Oh, and to those who are curious. It's defintely the transformer that's getting hot. The switcher chip is warm, but not hot. That transformer seems to heat up every component on the board.

 

My oscilloscope is about 44 years old. I made it as from kit.
It has an intermittent problem with its CRT where a weld is loose.
If I turn it on when it is cool then its works perfectly. But if I turn it off for a few minutes then turn it on again it is dim and very blurry and the intensity and focus controls do nothing. I hit the front of the CRT with a piece of wood to fix it.

 

Try adding a little 40mm fan to it. They run from a few mA and will extract all that heat out.

The reason the transformer runs hot is just as you would expect, to save money they use a smaller lower spec transformer and push it real close ot its limits (where it runs less efficiently and makes lots of heat). The transformer ferrite core and windings are 2 of the most expensive parts in the whole device.

 

Try adding a little 40mm fan to it. They run from a few mA and will extract all that heat out.

Know any that can run from 3.3V? (that's the voltage across the caps.)

The reason the transformer runs hot is just as you would expect, to save money they use a smaller lower spec transformer and push it real close ot its limits (where it runs less efficiently and makes lots of heat). The transformer ferrite core and windings are 2 of the most expensive parts in the whole device.

To be honest, I don't even know why they are using a flyback switching isolated power supply. Ethernet is already magnetically isolated to a few kV so there are no safety issues there. They could have got away with using a non-isolated supply such as a ViPer12A in a buck converter configuration or even a capacitive power supply. Why overcomplicate it with a transformer and optoisolation? (N.B. it is generally not recommended to deal with nonisolated supplies on your own, but it is safe to do them if they are in a sealed case, which they were, and with no nonisolated external interface (e.g. an audio output.))

 

I hit the front of the CRT with a piece of wood to fix it.

Can I borrow your bit of wood?

 

Know any that can run from 3.3V? (that's the voltage across the caps.)

I've gotten a computer power supply fan to run at 3.3V, you may have to short an internal cap to get it to work with non-pulsed DC though. Or you might be able to add a 555 to pulse it, or do some fancy things with switching caps in series/parallel, I don't know a whole lot about the area, but it sounds do-able.

Ethernet is already magnetically isolated to a few kV so there are no safety issues there.

Ethernet uses kV!?!
is this like the cat 5 Ethernet cables you're talking about??

 

I've gotten a computer power supply fan to run at 3.3V, you may have to short an internal cap to get it to work with non-pulsed DC though. Or you might be able to add a 555 to pulse it, or do some fancy things with switching caps in series/parallel, I don't know a whole lot about the area, but it sounds do-able.

Yeah, but it's not likely to start at low voltages, especially at high temperatures.

Ethernet uses kV!?!
is this like the cat 5 Ethernet cables you're talking about??

No. But on systems with various grounds, isolation is critical, as systems may float at various voltages. Ethernet is actually only a few volts IIRC. The little plastic brick you see next to the jack, that's the isolation transformer (two of them in one device.) You'll often see them in Ethernet adapters unless the jack has built in magnetics (MagJack), which is getting more common. Similar isolation is applied for the telephone network.

 

Yeah, but it's not likely to start at low voltages, especially at high temperatures.

Hm... well then, good luck with that

No. But on systems with various grounds, isolation is critical, as systems may float at various voltages. Ethernet is actually only a few volts IIRC. The little plastic brick you see next to the jack, that's the isolation transformer (two of them in one device.) You'll often see them in Ethernet adapters unless the jack has built in magnetics (MagJack), which is getting more common. Similar isolation is applied for the telephone network.

Ok, I would have been worried if the Ethernet cords carried that much, as for everything else... I'll take your word for it.

 

You can get the little 40mm fans in 5v DC versions, I'm guessing they would start and run from 3.3v.

Of course you could also modify the enclosure to greatly increase passive (convection) air cooling, just be sure to make it VERY SAFE as most of those PSU parts are at lethal mains voltages!!!!

 

You can get the little 40mm fans in 5v DC versions, I'm guessing they would start and run from 3.3v.

Of course you could also modify the enclosure to greatly increase passive (convection) air cooling, just be sure to make it VERY SAFE as most of those PSU parts are at lethal mains voltages!!!!

Honestly, I think I'll just replace parts as it breaks. There is absolutely no space left in the case for any size of fan.

 

Honestly, I think I'll just replace parts as it breaks. There is absolutely no space left in the case for any size of fan.

Maybe move it to a bigger case and you won´t need a fan. Depends if the original was made to conduct the heat away like in laptop adaptor, then it could actually get worse.

 

Maybe move it to a bigger case and you won´t need a fan. Depends if the original was made to conduct the heat away like in laptop adaptor, then it could actually get worse.

How am I going to make a custom injection moulded case with the slots for the connectors and the slot for the plug connector? The original case has holes cut in it for ventilation, but obviously they aren't working.

 

I didn´t mean custom injection moulded, just any bigger box from anything else that it could fit into

 

I didn´t mean custom injection moulded, just any bigger box from anything else that it could fit into

Yeah, would be nice, but it's really a custom job. Like the bottom of it has an interchangeable power connector that connects to the figure-of-8 on the board. It's not really something I can just find lying around, and I'd need two of them (one for upstairs, one for downstairs.)

 

I used my *new* IR thermometer to measure the temp of various components. The case is about 50°C. Transformer his 75°C. One heatsinked IC is running at 70°C. Why so hot???

 

Because there is no fan.

If you drill some holes on the top of the little plastic case and put a fan on top, to extract hot air out, it will run MUCH cooler.

 

Why so hot???

harmonics!

If you drill some holes on the top of the little plastic case and put a fan on top, to extract hot air out, it will run MUCH cooler.

good point : )

 

Sucking air out with a fan will not work unless there are more holes for fresh cool air to enter.

 

Sucking air out with a fan will not work unless there are more holes for fresh cool air to enter.

Good point - a fan is not a magical cooling device. It will only work if there is some way for the air to flow. Airflow pulls heat from devices, but it is so cramped inside there, I doubt it would have much of an effect. It could have the reverse effect, it might pull more current from the 3.3V power supply, leading to more heat being produced that it would otherwise remove.

It's doing well so far, but we shall see how long it lasts...

 

I have a modem on my BBS that is buried under stacks of floppies, so I placed a heat sink on top of the case to help suck the heat out, then added a people fan to have air flow over it (and cool the person sitting at that workstation). Works like a charm.