Tektronix 565 repair
On Monday, March 12, 2018 at 10:53:38 PM UTC-5, GS wrote:
Been playing with this scope for some time. Original problem, blowing 10
ohm 2 watt surge resistors in supply. Replaced a couple caps so far, not
sure if they had problems. I can bring up on variac and run, no problems.
200 ma. Draw on each of 3 resistors. I still burn resistors on full power
up. Not sure if replacement resistors are suitable. Thinking now of upping
power from 2 watt types.
If you are talking about R 660 and 680 I would be suspicious of C 661 and 681. There is a slight possibility that one of them, maybe more likely 661 has a weird failure mode in that there is leakage at zero volts which is "blown" clear once the voltage gets to it. It is a bit rare but at the age of that thing you can get failure modes that are not usual, as with anything.
Looking at the circuit I see nothing else that could be the problem. And as for them being the wrong type of resistors, why did the originals fail in the first place ? i don't know about you but I haven't replaced many resistors in that place in a circuit without finding some sort of a short.
The direct, unregulated supplies go through R 677 and 697 which limits current to where it should not blow your inrush limiters even if all four outputs are shorted. That would be 500 ohms and being much higher than 10 ohms those resistors would blow first.
The only other thing hanging off those supplies are tube plates, and you can eliminate them as a possibility by simply removing V 667, 677 and 697. Everything else is to high value resistors and no matter what the possibility of those shorting out is so remote it isn't even worth consideration.
The possibility or the wrong replacement resistors being the trouble while remote, is a possibility, but only if the replacements are smaller in size and therefore have less thermal mass. Some newer resistors can take higher temperature thus being able to dissipate more power when they are physically smaller. That situation might be ameliorated by using with 2 5 ohms in series or 2 20 ohms in parallel.
In theory raising the value should not work as overall dissipation will be higher, and lowering the value which might save the resistors will put more stress on the input filters and/or the rectifiers.
So the first thing I would do is to disconnect the original filters and wire in some 470 uF @ 250 volts and see what happens.
Worth a shot I would say.