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.