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Mirco 'Challenge' 4 Player Pong - PCB Repair Log #2 - Dave's Board


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Cz0fVM1.jpg

 

I had been trying to get my PCB to a fully working state so I could take references to compare with this board.

After a few hours I thought I'd take a break to see if I could at least get his board to a state where it's putting out some video on screen. After getting him to check the voltage on his cab he found that there was no 5v volt supply. Hmm... not good.

The first thing was to go over the tracks and check for any damage, I repaired some obvious broken tracks that were caused by the board being scratched.

The board had been messed with by someone (around 1992 by the date code on the 555's that were replaced) before he got it so I thought it prudent to recheck their work as the quality of the repairs was a little average. The first cap I checked at 3H had the wrong value, 100uF compared to my board which still had the original caps in place which was 5uF.

I found another at 9F which was 220uF and on my board it was 35uF then next to the P2 plug there was a 220uF instead of 33uF.

This cap was connected in reverse across the +5v and Ground where the supply enters the board and had let the magic smoke out, fortunately it was the newer vented type and didn't go into nuclear meltdown.

How do you miss that big arrow with a - pointing to the + on the PCB ??

 

 

FGtBV7M.jpg

 

That probably explains why his power supply wasn't putting any voltage out.

 

Here's what happens when you connect a cap in reverse...

 

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After about 15 hours working on this board I've had some success. I still need to work out an issue with the 4 Player side of the game, the second set of bats don't become visible when you add a second credit but the 2 Player game is now working perfectly except that the ball speed is supposed to increase after hitting the bats a few times.

 

I foung 11 faulty chips and a few 74161s that read as 74163 on my tester where brand new ones read 74161.

A couple of them were running hot so I replaced them anyway so possibly 16 faulty chips.

 

m4sb1qe.jpg

 

 

I haven't tested the sound yet and will wait until I get the board plugged back into my cab but need to sort out the voltage regulator issue before I do that. It was running at 6.38v when I checked it earlier this week.

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This PCB repair is now completed, all functions have been restored.

The speed up occurs at hit 4 and again at hit 12, the game really becomes brutal at that speed.

 

 

The entire board containing over 90 chips will be socketed allowing for easy replacement and testing of chips in the future.

All the 40 year old chips will also be replaced with new ones and hopefully this thing will keep going for another 40 years.

One of the most challenging parts of this repair was reading the chip numbers, they are mostly manufactured in 1974 so all date codes start with a 74. Easy to get caught out when you have a date code of 7404 and the chip is a 7408.

Edited by Kaizen
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Great work Adam.

Can I grab the old working chips off you when you replace them all as I'd like to build a replica Apple 1 computer one day and period correct chips would be a nice touch albeit a dangerous one.[emoji6]

Cheers Trev

 

Sent from my SM-G930F using Tapatalk

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sockets are for whimps and newbies :lol

 

Ha ha, yeah :newbie:

Socketing the entire board was done at the owners request so that was their call and it was paid work so I just did what the customer requested.

 

 

 

I worked out why there was no increase in ball speed, at some stage someone decided it was too difficult at the faster speed and disabled it by adding two jumpers, see image below.

 

2K1E14x.jpg

 

 

 

Great work Adam.

Can I grab the old working chips off you when you replace them all as I'd like to build a replica Apple 1 computer one day and period correct chips would be a nice touch albeit a dangerous one.[emoji6]

Cheers Trev

 

Sent from my SM-G930F using Tapatalk

 

 

 

The owner of the PCB wants to hang on to them for spares but if you offer to replace them with new ones he may be tempted. There's about $200 worth of IC's on the board if you get them locally from Jaycar or Altronics.

Edited by Kaizen
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Ha ha, yeah :newbie:

Socketing the entire board was done at the owners request so that was their call and it was paid work so I just did what the customer requested.

 

 

 

I worked out why there was no increase in ball speed, at some stage someone decided it was too difficult at the faster speed and disabled it by adding two jumpers, see image below.

 

2K1E14x.jpg

 

 

 

 

 

 

 

The owner of the PCB wants to hang on to them for spares but if you offer to replace them with new ones he may be tempted. There's about $200 worth of IC's on the board if you get them locally from Jaycar or Altronics.

 

No worries.

Cheers Trev

 

Sent from my SM-G930F using Tapatalk

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Now that I have Dave's board fully working, I wanted to plug it into my cab but the 5v supply is all over the shop so I've been trying to nut out what's going on.

Here's the regulator circuit.

 

UpvfxJs.jpg

 

eF2fB6J.jpg

 

I've been having a bit of a challenge so I'm hoping someone here might be able to help.

When you power the cab up with the board connected it outputs 1.1 volt, at other times it will output 8 volts (not good for TTL's).

I've been doing a bit of testing and here's where I'm at.

I've replace the original Op Amp at IC1 (T3L) with what I've cross referenced to a TL071, I socketed IC1 and have tried with both and get the same results. I matched the Regulator Driver transistor at Q19 to a TIP31C and also tried swapping that out with no joy. The original components I removed have been put back at IC1 and Q19.

If you power the cab up with the board connected, you get 1.1 volts.

If you disconnect the board and power it up you get 1.1 volts then when you switch it off the reading goes up to 2.45 volts, if you switch it back on again it will rise to 5.0 volts. If the cab is left powered up and I plug the connector into my board it runs okay (apart from minor issues I still need to fix but it displays the net and ball etc), when I plug Dave's board in the image is shifted downward by a half do that the score is in the middle of the screen and the bottom is over scanned to the top.

 

I'm getting the 6.2v at the output of the Op Amp (pin 6) and 5.2v is at the base of Q20. The 10.8v from the supply also goes to the audio amp and the audio plays fine.

 

 

So an uneducated guess is that Q20 is failing under load but I'm not confident in that conclusion so I'd like to hear what others think.

Edited by Kaizen
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I now believe that there's no issue with the regulator circuit.

I had a spare 2N3055 and swapped it out at Q20 to be sure, I also changed the 4.3v zener with a new one. I disconnectoed the cap at C47 and connected a 1000uF 200v one stolen from a spare chassis and still had the same issues.

 

To isolate the problem I decided to make a small 5v 1.5A regulator board on vero using a 7805 and other parts from an old scrap PCB.

 

y3n2x0v.jpg

 

I tested it using the 12v from a PC power supply and it put 5v out no problems.

 

To test it under load on the cab I used two pairs of 6.3v 250mA globes so each pair will draw a little over 500mA, the 7805 is rated a 1.5A max (the same as the monitors regulator).

 

With no load the supply coming from the bridge rectifier is at 12.27 volts and the output sits at 4.98 volts.

My multimeter has a missing segment so I'll state the supply and output voltages for clarity.

 

ELZ5wE6.jpg

 

When two lamps are connected the supply drops to 8.67 volts and the output is at 4.55 volts.

 

360nqsm.jpg

 

With four lamps connected the supply then drops to 6.71 volts and the output voltage is too low to be read (around 1.1 volts) by the small LED meter.

 

kgeyqcG.jpg

 

It's easy enough to replace the four diodes in the bridge rectifier but before I do that I would mind hearing some peoples thoughts.

From the way I see it the can only be two possible things at fault, either the bridge rectifier or the transformer.

 

OItPiV7.jpg

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Its possible that one of the reci diodes are crapping itself as it loads up. I must admit I have never seen that happen.....But I have seen diodes meter out correctly, but then breakdown when high voltage is applied. But you are not really applying that higher voltage ! It could be the transformer not being able to deliver a high enough current due to age, but again this would be a bit odd . It would be simple enough to piece together a temporary bridge and then discount the diodes out of the problem. Then It can only be the transformer ! It will be interesting to see where the problem lies. Good Luck
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Its possible that one of the reci diodes are crapping itself as it loads up. I must admit I have never seen that happen.....But I have seen diodes meter out correctly, but then breakdown when high voltage is applied. But you are not really applying that higher voltage ! It could be the transformer not being able to deliver a high enough current due to age, but again this would be a bit odd . It would be simple enough to piece together a temporary bridge and then discount the diodes out of the problem. Then It can only be the transformer ! It will be interesting to see where the problem lies. Good Luck

 

Yeah that's exactly what I'm thinking, I've worked on electric forklifts for many years dating back to the old SCR motor controllers that have huge plugging diodes that sometimes fail under load but seem okay when you put a meter across them and my first though was that one of the diodes is breaking down under load.

I don't know a lot about transformers or electronics in general but apart from poor connections I can't grasp how a winding can break down under load (maybe someone can elaborate if it's possible). I have plenty of diodes and bridge rectifiers on parts boards so I'll go and take care of that side of it now.

 

Thanks for the replt.

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Mystery solved...

I checked the four diodes with a meter before I went to remove them and they all checked the same. As I cut the legs off one of them it broke in half.

 

M6oNvf9.jpg

 

 

You can see where it had been arcing internally.

 

GFaoKLD.jpg

 

 

 

I salvaged an 8A 400v bridge rectifier from another parts board and will test it out shortly.

 

A42uQjh.jpg

Edited by Kaizen
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After replacing the bridge rectifier I also reconnected the original cap at C47 the reassembled the monitor then loaded up the 5v with the four globe and got 0.05 drop which was acceptable.

The monitor went back in and I cleaned up all the wiring, plugged Dave's repaired PCB back in and dropped a 1967 20c piece into the coin mech.

I had an awesome game, well crappy really as I was playing the game through the image on the phone while recording it but it was awesome to be able to play a full game on the cabinet for the first time.

 

 

 

Player vs Game

 

VolfMFu.jpg

 

 

Player vs Player (4 Player)

 

ZaX8uV2.jpg

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I'd like to say thanks to everyone who gave some constructive input and encouragement while I was working through this issue. I can usually sort most issues out with logic, commonsense and the limited electrical/electronics skills I have.

I try not to ask for help as I like to work things out myself (with the help of Google and Youtube of course) but after a day of going around in circles with no joy I though I better put my hand up.

A big thanks to @taito who spent over an hour on the phone with me while I worked through the plug in card that does most of the regulation and helped me to understand a little more about electronics.

Thanks to the advice from @thegrunta666, I went straight to the source of the problem and got it sorted rather quickly.

 

To me, that's what this forum is all about, we have a community if like minded people who all have different specialised areas of skills and if we all pool our resources and knowledge there's nothing we can't fix.

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No happy dance I'm afraid but there was a fist pump and a loud "YES!".

@raysco, you're welcome drop around any time and let me beat you at a few games.

Dave's cabinet should be good to go as soon as I return the board to him so there'll be two of these up and running here in Brisbane.

I still need to get my game PCB sorted but there's only a couple of issues with it and after repairing his board I don't think it will be too hard to sort out.

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