The problem with working on multiplex adapters, is that they can be "working" perfectly, as in there are no defective components and all voltages are correct, but if they become misaligned, they simply produce a mono signal out of both stereo outputs.
FM stereo works by using a L+R main channel, and L-R sub-channel, that is contained within the total modulation envelope. The L-R is then broken into two such signals that are 180 degrees out of phase with each other, producing L-R, and -L+R. With a proper mixing circuit, these three signals (L+R, L-R, and -L+R) can then be combined so that +R and -R cancel each other out leaving 2L in one channel, and +L and -L cancel each other out leaving 2R in the other channel. The relationship these signals have with the 19 kHz pilot "timing" signal then determines which channel is L and which is R.
I mention all of this hardly to impress, but to show how the whole process is ultimately based on cancellation and addition. When these processes don't happen, the whole house of cards falls down, and you get nothing but mono out of each side. This is why I have always said that for proper FM stereo operation, the alignment of the multiplex adapter is rather critical for the "math" to work properly. In particular, if the phase of the various signals is altered by much at all -- as determined (in part) by the adjustment of the various tuned circuits -- then proper cancellation cannot happen, even if the amplitudes of these signals are correct. In normal operation, a properly operating adapter will display at least 30 db of separation, while it takes little mis-adjustment to have this deteriorate down to 10 db or less.
The bottom line is that to check all of these things properly takes a very good scope (as in one with ideally zero phase shift difference between the X and Y channels, and a good multiplex generator, so that the amplitude and phase of the various signals employed can be precisely known. Without proper test equipment, it is almost impossible to adjust an adapter for proper operation by ear. You might take a look at a recent thread I posted about "A different kind of Fisher" where I recount my recent efforts in aligning the adapter in my Fisher 400. Apparently, the pics I provided for that thread were lost in some of AK's new server work, so I will try to repost them to give you an idea of the types of signals being discussed here.
Without proper test equipment, the one thing I would have you carefully recheck, is the connections of the diode modulator circuits. The connections for ground, and application of the composite signal are obvious, but the connections to the 38 Khz doubler transformer is critical. Normally, these are connect one way for one diode set, and the opposite way for the other. Therefore, if these become reversed in one diode set, you will effectively end up with the same single channel being produced at the output of both stereo outputs, which would certainly have no separation.
Ultimately, the diodes -- contrary to what some might like to think -- rarely go bad. It is unbelievable how many people "tinker" with the adapter to try and resolve some issue they perceive, and almost always end up making it much worse. If the adapter was not working before you started your work, and there are no errors in your work or defective components installed, it is almost a certainty that someone tinkered with the settings, which if this is in fact the case, then no amount of component replacement will ever bring the stereo back again. The unit will need to be evaluated with proper equipment to ultimately determine its status, and readjust it for correct operation again. The good news is that once set, the units rarely give any concern, except when someone -- with the best of intentions -- tries to "upgrade" all the components installed. The ONLY components that should be looked at in this light are the input coupling cap, and the two output coupling caps. The composite signal coupling cap (a 1 uF electrolytic) should also be replaced as a matter of good maintenance. However, all other components should be considered as off limits, and are not handling audio signals anyway.
This may not be what you wanted to hear, but it is honest information. Double check your work closely, but be prepared to send the unit off if no mistakes are found. The good news is, that a capable shop SHOULD be able to align the adapter by itself, making a little more work for you in its removal and installation, but making shipment much easier. Of course, it's always best to align the entire receiver in one complete setting, as a poorly aligned tuner section will also produce bad FM stereo, and the very best results will always be obtained when the tuner and multiplex units can be tested together. However, very good results can be obtained from aligning the adapter by itself as well.
I hope this helps!
Dave