Vertical return is used because it is an actual sample of yoke current for the feedback circuit, the same way the convergence circuit operates. The difference is that the vertical IC usually supplies it's own "commutated" power supply to the output stage. Sometimes that boost pulse is used for blanking or something, but not for protection.
The RET means return. all three yokes are in series, so if any yoke gets unplugged or open circuited for any reason, there will be no signal on the RET. Since they are in series, any open can cause screen burn in all three CRTs quite quickly. So using the RET to have the signal for the shutdown circuit that all is well, they use the RET. There will usually be a ramp or something that resembles a parabola waveform there, and it is rectified to usually saturate a transistor that if becomes turned of by loss of input, will either trigger a hard blanking circuit or the shutdown.
In the RET line sense circuit, electrolytics can be critical. Engineers deem the saturated transistor to be the best way to tell the shutdown that all is well. This is because of a few reasons, first of all if a connection is lost to any of the terminals of a bipolar transistor it acts as an open circuit. Secondly, in this circuit, the collector is not subjected to any great current or voltage, and is very unlikely to short out, and is not connected to an inductive load either. These factors are what made them choose the rectified base current method for this mode of protection. It is the best, surefirest way to assure there is vertical deflection.
There is a similar scheme for horizontal deflection, but it is not as surefire, because the horizontal windings are in paralell, not series. In sets with normal reactive scanning, if any yokes come disconnected it causes a decrease in high voltage. Sets with discrete HOT and HVOT usually have a quite sensitive circuit to detect any yoke disconnected, but even then it does not always work. However, these sets do not shut the HOT down when needed, only the HVOT. That's why the owner's manual says to unplug it.
At the high side of the vertical yoke you should see something like _/ ^|_/^|. that's the best I can do it in text. If you see |_ |_ |, like a square wave, you have an open yoke circuit. Some sets depend on DC feedback through the windings as well, so on chassis make sure you got the same DC voltage on the vertical output as the RET. It should always be the same. You might have to isolate your voltmeter with an R/ C filter, like a 10K resistor and a 10uF cap. It depends on your meter.
I know I have responded on this before, but right now I don't remember the whole thing, I'll have to go back and read it. But you asked what you asked now, so that is the answer. The RET is used for feedback as well as shutdown.
Now on Pioneers, sometimes the vertical circuit is on the convergence board, which would be an STK 4274 I think. If you got one of those it is push pull, alot more fun. It is very hard to get them anymore, but there is a new STK that can be adapted. By shorting a couple of pins together I think you can use an STK 391-020. However the mounting dimensions are different. Personally if I have to do this, instead of drilling new holes I would find some sort of clamping method, like used in newer Mitses for example.
Actually you need a scope on the B+ line, if it does not go above spec, jump the relay and see what you get.
Damn, I need to know if this one has seperate HOT and HVOT or uses that funky circuit to compensate. Tell me this, where the yokes plug in, do the foils go to the flyback at all ? (off the red wire) There may be a seperate little transformer. We need to know this.
And last but not least, if you have a print, where the hell did you get it ?
Weekend is coming, there will be calm times, perhaps I'll refresh my mind with the whole situation. I know Pioneers are a pain in the ass, but pain in the ass is my business.
JURB