Have a quasar wp3931aw 13" (1986) that I got going, (new flyback, etc).

After finally getting it up and going, there was no horizontal lock at all. Found a small 47mfd/35v cap that was blown out the bottom, swapped it. same result. Removed it and tried the set again (to see if that is the issue), and lo and behold, we have horizontal lock and stability!

Put the cap back in (a good replacement), and all lock is lost. fiddling with the hold control does nothing. Remove cap, all is good again. Right now I am running it with the cap out and the picture is rock solid. Is there some difference in the new flyback that's causing my set to run without this cap? (I bought it from electronix)

Were is the cap in the circuit?

I would be looking for a shorted diode or something like that causing the cap to load the circuit down when in place.

Did you check the B+ to make sure it's within limits. Usually, the regulated B+ is never over 135VDC. Most solid state sets from the '70's on have a fail safe circuit that will either throw off horizontal sync or kill the horizontal oscillator completely in the event of excessive HV, which is usually caused by high B+. In many of these sets, a flyback failure would short the HOT, which would then short the B+ regulator. If the B+ is too high and you keep running the set, you may cause excessive X-radiation, as well as ruin the new flyback, the CRT, or other parts.

Did you check the B+ to make sure it's within limits. Usually, the regulated B+ is never over 135VDC. Most solid state sets from the '70's on have a fail safe circuit that will either throw off horizontal sync or kill the horizontal oscillator completely in the event of excessive HV, which is usually caused by high B+. In many of these sets, a flyback failure would short the HOT, which would then short the B+ regulator. If the B+ is too high and you keep running the set, you may cause excessive X-radiation, as well as ruin the new flyback, the CRT, or other parts.

I had replaced that part as well (STR3130)

Anyone have a SAMS or similar manual on this set I can thumb through?