New 388 problem

  • Thread starter Thread starter famous beagle
  • Start date Start date
Don't yoink the PHONE AMP PCB until necessary.

The amplitude of the signal isn't so important (in other words 187mV is fine)

Can you get to pins 3 or 5 of U103?

I want you to do the same test as the last one, except compare voltage between P101 pins 1 & 2 to P101 pin 2 and U103 pin 3 -OR- U103 pin 5. I hope that makes sense.

We are checking voltage at P101 pins 1 & 2 to get our known voltage, and then comparing that to the voltage at the input of the monitor output buffer amp U103 (which we can measure by putting the - probe on P101 pin 2, the signal ground, and the + probe on U103 pin 3, the L channel input to the opamp, or pin 5, the R channel input to the opamp).

The last test we tested signal attenuation from the signal input of the board to the output. Now we are checking signal attenuation from the input of the board to a point just after the monitor level pot at the input of the output buffer amp. We are trying to determine where signal is becoming inappropriately attenuated.
 
Ok, just did the above test.

Again, got 187 mV for P101, pins 1 and 2 (1kHz tone at 0dB).

For P101, pin 2 to U103 pin 3 or pin 5 (tried them both), I could only get 3 mV even when the PHONES/MON volume was cranked to the max.
 
Crap.

That's not right.

Let me double check the schematic with the pinout on that 4558 opamp...
 
Okay. So the schematic is correct.

You should have signal at pins 3 and 5 of U103. You're positive you are probing pin 3 and pin 5?

image.webp

Can you get to pin 3 of U101 or U102?
 
I'm pretty sure I had the right pins, but I'll double check tonight
 
Yeah. I only ask because I've made the mistake myself many times...either didnt see the pin 1 dimple in the right spot our counted clockwise instead of counter clockwise.
 
Ok, just double-checked and triple-checked.

Measured from P101, pin 2 to:
U103 pins 3 and 5
U102 pin 3
U101 pin 3

I got 3 mV every time.

Couldn't this mean that U103 is shot? Or are you saying that, with other evidence, there should be signal there, so it must be a measuring error on my part?

And just to be clear, I wouldn't at all take offense to that. It's certainly possible. I just don't know what I could be doing wrong at this point. It seems pretty straightforward.

I know it's not a meter error, because I have no problem getting signal from pins 1 and 2 of P101.
 
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Damn ... is that another error in the manual/schematic?

The PCB layout for the PHONE AMP ASSY says U102, but the schematic says U201.
 
famous beagle said:
Couldn't this mean that U103 is shot?
Click to expand...

I guess even if U103 is shot, there should still be some signal at U201 or U103, because it's a parallel circuit and not series, is that right? (I'm at the edge of my knowledge here.)

And ... looking further, it seems that it passes through this circuit en route to the phones jack, which is working. So maybe that's why you're saying that there should be signal?

Maybe I should just shut up and stop complicating things. :)
 
So you're doing all these tests with the monitor master pot cranked fully clockwise right? Wide open?

U102 vs U201...Geez I don't know...the proof would be how it's labeled on the actual board. It's like hunting for Easter eggs on a DVD that manual.

Yes, because the signal is good at the headphone jack, you should have signal at pin 3 of both U101 and U102/201...pin 3 is the signal input of an LM386, the two opamps used for the left and right channels of the headphone amp. The inputs of the headphone amp section are indeed parallel to the inputs of the monitor buffer amp section. Signal comes in via P101, goes to the monitor level pot, then it branches off, one side to the headphone amp and the other to the monitor buffer amp.
 
Ok, so last night apparently I forgot to crank the PHONES knob. :)

Just tried it again with knob cranked and got interesting results.

P101, pin 2 to U103, pin 3: 184 mV :)

P101, pin 2 to U103, pin 5: 3 mV :(

P101, pin 2 to U102 (or 201), pin 3: 3 mV :(

Here's the deal: U103 is easy to get to. U102 is a little harder and U101 is harder still. So it's harder for me to be 100% accurate with the probes on those two chips. I'd like to pull out the PCB if I can, but I can't seem to figure out how to do that.

I tried pulling up the knobs (PITCH, VARI/FIXED/EXT, and PHONES), but I'm not having luck. They seem to come loose but then they get stuck, and I'm not able to pull them off the shaft as easily as I would think. I haven't really pushed it because I don't want to damage anything.

Do you know if there's a trick to getting that PCB free?
 
Don't know about the pcb. But the knobs can be taken off with one or two spoons.

Them and a cloth for cushion. Just use the spoons as levers.

It's worked for me.
:D
 
RFR said:
Don't know about the pcb. But the knobs can be taken off with one or two spoons.

Them and a cloth for cushion. Just use the spoons as levers.

It's worked for me.
:D
Click to expand...

Ok thanks for the tip. Just to clarify, you're talking about the three knobs at the far right bottom of the transport section, right?

So, once they give way and are just loose but still don't want to come up, you just have to use more force still?
 
I dont know about those particular ones. Never had a 388.
I was talking more in general.
On my tascam m520 mixer, some came off easy, some stubborn, and some I had to get out the spoons.
All tascam knobs from that era are pretty much the same. Some have that metal insert that's 3/4 round then has that flat spot. What I've found is sometimes the metal gets a bit corroded making it tough to get off.

But I have no direct experience with your unit.
 
RFR said:
I dont know about those particular ones. Never had a 388.
I was talking more in general.
On my tascam m520 mixer, some came off easy, some stubborn, and some I had to get out the spoons.
All tascam knobs from that era are pretty much the same. Some have that metal insert that's 3/4 round then has that flat spot. What I've found is sometimes the metal gets a bit corroded making it tough to get off.

But I have no direct experience with your unit.
Click to expand...

Oh ok, thanks. Yeah these are not the typical cylindrical knobs. They're trouble-makers. :) They're the bottom right ones in this picture.
 

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You talking about the three knobs on the bottom far right?

I bet you'd have an easier time once you take the faceplate off. Looks to be just fastened with 4 allen screws.

For all we know there could be a flange at the bottom of the knob preventing it from coming all the way up. Who knows, im just guessing. But for sure. I'd take the plate off.
 
Those look like buggers!!! Pliers with duct tape around the jaws maybe?
 
RFR: I have taken the (transport) faceplate off. They come with it. In other words, the PCB to which those pots are attached is mounted to the bottom of that faceplate.

Bachelorb: I basically did that. It's strange ... they come loose, as if they're getting ready to just slide off the shaft (i.e., you can freely rotate them for the most part), but then they don't come off. It feels as though something else is stopping them. I haven't continued to yank really hard because I didn't want to damage them or something else.

I figured if anyone had taken them off, it would most likely be Cory (sweetbeats), so that's why I want to hear what he says (if he remembers).
 
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