Boss DD-2 Repair

[g2s]

I thought it was possible the 4066 switch is bad and that is why I am loosing so much signal there.

The control for all 4 switches are connected together and controlled by IC7(18), so they should all be either open or closed. Since I was getting signal through 4a and 4b, signal should also be passing through 4c and 4d.

I measured resistance between the individual switches, expecting about 100ohms per 4066 datasheet.

The 4066 measured:
Pins 1-2 8.7k
Pins 3-4 8.7k
Pins 8-9 86k
Pins 10-11 170k

Pin 7 (VSS): Ground
Pin 14 (VDD): 4.98V

Thinking the 4066 may be faulty I swapped it with a new 4066 from Mouser.

Now the switches are at:

Pins 1-2 8.7k
Pins 3-4 2.3k
Pins 8-9 245k
Pins 10-11 120k

EDIT: The controls for the switches have 0.3VDC on them. 4066 data sheet specifies should be 0.7x VSS for switches to be ON (3.5V or so). These are all being run from Pin 18 (SAH) of the main controller.

[g2s]

As a test I injected 3.5V on the control of the 4066 to get the switch to open.

The signal getting through to checkpoint 4 in the schematic shot up  around 150mVp-p. Still much lower than the schematic calls for and the wave form looks nothing like it should...

[g2s]

At this point I know everything up to the Main Controller is good. There is good signal going in. 

I know the memory DRAM chips are good.

I know the clock signal is good and in the correct range (1.4-5Mhz).

I know all voltages are good, have checked on literally every chip.

I know the delay time switch is good and sending the correct voltage to the appropriate pin on IC7.

I am still getting incorrect signal out of the Main Controller. No voltage on Pin18 SAH to open the 4066 switches and the signal at the end of the R2R DAC is not the delayed audio signal (check with scope, audio probe etc). The output of the CPU at the end of the R2R network does not vary with the input at all.

I am pretty close to diagnosing the Main Controller as dead. Is there anything I've overlooked?

[Slowpoke101]

You have ruled out almost everything else. The Main Controller (IC7) is looking as if it has been damaged. Usually they do survive when things going wrong power-wise but not always.

One thing to check is pin 7 - the HOLD function pin. Put the pedal in the hold mode and toggle pin 7 with the foot switch. If the output of the R2R array doesn't change then the Main Controller is toast.

Finding a replacement will not be easy but not impossible. Do not throw this unit away even though it doesn't work. They can be quite valuable to some collectors.

[g2s]

Thanks, good test.

I have confirmed there is no change in CPU output while toggling the hold function.

Whatever overvoltage event happened here was pretty significant, with a ton of burning on the pcb around D6. Much more than others I have seen online.

It seems the CPU didn't make it.

Thank you both for the help. I learned a lot about DD2's and R2R DAC. Super interesting.