Bally Pro Arcade

[nikola-wan]

BTW - I powered up my Bally Computer System and the VRs are working but I have no video out of the RF Modulator.

I even unplugged the RF Modulator and connected pin 1 of J1 (farthest away from the Line Filter) and ground to an RCA video cable to my little Sony TV and had no video.

According to the Bally forum - that is a common problem. I think I will need to build the Balcheck board or the SetScreen board to diagnose.

 

[falter]

Doh! Missed that. Twice.

Thanks muchly.

I think I may have found part of the problem. There is a 6.8uf resistor (C8) that appears to have very low (0.04ohm) resistance across it. To my eye it also looks swollen and slightly melted.

 

[Someone]

All of the voltage regulators are bog standard parts made by many different manufacturers.
They are often in the format 78xx for positive voltage regulators and 79xx for negative voltage regulators. (sometimes with a letter between the 78 and the xx for power capability)
National Semiconductor uses LM340-xx for positive voltage and LM320-xx for negative voltage.
For the schematic it would be 7815, 7810, 7805 and 7905.

Note: The resistors in front of the 7810 & 7905 share dissipating the unwanted power & could be their to also reduce inrush current.
Note 2: The pinout for the negative voltage regulators is different to the positive regulators!!!

 

[daver2]

The third diode up (as you have stated it) is CR5, and this is in the full-wave bridge rectifier feeding Q1 (providing the +5V supply) - so nothing to do with any of the other regulators.

One end of C9 looks a little scorched?

C8 could be faulty. This is on the INPUT to VR3 - but is common to the positive 10V, 12V and 5V regulators. As it is on the INPUT side of the regulators, it can't be responsible for any of the regulators smoking.

If you want to perform a diode check on the bridge rectifiers, I would unplug the transformer connection first. This would provide a low impedance path (via the transformer secondary windings) and could confuse the results.

Dave

 

[falter]

The third diode up (as you have stated it) is CR5, and this is in the full-wave bridge rectifier feeding Q1 (providing the +5V supply) - so nothing to do with any of the other regulators.

One end of C9 looks a little scorched?

C8 could be faulty. This is on the INPUT to VR3 - but is common to the positive 10V, 12V and 5V regulators. As it is on the INPUT side of the regulators, it can't be responsible for any of the regulators smoking.

If you want to perform a diode check on the bridge rectifiers, I would unplug the transformer connection first. This would provide a low impedance path (via the transformer secondary windings) and could confuse the results.

Dave

Thanks Dave. I'll give that a try. Does it make sense that only the 12v regulator is getting smoked here? If something were overloading/shorting the input side, with the three regulator inputs connected (I think?) shouldn't they all be smoking?

 

[daver2]

If something was shorting the input side of the voltage regulators NONE of them will be smoking!

Think about it...

Dave

 

[falter]

I swear I've been reading about this during my holidays. I really want to understand but it's really hard for a non-EE mind to grasp. I'm thinking about doing some scratch build stuff to learn, working with datasheets. That's the only way I might get it.

I've got the PSU disconnected and am rechecked diodes.. but I'm still not getting consistent results. I'm honestly wondering if my DMM just isn't up to the task. It should be simple, right? Put it in resistance mode, it should be low resistance one way and high resistance with the probes reversed? And then in diode mode it sends voltage one way but not the other? But each time I test the same diode multiple ways it comes back with different results.

 

[daver2]

Are you using a digital multimeter or an analogue multimeter?

You are correct, one way the diode should conduct, the other way it should be a high resistance (incidentally it is current that flows not the voltage - you need a voltage difference for a current to flow in the presence of a resistance (Ohm's Law)).

When you say 'different results' what do you mean (i.e. what readings are you getting)? Give me an example from one diode...

Dave

 

[falter]

Are you using a digital multimeter or an analogue multimeter?

You are correct, one way the diode should conduct, the other way it should be a high resistance (incidentally it is current that flows not the voltage - you need a voltage difference for a current to flow in the presence of a resistance (Ohm's Law)).

When you say 'different results' what do you mean (i.e. what readings are you getting)? Give me an example from one diode...

Dave

Yes I am using a digital multimeter which came from a local Canadian Tire.

So for example in resistance mode on one test with probes aligned one way it might show 2M+ ohm resistance, and little the other way. But then if you perform the test again later, probes oriented the same way as original, it might show 500ohm resistance that direction now instead of 2.5M. And I've really checked to make sure the probes aren't touching anything adjacent.. just the two leads from the diode itself.

 

[falter]

If you do a check in diode mode, it might show .7v going one way and close to 0 the other, but a retest might show .7v going one way and .6v the other, probes aligned same way.

 

[daver2]

Ah, now remember that there are some storage capacitors in circuit here as well.

It is just possible that what you are doing is charging up one of the storage capacitors when taking a reading and then the meter is actually reading not the resistance of the diode - but a combination of its own signal plus that from the charge accumulated on the storage capacitors.

This is sort of why I like to use a cheapo analogue meter for things like this, because you can see the capacitor charge leaking away as you watch it...

What your diode check is doing is reading the voltage drop across the diode. 0.7 Volts is about right for a forward biassed diode of course.

Dave

 

[falter]

Ah, now remember that there are some storage capacitors in circuit here as well.

It is just possible that what you are doing is charging up one of the storage capacitors when taking a reading and then the meter is actually reading not the resistance of the diode - but a combination of its own signal plus that from the charge accumulated on the storage capacitors.

This is sort of why I like to use a cheapo analogue meter for things like this, because you can see the capacitor charge leaking away as you watch it...

What your diode check is doing is reading the voltage drop across the diode. 0.7 Volts is about right for a forward biassed diode of course.

Dave

I do have an analogue meter somewhere.. I think I'll dig that out.

 

[falter]

Ok thanks for the suggestion on the analogue dmm! Much clearer.

And.. yeah, not seeing a problem. I'll try to add some pictures but basically one way there was very little resistance, the other, if you waited a while for the caps to discharge, showed around 3000ohm resistance (i think i am reading the meter right.. the switch for resistance mode is marked X1K, I assume a 3 on the meter is 3x1000?). Unlike the redrawn schematic, not all the diodes are IN4007, four of them are IN4004. Maybe differences in the later Astrocade machines. On the IN4004s, when the probes are oriented one way you see infinite resistance, the other, around 3k ohm.

I also checked other diodes as I found them including inside the power connector box. I note in there one diode is the reverse orientation of the others.

The pics I tried to order them.. so the first pic of the meter attached is with the probes attached as shown in the picture to its right, and so on.

 

[daver2]

I would expect a lower forward resistance than 3k... More like 600 Ohms.

You did short the multimeter probes together and ensured that the meter read 0 Ohms before you started didn't you?

Yes, 3 indicated would be 3k.

This is a really basic meter!

Are we convinced there are diodes inside that metal can? I am not... Those red and black things should be capacitors...

I suspect you are measuring the diodes again on the board...

Dave

 

[falter]

Hmm.. I just checked again and yeah putting the probes together is a bit above 0 ohm. So I recalibrate to 0. So now with the IN4007s I get zero ohm resistance one way, and still about 3k ohm the other.

The IN4004 which seem to be connected to the 11.5v, they are infinite resistance one way, but 2k resistance the other. I tapped the probes together to make sure they still hit zero and they do..

 

[falter]

Just for good measure I removed all of the in4004s to test out of circuit.. they seem consistent.. .6v one way, nothing the other.

 

[daver2]

That's the clincher then that they work if they are fine out of circuit.

Dave

 

[falter]

That's the clincher then that they work if they are fine out of circuit.

Dave

Alright.. back to hunting.. I was reading through some troubleshooting docs and they mention checking the transistor over there in case that is shorted..

 

[falter]

Okay, so I did some more checking, and I think *maybe* I've found a problem, if not the problem.

VR3 - a uA78GU1c. I've got the right datasheet here, the COMM leg and the IN leg are shorted. The COMM legs of VR3 and VR4 are tied together (is COMM another word for ground?). So, my undereducated guess is, we have the input voltage going across to COMM, and that is going straight into the GND leg of the LM342P-12, quickly frying it, as COMM and GND of VR3 and VR4 respectively are connected to each other by a trace.

How did I do?

 

[seat safety switch]

COMM would be short for COMMon in my mind, so I'd expect it to be ground. On a 7805 the pinout is usually I-C-O, for input-common-output, for instance.

If you can, I'd lift the input leg of the µA78G (datasheet, for everyone else) out of circuit and check continuity again, to see if the short goes away.