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2018/12/29
Recently I fix three Tektronix 4xx series Oscilloscopes, they are all beauty and deserve treat well. Amost All of them had power supply problem, it's reasonable for a 40 years scope. The amazing thing is, except dry capacitors they are almost same as first born.
Golden plating switches and transistor sockets is really awesome, so many years keeping good contact performance.
Keep Life Easier
Keep Life Easier
Keep this on hands, these capacitors are most suspicious one which won't work after several decades. Clearly Mark the position and polarity, maybe several yeas later, we forget anything about Tektronix 4xx, with this, really help to checking around.
White dummy resistor could use to isolate power supply with other board.
Replacement Guide (Came from eevblog, thanks)
Power Supply Fist Step
Power Supply Fist Step
Power supply is the most vulnerable part for a decades electronic device, Especially for Tek 4xx series Scope. Before dive too much into power supply part to tracking anything further, following steps is the FIRST OF ALL procedure.
Testing every DC TIP voltage to check if the voltage is in normal range. One or more is zero or low, yeah, Power Supply Fault.
Checking DC test point ripple, better with another scope. Too much ripple indicate one or more filter capacitor might dry, leading to wired sweep beam even BLACK.
Checking DC test point resistance to Ground : There may be somewhere with shorted capacitor.
Reference 1
Tek 465 supply TIP to Ground resistance:
+110V -> 17.1K
+55V -> 850 ohms
+15V -> 153 ohms
-8V -> 56 ohms
+5V -> 105 ohms
Filter Capacitors Failure
Filter Capacitors Failure
Power supply fault might had many symptom:
Totally Black, even the FAN is stop.
All kind of wired sweep line.
Dry Cap: The retrace beam is flickering visible
Dry Cap: the beam modulating by AC ripple (Thanks, Ron Childress)
This is 110V rail's ripple, 50V/Div, this is way too much ripple. It turns out C1512 is almost dry enough.
Another similar symptom, but with All rail had a large ripple, DC checking indicate every rail drop 20%, Scope is totally dead.
Normally, if 5V/15V/-8V/110V rail had too much ripple, it's own filter capacitor is dry.
But if 55V rail fault, all other rail been affected. because the other rail use 55V as it's voltage reference.
That why every other rail had same scaled ripple if 55V capacitor C 1513 is dry.
One of my 465, this C1513 is completely dry. the ripple is 120Hz, and the amplitude as much as the rail itself. every rail DC voltage dropped 20%.
How To Replace the Filter Capacitors
How To Replace the Filter Capacitors
You might broken the PCB if you try to remove the old BIG filter capacitors.
Because that need Higher temperature to de-soldering and Those capacitor soldering to PCB on bothe side, this is terrible part.
Overheating the PCB tear down the pads from PCB, you need jumper eventually.
Th art for capacitor replacement
Dirty quick method: Recommend easy, won't broken the PCB.
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