New 388 problem

Thanks again for the detailed explanations along the way. I really appreciate them.

Your next request is maybe possible depending on what needs to stay connected and what can come loose. I definitely can't get to it as it is now (with the M BUSS PCB still connected and all the cards connected to the front panel).

I can unscrew all the nuts on the front panel and pull the cards out, but I don't know how far I'll be able to pull them out with all the connectors still plugged in. (See photo.)

Can any of these connectors be unplugged while taking those measurements?
 

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famous beagle said:
Thanks again for the detailed explanations along the way. I really appreciate them.

Your next request is maybe possible depending on what needs to stay connected and what can come loose. I definitely can't get to it as it is now (with the M BUSS PCB still connected and all the cards connected to the front panel).

I can unscrew all the nuts on the front panel and pull the cards out, but I don't know how far I'll be able to pull them out with all the connectors still plugged in. (See photo.)

Can any of these connectors be unplugged while taking those measurements?
Click to expand...

What would be ideal is if you could clip a small flying lead into the 'G' terminal of Q101 or Q102. Do you have any of those small captive clip wires (not alligator clips)? That would be perfect for this...you would remove the M BUSS PCB, and disconnect any connectors to gain access, then clip the lead on the 'G' leg of the transistor, then put everything back together and probe the other end of your flying lead and the terminal on the M BUSS PCB.

I'm talking about this kind of thing...They come in different sizes and you can get them pretty small, usually in a pack and they don't cost much:

image.jpeg
 
sweetbeats said:
What would be ideal is if you could clip a small flying lead into the 'G' terminal of Q101 or Q102. Do you have any of those small captive clip wires (not alligator clips)? That would be perfect for this...you would remove the M BUSS PCB, and disconnect any connectors to gain access, then clip the lead on the 'G' leg of the transistor, then put everything back together and probe the other end of your flying lead and the terminal on the M BUSS PCB.

I'm talking about this kind of thing...They come in different sizes and you can get them pretty small, usually in a pack and they don't cost much:

View attachment 97777
Click to expand...

Ok ... First off, no I don't have any of those captive clip wires, but I could get some. From looking at the picture, I can't tell how they work, but I imagine it would be a bit more obvious if I had them in my hand.

Secondly, I see that the schematic shows G as the middle terminal on all of those JFETs. Does that mean it's for sure physically the middle terminal? I can't see an identifying mark on the PCB right now (the card is still installed), but I'm just wondering if there will be one since, remember, those four JFETs don't appear on the PCB layout in the manual.

Thirdly, those JFET leads are tiny and close together! :) I can see how an alligator clip won't cut it --- especially if it's the middle terminal I'm after. I've searched around for "Flying leads" or "captive clip wire" and haven't found what you showed. Could they be known by another name?

Or ... is there a reason that I couldn't just make a connection with a piece of hook-up wire by just wrapping it around the terminal (if possible)?
 
famous beagle said:
From looking at the picture, I can't tell how they work, but I imagine it would be a bit more obvious if I had them in my hand.
Click to expand...
I've not see these befpre, but oscilloscope probes often work the same way. The conductor is usually a hook inside the plastic sheath. You pull the plastic flange back and the hook is exposed. You can then hook it onto the wire or IC pin you want, with the minimum amount of conductive surface exposed, so there's much, much less danger of a short.

Unfortunately I don't know what they're called either.
 
jpmorris said:
I've not see these befpre, but oscilloscope probes often work the same way. The conductor is usually a hook inside the plastic sheath. You pull the plastic flange back and the hook is exposed. You can then hook it onto the wire or IC pin you want, with the minimum amount of conductive surface exposed, so there's much, much less danger of a short.

Unfortunately I don't know what they're called either.
Click to expand...

Hmmm ... interesting. Thanks for the info!
 
Yes, you squeeze the spring-loaded plunger which exposes a small 'J' hook...you hook the component leg and when you release the plunger, and because of the hook, there's no risk of your clip touching other contacts even in crowded situations.

You *could* use hookup wire, but I wouldn't...I'd be worried about it coming loose and contacting something else.

No, the schematic shows a standard diagram for a JFET, but different parts have a different pinout. Often the Tascam gear has the D (drain), S (source), and G (gate) labeled on the board. If it isn't labeled, or you can't get a visual on it you have to get the datasheet for the part. The body of the JFET is flat on one side right? That's how you identify the orientation and where pin 1 is. Sometimes it's on the left, sometimes on the right, and then the function of each pin (D, S, and G) varies from part to part. It's uber essential to mind the pin layout when replacing transistors.

I'm running from one flight to the next at the moment. Find the original part type in the manual and download the datasheet. It usually shows the pinout on the first page. It's a good idea to do this anyway even if the board is labeled.
 
sweetbeats said:
Yes, you squeeze the spring-loaded plunger which exposes a small 'J' hook...you hook the component leg and when you release the plunger, and because of the hook, there's no risk of your clip touching other contacts even in crowded situations.

You *could* use hookup wire, but I wouldn't...I'd be worried about it coming loose and contacting something else.

No, the schematic shows a standard diagram for a JFET, but different parts have a different pinout. Often the Tascam gear has the D (drain), S (source), and G (gate) labeled on the board. If it isn't labeled, or you can't get a visual on it you have to get the datasheet for the part. The body of the JFET is flat on one side right? That's how you identify the orientation and where pin 1 is. Sometimes it's on the left, sometimes on the right, and then the function of each pin (D, S, and G) varies from part to part. It's uber essential to mind the pin layout when replacing transistors.

I'm running from one flight to the next at the moment. Find the original part type in the manual and download the datasheet. It usually shows the pinout on the first page. It's a good idea to do this anyway even if the board is labeled.
Click to expand...

Ok thanks. Whenever you get a chance, if you could direct me to a source to buy those clip wires you're referencing, I can get some. I'm not finding anything on my own.

Thanks!
 
famous beagle said:
Just ordered some.

While I wait for them to get here, do you want me to replace that IC?
Click to expand...

Well that is a question isn't it...I don't know for sure U104 is the problem. And even after you get the test leads and do the next test that won't necessarily name U104 as the culprit...could be something with U101 or U102...or the solder joints or traces around those components...so, yes you could shotgun U104 and maybe the problem is fixed or maybe not. It's up to you. I'm just honing in on U104 because I had a problem with logic switching on my 388, and know of at least one other owner on this forum that had the same problem...and I think it was a failed hex buffer IC...the problem in that case IIRC was you could latch a L-R assign switch, but no signal would pass. The L-R assign switches just acted like mute controls. ICs like the MC14049 don't usually go bad, but since I've dealt with it twice in another section of the 388 mixer I'm suspicious of that IC being your problem here, and our tests have ruled out other possibilities. We are getting pointed to that U101/102/104 switching network.

If you do shotgun it, I just want to make sure you know you shouldn't have to remove all the mixer cards. Just remove the nuts for the BUSS B PCB and remove the M BUSS PCB...IIRC you can then unplug the other connectors to/from the BUSS B PCB and remove it with all other cards intact.
 
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sweetbeats said:
Well that is a question isn't it...I don't know for sure U104 is the problem. And even after you get the test leads and do the next test that won't necessarily name U104 as the culprit...could be something with U101 or U102...or the solder joints or traces around those components...so, yes you could shotgun U104 and maybe the problem is fixed or maybe not. It's up to you. I'm just honing in on U104 because I had a problem with logic switching on my 388, and know of at least one other owner on this forum that had the same problem...and I think it was a failed hex buffer IC...the problem in that case IIRC was you could latch a L-R assign switch, but no signal would pass. The L-R assign switches just acted like mute controls. ICs like the MC14049 don't usually go bad, but since I've dealt with it twice in another section of the 388 mixer I'm suspicious of that IC being your problem here, and our tests have ruled out other possibilities. We are getting pointed to that U101/102/104 switching network.

If you do shotgun it, I just want to make sure you know you shouldn't have to remove all the mixer cards. Just remove the nuts for the BUSS B PCB and remove the M BUSS PCB...IIRC you can then unplug the other connectors to/from the BUSS B PCB and remove it with all other cards intact.
Click to expand...

Ok, yes I had figured that out (about removing the M BUSS PCB and then being able to remove only the BUSS B PCB without removing all the other cards), but thanks for the tip. I appreciate any input I can get, and me knowing that is definitely not a given!

I just sent you pictures of the chip so you can hopefully identify whether it's legitimate or not, so I'll wait to hear back from you on that before I do anything.
 
So I'm an idiot! :) I ordered the wrong damn part. I ordered MC14049 instead of MC14069. Even when I was looking at them side by side, I didn't notice. The thing that tipped it off? The ones I ordered have 16 pins, and I need 14 pins.

However, I don't feel so bad, because this is another mistake in the manual! On page 3-23 of the 388's manual, it's a part list for the BUSS B PCB. And next to U104, it says, clear as day: MC14049B

But in the schematics, on page 4-6, it shows U104 as MC14069UBCP.

I'm going to have to stop trusting this manual and verifying everything. Apparently, whomever created the manual back in 1985 wasn't terribly detail-oriented! :)

So ... could you take a look at these to see if they look genuine. Looks pretty good to me, but maybe you'll see something I don't.

Moto MC14069UB DIP14 Qty 1 SHIP in USA Tomorrow | eBay

or:

5 Five New Motorola MC14069UBCP Hex Inverter IC U s Seller | eBay
 
Oh no!!

Geez, Beagle I should have questioned myself more...I think I knew it was the 14069 and when you ordered the 14049 I looked in the manual somewhere and saw the same thing you did, but didn't chase down the little voice in my mind that said something wasn't right. Shoot! That's a bummer for sure.

The first one for sure is a genuine part. The second one may be genuine Motorola, but maybe of later manufacture and outsourced? They look funky. Let me compare to my spares...
 
My 14069 parts are TI branded...another good option. I forgot about that.

Looking again at the second link I think those are fine. So either one...better deal to be had with the second one of course.
 
famous beagle said:
Thanks again for the detailed explanations along the way. I really appreciate them.

Your next request is maybe possible depending on what needs to stay connected and what can come loose. I definitely can't get to it as it is now (with the M BUSS PCB still connected and all the cards connected to the front panel).

I can unscrew all the nuts on the front panel and pull the cards out, but I don't know how far I'll be able to pull them out with all the connectors still plugged in. (See photo.)

Can any of these connectors be unplugged while taking those measurements?
Click to expand...

Ok, I got those clip leads in the mail yesterday, so I can possibly take the measures on those JFETs this evening. I don't think you ever answered the above question, but if you did, I missed it. I'm just needing to know what all needs to stay connected when I take those measurements.

Thanks!
 
I did answer it but in a roundabout way...the coarse instructions I gave for what we will do with those clip leads guided you to put everything back together after having the clip lead in place. So...the ideal would be to disassemble whatever necessary to get a lead hooked to a 'G' leg of either Q101 or Q102, and then connect everything back up and conduct the test.

So here is what you will do when you can get to it:

1. Connect a flying lead to the 'G' leg of either Q101 or Q102
2. Ensure all L-R assign switches are in the up/unlatched position
3. Power on the 388 and, with the + probe of your multimeter connected to the flying lead, and the - probe touching pin 8 of connector P102 on the BUSS B PCB (A-OK to probe the related solder joint on the M BUSS PCB), measure for DC volts
4. Latch any L-R assign switch
5. Measure for DC volts as in step 3
6. Report your measurements

I don't know exactly what voltages you should see...I'm not even sure of what state should result in positive voltage. What I DO know is Q101 and Q102 are like little gates...with voltage at a certain state at their 'G' terminals, the connectivity between their 'S' and 'D' terminals is pinched off. This is what should be happening when any L-R assign switch is latched. What I *think* is happening is the IC array U101, U102 and U104 is not functioning properly and Q101 and Q102 are always seeing the same voltage state at their 'G' terminals regardless of the state of the L-R assign switches. So rather than looking for a specific voltage in steps 3 and 5 above, we are looking to see if there is any significant *change* when any L-R switch is latched. We have tested to make sure power is getting TO U101, U102 and U104...we have tested to make sure audio is getting TO Q101 and Q102. So now we are testing to see if anything is happening at those control terminals 'G' of Q101 and Q102 when an L-R assign switch is latched. If there is little or no change between the voltage measurements at steps 3 and 5 above, that further reinforces something is wrong with the U101, U102 and U104 array, and because I have dealt with MC14069 failures on more than one occasion on a 388 in the past, my advice would be to go forward and shotgun U104.
 
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