Tron PSU - +5 problem

[guddler]

Ok, this is peeing me off now!

My Tron's +5v has always been very low but contrary to what everyone seems to think, MCR boards really aren't fussy about voltage at all, they'll run right down to about 4.2v or so. Consequently, I've never got round to doing anything about it in all the time I've had it.

Powered the machine on today and put the PCB back in (it had been out for months) and squat from the board. Voltage is now down to 3.7v so clearly it's now become too low.

I'm not too hot on PSU theory of operation so am looking for some suggestions.

Without a load the 5v is up at over 5.5v. As soon as you plug stuff in it's dropping to 3.7v under load. I've pulled and tested all the transistors that I thought had anything to do with increasing the load that the 5v could sink and they're fine. That therefore also covers dry joints. Done Q103/4/5, have also replaced the LM305 but it made no difference so I'm assuming the old one is still good and put it in my parts stock.

Have also tested the resistors in the 5v circuit and they seem ok, and none of the caps that sit between +5 and GND are reading a short.

I originally thought that the adjustment pot was faulty as it doesn't read right. I don't have a suitable 100 Ohm pot so I put a 1K pot in and just marked the range with a sharpie that gave between 100 Ohm and 0 Ohm. Figured that would at least prove a theory. Didn't seem to make things any better. If anything it made them worse.

SO, any ideas?? I guess I could use a suitable 100 Ohm pot.

No doing any more on it now as Xmas starts here but I'll take another look at some point over Xmas.

Circuit is given below (hopefully)

 

[PaulSwan]

Did you check that the voltage drop is not anywhere in between the output of the power supply PCB (on the PCB) and on the game PCB itself?
If you have 3.7V right on the power supply PCB itself then you should check the AC side. I'd check to see if the AC in has dropped out because of a bad connector, bad fuse (or fuse holder) or bad bridge rectifier.

Paul.

 

[PaulSwan]

...and that R110 is not blown, though it being all black is usually the give-away for that You should test for that by looking for the voltage drop across it rather than continuity in case it opens up under load - the general theme is to check the voltages under load because everything may look fine with no load. If you don't want to risk your Tron PCB you can shoe horn in a dead JAMMA bootleg as a dummy load

Paul.

 

[guddler]

I like your style!!

Erm, right - TBH, haven't done too much with live measurements. Simply for the practical reason that this is a Tron cocktail and it's nigh on impossible to get to anything in it with anything like the required degree of accuracy for prodding about with it switched on.

I've got a loom for working on the PCBs, but I'm only set up for Atari stuff when it comes to bench work on power boards.

So anyway, there is some voltage drop between the power board and the game boards, about 0.5v but I'd probably expect as much really on something like this. The 3.7v is taken from a logic chip on the top layer of the game PCB. It's about 4.1 - 4.2v at the output connector of the power board.

R110 is measuring the correct Ohms, but I've only done so without power, on the bench.

It's basically behaving like the 5v is coming straight from the LM305 and Q103/4/5 are being ignored, which I assume are there to up the capacity of the 5v to the given 7.5A, so the theory of R110 sounds good. It's not cooking hot, just warm, which I guess for a 15W beast is normal.

Maybe after tomorrow I'll see if I can make up an extension cable for the AC input so I can work on it live...

 

[P-Man]

Is VR102 cranked? do you actually get any voltage movement at all when you adjust VR102? I would check the values of the pot/resistors connected to VR102 as well after doing what paul says above..

 

[guddler]

I'll certainly double check but yeah, seem to remember I was getting voltage movement. And it's certainly turned right up.

 

[guddler]

Right, been tackling this one a bit today as the cab needs to go back where it lives now the Xmas tree is gone. Didn't really want it to go back non-working as it'll still be non working next Xmas if I do that. Plus I'd like to rack up some high scores...

SO, VR102 is definitely duff. Until I can find a replacement I've swapped it with the one from the 12v circuit as that voltage is only for sound so not so critical.

I then had a small amount of adjustment on the 5v line but the voltage was still very low.

I went right back to the transformer and checked the 8-0-8 VAC output. It was 6-0-6VAC so that pretty much explains that methinks. At this point I'm keeping everything crossed that it's the piss poor monitor isolation transformer that the cab is being run off of at fault. I didn't fit that I might add, the cab was originally imported by a certain company in Castleford so I would assume it was them. Regardless, whoever did it was sh*t at soldering and didn't know which colour is which in the mains cable!

Tested the iso xformer and it's only kicking out 100VAC. SO, there's a second item for the shopping list. A decent US->UK stepdown from someone like Airlink. Temporarily, for fault finding I decided to see how many volts the iso was kicking out from the 128v lug (it was on the 120v lug) and found that it's just about managing 108v on that one. That'll do just for the moment.

Now I'm getting 8-0-8v under load. Probably still a little low but it should suffice for now. I'm still only getting 4.12v at the game board though. And now it looks like I've lost my adjustment too.

Not given up quite yet though for today. I've pulled the power board back out again and am just checking that again.

One note - the fact I had adjustment before and now haven't, could be down to my meter(s). I've been battling with them all day. I couldn't find my usual one so was using my auto ranging one that I don't desperately like but the battery light was on so had to stop using that. Next, I tried my AVO. Seemed ok for resistance and such but AC volts were all over the place. It reckoned my line volts were about 180v and the iso was kicking out 60v or so which clearly wasn't the case, so maybe the AVO needs looking at too. Finally found my main meter so possibly the reason for the differing readings.

 

[guddler]

Ok, am getting adjustment on the wiper of the pot (and pin 6 - feedback - of the LM305) between 1.20 - 1.48v, but no variation in the 5v output.

Giving up for today now, had far too much of battery rot for one day.

 

[P-Man]

If you have low output from your transformers, have you thought to look on the input? what is your input line voltage? what is the voltage actually reaching your transformers?

 

[guddler]

I did indeed. Have covered all that above. Line voltage into the cab is fine, voltage out of the 'step down' (AKA monitor iso) wasn't. It's ok now as is the output as I moved to the other lug but still not really what it should be so I need to order a proper decent step down from somewhere link Airlink.

I think really I'm battling battery rot on the power board. Bloody thing!

 

[Judder]

Excuse the joining of the Tron PSU discussions thread but I (foolishly?!) bought a Tron upright to go with our Defender project and the previous owner state it was a Canadian import, so I'm guessing runs 115V natively.

Can't seem to find any discussions on modifications to the MCR PSU to convert US to UK 240V so wonder if anyone has any tips?

There's a good PDF on measuring the voltages once I can get power to it I found here - http://users.rcn.com/jenison/mars/trouble/part/powersupply/MCR/mcrPowerSupply.pdf

Many thanks again

 

[DanP]

The power brick for MCR games is native to each country it's supposed to run in (MT89 or MT90 iirc). If you can't find a UK spec one then you have no choice but to run it with a step down transformer. The PSU PCB takes it's input voltage from the brick so it will run just fine once you sort that bit out.

HTH

Dan

 

[Judder]

Hi Dan

Many thanks for the reply!

OK well that kind of makes sense (I've been rebuilding the Defender transformer for 240V so was expecting everything else to be similar ) and if anyone has a UK power brick spare for sale please drop me a line.

Otherwise it looks like I'm going to need a Switching Power Supply Adaptor like this one...

http://www.arcadeshop.com/mcr/mcr.htm

 

[P-Man]

Well you will need the stepdown transformer regardless, the switching psu adapter is your own choice if you dont want to run the pcb with the original psu. You still need to run your monitor/lights etc whcih will all be 110v, and you'll start to really hack things if you try an run all those ontheir own or change them.

Its safer/easier/cheaper to buy a suitable stepdown to run the cabinet from. Mount it inside the cabinet if you want then its seemless.

 

[Judder]

Yep - very good point about the lights etc. (thanks Andy) and a step down transformer therefore definitely seems the way to go.

Any idea of the Watts one of these cabs will require - step down transformers seem to come in 50W, 100W, 200W and 300W capacities and rather not plug say a 50W in and watch it melt!

Many thanks

 

[P-Man]

not to mention the monitor etc... just dont go there...

Get a 300W stepdown and you will be fine. Original spec is about 200W, 300 for some breathing room. Check your lighting ballast transformers, look at the markings, because if any of them just say 115v 60hz then you will need to change them for 50/60hz ballasts or the 60hz ballast will catch fire eventually.

 

[Judder]

Will do - thanks again

 

[bbz149]

I have the correct size 100 Ohm VR that I need to swap out
with the smaller pot I had to make do with at the time. I ordered a selection
of pots for my Tron and other monitors and will check if I have a spare for you m8

 

[Judder]

I have the correct size 100 Ohm VR that I need to swap out
with the smaller pot I had to make do with at the time. I ordered a selection
of pots for my Tron and other monitors and will check if I have a spare for you m8

That'd be great - many thanks!

 

[Judder]

Get a 300W stepdown and you will be fine. Original spec is about 200W, 300 for some breathing room.

On offer at Maplin for £19.99 in case anyone needs one - 300W step down.

http://www.maplin.co.uk/300w-uk-to-usa-voltage-convertor-265288

Now just to wire a US plug on to the mains lead where the shippers smashed the original one off...