Tek TDS 400 PSU ouputs high

[james_s]

I'm working on a power supply from a TDS 410, it was completely dead when received. I replaced some capacitors and then it would blip briefly when power was first applied. After finding some tips online, I disabled the OVP and now it comes up but all outputs are significantly high, 5V is around 7.5V, 15V is close to 20V, something is clearly not right. I replaced the rest of the electrolytic caps, and while I had the vertical daughterboard removed I replaced the op-amp and comparator ICs on that as removing the board to replace them later would be a pain. Unfortunately the symptom is still exactly the same. Anyone encounter this before? From what I can tell, the regulation is accomplished via a separate secondary winding on the transformer that feeds back to the hybrid controller, it appears that the circuitry on the outputs is only for over-voltage and over-current protection.

I also have a very similar power supply for a TDS 460 that suffered a flashover that seems to have damaged the hybrid. I'm working on a replacement hybrid for these, found a reverse engineered schematic but it doesn't quite match what I can see in an xray I took of the module so I'm probably going to have to grind off the coating to get a better look.

[Armadillo]

In the tek forum, some found the problems to be the optocoupler and the LM339 for overcurrent shutdown.
" the opto photo-transistor was damaged and would not allow the CE voltage to go higher than about 2.0v."

[james_s]

I don't think that's the problem, I can disable the OVP by bridging across the LED in the optocoupler and that prevents it from shutting down but the outputs really are too high, the OVP is working correctly when it shuts it down.

[Armadillo]

I don't think that's the problem, I can disable the OVP by bridging across the LED in the optocoupler and that prevents it from shutting down but the outputs really are too high, the OVP is working correctly when it shuts it down.

In operation, I don't think the LED is either ON or OFF but at operational biased intensity. So at this operational intensity does it trigger a shutdown, - implying;

You could try remove the bridge and measure the LM339 to determine which one of the opamp output is low that drive the LED [overcurrent, overvoltage or temperature?], OR actually the optocoupler transistor is half-dead that even at operational intensity, it triggers the shutdown.

[james_s]

The outputs really are high though, this is not a malfunction of the OVP circuit, the OVP circuit is working correctly and shutting down the PSU. The problem is that the output voltages are too high, it seems to be running flat out and not regulating. The opto is for shutdown only, it is not part of the regulation loop.

[Armadillo]

Did you missed checking the Ref2 / E504 voltage? That is around the VR501. It should be 2.5v
Also VR501 is 5.1v. It is the VCC voltage.

[Edit: I have read the whole power supply circuit including the hybrid module  . highly likely culprit Ref2 voltage, all the LM324 takes reference from it, next but uncommon, the hybrid PWM oscillator caps].   

[james_s]

I hadn't but I checked the ref2 just now and it's exactly 2.5V.

I think I may be onto something though. Most of the outputs are high, +15V is almost 20V but -15V is 14.6V, definitely have a few things to check.

[Armadillo]

1, Those overvoltage rails, its associated shunt transistors should have been fired "ON" by now, trying to pull the voltages down. Associated transistor or resistor of the shunt could be open thus voltage remain high "OR"

2. Those rails with voltage dipped like the -15v rail may be overloaded, causing the PWM pulse width to increase the other voltages beyond shunt transistor regulation. You could unplug all dipped rails from circuit to confirm 5v and 15v power supply returns to normal.

above are just suggestion.

[james_s]

So it turns out the pot "V4" that adjusts the -15V works and I can adjust that voltage within spec. The other 3 pots have little or no effect on their respective outputs though. This is one of the strangest power supplies I've ever worked on, I haven't quite worked out how the individual regulators even work. The similar schematic in the LeCroy service manual has got to have errors too, some of the output filter capacitors are drawn backwards.

[james_s]

So I think the fact that the adjustment of one of the outputs works, and the fact that it appears the reference is different for one channel with 3 fed off another may be a clue. I'm sure the fault will turn out to be something really simple, it's difficult to trace the circuit the way the schematic is organized and the reference designations on the daughterboard are directly under most of the parts so I can't easily tell which is which.

Can anyone explain how these regulators work? The circuit is not all that complex but I'm just not grasping it for some reason 

How is a little MPSA06 regulating the outputs? It appears that the regulation of all four outputs is done independently on the secondary side.