General Stuff(tm)

[stephen_usher]

Putting a load on the power supply and looking at it using a scope showed that the output smoothing cap isn't doing its job any more and there's massive ripple.

Any chance you might be able to replace just that capacitor if the potting compound plays ball?

Though my temptation would be a full re-cap.

Unfortunately the PCB is the “right way up” so the PCB is on the bottom. All the potting compound will need to be removed or liquified to get it out of the case and access the components.

[rubber_jonnie]

Any chance you might be able to replace just that capacitor if the potting compound plays ball?

Though my temptation would be a full re-cap.

Unfortunately the PCB is the “right way up” so the PCB is on the bottom. All the potting compound will need to be removed or liquified to get it out of the case and access the components.

I do so hate potting compound

[stephen_usher]

I think what I’m going to do is get a disposable baking tray and put the unit on stands so that the putty can run down into the tray and then bake it in the oven at about 100C and see if gravity can be my friend.

I can’t do that until at least tomorrow though.

[PhilC]

I'll be looking on with interest as I've got a couple of PSUs with similar. The C64 ones might be different though as it looks like a resin.

[stephen_usher]

Well, using the oven to try to melt out the potting compound failed dismally.

Although at 120C the potting compound started to flow *VERY* slowly the ABS plastic deformed far more readily. Oh well. It was dead anyway.

[rubber_jonnie]

Well, using the oven to try to melt out the potting compound failed dismally.

Although at 120C the potting compound started to flow *VERY* slowly the ABS plastic deformed far more readily. Oh well. It was dead anyway.

Oh that's rubbish

[PhilC]

Shame about the case.

Wondering if there's some highly toxic chemical way to remove it?

[rubber_jonnie]

Shame about the case.

Wondering if there's some highly toxic chemical way to remove it?

I expect Acetone would have a good go, though it might do for a bit more than the potting compound...

[stephen_usher]

Shame about the case.

Wondering if there's some highly toxic chemical way to remove it?

IPA seems to be able to remove it from the tools I used to try to pry the gooey stuff out yesterday, so maybe put it into a bucket of IPA for a week or so?

Acetone will just dissolve the ABS plastic immediately and will damage the plastics in the transformer.

[stephen_usher]

Well, well, well...

On Saturday I had to up the voltage on the replacement power supply on the Enterprise to keep it stable. And today I had to do the same with the new power supply as otherwise the 14MHz clock didn't work properly.

Investigation today shows that the buck booster circuit on the machine, which generates the higher voltage which is then regulated down to 12V, isn't working. There's no oscillation. So, either TR2 or TR3 died before I plugged things in on Saturday or the very ripply 12V from the original PSU killed one or both or the 17V I had to use killed one or both. I just don't know. Well, I'm guessing that it's the transistors which have gone. I'm going to have to desolder them and test.

Increasing the input voltage to the machine to 15V (still well within the 7805 and 7812 regulators' range) allows the machine to run normally.