Fault: left channel audio was degraded and there was a loud “pop” when bypass was enabled/disabled.
No schematic available.This particular PCB is dated 1999.
Basically this unit is a digitally-controlled analog compressor, with (I think) a fully stereo pathway, based around 2 single-channel VCA chips and a dual op-amp.
Initial measurements showed +12 volts DC on the Channel B audio output pin of the dual op-amp buffer (OP275, pin 7). This should normally be close to zero volts.
Looking first, as always, at the power supply (NOTE: this unit requires 9 VOLTS AC voltage input, NOT DC!! Do NOT use a footpedal PSU with this unit) shows robust +/- 15 volts and + 5 volts in all areas that require it.
There appear to be decoupling tantalum caps near the analog audio section, but these were not short and not hot.
Caps and resistors around the op-amp feedback circuit seemed intact.
Replacing the op-amp (in the pictures I have pre-installed a socket) resulted in NO FIX.
Replacing the VCA (THAT 2155, single inline 8 pin chip) resulted in a FIX.
However, see THAT Corporation Design Note 137: the 2155 chip is obsolete. It is easily replaced by a 2180, however, a couple of resistors need to be removed, and pin 4 floats free, unconnected - see my pictures below where I have removed the 150kOhm resistors from the PCB, and installed a single inline series of sockets for the THAT 2180 chips, excepting pin 4.
Both THAT VCAs were replaced because the unit is stereo, and the response of the older 2155 chip (both to audio and CV) compared to the newer 2180 is very likely different. Would the difference be significant? I don't know.
This also begs the question: are the control voltages generated separately for left and right? Again, I don't know. One would think that they would have to be, otherwise you would get stereo imaging problems in certain cases, and these 1773s are well-regarded units for their price.
Comments
Post a Comment