New 388 problem

sweetbeats said:
I figured as much.

Okay. So next we need to move upstream one step and see if we have signal at the input of the switching transistors. This may get tricky with just standard probes because I can't see how easy it is to get to the transistors.

You are looking for Q101 on the BUSS B PCB. It is a little JFET transistor. It'll have 3 legs and the legs should be labeled D, S and G on the board. Leaving all conditions the same as the previous test, I want you to put the + probe on the 'S' terminal of Q101, and leave the - probe on pin 2 of P108.

What do you get on the multimeter?

-OR-, put up a pic of what you are working with if you can't get to stuff, and we'll see if we can ascertain a work-around.

This is a JFET:

View attachment 97673
Click to expand...

Ok, I've hit the first hurdle. I think I may have to take the knobs off the front so I can pull the monitor assembly out a bit because I can't seem to locate Q101. I found a Q104 at the end of a line of four JFETs, but I can't see the names of any of the other ones, and there's an IC in the way.

Like I said in the PM I sent you, my manual is missing several pages from the maintenance portion, and the layout for the BUSS B PCB is one of them. :(

Oh, and I should still be measuring AC voltage, correct?
 
Yes, AC voltage.

Oddly enough I looked at the PCB layout in my pdf and paper 388 manuals and I can't find Q101~104 at all...maybe it's just been a long day.

Surely Q101 is amidst that array of 4 JFETs you found though. If it's too much of an impediment let me know and we'll try to find an alternative, though testing at the JFET would be best.
 
sweetbeats said:
Yes, AC voltage.

Oddly enough I looked at the PCB layout in my pdf and paper 388 manuals and I can't find Q101~104 at all...maybe it's just been a long day.

Surely Q101 is amidst that array of 4 JFETs you found though. If it's too much of an impediment let me know and we'll try to find an alternative, though testing at the JFET would be best.
Click to expand...

It would make sense that it's most likely the one at the other end of the line of 4, but even if it is, I can't tell which is the S terminal from here and most likely wouldn't be able to get a probe to it if I could.

Am I right in thinking that, if remove all the knobs and nuts from the pots on the front side, all of these PCBs will pull away?

It looks as though maybe the MONITOR PCB ASSY (the one with the 8 record function buttons) has at least two screws holding it to the chassis---one at each end. I can't tell if they are or not.
 
somewhere I think I have pictures of the 388 mixer section cards, both sides. I'll look for them as soon as I get a chance and maybe I can give some better advice on what to do.

I do t think you have to remove all the cards to get to any individual card.
 
But yes, if you remove all the knobs and pot nuts all the cards can be separated from the dress panel. And I do recall there are, additionally, two screws on the MONITOR PCB associated with the REC function switchrack.
 
Here's something else to try in the meantime.

1. Ensure all L-R assign switches on mixer channels 1~8 are in the up/unlatched position
2. Locate P101 and P102 on the BUSS B PCB; P101 and P102 are the connectors where the M BUSS PCB (the one that goes across the bottom of all the mixer cards) connects to the BUSS B PCB
3. Set your multimeter to continuity
4. Measure for resistance between pin 9 of P101 and pin 8 of P102.

It *should* be open circuit (no connection). When you latch any one or more of the L-R assign switches, the STEREO CONTROL circuit found on pin 9 of P101 becomes connected to ground...it looks like doing so kills power to the PGM BUSS amps and also, through U101, U102 and U104, pinches off the output of the MONITOR mixer through Q101/102. If P101 pin 9 and P102 pin 8 are connected with all those L-R switches up, there is a short somewhere telling all that muting circuitry to do its job when it's not supposed to.
 
sweetbeats said:
Here's something else to try in the meantime.

1. Ensure all L-R assign switches on mixer channels 1~8 are in the up/unlatched position
2. Locate P101 and P102 on the BUSS B PCB; P101 and P102 are the connectors where the M BUSS PCB (the one that goes across the bottom of all the mixer cards) connects to the BUSS B PCB
3. Set your multimeter to continuity
4. Measure for resistance between pin 9 of P101 and pin 8 of P102.

It *should* be open circuit (no connection). When you latch any one or more of the L-R assign switches, the STEREO CONTROL circuit found on pin 9 of P101 to ground...it looks like doing so kills power to the PGM BUSS amps and also, through U101, U102 and U104, pinches off the output of the MONITOR mixer through Q101/102. If P101 pin 9 and P102 pin 8 are connected with all those L-R switches up, there is a short somewhere telling all that muting circuitry to do its job when it's not supposed to.
Click to expand...

Ok cool, I'll give this a shot and report back. Thanks
 
sweetbeats said:
Here's something else to try in the meantime.

1. Ensure all L-R assign switches on mixer channels 1~8 are in the up/unlatched position
2. Locate P101 and P102 on the BUSS B PCB; P101 and P102 are the connectors where the M BUSS PCB (the one that goes across the bottom of all the mixer cards) connects to the BUSS B PCB
3. Set your multimeter to continuity
4. Measure for resistance between pin 9 of P101 and pin 8 of P102.

It *should* be open circuit (no connection). When you latch any one or more of the L-R assign switches, the STEREO CONTROL circuit found on pin 9 of P101 becomes connected to ground...it looks like doing so kills power to the PGM BUSS amps and also, through U101, U102 and U104, pinches off the output of the MONITOR mixer through Q101/102. If P101 pin 9 and P102 pin 8 are connected with all those L-R switches up, there is a short somewhere telling all that muting circuitry to do its job when it's not supposed to.
Click to expand...

Ok, just tested this. Let me get a few disclaimers out of the way.

1. These pages (BUSS B PCB and M BUSS PCB) are missing from my manual, so I couldn't verify on the layout.
2. I'm pretty sure I know which connectors are P101 and P102 because of what you described (they connect to the M BUSS PCB), but I can't see any identifying numbers on the actual PCB to verify this.
3. You mentioned pin 9 of P101 and pin 8 of P102. Since one of the connectors only has 8 terminals, I'm guessing that's not P101.
4. Knowing all this, I'm assuming that P101 is the connector with 11 terminals and P102 is the one with 8.

Is this correct?

If so, the result of the test (I made sure all the L/R buttons are up) is that I am getting continuity between those two points. It was pretty hard to only touch terminal 9 on P101 (terminal 8 on P102 was pretty easy because it's at the end), but I'm very confident that I got it right after several tries.

By the way, I don't know if you remember or not, but the reason that I needed to purchase your side panel for this unit back in the day was that it had obviously suffered a blow at some point (see attached photo). I'm wondering if that has something to do with these issues.
 

Attachments

  • 388 damage.jpg
    388 damage.jpg
    1 MB · Views: 0
  • 388 front.jpg
    388 front.jpg
    1,001.5 KB · Views: 0
I do remember that.

It *could* be a causal factor...or maybe not.

As soon as I can I'll get some clarifying info up so you can double check.

I think the next step though is going to involve you removing the M BUSS PCB and checking each input channel for continuity between ground and the L-R switch. More instructions to follow.
 
Don't remove the BUSS B PCB or any of the mixer cards yet...we're not going to worry about Q101 at the moment.

Further instructions to follow which will involve you first removing the M BUSS PCB
 
First we're going to test to make sure each of the 8 L-R assign switches are appropriately making and breaking connection to the ground buss.

1. Remove the M BUSS PCB; work carefully from one end of the PCB to the other, rocking it at each card to work it loose a little at each card and on down the line, start back at the first card and rock it loose a little more, repeat until it is free from each card.
2. Locate pin 9 of connector P104 (the 11-pin M BUSS connector), and pin 8 of P105 (the 8-pin M BUSS connector)
3. Set your multimeter to continuity
4. Starting with the channel 1 input card, measure for resistance between P104 pin 9 and P105 pin 8 with the L-R assign switch in the up position, and then in the down (latched) position
5. Repeat step 4 for channels 2~7.

When the L-R assign switches are in the up position there should be no continuity between P104 pin 9 and P105 pin 8...nothing. When the L-R assign switches are in the down (latched) position there should be a dead short (very low resistance...something less than 1ohm) between P104 pin 9 and P105 pin 8.

Test and report back and depending on the results I'll have next steps for you.
 
sweetbeats said:
First we're going to test to make sure each of the 8 L-R assign switches are appropriately making and breaking connection to the ground buss.

1. Remove the M BUSS PCB; work carefully from one end of the PCB to the other, rocking it at each card to work it loose a little at each card and on down the line, start back at the first card and rock it loose a little more, repeat until it is free from each card.
2. Locate pin 9 of connector P104 (the 11-pin M BUSS connector), and pin 8 of P105 (the 8-pin M BUSS connector)
3. Set your multimeter to continuity
4. Starting with the channel 1 input card, measure for resistance between P104 pin 9 and P105 pin 8 with the L-R assign switch in the up position, and then in the down (latched) position
5. Repeat step 4 for channels 2~7.

When the L-R assign switches are in the up position there should be no continuity between P104 pin 9 and P105 pin 8...nothing. When the L-R assign switches are in the down (latched) position there should be a dead short (very low resistance...something less than 1ohm) between P104 pin 9 and P105 pin 8.

Test and report back and depending on the results I'll have next steps for you.
Click to expand...

Ok, I'll get to this sometime tonight I think. Thanks!
 
Hey Cory...I see you're going ahead with parting out your MCI...over in the GS classifieds and here.
Time to let her go I guess. :(

I'm curious...have you tried selling it complete? I would think some of the guys on GS might go for that...?

OK...sorry for the segue. :)
 
I just edited the GS post...yeah way interested in selling it complete...or parting it out. Anything.
 
sweetbeats said:
Test and report back and depending on the results I'll have next steps for you.
Click to expand...

Hey Cory, ok just tested it. Everything was as you said it should be.

No continuity when the L/R switches were up.

When they were down, there was between 0.3 and 0.6 or so ohms of resistance.
 
Okay. So just humor me and do this:

1. Set your multimeter to continuity
2. With the M BUSS PCB still out of the machine, and with the machine powered, measure for continuity between P101 pin 9 and P102 pin 8 of the BUSS B PCB; you may need to increase the range setting of your multimeter to achieve the actual resistance measurement
3. Switch your multimeter to DC volts, and with the - on P102 pin 8, and the + probe on P101 pin 9, note the voltage reading
4. Shut off the 388
5. Unplug the 388's power cord from the wall
6. With the 388 unplugged from power switch the 388 power switch to ON for about 5 minutes
7. After about 5 minutes switch the 388 power switch to OFF
8. Set your multimeter back to the continuity setting
9. With the 388 still switch OFF and unplugged from the wall, repeat the test in step 2

Report the results of steps 2, 3 & 9.

I'm not exactly sure what I'm looking for. We are starting to get a little out of my wheelhouse here, but I do know you shouldn't measure anything *more* than 1.5K ohms. I'll have to figure out what the voltage *should* be.

Report back or ask for clarification if needed.
7.
 
Last edited:
sweetbeats said:
Okay. So just humor me and do this:

1. Set your multimeter to continuity
2. With the M BUSS PCB still out of the machine, and with the machine powered, measure for continuity between P101 pin 9 and P102 pin 8 of the BUSS B PCB; you may need to increase the range setting of your multimeter to achieve the actual resistance measurement
3. Switch your multimeter to DC volts, and with the - on P102 pin 8, and the + probe on P101 pin 9, note the voltage reading
4. Shut off the 388
5. Unplug the 388's power cord from the wall
6. With the 388 unplugged from power switch the 388 power switch to ON for about 5 minutes
7. After about 5 minutes switch the 388 power switch to OFF
8. Set your multimeter back to the continuity setting
9. With the 388 still switch OFF and unplugged from the wall, repeat the test in step 2

Report the results of steps 2 & 7.

I'm not exactly sure what I'm looking for. We are starting to get a little out of my wheelhouse here, but I do know you shouldn't measure anything *more* than 1.5K ohms. I'll have to figure out what the voltage *should* be.

Report back or ask for clarification if needed.
7.
Click to expand...

For steps 1-2, I measure 1.409(ish)K ohms.

I'll take the other measurements tomorrow. Thanks
 
Oops...editing my last post...where I said "report the results of steps 2 & 7" that was wrong. :drunk:

It should refer to steps 2, 3 & 9.

So are you saying you measured about 1.4K ohms for steps 2 & 9?
 
You must log in or register to reply here.
Back
Top