Tektronix 464 mains fuse blows when turning on

[Ja-no]

Hi, I have a tek 464 that I had been using for about a year. Some months ago I turned it on and it blew the mains fuse. I set it aside as I don't use it much, but now I'm trying to repair it.

I read the Tektronix oscilloscope troubleshooting guide and it says that if the fuse blows when power is applied then I must check for shorted filter caps or shorted diode bridges. I checked them and they seem to be ok, so maybe it's a short in the transformer?

I measured continuity from line to neutral in the mains input and it's 14.2 ohms, no continuity neither from line to ground nor from neutral to ground.

I measured continuity of the different power rails to ground and got this:

   5v  : 303.8 ohms
  15v : 14.92 ohms (this one seems to be too low, but I don't know what is the normal reading)
  65v : 3.92 kohms
140v : 21.8 kohms
   -8v : 88.2 ohms (this one also seems low)
 -15v : 2.72 kohms

I checked components near the bridge and filter cap in the 15v rail and they seem to check ok.

Perhaps I should put a bulb in series with the mains input and see which voltages are present?

[jaycee]

How did it blow ? Did it just open or was the inside of the fuse blackened ? I would try another fuse, as the old one may have just been "tired" and blew due to that. If the replacement fuse blows, then start investigating.

However if the original fuse blew and was left blackened, then there is a fault. A dim bulb test will help.

[Dacke]

I haven't been doing this long but in the 6-7 years since I started repairing electronics I've yet to see a fuse blow just due to age,  though I've heard people tell me that it can happen.  Personally I would not put another fuse in there just to blow as this could potentially cause more damage.  You could use a dim bulb tester, variac, etc.,  but you can also do checks for gross shorts.  If you've checked all the rectifiers and main filter caps already,  I would check the big power transistors in the regulator circuit,  these are the ones attached to the chassis heatsink.   I believe this would be Q1756, Q1766, Q1776, Q1788.  Also, check the tantalum capacitors in the power supply,  they can fail short.

I'm rebuilding a Tektronix 475A myself that has multiple issues and it has a very similar power supply.

[med6753]

As I recall in troubleshooting the power supply in a similar 465 your 15V supply is indeed reading too low. Most likely due to a shorted tantalum capacitor. In the troubleshooting section of the manual it should describe how to isolate each board in order to track down the short.

[Ja-no]

Some news (I still don't know if good or bad)

I started disconnecting the different modules to try isolating the short to one of them. I disconnected almost all of the modules except for the digital storage one to no avail.

I started unscrewing both the power and storage modules since it seemed that I had to remove them to disconnect them. With all the screws removed I measured the resistance of the power rails and got this (higher) readings:

   5v  : start at 1M ohms and decrease to 115k ohms
  15v : start at 1M ohms and decrease to 108k ohms
  65v : start at 250 kohm and decrease to 150k ohms
140v : 22.5k ohms
   -8v : 113k ohms
 -15v : start at 28k ohms and increase to 72k ohms

I started putting back the screws one by one and measuring resistance in the power rails. The readings stayed the same until I put in the screw in the place that can be seen on the first picture, then I got the 14 ohm reading on the 15v rail and the lower readings on the -8 and 5v rails. Without the screw in I started connecting the different modules and the readings remained similar.

The first picture shows the screw location (between the danger label and the GND test point). I measured continuity between the lip(?) the screw was gripping and ground an they are connected. It screws onto something that seems like a plastic isolator from the chassis.

I don't know how that screw can affect the readings unless it was connecting the ground to some other rail or component. at the bottom of the isolator of the screw is a large thick coil that I think is used to modify the trace rotation (part of which is seen through the hole on the second picture).

I was thinking on doing a dim bulb test of the scope without that screw and see if the problem persists or the behaviour of the scope changes.