Tektronix 2230 PSU cycles

[Floopy]

Hello, I'm a bit stumped and this is getting out of my very limited domain of knowledge. I tried to turn on my Tek 2230 and all it did was turn on, but turn off very shortly after and then it repeats until I pull the plug. Why would it do that?

I unplugged boards until something turn on, I noticed if I leave the GPIB board plugged in, it doesn't start. With it, it power cycles?

I installed a GPIB board in it about a month ago and it was working at first.

Anyone have a hint on where I should head?
Thank you.

[jklasdf]

Have you ever replaced the electrolytics? These scopes are getting quite old now. There is an SCR crowbar circuit for the switching power supply which will trip sometimes and show the behavior you're seeing where it's constantly power cycling after a while. Usually old/drying electrolytics are the cause. Be careful when probing the switching supply though, you really need an isolation transformer as the service manual recommends.

[Floopy]

Thank you, I do have an isolation transformer. I'll poke around, I already replaced the fan years ago so I'm not worried about taking it apart.

[bdunham7]

I unplugged boards until something turn on, I noticed if I leave the GPIB board plugged in, it doesn't start. With it, it power cycles?

I installed a GPIB board in it about a month ago and it was working at first.

Anyone have a hint on where I should head?
Thank you.

Are you saying that if you unplug the GPIB board that the scope powers up normally or that it won't come on at all, even power cycling?

If the former, then get a manual and look for the power supply voltage test points and values (there's a little chart in the Maintenance section that shows you the DC and ripple limits) and then measure each one. 

If the latter, look over all the wiring and connectors and see if you have disturbed something either now or when you installed the GPIB board.

[David Hess]

The switching controller has an input RC circuit which charges from the rectified DC input to start the switching controller and then power is supplied from an extra winding on the bucking inductor.

If power is never supplied from the bootstrap winding on the bucking inductor, then once the startup capacitor discharges the cycle will repeat.  This could happen if current limiting is activating or if the buck inductor is damaged.  I think it also happens if the output capacitor is worn out which is likely by this time.

[Floopy]

Thank you, I'll work with that, your knowledge is precious!

[Floopy]

Working on it....

The crowbar circuit seems to be switching on whenever I have boards plugged in. I unplugged part of the front panel board and it works even with the GPIB board in there, meaning that the GPIB board is fine. Probing around, the SCR on the crowbar seems to be turning on.

So, what capacitors should I replace? I'm wanting to do the bigger electrolytic that are in the power supply. Should I bother with the HV wax capacitors? Should I bother with the 75 uF big can?

[jklasdf]

Working on it....

The crowbar circuit seems to be switching on whenever I have boards plugged in. I unplugged part of the front panel board and it works even with the GPIB board in there, meaning that the GPIB board is fine. Probing around, the SCR on the crowbar seems to be turning on.

So, what capacitors should I replace? I'm wanting to do the bigger electrolytic that are in the power supply. Should I bother with the HV wax capacitors? Should I bother with the 75 uF big can?

The crowbar tripping is a common symptom of the electrolytics drying out on these scopes (although there could still be other causes). I wouldn't bother with the high voltage capacitors, just replace the electrolytics.

[Floopy]

Finally got some parts in, I've desoldered some capacitors and reading the values I can tell they aren't on the juicy side of the scale. So yes, they are getting replaced. Thank you for the suggestions!

[Floopy]

Well, got everything changed out, now it turns on once in a while. I guess that's progress. I finally got it nailed down to the current limiter keeps tripping. Now things went from the PSU to almost anywhere in the machine. I'm very tempted to do something temporary like take the fan speed down to prevent it from tripping, but I know it'll come back later if it's a component that's degrading.

[Floopy]

More probing, the -8.6 line is right on, but the +5 V line is sitting at 5.17 V. There is only a -8.6V regulator. taking it down seems to be solving problems. What could cause the +5V line to go high? According to the schematic if the -8.6 is regulated right than every other line should be fine.

[jklasdf]

What do you mean it turns on once in a while? Does it work fine when it does turn on? What happens when it doesn't turn on? Also what current limiter are you talking about tripping? One thing you can do to help narrow things down is to bypass the preregulator circuit, and power the inverter by supplying 43V directly.

Page 5-5 in the service manual has the allowable range for supply voltages, and 5.17V is within limits for the +5.0V supply. There's a fairly large range allowable (all of the outputs of T498 are regulated as a group).