Tek 475 -8V supply

[Carl E]

Hi,
I have a Tek 475 that was salvaged from the boneyard that I'm trying to save. The voltages are all present except the -8V. The other voltages are OK, except the -15 is at -19 which I will fix later.. The -8v is .04v. Ive changed the op amp and all the transistors in that circuit but no -8v. The supply to Q1468 (MJE2801) is 13.6v. I don't see any ripple on the 13.6v which (I assume) means the filter cap after the bridge is OK.  Schematic is on pg 172 of the service manual.  Anyone have any suggestions?
Thanks  Carl

[David Hess]

Hi,
I have a Tek 475 that was salvaged from the boneyard that I'm trying to save. The voltages are all present except the -8V. The other voltages are OK, except the -15 is at -19 which I will fix later.

What -15?

The -8v is .04v. Ive changed the op amp and all the transistors in that circuit but no -8v. The supply to Q1468 (MJE2801) is 13.6v. I don't see any ripple on the 13.6v which (I assume) means the filter cap after the bridge is OK.

I would start with the assumption that Q1468 is open and it was caused by a shorted solid tantalum capacitor somewhere on the -8 volt line.

[Carl E]

Thanks for your reply David,
I replaced Q1468 with 2sd1707. No change. I noticed the op amp gets hot. The -15 is associated with Q1478. i've attached the schematic with the voltages measured.
thanks

[bd139]

Turn it off and measure resistance from -8v to ground. Probably a shorted tant.

[med6753]

As bd stated probably a shorted tantalum capacitor. The -8V should measure no lower than 32 ohms to ground. If it does you'll have to start isolating the boards to determine which one has the short. The 465 manual gives an excellent detail on how to isolate the boards but unfortunately the 475 manual does not but is somewhat similar in configuration to the 465.

[MathWizard]

Looking at the way tektronix draws bridge rectifiers, reminds me of a few years when even that threw me off.

I have a beautiful old Tektronix scope that I turned on once, before I had a variac or even light bulb limiter, and I think something fried, it lost the trace.

For a long time I've been looking forward to taking it all apart, like a kit, test every part, then rebuild it.

Hopefully this winter when i have a lot of free time, I will.

[Carl E]

Thanks to all who responded. The -8 measures .4 Ohms to ground. I'll download the 465 manual and find the short and report back.
Thanks
Carl

[David Hess]

I replaced Q1468 with 2sd1707. No change. I noticed the op amp gets hot. The -15 is associated with Q1478.

I would have tested the transistor after pulling it.

i've attached the schematic with the voltages measured.

These power supplies use the +50 volt rail as the reference for the other outputs which then track the +50 volt output, so the ratio of R1464 and R1465 sets the output voltage.

The voltage at the base of Q1468 results in 0.73 volts at the base of Q1464 so the current limiting is turned on hard indicating a fault.

If I am reading it right, the output current is 333 milliamps and the load is 0.15 ohms so that definitely looks like a short on the -8 volt supply somewhere, likely from a shorted epoxy dipped tantalum capacitor which is hardly unusual.  C1468 is suspect but there are others to check as well.  I might just replace all of them on the -8 volt rail.

Also make sure CR1468 is not shorted.

[bd139]

If you have a decent enough DMM i.e. very low ohms range then you can usually work out which tant is short but I agree with David Hess here. You fix one and another one will go a few days later. Might as well do the lot! I've done that on a couple of 4xx scopes.

Bastards all of 'em.

[David Hess]

If you have a decent enough DMM i.e. very low ohms range then you can usually work out which tant is short but I agree with David Hess here. You fix one and another one will go a few days later. Might as well do the lot! I've done that on a couple of 4xx scopes.

The bad capacitor is probably hot so it can be easy to find if you have some way to measure temperatures.

Just to be clear, I do not suggest replacing all of the epoxy dipped solid tantalum capacitors.  But I might replace all of the capacitors on the -8 volt rail if those capacitors all have the same voltage rating.

[bd139]

Tried that before. They don’t get hot enough to be able to find usually. Either that or they catch fire suddenly.

Last time I did one I used my 8012B which had low ohms mode and poked all of them until I found the one with the lowest resistance. That was then snipped out (leaving some legs left to desolder properly) and the unit powered up. Came straight up!

Bob Pease described a nice little “short finder” in Troubleshooting Analogue Circuits for this kind of thing too.

[Carl E]

Hi, I found 1 shorted tant and the -8 to ground measured 32 ohms. I thought I got lucky so I replaced all 3 transistors and the op amp again. Still no -8. I'm going to replace all the tant on the -8 supply and maybe this time I'll get lucky. Is it safe to disconnect any board and power up to see which board may have the problem?

[bd139]

Yes you can do that safely.

[Carl E]

Well, I still have a few hairs to pull out but not many.  I still measure 32 Ohms from -8 to ground. I isolated 2 boards and the short still exists. The voltage jumped up to .7. I'm ready to give up, too bad because I see a trace right after it is power cycled so the tube and high + is good but the trace disappears without the -8v almost instantly..  Ughhhhh.