Euro Robotron powersupply question *Fixed again*

[Vond]

I've been working on our Robotron the last few days and ran into some issues with the power supply.
I was wondering if the schematics i'm using aren't the wrong ones or if there's something really funky going on with my power supply.

I was using the robotron_upright_drawing_set_mar_82.pdf file from http://www.robotron-2084.co.uk/manualsrobotron.html and found some
differences in the schematics and my measurings (measuring at the transformer to be exact). Now i'm not sure the drawing I was using is
a US or a EU one and even if it makes in difference in that regard.

On the grey/yellow - grey/white cables coming from the transformer I am measuring a solid 26.6vAC while it should be 20vAC according to the
drawings I'm using.

P.s. I also blew the fuse F4 (responsible for the -12v on the power supply) while testing some of these voltages, might be related?




P.p.s. I've been working on it a few days now and already booked some results. Went from a dead on arrival machine to one trying to display something on the screen
and making sounds, sometimes the startup sound on its own and always with a little help from the sound test button, so all in all I'm really satisfied with my results so far considering im quite the newbie concerning electronics ghehe. There is still some life left in the old lady!!

[level42]

That's normal. You are measuring an unregulated AC voltage. I assume you measured it without any load on it ?

I you load it (connect the PCB) the voltage will probably drop a bit towards the 20 VAC.

[Vond]

I actually did measure it while in load and got the 26.6vAC on the transformer
I recently went full shotgun and installed all new caps on the power supply board and exchanged the 2N6057 and 2N3055 (both mounted on the heatsink)
so there should not be any problems there.

[Etienne MacGyver]

that value is not a big issue as long as the output from the powerboard after rectification and regulation has the correct value, and the input for the regulator is not above the range

many times the regulators can be fed with a range of voltages, for example a 7812 has a regulated output of 12 volts, and the imput range can be between 14 Vdc - 27 Vdc or something like that

[Vond]

Just did some research, agreed!
Also it could be that this is happening due to the Dutch electricity net changed from 220v to 230v since 1985?

Anyways, its back to the schematics for me to figure out why my power supply is giving me the finger on certain voltages.

Thank you very much for the help As always, appreciated!

I'll keep you updated as to what other ineffable arcane mysteries I might uncover dwelling deep inside the power supply

[Etienne MacGyver]

Just did some research, agreed!
Also it could be that this is happening due to the Dutch electricity net changed from 220v to 230v since 1985?

That could very well be the case, but dont forget that transformers have also some tollerance.

anyway, nothing wrong with that value IMHO

[Vond]

Alrighty then!

My problem was a (near) lack of voltage on the 12v power section of the powerboard.
After some measuring I verified that the issue was due to a bad IC, namely IC1 (Voltage regulator 723) and replaced it on the power supply board.

Succes!


Well, at least if there isn't any load on it. It measures solid voltages exactly as it should be on all the testpoints on the power supply board.
When loaded however I blow fuse F4 (-12v Unreg.) However, since this is only used for the soundboard as far as I can tell, we will try to boot the
Robotron again this evening with the soundboard disconnected *fingers crossed* 

[Vond]

HAH!

Apparently the fuse that was installed in place of F4 on the powerboard was a wrong fuse all along...
It should have been a 7Amp S.B. and the one installed (from the moment I got it) was a 4Amp.
I took the "original" out and replaced it with the same type without bothering to check the drawings.....lesson learned the hard way....
While I was searching for the cause...it turned out the fuse itself was wrong all along...

Replaced it with a fuse of the correct value and :

BAM!


A working Robotron2084 !!!




Still have a issue with the 5v failing sometimes (indicator led also turning off etc), Monitor giving a messed up signal (vibrating screen/colors shifted) and a broken microswitch in the right-hand joystick.
But I'm sure these issues will not cause too much trouble...

Feeling quite satisfied with the result I've got so far!
I'll keep this thread open as I might need some help with the 5volt issue...Gonna give it my best shot first though 

[Q*ris]

Aaaaand it´s alive!
Good job Vond!!

[vernimark]

Wow great job! It's a pleasure to know a robotron is working again!

[level42]

Wow, one of those rare "it's just a fuse" cases....well almost.
You're very lucky to find a Robotron and even more to have a working boardset

Congrats !!!!!

[Vond]

Haha, well thanx but it was short lived.
Having issues with the +5v now =(

Was working correctly, but dropped every now and then, and needed to cool down before becoming active again (the 5 volt that is)
now it seems it disappears completely when the PCB is connected (ergo, under strain)

So, still have work to do! (will remove the *fixed* from the forum thread, sigh...)
Feeling quite optimistic though, the old lady _was_ working absolutaly fantastic, good graphics, awesome gameplay!

[Vond]

Turned out this was a faulty rectifier bridge, just installed a new (beefier) one today and lo and behold.
The old lady wants to dance again

Now I only have to fix the pot-meters on the monitor and do a cap kit just to be sure and it should
be working 100% (now i'm getting some hold issues now and then)