1.2 ohms) so I'll replace that too

    **My question 2 - Is the NTE182 the best match? Probably hard to answer without an original datasheet...

    The identified failures don't seem to be the source cause of the distortion, as with them out of circuit completely, i still hear it, so it must be coming from an earlier stage. I attempted to set the bias but couldn't get anywhere close (on the right channel) with the current components.

    ** My question 3 - Why is audio coming out of Q802 in different spots then the left channel counterpart?
    ** My question 4 - Thought what to investigate next? Components to check out?
    ** My question 5 - Last one in this message - I promise I'll create a new message for others :) During probing, I inadvertently broke CR803 (on my good channel of course, figures). Channel still seems to be working though. The part # SIT51B096-5 goggle says replace with NTE116 1A 600V rectifier. I don't have any of those but I do have a handful of 1N4003 1amp 200V. Could that be substituted instead?

    Fisher250tx_Driver board_Voltages.jpg
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    http://www.vetco.net/catalog/product_info.php?products_id=2558

    You will find the equivalents listed on that page for the NTE part. When you look up the part, pick that distributor, vetco.net. That will help you a little.

    On edit:

    Not necessarily. Emitter followers the output is taken at the emitter.

    Are you talking about the physical PCB below the schematic? They look like mirrored PCBs.

    Look for anything obviously burnt or overheated. Look at the solder joints to make sure they are good. Once you replaced the capacitors and the suspected components, you can do the voltage comparisons with NO input signal to the board. If they look good in the quiescent state, then apply a signal.

    1n4004 is the 1 amp 600V diode.


    Attached is the full schematic of the power out of one channel :
     
    Last edited: Jul 14, 2016
  3. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    didn't realize you had an edited response! I got notice on the first response but as there was nothing really there I kept digging on the board myself. I never got notice on the edit so sorry for my late response back!

    A few follow ups to your answers

    #1 - Got it. New education for me. I like to be educated. That's exactly why I'm here!
    #2 - I've ordered an NTE182 (along with new 5watt emmiter resistors as mentioned above)
    #3 - I was referring to the schematic, trying to understand how there be audio coming out (or at least present) on the emitters on the good channel and collectors on the bad channel.
    #4 - In the interim, I had checked the 1uf and up caps on the board. I did find C804 (4uf) and C806 (25uf) to be substantially out of spec so I had replaced those. I also checked ALL the transistor on that channel. They all checked out good (other than the 1st one that is already being replaced) except when I got to Q802 (TR01051-5) it was stubborn coming off the board and not one but TWO of the 6 leads broke off, right at the body. Unusable, and not even testable now. OK - Now I need to order that. Ironically, it was the very first part that I suspected as failed and I had already scoped out it's replacement NTE81. Just would have been nice to know it was really needed before I broke it. Which brings me to
    #5 - Diode. Two more broke off during all this. So now 3 of the 4 need to be replaced (and I'm replacing the 4th as well). Very delicate and for all I know one or more could have been broken the whole time. But back to the original question. Which was not what is the equivalent part (thank you for that part # though) it's "can" i use the parts I have? 200v vs 600v - are we ever going to see that voltage level regardless? If the only difference is the max voltage, I guess I'm not understanding why the part I have now work work in place?
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
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    The one link to the NTE showed all the parts that the NTE182 replaced. You will find those parts substantially lower in price.

    With respect to your question number 5, the max the signal can go is from negative 32 volts to positive 32 volts. You probably will do ok with the 200V diode, but I would just get the correct one, a 1n4004 which is rated at 600 PIV. They are relatively inexpensive. I don't know where you are located, so I can't offer any advise on where you can procure such items.
     
  5. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    1n4004 is 400v. So I think you meant 1n4005
     
    JoeJester likes this.
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
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    your right ...
     
  7. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    Parts all came in, installed and of course, now both channels sounds like complete crap. UGH! Heavy distortion on both. The ONLY thing I changed on the left channel was those diodes (CR801/CR803) with the new 1n4005. But looking at the schematic again, I now see it has what looks like 4 in series, yet the part that came off was a single diode, so I put a single diode on in it's place. I guess the way I interpreted the substitution was that it was a 1 for 1 part exchange, not a 4 for 1. Should I have connected 4 of those together in series for the substitution? If so, I hope this hasn't caused irreparable damage....
     
  8. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    Found Left channel Q809 to be bad and leaky. Wasn't until now so maybe just murphy working against me. I'll get a NTE183 replacement then see if that channel clears back up and get another baseline, since left is still not resolved.
     
  9. jimkarl

    Thread Starter New Member

    Oct 10, 2014
    21
    2
    New power transistor installed on left channel. No change. Heavy distortion still present on both channels. Jumping back the the right channel for continued analysis, since it has the most new parts installed. Attached is an updated pic of the schematic voltages captured. A couple things I find disturbing are:
    1) the -1v between C812 & R826 (schematic shows -13v)
    2) The base and the emitter of Q812 are the same. That would indicate a short but both Q812 and R832 are brand new and have been tested and are fine. So how is that possible?
    3) Attempt to bias to 15mv per manual had zero effect. In fact, changing the trimmers basically doesn't even move the voltage reading at all, or only a mv or so. Previously, before all the diode & transistor replacements the trimmers definitely had an impact and I was able to bias (even though I still had the distortion)

    Unrelated but another question - I saw the C813-C1816 "220" caps on the schematic but on the board they are small brown disc caps, so these are really PF caps, not UF correct, even though the schematic does not state that? Pic attached. I pulled them to test and a bit out of spec or close so plan to replac them, one destroyed in the process of course....
     
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