Williams Robotron PSU help please

char1ie

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Hi all, looking for some help.

I just did a Bob Roberts rebuild on a Robotron power supply. The cap kit was fine - I re-fitted the board and it worked fine.

So far so good.

Then I did the three bridge rectifers and the big 18000uf cap at C10. When I powered it on it ran for a few seconds, made a hissing sound then the fuse at F4 (-12v) blew and the machine died. Which of my three BRs has failed and would anybody like to predict which component I may have installed the wrong way round?

There was lots of heat around the big metal BR.

Thanks all for your expertise.

Charlie
 
Thanks. And if BR1 is brand new and installed right it's unlikely to have failed?

Are BR2 and BR3 related to the -12v supply?

As far as I can tell the big 18000uf cap and the three bridge rectifiers are all installed right.

Any suggestions as to what I should check? Or should I just get a new rectifier for BR2 assuming that a new one from Bob Roberts has failed?

Thanks,


Charlie
 
I would check to make sure the 4700uF capacitor (the second biggest one) was installed correctly. That provides the stability to the -12V circuit and if installed backwards, it is fried. Fortunately it is not difficult to replace.

The other likely culprits are the SCR and the zener diode on the -12 V regulated over voltage circuit.

Lower down in probability, but still possible are the resistors that form the division lattice around the voltage regulator IC or the voltage regulator itself.

ken
 
Thanks Ken. 4700uf cap is installed correctly. Whats the SCR?

I'm showing my inexperience here: On the 18000uf capacitor I got from Bob Roberts it has the big stripe down one side which on other capacitors indicates the negative terminal. It has + markers alongside it so I assumed it was the positive terminal. Does a big black stripe along one side always indicate negative? If so, I have installed that round the wrong way.

Charlie
 
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OK, I think I may have answered my own question with a google images search:

THA%20SERIES%2060,40D.jpg

See the + symbols? They DO NOT mean that's the positive terminal do they? No. Duh!

I have installed my 18000uf cap round the wrong way. What have I blown? Can I install it the right way and expect it to work or will I need a new one?

I'll try it and see what happens.

Thanks all,


Charlie
 
Sorry, I'm tired. I was thinking of the +12V regulated circuit. The hissing sound was probably the 18,000uF cap frying. You can get replacements from www.greatplainselectronics.com . Ed is a great guy to work with.

It is not likely that the BR1 blew, although it is possible.

It is not a great picture, but you can see the correct way to mount the big cap in this picture:

attachment.php


ken
 

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Aaargh! Just had a good game. Around 800k and the machine reset, displayed the coloured carpet, told me it was operational and started up again.

I have done the caps and rectifiers on the power supply. What else should I do to stop this happening?

I'll get the voltmeter out and start checking the 5V supply. Am I right in thinking that a low 5V supply causes the reset?

Thanks,

Charlie
 
Aaargh! Just had a good game. Around 800k and the machine reset, displayed the coloured carpet, told me it was operational and started up again.

I have done the caps and rectifiers on the power supply. What else should I do to stop this happening?

I'll get the voltmeter out and start checking the 5V supply. Am I right in thinking that a low 5V supply causes the reset?

Thanks,

Charlie

Low 5V can cause resets... RAM farts can cause resets, loose ribbon cable can cause resets, bad connectors/cracked solder joints can cause resets, shooting into the corner can cause resets, thinking about resets can cause resets, a mouse farting in the next room over can cause resets...
 
The main power connection from the transformer to the board can build up resistance and heat up, burn, cause resets. I'd replace that connector and the header pins on the board and double up the ground on it. Solid power to the board itself is very important.
 
Aaargh! Just had a good game. Around 800k and the machine reset, displayed the coloured carpet, told me it was operational and started up again.

I have done the caps and rectifiers on the power supply. What else should I do to stop this happening?

I'll get the voltmeter out and start checking the 5V supply. Am I right in thinking that a low 5V supply causes the reset?

I had this problem, I did the following:

Reflow the connector pins on each of the boards and replace the IDC connectors on the harness (Bob Roberts has a Williams connector kit).

Alternately, I believe Dokert makes new Williams harnesses for a good price.
 
Quick update to say that I had a couple of Ram 16 errors so replaced the IC and the game seems to be fine now, running without resetting for some time.

Thanks all.


Charlie
 
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