TEK 475 PSU Fault

[tombi]

Similar fault to the one discussed in the forum before
https://www.eevblog.com/forum/beginners/textronix-475-needs-repair/msg182079/#msg182079

In my case there was power to some things like the graticule illumination and other bits but no screen. I pulled the cover off the HV components and the neons were not lit. Then I did the obvious and poked around the supply lines and found the -8V line is at +800mV. Not surprisingly the -15V line is at -23V (given the ground line for U1464 is the -8v line this makes sense).

The question is where to start.

Someone already replaced the filter cap on the 5v line (it's blue and there is a wire bridged over to the earth on the component side for the third leg).

The big transistor Q1468 looks Ok when I poke it with the DMM in diode mode.

I can't easily poke the bridge rectifier as I need to get down deep into the unit.

The voltage at the collector of Q1468 is about 660mV when the unit is on. On my other scope it looks like a high frequency sine wave with around 800mV P-P

So is it the bridge or the cap? How do I tell?

[Paul Price]

The ground line for U1464 is ground.

if the collector of Q1468 is at 800mV then the -8V line should be much more negative than -8V unless a trace from the bridge rectifier or the connections from the transformer are open due to a bad solder joint/vaporized PCB trace..

The schematic you've posted is truncated and doesn't show the whole circuit.

[tombi]

Thanks Paul,

I was struggling to get the scan to fit within the forum size limits. Here is another scan but of the while page.

Sorry not the ground U1464 but VSS is connected to the -8 line. The second opamp in this package is used in the -15 circuit. This is what made me thing the -15 being at -23 could be related.

It's really hard to physically access the backside of the board and the transformer so I can't tell if the voltages coming into the bridge look sane.

My thoughts were that the bridge or the capacitor have shorted and so the voltage drop from the collector to the -8v line is quite small (maybe zero). I know bridges failing with a short are a common failure mode for these scopes. The 800mV could be from the HV circuit as I can see on the scope that it is a 60Mhz sine wave with a positive offset.

It looks like the 5v regulator was replaced by destroying the package and soldering it to the component side (to avoid having to try and get the board out). Very tempting to do the same but I want to be a bit more certain before I leap.

Not quite sure what I am measuring as the caps have 3 legs (an earthed can I believe) but my DMM says they are about 0.1ohms or less. I thought that looked Ok.

[Paul Price]

when caps measure on a DMM .1 ohm they are resistors, no longer capacitors.

If you in fact have a shorted bridge rectifier(uncommon, but much more common than a shorted electrolytic) then be very careful to not leave the scope on very long or you will fry its power transformer.

You need to have access to the bridge rectifier/transformer connections and make some resistance and pcb trace checks.

[tombi]

It actually turned out to be an open bridge rectifier! I took the plunge and used side-cutters to destroy the thing and then a solder sucker to remove its legs from the holes. I then soldered in a new bridge (from the component side) and now -8v and -15v are working.

But still no HV 

Sigh...