PETulant PET Power

[crock]

Hiya all,

I have a recently restored an 8032 that has been giving me sporadic problems with blowing fuses. When I first got it, it was completely dead. After cleaning it all up, replacing one of the rectifiers, some RAM, a burnt out trace and removing the line filter, it's now working well. Most of the time...

About once every 10 power-ons, it blows the fuse. Only on power-up though, never once its running. When running normally I can measure that it's drawing around 0.4 amps at 240 volts, around 60 watts, which is well under the rated fuse value of 0.8 amps. For most of the problems I can think of I would expect it to always blow (shorts on the board, dried out electrolytic caps, failed transformer) but once this one is running, it's rock solid. I also know it's not the monitor, as it was doing the same when the monitor was disconnected. I am a bit stumped as to what could be causing the issue.

Any suggestions chaps?

Rob

Zurich, Switzerland

 

[KevinO]

Hiya all,

I have a recently restored an 8032 that has been giving me sporadic problems with blowing fuses. When I first got it, it was completely dead. After cleaning it all up, replacing one of the rectifiers, some RAM, a burnt out trace and removing the line filter, it's now working well. Most of the time...

About once every 10 power-ons, it blows the fuse. Only on power-up though, never once its running. When running normally I can measure that it's drawing around 0.4 amps at 240 volts, around 60 watts, which is well under the rated fuse value of 0.8 amps. For most of the problems I can think of I would expect it to always blow (shorts on the board, dried out electrolytic caps, failed transformer) but once this one is running, it's rock solid. I also know it's not the monitor, as it was doing the same when the monitor was disconnected. I am a bit stumped as to what could be causing the issue.

Any suggestions chaps?

Rob

Zurich, Switzerland

Rob, I know you're usually on top of these things, so I don't know if I really have anything of value to add, but do you know for sure it has the right fuse in it? I have never checked my PETs to see what fuse is in them (it would be different since I'm in the US anyway), but should it maybe be a slo-blow instead of fast acting? Could it be something like that?

Assuming that's not it, then I would still suspect the big electrolytic of being leaky. Do you have a cap checker that tests leakage?

 

[KC9UDX]

What you can't (easily) measure is the inrush current at power-on, which is when your fuse blows. Like KevinO says, I would recommend a slow-blow fuse, if you aren't using one already.

If that doesn't solve it, I would probably put in a 1A fuse. Not that I actually recommend that, though.

 

[crock]

Hey Kevin,

It's got me stumped, so I'm open to any suggestions.

... but do you know for sure it has the right fuse in it? I have never checked my PETs to see what fuse is in them (it would be different since I'm in the US anyway), but should it maybe be a slo-blow instead of fast acting? Could it be something like that?

The normal rated fuse is 250v, 0.8A and the ones I'm currently using are rated at 1.2A slow blow.

Assuming that's not it, then I would still suspect the big electrolytic of being leaky. Do you have a cap checker that tests leakage?

The big cap checks out OK with an ESR meter - 27000uf. It would usually tell me if it's leaky. I guess my next plan will be to recap the whole board. :-(

Rob

 

[MikeS]

Does it ever blow fuses with the main board disconnected?
I assume that the line filter is in the circuit before the fuse?
Can you borrow a main filter cap from another system? Sounds like something intermittent that your meter could miss.
Not really many caps on the main board that are likely to blow the main fuse, but can't hurt I suppose...

 

[crock]

Good evening Mike,

Does it ever blow fuses with the main board disconnected?

No, not that I've noticed but admittedly I haven't tried more than once or twice.

I assume that the line filter is in the circuit before the fuse?

It was, but that was one of my initial suspects so it's now removed. I never trust them and I routinely remove them if I ever need to get inside the power supply on a PET or CBM-II.

Can you borrow a main filter cap from another system? Sounds like something intermittent that your meter could miss.

Its possible I suppose. I will take one from another PET case.

Not really many caps on the main board that are likely to blow the main fuse, but can't hurt I suppose...

Right, but assuming the big cap is OK, it must be something on the main board.

cheers, Rob

 

[3pcedev]

I would start by replacing the PSU capacitor(s). It is also possible you have a shorted winding in a (large) inductor somewhere in the power supply. A transient fault like you have described is caused by a reactive part of the circuit (inductor/capacitor). My money is on capacitor as once it is charged and the system is running you have no problem.

My money is on something like the line filter capacitor in the PSU - it could definitely cause this fault.

 

[MikeS]

I would start by replacing the PSU capacitor(s). It is also possible you have a shorted winding in a (large) inductor somewhere in the power supply. A transient fault like you have described is caused by a reactive part of the circuit (inductor/capacitor). My money is on capacitor as once it is charged and the system is running you have no problem.

My money is on something like the line filter capacitor in the PSU - it could definitely cause this fault.

The older PETs used a pretty basic brute force linear supply; only one small inductor in the unregulated 9V line AFAIK, no nasty tantalums and only 9 electrolytics: one huge one for the 5V supply, a couple of smaller ones for +12V and -5V, and half a dozen on the 5V regulators and the 9V line. Lots of small bypass caps at the chips but they don't usually cause any problems, and if they did they'd probably blow before the fuse.

As to the line filter, it's usually before the fuse and in any case Rob's already removed it.

So that leaves an intermittent short in the transformer, a bad filter cap, or something on the main board; could be hard to pin down considering how intermittent it is. I think Rob's got one or two other PETs, so I'd start by swapping the off-board parts, i.e. the transformer and the main cap, and see what that tells us (if anything ;-) ). Might also make sure no wires are touching anything they shouldn't underneath the transformer, nor anything shorting to the case anywhere else.

 

[crock]

Solved.

In the end it was the transformer itself. The windings don't appear to be shorted, as the voltages seem about right, but it is very noisy when running. I don't really know much about typical failure modes of a transformer, so I guess it goes into the scrap iron bin. Has anyone found a compatible replacement for the PET transformers?

Rob

 

[KC9UDX]

Im not surprised the transformer is the problem, nor that it is noisy, but somewhat surprised the two are related.

I wish I jad schematics haandy to be sure, but I reckon Hammond makes something suitable.

An *analog* ammeter would be very helpful in being certain the transformer is at fault, if you haveen't done that already. I'd hate to see you spend a small fortune on aa traansforrmer only to find out it isn't really the problem.