Uh oh...he's done it again...geez...

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ModRob
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Re: Uh oh...he's done it again...geez...

Post by ModRob » Mon Oct 11, 2004 7:59 pm

I've checked upteen times for the cap's polarity, and I'm sure they are oriented right. But let me say that all these new caps had the little "minus" symbol with an arrow band that pointed towards the end of the cap, away from the cap end that has an indented ring around it. So I figure this indented end is the positive end, since the arrowed-band points toward the opposite end. I just went in and checked all of them again, and still yet, all the indented ends of the caps are soldered into the pad with a "+" symbol at them.<p>Just a few minutes ago, I hooked up all again, and I'm concerned that the big D7 and D8 diodes are getting hot, almost too hot to hold on to. I haven't left it powered up long enough to see if it gets even hotter...and still nothing yet even flashes on the monitor. At least at the beginning of all my troubles, I was seeing a brief flash of game on the monitor when I hit the power off switch, and I believe I saw it sometimes when I turned it on. But now, nothing at all...<p>Just for the heck of it, I'm going to replace both of those diodes...<p>Sorry guys, I wished I was more technically-minded to aid you...

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Re: Uh oh...he's done it again...geez...

Post by Enzo » Mon Oct 11, 2004 8:42 pm

The diodes are cooking because something is shunting the current to ground from their cathode end. Did we explore the wiring to pin 5,E on the edgeconn? I mentioned it, but I don't recall a result, we want to make sure nothing external is loading down this circuit.<p>The indented end of the caps is bound to be the pos end, and the arrows with the minus sign are neg. So the indented end should be the end away from the edge of the board.<p>My personal troubleshooting flow is different from what we are discussing here, but if I got to this point I would be disconnecting things - and taking notes as to what they were. And assume nothing. As soon as you say to your self, well THAT thing I KNOW is OK, so I don't need to recheck it, you are asking for trouble.<p>For example here we have the rectifiers and the two filters. COnnected to them is the transistor on the heatsink, the 10 watt resistor. the tantalum cap, and D4, plus the tracec back to the edgeconn 5,E. That is it. I would be lifting the resistor, the tantalum, the D4, and removing the transistor. Now all I have left is the rectifiers and the caps. If it still only charges the caps up to 1V, then the trouble is either a short I can't see or the caps are bad, assuming there is nothing connected to 5,E.<p>If at that time the 7.5 has come back up, then I restore the transistor and resistor, if it still holds up, then either the tantalum or D4 are involved. If the resistor and xstr drop it, then they are in the current path to the trouble source.

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ModRob
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Re: Uh oh...he's done it again...geez...

Post by ModRob » Mon Oct 11, 2004 8:44 pm

Ok here's more...reading back through all of your hints I've printed, I've done the following:<p>Measuring AC..harness attached and game powered up...
voltage across both anode sides of D7&D8 is 14 volts.<p>voltage across anode sides of D3 & D4 is about 25 volts.<p>Measuring DC voltage:
I placed the ground probe on the a1&b2 ground points on the edgecard, and tested;<p>measured both sides of R52 and instead of 12 volts, found very low readings on either side. AMENDED NOTE: THIS CORRECTED NOW...<p>Measurements at C2 and C3 were 5volts. Resistor R53 does get very warm. Measured from ground to BASE pin of Q6 (D44mv4) and got 5.82 volts.<p>Holding up at this point (did not replace the diodes as mentioned earlier yet). AMENDED: DID REPLACE THE A15 DIODES...<p>[ October 12, 2004: Message edited by: ModRob ]</p>

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Re: Uh oh...he's done it again...geez...

Post by rshayes » Mon Oct 11, 2004 11:11 pm

It sounds like the power supply is working. You are measuring about 5.8 on the base of the power transistor. This would put the emitter at about 5 volts.<p>The peak voltage of the 7 volt transformer winding is about 9.9 volts. The DC level of the rectified voltage is given as 7.5 volts. A rough estimate of the rippple voltage is 2.8 volts peak to peak. Assuming that this occurs in 1/120 sec, the current is in the neighborhood of 6 amps.<p>The rectifier diodes are probably rated at 3 amps. Diode ratings are usually based on maximum junction temperatures. Each conducts half the current, so using them to provide 6 amps is right at their maximum rating. I would expect them to be warm.<p>I suspect that the original fault was bad contact in the edge connector. This would overheat one diode, increase the ripple voltage, and reduce the average voltage.

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ModRob
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Re: Uh oh...he's done it again...geez...

Post by ModRob » Tue Oct 12, 2004 8:35 am

Well, I went ahead and replaced the big diodes anyway...did more testing, and all readings seem to appear ok. Big diodes are a bit on the warm side, and the big 4ohm 10watt is a little hot. But otherwise it seems to be ok having been powered up during all my testing.<p>I still have no video. But in further research on the net, I find that if I happened to put the Z80 Sync Buss controller PCB in upside down, and power up, then pfft! No more. I know for sure that I did try it both ways (should have known better). So I'm assuming that this would now be my problem, since the website says it's a common mistake. But I still have no game at all. No sounds. NO nothing. I eliminated the aux. board, and plugged in the main chip into the main logic board too. (According to others, this would make the machine act as a PacMan instead of Ms. Pacman.)<p>I'm at my wit's end...I might just stop by the local hardware and purchase an old analog Sledge Hammer, and give it a little "adjustment"...

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Re: Uh oh...he's done it again...geez...

Post by Edd » Tue Oct 12, 2004 12:10 pm

Sounds like you now have a working power supply again by the last readings you gave, excepting for your statement :<p>Measuring DC voltage:
“I placed the ground probe on the a1&b2 ground points on the edgecard, and tested;
measured both sides of R52 and instead of 12 volts, found very low readings on either side.”<p>We didn’t leave our meter in AC range did we….hmmmm….hmmmmm….. since the 16 volt supply is workingO.K., as is everything else in the power supply. With the one exception of , possibly, an excess pull on the 7.5 raw power buss by external loading(s).<p>“I still have no video. But in further research on the net, I find that if I happened to put the Z80 Sync Buss controller PCB in upside down, and power up, then pfft! No more. I know for sure that I did try it both ways (should have known better).’’ <p>That certainly may just be your problem now, in which case you are in deeeep doo doo….unless just a diode or zener was popped by that action.. ALWAYS mark your boards and connectors ends with codings from a magic marker.<p>73's de Edd
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Re: Uh oh...he's done it again...geez...

Post by haklesup » Tue Oct 12, 2004 12:50 pm

Forgive me for coming in at the end and not reading all the posts that led up to here.<p>Since you suspect that plugging in a board backward may have caused a failure, I want to give you a few general tips about that subject.<p>When you insert a device or a board backward you will most likly have applied VDD (16V) to a ground or input pin while other pins were grounded. This causes a large amount of current to surge into the overbiased pin and through the input protection diodes. Some devices might tolerate this for a few seconds but for many more it will result in a pin that is either shorted or open. (shorts eventually turn to opens usually if left powered up)<p>Generally, you can use a DMM in diode check or continuity mode to probe each pin with respect to its power and ground pins. Most pins will show a small resistance or voltage drop (diode check mode) if they are good. If you find a dead short or completely open pin on any IC, you have a suspect.<p>Before you can pronounce the part dead, you still need to look at a data sheet or compare it to a good device or similar pin on the same device (address pins should all look alike for example). Because some pins do look like shorts or opens and other passive components on the board also interact.<p>A MultiMeter is effective but crude tool for this type of troubleshooting. A more optimized tester might be like this one www.curvetrace.com (download manual for more T/S tips) or www.huntron.com <p>Slipping an ammeter in series with the board and supply can also be telling, especially if you know what it should be. If the supply is current limited and the board is consuming max current, it is safe to say you have a short on the board.<p>It is not unusual for a circuit like that to consume significant current (several hundered mA) while idling. Efficiency wasn't a big thing back then.

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ModRob
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Re: Uh oh...he's done it again...geez...

Post by ModRob » Tue Oct 12, 2004 2:32 pm

Yep, Edd..you got it right...evidently I had the meter in the wrong range..the resistor now measures correctly.<p>I found all the chips to rechip the sync bus controller...I got three-quarters of the old chips out (dang they are tedious work, and already tore a trace--have to run me a direct wire for that one). Back to the regular grind tonight, so maybe in the morning I can finish up the rechipping, THEN see where we stand...

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Re: Uh oh...he's done it again...geez...

Post by Enzo » Tue Oct 12, 2004 4:08 pm

Yep, you can't plug stuff in backwards.<p>With the power supply apparently working now, you move on to troubleshooting the lack of function. With good parts in the plug-in, we can start to check for clock and also interrupts and reset lines.<p>Did we determine if you have a scope or not?

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Re: Uh oh...he's done it again...geez...

Post by ModRob » Wed Oct 13, 2004 4:56 pm

Enzo...haha...remember, I'm just a novice--or lower--and don't have any sophistated equipment other than normal everyday tools, and both an analog and Digital meter.<p>Didn't get to finish replacing chips on Sync Bus card, but should get to it tomorrow..then see what happens. I now know which way it plugs in...haha...<p>More later.<p>Timbo

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Re: Uh oh...he's done it again...geez...

Post by Enzo » Thu Oct 14, 2004 3:08 pm

We all start somewhere, good luck.

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ModRob
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Re: Uh oh...he's done it again...geez...

Post by ModRob » Thu Oct 14, 2004 4:49 pm

"sophistated"? Hmmmm...I really got a LONG way to go....haha

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Re: Uh oh...he's done it again...geez...

Post by ModRob » Fri Oct 22, 2004 7:33 am

Well, failed again....arrgggggggg...<p>I rechipped the sync bus daughterboard, looked it over good, installed the correct way, powered up, and still nothing. No flash or anything on the monitor. After reading some other info, I don't notice any kind of action on the tube--no noise, no kind of glow of any kind in the neck of it...and still no game sounds whatsoever. I'm going back to square one and check all the readings again. I noticed on one site where a gentleman said for a dead game, he replaced resistor R52, and all worked again. I remember at one point getting an odd reading on it too. So what the heck, I'm running out today and picking up another and try it anyway, plus one other chip he recommended.<p>More later.

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