Atari 1050 12VDC internal power supply

[Geoff78]

Hello everyone.

 

just wondering if someone could offer a bit of advice about the internal 12 volt dc side.

 

I've got this drive which has perfect stable 5 volt dc, power led illuminates etc. The 12 vdc is another story with only 0.70v at TP14 and 18.36v coming from the doubler. I replaced Q8, tested voltages again which were the same. I then replaced CR15 &CR16 and still no change.

 

Does anyone have any ideas? The only thing I've not changed is C71 and C68, the big electrolytics but I have ordered some more just in case one or both are bad.

 

Cheers.

 

[1050]

Only thing that makes any sense is that by happenstance

the replacement Q8 was bad to start with.

 

Try another?

 

These work but there is the boat time lag to consider.

swtiching 7805 7812 voltage regulator

http://atariage.com/forums/topic/268721-ac-output/?do=findComment&comment=3824750

[_The Doctor__]

I'd bank on the caps first, then double check you previous work.... This has been the case many times!

[Bryan]

Boy, that's weird because anything shorted would pull down that 18V point. Do you have continuity from TP14 to the OUT pin of the 7812? Does the regulator get hot?

[_The Doctor__]

I'm also worried and about c65 and c70 as well as the 7812.

Edited January 6, 2018 by _The Doctor__

[Geoff78]

Thanks for all the replies.

 

Just changed the two big electrolytics C68 and C71 and still the same voltages.

 

Q8 gets hot enough to fry an egg on.

 

There is continuity between tp14 and the output of Q8, however, there is also continuity between tp14 and the ground of Q8. Guess this indicates that Q8 has shorted, or my handy work is bad. This is the fourth 7812 I've put in and each time showing the same faults.

 

I've double checked my soldering with a lamp and magnifying glass and I cant see any bridged joints.

Could it be the 7812's I've bought?

 

Cheers.

[guus.assmann]

Hello,

 

Sounds more like a short somewhere "further down the line"

I'dd suggest to remove the output leg of Q8 and measure if the 12V is there.

I guess it will be.

Next, chech if there are any other (small) capacitors and their condition.

Also, the transistor for the drive motor could be defective.

 

BR/

Guus

[Geoff78]

Thanks Guus,

 

Bit more fault finding required then. I'll try removing the output leg on Q8 and go from there. It'd be a shame to give up on it now.

 

Cheers.

[Level42]

Yes check the 7812 unloaded.

 

But yes, there are known bad (fake) 78xx regulators around too, but testing it without load will show what's wrong quick enough....

[Geoff78]

As suggested, disconnected the output of Q8 from the rest of the circuit and sure enough - nice smooth 12 volts DC.

[1050]

So Q6 next target?

 

It's a NPN darlington and mounted to the heat sink as well.

 

https://www.onsemi.com/pub/Collateral/TIP110-D.PDF

 

Gives the part number on the schematic, but it would

help if I wasn't confusing it for TP 110 while wondering

if there really are 110 testing points on that board.

Homer moment, D'oh.

[Geoff78]

So far I have taken the following components out of the circuit :

 

C65

C70

Q4

Q6

 

Still no 12V at TP 14. Starting to worry now.

[1050]

Try unplugging the stepper motor, not sure if

backwards would short this supply but it just

might.

 

Later edit: Shorted C43 would do it especially

if Q6 is MIA.

Edited January 7, 2018 by 1050

[Geoff78]

To be honest, I've got everything unplugged. I have the board sat on my bench out of its case so I can work on it more easily.

 

The service manual suggests U5 and VR2 - troubleshoot and replace defective components. Has anyone ever had trouble with either of these two components?

 

I don't fancy my chances de-soldering U5. VR2 looks tricky enough

 

Cheers.

[_The Doctor__]

Your chances increase with good rosen flux on both sides of the pcb and fresh touches of solder then suck it out use braid for clean up or hot air to blow it out.. a few

[1050]

I can't recall even hearing about troubles with either

U5 or VR2, sorry.

 

Just a snafu check... you got a mica insulator on Q6?

And plastic mounting screw or otherwise total isolation

from the grounded heat sink? Because the heat sink tab

on Q6 is output pin just the same as pin 2 in the middle.

 

Heat sink is grounded by both 7805 and 7812 by their

middle pin 2 being direct connected to their heat sink

tabs too. So no mica insulator on Q6 is a dead short to

ground.

 

And for everybody, it's a common theme for this 220 case

style to have the heat sink tab connected to the middle

pin 2 just about every time. What isn't standard is what

that pin 2 is or does.

 

Sorry it took so long to find a motherboard I could gaze

upon. Without thinking about it, in 2 seconds I noticed

the mica on Q6 and went with it, double checking

datasheets. Should be some mica insulator sheets,

washers, and collars for bolt that come with decent

7812 package - you know just in case it's needed.

 

You can always blame it on the guy before you if it works

out that this is it. How could you know, it was gone

before you saw it for the first time.

 

Old standard question, any joy is probably going to

be met with at least some mumblings if my guess is

right, but in the long run - joy?

[Geoff78]

Hello everyone,

 

It's back to life at last. It was C43, the only electrolytic I didn't check as I thought it was part of the 5 volt side (which was working)!

 

Anyway, it reads fine and loads up all the games nicely so thank you all for the advice.

 

I just need a spare power brick now as I only have one. I guess they are hard to come by unless you buy one with a disk drive.

 

Cheers.

[1050]

Great news there. I recall a recent thread where one

was found on your side of the pond. Just match the

output at 9 Volts AC at 3.5 amps and it should work

fine for you.

 

I too had a lot of wrong thoughts on this one, for

several hours I was tracing output from the wrong

regulator.

My turn to get scared as I found U22, U23, and U24

all have direct connection to that 12 volts by the

schematic and any one of them could have been blown

doing it. Haven't confirmed power connection via

actual tracing yet though and probably won't now

that it's fixed.

[Geoff78]

Yes, I saw that too so disconnected R25 in an attempt to isolate U22-U24. I was quite relieved when it made no difference then started working back over, that's when I found C43 the little blighter.

 

I'll keep a look out for a spare adapter or one of a similar rating that I could put to use. I could always make one if I get bored.

 

Thanks again for all the help.

 

Geoff.

[_The Doctor__]

Cool and so it unfolds... yet another dead capacitor. Did I say to bank on the caps being bad? Nice to here you got it. It might have been overlooked but 1050's note might have reminded or even nagged at the brain a little bit to get you to go back there and check

 

Good job saving another 1050, that's my happy for the day!

Edited January 8, 2018 by _The Doctor__