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Not the heads!
The heads were not guilty! (I suspected that because the tapes recorded in the machine did PB in color in another VTR). With the aid of the service manuals I got from elektrotanya, I did trace the PB signal. Near the head drum there are PCBs for the REC RF amplifiers and PB RF preamplifiers (much like the circa-1968 1Ļ open-reel Sony EV-300 series). But in the Betacams there are two boards, because the format uses separate pairs of heads for luminance and chrominance.
The four RF packets come out alive and well from these boards. One of the channels in the chrominance system has a slightly lower level, nothing that can not be adjusted later. But the trouble is in the DM board that comes later in the chain.
In this board the signals from the heads are selected (the 70 has TWO pairs of heads for each luminance and chrominance, the second pair is the "confidence" head while recording).
The process is the same both for Y and C circuits. After selected, the PB/Confi signals are amplified and DC-corrected, and then switched so the "active" headīs signal is further processed.
In the Y circuit I get both A- and B-channels perfectly, but in the chroma path only the A-head signal is present.
So the trouble lies between the input of the MD board and the test point I am measuring from.
These test points are located (very cleverly) at the edge of the board, so they are accessible without removing it.
I donīt yet have the extender board that will allow to operate the machine with the MD board out of its slot, so will have to wait until I get the extender board. Then I will have access to the full surface of the board.
I could solder temporary long wires from the points I want to measure from, reinsert the board and take the measurements from the wires, but with so much soldering and resoldering I could easily ruin the board. I am not taking chances with this machine.
So for the time being, I will put myself in a brief "stand-by off" mode. I will let you guys know very soon how this story ends.
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