Took apart and cleaned all my ribbon cable connectors (all off at once). Also took off and cleaned and reseated all molex connectors. Now back together but....solo light on all channels is lit with nothing soloed and soloing or muting a channel has no effect. Something crossed in ribbons or through the path of the molex connectors?

I would suggest taking it into the nearest factory authorized repair center. Go to the Technics web site to find one in your area.

According to the schematic, once the signal is initially amplified (also where the LED shows signal), the audio is sent to the send/return jacks (shown on pg3-C7). A quick test to see if audio exists at the insert point will require an insert cable. An insert cable can be a balanced 1/4" cable which splits out to two mono 1/4" connectors or a pair of RCA's. Connect one of the insert outputs to a console (if 1/4" cable) or a tape loop on a home stereo receiver (if send cable splits out to RCA's) and see if you've got sound when playing your guitar (with LED on the Marshall showing signal). If no sound, go to (A) below. If you have sound, go to (B) below.
(A) Ten pin connector labelled CON2 (shown on pg5-B1). Make sure connector is tightly seated and that the pins on the jack and the female connectors in the plug are clean and that good contact is being made.
(B) Twelve pin connector labelled CON1 (shown on pg5-B4). This is the DFX board - make sure this is tight and well seated.

According to the schematic there are voltages seen at several of the connection points previously mentioned. If you have a multimeter and are qualified and careful - check to make sure the voltages are present as shown.

If you're not qualified - meaning you have little or no experience - STOP and refer this repair to a qualified bench tech. They get paid to keep the magic smoke inside the amplifier. You may end up damaging things worse or even beyond repair or even damaging yourself.

Are you able to get a dry mix under these circumstances? If it is simply a dry version of the wet signal as you have turned the effects off yes it is possible a ribbon cable has come loose but it is more likely that an IC has died, this is not uncommon in these and they exhibit a range of symptoms including yours. If yea are happy to take apart and have a tame service centre this should save you quite a bit as the biggest cost is dismantling and rebuilding. As this is not unusual issue they should be able to fix quite easily

It is VERY easy to not have gotten the cables back and seated completely for the floppy disk. ALSO it is very easy to damage the ribbon cables on floppy drives by pulling at the edges of the ribbon to pull connectors off. One MUST distrbute the force all across the cable as it is easy o damage the connections in the connector for the wires near the edge. It is also easy to damage the pins on floppy drive connectors. You will need to reopen the unit and check the cables INCLUDING the little power cable to the drive. If you have damaged the ribbon cable you can either replace it or possibly get another connector and put it on the cable right next to the one with the problem and crimp it in a vise... These are an insulation displacement type so you need to get it on straight and ALSO verify the index is the right direction before crimping. For your problem think cables, cables, cables...

Your key contacts have gotten dirty. Two contacts per key are used to sense velocity and if one doesn't work the sound will be full velocity and loud.

The contacts are conductive rubber "pills" in silicone rubber domes that the key presses down onto circuit board traces under the keys. Clean both the "pills" and the black portion of the circuit board GENTLY using ONLY 99% isoprophyl alcohol and QTips. There is a lot of disassembly required and one has to keep track of where the screws go and length and thread type to re-assemble after cleaning. Also great care must be taken to not damage the ribbon cablles connecting the top and bottom halves of the case.

I have read that the problem is in the FX send/return jacks which oxidize if not used much. All you have to do, apparently, is ram a jack plug in and out a bunch of times. This breaks up the oxidation, and allows the amp to work properly again.

Many of those connections REQUIRE using TRS cables (stereo style) rather than TS cables (mono style). It is possible you have mixed the cables if it worked before.

Any cable plugged into an "insert" should be a TRS (tip-ring-sleeve) cable.

Look up these connections in your 1604 manual.

You either did not get the ribbon cable seated OR got it upside down.

If this cable is the flat ribbon with exposed solder plated points of contact, OFTEN those connectors have a latch to release by GENTLY moving tabs on eaither side in the direction of pulling the cable out. This lock MUST be moved out slightly to allow the cable to seat properly. The solder plated contacts will be FULLY into the connector when seated properly.

These cables ALSO can be inserted backwards and NO connection will be made.

Unless you are in super humid environment, this is more likely a crack in the circuit board OR a bad connector on a ribbon cable. Also BROKEN or cold solder joint for a connector pin through the circuit board.

Good way to cause this with ribbon cables is to NOT distribute the load when pulling them off. Often the wires near the edge are stretched and make poor contact with the little forks that pierce the insulation.

CRC226 is a good product to use for contacts. Available at Home Depot in the electrical department. Blue spray can. Use sparingly.