DigiTech GSP 5 repair: generates tone / noise

[obiwanjacobi]

Hi,

I have an old DigiTech GSP 5 to repair. After I put in a new DAC it now produces hi pitched tones and noise (sounds like distortion).
The analog part seems to work fine, however something in the digital part is generating data to the DAC.

Anyone any experience with this?

Thanx
Marc

[Enzo]

Perhaps a missing connection due to an address bit or a data bit not having a good solder connection.

[obiwanjacobi]

Yeah - I will check out the solder connections.

I found the schematic - for those interested. http://www.ampix.org/albums/userpics...itech_DSP5.pdf

I have replaced the 2 74HC374 buffers (U10/U11) that drive the DAC straight from the DSP processor with ICs from elsewhere in the device (there like 10 of them in there) but that didn't fix anything.
I just hope its not the DSP processer - I can't find that thing anywhere and has DigiTech branded on it...

[Mick Bailey]

Was the original DAC faulty?

[obiwanjacobi]

Yeah it ran pretty hot and output a garbled analog signal... I think some bits were stuck.

[obiwanjacobi]

Sorry it took a while...

I examined the board and there are no obvious bad solder joints I can find (with magnifying glass)...

I noticed that the dynamic RAM chips (U1-U5) are getting warm. Not hot, but definitely warm.
Is that normal for these chips? Could they be faulty and feed the processor with bad data that causes the signal to appear?

[g1]

In my experience, the DRAM run warm and bad ones run much hotter.
One end of the bank will be MSB, the other end will be LSB.
You can remove them one at a time (re-installing after each test) and listen to the results. It will give you an idea of what bad DRAM sounds like. With LSB out. least effect, MSB out greatest effect.