Tascam 34 - meters doing strange things

  • Thread starter Thread starter rob aylestone
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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.
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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.
 
frnkfrkl said:
There's a link to the Service Manual somewhere back in this thread. That's where i got it. Does the "mute" function mute everything? I mean there's no output, sync or repro, nor input (like no signal getting through in record mode). It's got to be something just after the heads. It's like all the heads are completely non-functional. Now I know that three separate heads are not going to just blow up spontaneously at the same time. What controls ALL the heads and their signal flow? I think that's the key.
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Think about this…how can it be the heads, or only the heads? You connected a source to an input and got nothing at the output. No heads involved there. So logically the issue is further upstream.

Yes the power mute circuit mutes all audio at all outputs for a couple seconds +/- at power up…mitigates the power thump in the outputs. After reading the operational theory in the manual for the circuit, we learn it works by delaying power to all the K102 relay coils on the amp cards. These relays are normally open, until the coils are energized and then they close passing signal. If there is something wrong with the delay circuit or with the power it passes and the relay coils don’t energize, well, no sound. So I’ve asked you, after the delay period, to check and make sure you have power at pin 3 of J171, which is the pin the passes power to the motherboard to the power mute pins of all the amp cards…to energize the K102 relay coils. If there is power there I want to know what voltage, and if that’s good then we need to next check that it’s making it across the motherboard to the amp cards.
 
Too many posts. I can’t keep up with the blasts. Please let’s take things one step at a time.
 
600mV is very not right for pin 3 after startup.

Yes, reference to pin 2 for ground is correct.

Did you get that measurement with J171 connected or disconnected from the motherboard?
 
Measure for DC volts between J171 pin 2 and the base of transistor Q801. The base pin is the first one, the one closest to the edge of the power supply PCB.
 
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