Tascam 34 - meters doing strange things

  • Thread starter Thread starter rob aylestone
  • Start date Start date
sweetbeats said:
You’ve got your hands full and I’m not sure if I’m going to be able to help. Again if it is global then it has to be something high up in the chain. I would verify power is actually getting to the opamps…like check for DC volts at pin 4 or pin 8 of any of the 4558 or 4559 opamps. The other though is a faulty power mute circuit but I don’t know how the one in the 34B works. If I have time I might see if there is a description of the circuit in the manual.
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OK, I'll have to find those. Any particular pin for reference to ground or will chassis ground do the trick? Perhaps that will become clearer once I actually locate them.
sweetbeats said:
You’ve got your hands full and I’m not sure if I’m going to be able to help. Again if it is global then it has to be something high up in the chain. I would verify power is actually getting to the opamps…like check for DC volts at pin 4 or pin 8 of any of the 4558 or 4559 opamps. The other though is a faulty power mute circuit but I don’t know how the one in the 34B works. If I have time I might see if there is a description of the circuit in the manual.
Click to expand...
Ok, are we looking at J102 or J106? There are 4558 and 4559 op amps on the rec/play amp PCB but, if I'm not mistaken, there are 4 of those, one for each channel, and they would all have to have failed at the same time. Shouldn't I be looking more at the MONITOR AMP PCB? Like maybe J203?
 
Ok, let's see if I can sort this out in my head. J171 (pin 3) connects to P102 which connects to J203, via pin 5, on the MONITOR AMP PCB. Am I following the logic correctly? More info, if useful: Referenced from PIN 2- J171:

pin 3 = 600mVdc
pin 4 = 5Vdc
pin 5 = nonsense in mV (just meaningless fluctuations in the range of +/-1-2mVdc)
pin 6 = 24Vdc.

So pin 5, J171, is the only one that doesn't show at least some stable voltage. J171, pin 5, ends up connecting to P205, via its pin 6, on the MONITOR AMP PCB. Logic would lead me to think that either pin 5 on J171 should have some voltage on it and doesn't, which would be a problem from the POWER SUPPLY PCB, OR something is fishy on the MONITOR AMP PCB section P203 (if 600mV is not what we want to see on J171, pin 3) or P205, MONITOR AMP PCB because no meaningful voltage on pin 5, J171...........?

Ok, I'm not a tech and my brain is fried for today. I'm starting to get the hang of the architecture, though.
 
Are you thinking bridge rectifier D801-804? Or the transistor Q801? There are some pretty old caps there, too. I'm suspecting that 600mV is not at all what we want to see on pin 3, J171, right? And that circuit is fused to F501, the one that blew. It's not blowing now, so not a dead short, I am guessing. Something just went open circuit (better, I guess than a dead short). Am I (you, we) on the right track here? That would be so cool. I've got the value for all the components, I can figure out how to get to the board and I can solder. I just looked again. It can't be the rectifier because I AM getting 6Vdc on pin 1 and that's after the rectifier. So, I'm betting on the transistor. All I'm seeing after the 6V tap to pin 1 is one resistor, one cap and one transistor and that's fed to pin 3.
 
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