Boss DD3 Repair Mystery - Signal not passing IC

[clarisso11]

Hi everyone,

I've been working on repairing a Boss DD3 delay, here is the service manual: https://www.synthxl.com/wp-content/u...vice-Notes.pdf
I think I narrowed down the problem, but just can't figure out what is causing it.

Symptoms:

• No direct signal coming through Mono out when off
• When on, only wet signal coming through Mono out
• Dry signal reaches IC1a (5218), pin 2, but doesn't make it through to pin 1

Tried so far:

• Checked all electrolytics and found some bad ones, replacing them restored the wet signal fully
• Replaced IC1 with a new 5218 chip. I thought I could hear signal coming out pin 1, but after a few minutes it was gone again
• Replaced IC1 with 2 different NJM4558l (same pinout etc), still no signal at pin 1
• Measured voltages of IC1, and compared to a working Boss DD3 unit. All IC1 voltages seem fine.
• Confirmed dry signal reaching direct out

Question:
I just can't figure out why the dry signal would reach pin 2, but not pin 1, especially once I confirmed good voltages and replaced the IC.

Any help would be greatly appreciated!

[g1]

What are DC voltages at pins 1,2,3, of IC1 ?

[clarisso11]

Hi g1,
Voltages are 4.2V for those pins (1/2Vcc, pedal running on battery), same on the working unit.

[g1]

How are you measuring signal at pin2, with scope?

[clarisso11]

How are you measuring signal at pin2, with scope?

So far I’ve only measured signal with an audio probe, but will try a scope next- should probably compare that with the working unit as well.

[Grrrg]

If I read the schematic correctly: isn"t the wet signal going through IC 1 as well? Then IC 1 should be fine. Maybe the switching circuit is responsible? Q5?

[clarisso11]

If I read the schematic correctly: isn"t the wet signal going through IC 1 as well? Then IC 1 should be fine. Maybe the switching circuit is responsible? Q5?

Good point, I think your right! I can hear the wet signal being amplified, between pin 2 and pin 1 of IC1, on both the non-working and working unit.
I checked and dry signal passed Q5 in both with the audio probe, but I noticed that Q5 voltages are different:

Working:
S: 4.7V
G: 50mV
D: 4.7V

Non-working:
S: 4.7V
G: 0.5-0.6V
D: 4.7V

I hope I labelled the pinout of Q5 correctly, "G" is where D1 is connected to on the schematic.

[clarisso11]

So far I’ve only measured signal with an audio probe, but will try a scope next- should probably compare that with the working unit as well.

Here are the waveforms at pin 1 of IC1. My scope isn't the best and I'm still learning how to use it, so I was struggling getting anything at pin 2..

[Grrrg]

[clarisso11]

Worked on this for a few hours more today, to no avail.. Some notes:

• I replaced D1 which brought Q5 voltages to the same as on the working unit
• I had sound cutting in from time to time while moving the PCB, so I checked all wires, replaced a few, tightened up the jacks
• I realised that some of the sound cutting in while moving was from Q8 touching the enclosure
• Still get effect only, dry signal stops at IC1 pin 2. I had dry signal coming through at some point, for a few seconds, but that stopped after I moved the PCB around. Can't reproduce now.
• Signal level, at least on the audio probe, is the same at IC1 pin 2 between non-working and working unit
• Voltages of all the transistors and ICs I tested were the same between the two units
• Can't see any broken solder joints

I'm really lost on this one!

[g1]

You say you can hear amplification between pins 2 and 1 for both units, but scope does not confirm this. Wet signal can still bleed around IC1a via R16.
Is E.Level pot working when you hear only wet? You should be able to kill the wet signal with the E.level pot.
We used to get some of these where the jacks got swapped around or mis-wired. Double check against your good unit just to be sure.

[Helmholtz]

Signal at pin2 is extremely low as this is a virtual earth node by the inverting opamp configuration.

You should be able to scope the dry signal before R13. If you see signal there check R13 and/or look for a broken board trace.

[g1]

In stereo mode (when something plugged into direct out), mono out becomes wet only. This is achieved by turning off Q5. If Q5 off, then also no signal in bypass mode.
So agree with Grrrg that Q5 is a likely suspect.
If it is bad, swapping Q5 and Q8 will probably result in all dry, no effect.

[Helmholtz]

Q5 is used as a switch. As drain and source voltages are the same it should be on.
In case of doubt remove it and wire a jumper between drain and source connections.

[g1]

Q5 is used as a switch. As drain and source voltages are the same it should be on.

You can't go by the DC drain and source voltages as they are both sitting around 1/2 VCC at all times (unless defective IC1).
Agree that shorting drain to source easier than swapping with Q8.