New 388 problem

  • Thread starter Thread starter famous beagle
  • Start date Start date
Well there ya go!
You need some 'been there, done that' input.
That would be light years ahead of my uninformed speculation.
:D
 
Well gotta try i guess.

Have you had that control section out?

Why do you need those knobs off?


There has to be away to get the faceplate separated from the pcb
 
Yeah, Beagle I'm wracking my brain to remember how that comes apart...I don't recall it being that complicated...but it's been 7 years or so...

Gonna look at the exploded view in the service manual to see if that jogs my brain.

I was going to suggest the next thing you do is get the PCB out and take some high quality pics of both sides of the board and either post here or email them to me.

Things are not adding up with the test results and I'I think we need to rule out discrepancies with the schematics and the actual assembly before I suggest anything further.
 
I remember now.

The face plate #33 attaches to the steel sub-panel #40 to which the the pots of the PHONE AMP PCB #46 are attached.

Remove the face plate and from there you should be able to remove the knobs and access the pot nuts.

image.webp
 
Ok cool thanks. I did notice that steel sub panel when I pulled up the face plate. I'll try to get to that this weekend.
 
Ok, got the PCB out. Yeah the issue was that those three little knobs flair out at the bottom (which you can't see while the face plate is on). That's why they would seem to give but then wouldn't come off. They were slipping off the pot shafts, but then the knob itself wouldn't fit through the opening in the face plate. Once I got the face plate off, they came off like normal knobs.

Here are hi-res pictures of the PCB front and back:
https://www.dropbox.com/sh/vekq4y7uu776slj/AADbRKMdJJB7igL1_zITb_Soa?dl=0

I guess I'm going to now try to reassemble it with the PCB still accessible so I can redo those tests and make sure that my probing is accurate.
 
RFR said:
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.
Click to expand...


Aha ! I was right about something for a change. :D


Good you got it apart. :thumbs up:
 
RFR said:
Aha ! I was right about something for a change. :D


Good you got it apart. :thumbs up:
Click to expand...

Holy smokes, I totally missed what you said about the flange on the bottom. You're a genius! :)
 
Well damn .. that was an exercise in frustration. I have the PCB loose with everything hooked back up (see pictures), but I'm still not able to get reliable measurements. I'm getting stuff all over the place.

I tried using the trace side, but I can't get a solid hold on anything because the PCB isn't mounted to anything. I'm always getting 3mv when I try that way. So I tried the component side. I can get a really solid contact to the chips that way, but I'm not positive that I'm making good contact with P101, pin 2. I can't tell whether my probe is reaching far enough into the plastic slot to make contact with the terminal or not.

FWIW, there were times where I seemed to be getting good strong signal (around 250 or 300 mV) at pin 3 of all the chips (U103, 102, and 101), but I couldn't recreate it.

I would try to measure from pin 1 to pin 2 of P101 from the component side and would only get about 120mV (whereas before, when it was mounted and I was using the trace side, I was getting 187 mV consistently). When I tried to measure those on the trace side, I couldn't get anything other than 3mV.

I tried clipping a flying lead onto the trace side of P101 pin2, but there's not enough lead poking through, and it keeps slipping off.

I need to leave it alone for the rest of the day and come back refreshed I think.
 

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Nice pictures. Thanks for uploading those. They are helpful.

The PCB assembly looks really clean, so that's good.

Questions:

• When is the last time you replaced the battery in your multimeter?
• do you have the same problem with relatively low output via the monitor out jacks as compared to the headphone jack when the source is the AUX or the EFF buss, or does the problem only occur when sourcing the STEREO buss?
 
sweetbeats said:
Nice pictures. Thanks for uploading those. They are helpful.

The PCB assembly looks really clean, so that's good.

Questions:

• When is the last time you replaced the battery in your multimeter?
• do you have the same problem with relatively low output via the monitor out jacks as compared to the headphone jack when the source is the AUX or the EFF buss, or does the problem only occur when sourcing the STEREO buss?
Click to expand...

I don't think I've ever changed the battery, so I went ahead and put a new one in there (it uses a 9v). Once I did, I measured the old battery, and it showed to be full, so I don't think that was it.

I've built a little shelf out of stacked cardboard boxes so I can lay the PCB on something solid when I try the measurements again. I imagine that the trace side will still be my best bet. It was just most likely that, since the PCB wasn't anchored anywhere, I wasn't able to get a good, precise connection with the probes.

I'll also test for your other question regarding the AUX and EFF buss when I get a chance.

I think it will probably be next weekend before I can get to this. I'm flying up to Milwaukee on Tuesday to shoot some videos for guitar instruction books and won't be back until Saturday. I'll most likely be doing last minute prep for it tonight and tomorrow night.

But I'll get back at it as soon as I can! Thanks
 
Ok, finally getting back on this now.

I just checked the AUX and EFF busses, and yes, the signal level out of the MON jacks is equal to that of the STEREO buss. I played the same source through CH 1 and 2, matched the levels so that the EFF, AUX, and STEREO were all registering around 0 on the meters, and when I did that, I could freely select the AUX, EFF, or STEREO button next to the monitor knob, and the levels were all basically the same. (Though I did make sure that I was in fact hearing different mixes by altering the volume of each.)

So now I'm going to try measuring those voltages again from PIN 2 of P101 again, now that I have the PCB laying on something sturdy.

More to come soon...
 
Ok, just performed the voltage test and got stable results this time. Here's what I did. (Since it's been a while, I want to check my work):

1. Ran a 1kHz tone into channels 1 and 2, assigned them both to 1/2 and panned hard left and right.
2. Adjusted level so the meters on CH 1 and 2 were around +0.5 or so, and the Master meters were at 0dB.
3. Measured voltage between pins 1 and 2 of P101 = 328 mV
4. Measure voltage between pin 2 of P101 and pin 3 of U103 = with phones knob cranked, I got 328 mV as well.
5. Repeated step 4 with pin 5 of U103 and got the same thing.

Hopefully this is more what you were expecting? :)
 
famous beagle said:
Ok, finally getting back on this now.

I just checked the AUX and EFF busses, and yes, the signal level out of the MON jacks is equal to that of the STEREO buss. I played the same source through CH 1 and 2, matched the levels so that the EFF, AUX, and STEREO were all registering around 0 on the meters, and when I did that, I could freely select the AUX, EFF, or STEREO button next to the monitor knob, and the levels were all basically the same. (Though I did make sure that I was in fact hearing different mixes by altering the volume of each.)

So now I'm going to try measuring those voltages again from PIN 2 of P101 again, now that I have the PCB laying on something sturdy.

More to come soon...
Click to expand...

I'm confused.

The problem is the signal at the MONITOR OUT jacks seems lower then it should be compared to signal at the STEREO OUT jacks, yes?

But when monitoring the AUX or EFF buss via the MONITOR OUT jacks the signal is normal? It's only when you are monitoring the STEREO buss there's a problem?
 
famous beagle said:
Ok, just performed the voltage test and got stable results this time. Here's what I did. (Since it's been a while, I want to check my work):

1. Ran a 1kHz tone into channels 1 and 2, assigned them both to 1/2 and panned hard left and right.
2. Adjusted level so the meters on CH 1 and 2 were around +0.5 or so, and the Master meters were at 0dB.
3. Measured voltage between pins 1 and 2 of P101 = 328 mV
4. Measure voltage between pin 2 of P101 and pin 3 of U103 = with phones knob cranked, I got 328 mV as well.
5. Repeated step 4 with pin 5 of U103 and got the same thing.

Hopefully this is more what you were expecting? :)
Click to expand...

So that's all normal.

now, with the monitor level pot fully counter clockwise measure for AC volts at one of the MONITOR OUT jacks...negative probe on the jack ground, positive probe on the center pin.
 
sweetbeats said:
I'm confused.

The problem is the signal at the MONITOR OUT jacks seems lower then it should be compared to signal at the STEREO OUT jacks, yes?

But when monitoring the AUX or EFF buss via the MONITOR OUT jacks the signal is normal? It's only when you are monitoring the STEREO buss there's a problem?
Click to expand...

No ... sorry, I wasn't very clear. What I meant is that the problem with the MON OUT jacks is the same whether I monitor the STEREO, AUX, or EFF buss. The signal is always quieter than when coming out the STEREO OUT jacks.
 
sweetbeats said:
So that's all normal.

now, with the monitor level pot fully counter clockwise measure for AC volts at one of the MONITOR OUT jacks...negative probe on the jack ground, positive probe on the center pin.
Click to expand...

Do I still need to run the test tone to the monitor buss?

I've never probed a jack before. The ground is a no-brainer, but is there any trick for the center pin? Do I just stick it in there at an angle so that it's making contact with the side?
 
famous beagle said:
Do I still need to run the test tone to the monitor buss?

I've never probed a jack before. The ground is a no-brainer, but is there any trick for the center pin? Do I just stick it in there at an angle so that it's making contact with the side?
Click to expand...

If you're actually sticking the probes into the jack socket, that's probably not a good idea. The best way to test a female jack IMHO is to get a male-to-male TRS cable, and use crocodile clips on the plug.
 
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