Tapco Blend 6 sound mixer blown (?) mic in

• Tapco Blend 6 sound mixer blown (?) mic in
  Posted by jtu on 03 Jan, 2018 22:24
• Hi,
  
  I've got another repair opportunity. I have Tapco Blend 6 sound mixer with two channels (1 and 2) not working (properly).
  
  The reason (most probably) being that audio output level signal has been applied to mic in [1] or inst in [2] on one or both channels, not known exactly . I'm referring here to the block diagram found also in the gallery linked at the end of this post.
  
  This is one of the challenges - I can't find the schematics for the product. Google gives only user manual, pics, etc, but not any schematics, service manual, repair stories. I'd appreciate if somebody could point me somewhere! Reverse engineering seem possible, double sided board with SMD and through hole components. But having schematics would save a lot of time, there are quite dense areas on the board, trace visibility not very good. Especially in the mic in / inst in preamp area which is the prime suspect. With schematics / service manual it should be easy, board is covered with marked test points.
  
  The symptoms and the design seem quite straightforward. Therefore the repair should be possible even without the schematics. I'm writing here because I have never really tried repair on audio equipment, just thinking aloud.
  
  Ok, never, except for building and poking around a few very simple AM radios 30+ years ago when AM was still a real thing
  
  
  
  Symptoms. Having mic input signal applied to [1] or [2] of channel 1 or 2 you should be able to adjust gain (opamp) potentiometer [3] until level LED indicator [4] barely lights up. That does not happen anymore, I tried with mic and guitar. It is possible to have some signal through/out if gain adjustment potentiometer is turned close to max, but any regulation is barely possible in that state. When gain adjustment potentiometer is at max, signal is distorted to unusable.
  
  Power supply seem to be ok, gives good ±15V and +60V (which must be coverted somwhere down to +48 for Phantom.
  
  My guess is that one or several opamps are blown. Opamps are: one JRE 072B, several JRE 4565's and one LM339 shared between channel 1 and 2. I'm thinking about applying some audio level signal to the input and poke around with scope while reading opamp datasheets. Suggestions and hints appreciated.
  
  I have an entry level Siglent SDS1202X-E scope. I don't have anything close to that level in signal generator department, but I have 2 choices: cheap-ass toy from China
  
  
  
  and Soviet scientific laboratory grade seem-to-be-working one from 70ties I inherited from my father
  
  
  
  I have not learned to use the beast yet. It's primary function seem to be 0.1-35MHz with with 400Hz, 1kHZ or external modulation, I guess it should be able to give 400Hz, 1kHZ as output too. Not sure about that though. The difference between the two is output amplitude, toy's minimum is ~50mVpp, beast is promising uV range. I'm a little bit afraid to apply tens of mV from the toy because I'm dealing with mic input which to my (not trusted) understanding should be designed for a few mV, an order less. I tried simple voltage divider on the output of the toy, but that gave me something too noisy to my mind. Toy is designed for volts, not mV output, so putting it to minimum creates blurry sine already. Mixer seem to be very good quality, I don't see any noise added by it, but it is also not cancelling noise on input, so I would like to have as clean input as possible for testing it.
  
  Or, maybe I should not worry so much about uV vs mV for testing on the input, just ensure currents are not exceeding some value?
  
  As I said, thinking aloud here, suggestions, hints and lessons are welcome!
  
  Photos here.

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• #1 Reply
  Posted by jtu on 05 Jan, 2018 22:31
• Ok, applied the 50mVpp sinewave I have to the input in question, tried to poke around with my scope, no luck. I kind'a can follow the signal, but can't figure out what's been damaged. My lack of knowledge and experience shows. Also, board is a bit too dense to follow tracks. Schematics would be of real help. Anybody?
  
• #2 Reply
  Posted by Seekonk on 05 Jan, 2018 22:53
• Any audio source would be fine. I'm thinking a tuner with a voltage divider pot to drop the signal level with an initial resistor to make a significant drop in signal level.  Any distortion should be obvious with clipping that isn't uniform.  Form follows function.  You mentioned op amp.  Look up the data sheet for the op amp. ven an unmarked op amp generally follows pin outs of the common ones.  There should be gain from input to the output.  Output should not be sitting near the voltage of the IC power pins. The + & - inputs should be at the same voltage within a couple mv. If there is only a + supply voltage the output should be at about half of that.  If there are any diodes at the input, check for shorts.  These are likely protection diodes that got damaged by a high input voltage.  This is a start.
• #3 Reply
  Posted by jtu on 05 Jan, 2018 23:13
• Thanks for your suggestions!
  
  As I said, I have two candidates for a signal source, I just do not know the levels an audio mixer can take / mic or an instrument or any audio out (CD, tape, etc) make. If I would known, I could have measured the test source before applying.
  
  Of course I looked up datatsheets. The problem is that I do not know which opamps do what exactly, how they are implemented. Obviously there are opamps implemented as amplifying (with gain) and not amplifying. It is practically impossible to figure it out from the board without schematics.
  
  Is there a way to distinguish blown opamp IC from the good one other than checking supply voltages? Are you saying by "Output should not be sitting near the voltage of the IC power pins. The + & - inputs should be at the same voltage within a couple mv." that if it is, it's blown?
  
  All IC supplies are dual +/-16V.
  
  Will check the diodes, there is a block of 4 diodes near the each low signal input. Thanks for this clue!
  
• #4 Reply
  Posted by jtu on 06 Jan, 2018 21:31
• Ok, I have poked through all opamps with multimeter. There is one LM339 chip with all outputs showing near supply voltage and +/- inputs show voltage diferences in volts. I assume this one is dead, according to advice given.
  
  

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• #5 Reply
  Posted by jtu on 12 Jan, 2018 19:30
• No, my assumptions based on the advice given did not work out. I replaced LM339 and no change whatsoever.

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• #6 Reply
  Posted by jtu on 12 Jan, 2018 20:11
• However, poking around gave me another clue.
  
  I recognized that I actually did not try MIC IN [1], but INST IN [2] all the time, because I don't have XLR connector. I jacked mic with random wires and it appears that MIC IN works! Gain LED [4] works, so I deduce that potentiometer and opamp [3] and the following signal path is ok.
  
  What does NOT work is INST IN, which is meant for high impedance instruments (as opposite to low/lower impedance mics). So my next candidate is JRC 072B opamp which is physically close to INST IN and is shared between channels 1 and 2. Whatever the configuration of the opamp is named, seem that it is meant to manage the high impedance of the input signal. The pity is that IC is not easily accessible, I'll have to unsolder the surrounding parts to get to it.
  
  

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