Help with Didgitech effects processor repair

[Beergod]

I am trying to diagnose and repair a 1980's vintage Digitech RDS 1900 rackmount delay unit. It has been packed away for several years and was working previuosly, but now two functions are inop. I found the schematic here:
http://ericarcher.net/devices/digitech-rds/
It is complicated beyond my ability and need help identify the possible fault.

The device enables four functions - Flanger, Chorus, Doubling and Echo (Delay). The Flanger and Doubling work fine. The Chorus an Echo functions do not produce an output signal. However the led's associated with discrete function select switches light up.

I've been looking for a component shared by the Chorus and Echo without success. The HC139 is a Demux/Decoder which is suspect, but I can't justify why the led's would light if the HC139 is faulty.

I've spent hours on this and getting nowhere. The manual and schematic pdf's were too large to attach. Sorry for the inconvenience.

 

[Harald Kapp]

For starters I suggest you test all operating voltages.
Chances are that after long storage some electrolytic capacitors in the power supply have lost most of their capacitance. Sometimes you can even see this if the aluminum housing of the capacitor bulges outward.
Once you've verified that all operating volatges are in good order, we can continue the search for a possible defect.

 

[Beergod]

Thanks for your reply. Yesterday was the first chance I had to work on your initial suggestion. Th PS caps seem to be in working order. The +5, +12 and -12v points around the board are correct.

Once again, this url: http://ericarcher.net/devices/digitech-rds/
is where the supporting documents for this device can be downloaded. the pdf files are too large to include as attachments.

 

[Harald Kapp]

Do you have an oscilloscope? Or a signal tracer (you could build a simple one yourself?

Apply an input signal to the processor. Follow the signal along its path (use the schematic to guide you). At some point you will loose the signal. Go back and verify that the signal is present befor that stage. Now you have located the stage where the signal is lost.
Exampel assume thge signal is present at the input potentioometer 100k (connected to C66 in the schematic), but is not present at the output of U9A (C4). The we can anylyze the stage in question in more detail (like measuring resistors, looking for bad solder joints etc.)

If the fault is within the digital section, it's going to be more difficult and I'm probably not up to remote diagnosis.

 

[Beergod]

I do not have a scope. I do know that the analog circuit is complete because the Flange and Double signals are at the mix output jack.

So. if we look at the bottom of the schematic on page 1, and if I am reading this correctly, it appears possible that U43 (74HCO4) is not passing signals for the Chorus and Delay functions. I arrive at this conclusion because:
1. All four led's will light when their respectitve switch is is selected at U46 (left side of schematic). And these led's are connected immediately before the U43 inputs.
2. All four functions share a signal path beyond U43.

Does this logic make sense to you or am I missing something else???

 

[Harald Kapp]

Possioble, but not definite.
Look at the inputs and output of U43. The output has to be inverse to the input. If they are, the fault is elsewhere. If they are not, replace U43.

What happens when you bias Q4 and Q11 with a small positive base current? Try a 10k resistor from 5V to the common base connection (R84, R85). Does the output of the mixer react?

 

[Beergod]

I see proper inverted output on U43 except when the Echo function is activated pins 10 and 11 are both +2.5v. ???? Could this be correct?
Pin 10 feeds one input of the NAND gate (U33), which I'm guessing controls the flash rate of the Echo led which corresponds to the delay length as selected by the delay time pot on the front panel. I cannot seem to find this pot on the schematic either.

I have not yet biased the Q4 and Q11.

 

[Beergod]

I fired the processor up again today. I'd like to be able to say that I repaired it, but can't honestly do so because I don't know exactly what i did to get it working again.

Regardless, all functions are now working properly. Yea!!!

Thank you, Harald for your kind and patient assistance!