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Grandstand Firefox F7 - Sound chip very hot
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Quaxo76
Mattel Football


Joined: 18 Jun 2013
Last Visit: 23 Oct 2013
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Location: Faleria (VT), Italy

PostPosted: Sun Jun 23, 2013 8:35 am    Post subject: Grandstand Firefox F7 - Sound chip very hot Reply with quote

Hello,
I just recently acquired two nice Grandstand games, the Firefox F7 and Scramble. I have a "soft spot" for VFD or LED games, and for vintage electronics in general...
Anyway, they're in good physical condition, no corrosion or obvious damage, so I powered them up with a standard 5V power supply (I know they can run OK as low as 4V). The Scramble worked perfectly, while the Firefox's display was really dim. Suspecting a low voltage on the display, I checked the input voltage, and it had dropped to about 2V - no wonder the display was dim, I was surprised it worked at all!
Turns out, the power supply I was using was unable to provide enough current to the Firefox, but was adequate for the Scramble. So I opened it up, and noticed that the sound processor (the D1771C) runs very hot.
The other chip, the main processor, is totally cold (ambient temperature) even after a long run.
The chip's temperature is around 50°C, and its supply voltage is 5.8V.
So, I'm wondering: does this chip have to get so hot? Or is it failing somehow? Could it be an external component? The sound itself appears to be OK, both the piezo "beeps" and the synthesized effects.

Here are some current readings I took:
Firefox, standby mode: 290 to 300 mA, @ 6V.
Firefox, game mode: 310 to 330 mA, @ 6V.

Scramble, standby mode: 230 to 240 mA, @ 6V
Scramble, game mode: 240 to 250 mA, @ 6V.

The Firefox appears to be using about 40% more power, and I suspect that's what gets converted to heat in the sound chip.
Do these readings make sense? Can anybody check the temperature and power consumptions on this game, or is there some check I could do? I'd hate to have the chip blow up due to overheating... not to mention that at these rates, it will use the batteries in a very short time.

Thank you in advance,
Cristian
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blanka
Atari Cosmos


Joined: 14 Dec 2010
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PostPosted: Sun Jun 23, 2013 11:15 am    Post subject: Reply with quote

The sound chip is a microcontroller as well, not an amp or so, and it should not get hot. I think one of your Zener diodes is broke. These regulate the voltage on different sections. The main micro and the sound chip both have a different zener to control Vic. Use a multimeter to check the voltage over the different zeners. It might also cause the dim thing, I don't believe your lower voltage is to blame. If the voltage drops below a certain point, it just stops working.
The higher power use will be quite normal: you have a second microcontroller, double the amount of ROM/RAM, a bigger display with faster refresh...

Zener 1: Between pin 5 of the main chip and the transformer: +/- 6V
Zener 2: Between one of the 220µF 16V capacitors/pin 42 on the main micro and the 6.8kΩ resistor that goes to the 6V input: +/- 12V
Zener 3: Between pin 4 of the main chip and the ground from the batteries: +/- 9V

Another trouble maker can be the other 220µF 16V capacitor directly between the batteries. If that one is broke, it can short circuit the batteries/power adapter and will also cause a big drop in the measured voltage on the supply. Does the top metal cap pop out a bit?
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Quaxo76
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Location: Faleria (VT), Italy

PostPosted: Sun Jun 23, 2013 1:22 pm    Post subject: Reply with quote

Hi blanka,
thank you very much for your reply!

blanka wrote:
The sound chip is a microcontroller as well, not an amp or so, and it should not get hot.

So my guess that something was wrong, was right! :/ Even if it was an amp, it had no reason to get hot since the sound is mostly silent anyway, and low power.

Quote:

I think one of your Zener diodes is broke. These regulate the voltage on different sections. The main micro and the sound chip both have a different zener to control Vic. Use a multimeter to check the voltage over the different zeners.

I'll check it tomorrow, thank you! Do you think 5.8V for the sound processor is too much?

Quote:
It might also cause the dim thing, I don't believe your lower voltage is to blame. If the voltage drops below a certain point, it just stops working.

Actually, no, when I tried it with a more powerful power supply (one that was able to cope with the higher drain) the display went back to normal. I used a variable voltage, stabilized power supply, and once the voltage goes above 4V, the display stays at the right brightness. Between 2 and 4 volts, the display is dimmer but the game still works; under 2 volts, it quits. I have to test the same thing with the Scramble, to see if it behaves the same.

Quote:

The higher power use will be quite normal: you have a second microcontroller, double the amount of ROM/RAM, a bigger display with faster refresh...

You're right, but I thought that about 80mA were a bit much for just an additional chip. And I didn't know about the fast refresh.

Quote:
Another trouble maker can be the other 220µF 16V capacitor directly between the batteries. If that one is broke, it can short circuit the batteries/power adapter and will also cause a big drop in the measured voltage on the supply. Does the top metal cap pop out a bit?

I think it "looked right" but I don't have it in front of me now, I'll check tomorrow. But I repeat, once I use a better power supply, I don't see a voltage drop anymore.

Now I have another question: is there a source of technical info about these games? I'd like to have the schematics, and possibly some info about the processor. It would be really interesting to actually extract the ROM code (it has been done in the past, by decapsulating the chip from a broken unit), but I have no idea of what machine language they use...

Cristian
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Rik
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PostPosted: Sun Jun 23, 2013 5:48 pm    Post subject: Reply with quote

Quaxo76 wrote:
Now I have another question: is there a source of technical info about these games? I'd like to have the schematics, and possibly some info about the processor. It would be really interesting to actually extract the ROM code (it has been done in the past, by decapsulating the chip from a broken unit), but I have no idea of what machine language they use...

Cristian


Schematics can probably be drawn up pretty easily... None of the games have multi-layer boards or anything, so you can pretty much just follow all the traces... Just takes time.

As for getting the ROM code, I'm already working on that. There's a hobby group that decapsulates chips and then writes emulators for the chips based on physically looking at them. (see www.visual6502.org) After they decapsulate and take electron-microscope photos of the chip, we'll be able to manually extract the ROM (which shouldn't be _too_ bad on these tiny little chips). And we might even be able to code perfect emulators for some of the games usual the visual-chip technique. And possibly even 'watch' the chip work while playing the game... That would just be way too cool... Cool

I'll have to prod the team and see what the status is on the games I sent (Just some random ones to start with- Mattel Football, Microvision console and cartridge, Tiger Pinball I think... I wanted to see how it went first before committing rarer games to their death, even if it is in the interest of science. Cool ) The nice thing is that a lot of games use the same basic CPU (like the Hitachi HD38800), but just a different mask-ROM. So if we get the CPU working, all we need is the ROM code from other games, and the graphical interface, and we have the game working perfectly...

The programming language would, of course, be assembly language specific to the processor chip used in the game. Programming documentation for some of these CPUs would be nice to track down too... But knowing the language wouldn't be necessary for emulating the games, you just have to type in the 0s and 1s correctly from the photos.
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blanka
Atari Cosmos


Joined: 14 Dec 2010
Last Visit: 13 Oct 2022
Posts: 561
Location: Eindhoven, the Netherlands

PostPosted: Mon Jun 24, 2013 12:58 am    Post subject: Reply with quote

The NEC form the Firefox and the Hitachi both have instruction set information available, so understanding the code is no problem. This specific NEC, and the HD38800/38820 are one of the few very well documented micro's.
Now find some decent info on the NEC D552/553! The other heavily used controller. It might be a simpler version of the Firefox one though.
I have the schematics of the Firefox. I'll pm you it.
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Quaxo76
Mattel Football


Joined: 18 Jun 2013
Last Visit: 23 Oct 2013
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Location: Faleria (VT), Italy

PostPosted: Mon Jun 24, 2013 2:08 am    Post subject: Reply with quote

Rik,
I know the visual6502 site very well, the 6502 is actually the cpu I know best! Smile But what I was thinking isn't actually for an emulator, it's just that it would be interesting to have a listing of the actual code, to see how those games were programmed. But of course an actual emulator would be the "best of both worlds".

Blanka,
thank you! I'll have to open those images with my other computer with a higher resolution screen.

I'm disassembling the game again right now, to check the zeners and capacitor. I'll post when I find something!

Thanks again,
Cristian
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Quaxo76
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PostPosted: Mon Jun 24, 2013 3:07 am    Post subject: Reply with quote

OK, I took some readings.
I was unable to find out which diodes are actually "Zener 1" and "Zener 2" in your list (I need to see the schematic with a higher resolution screen to do so), but I measured the voltages across all the diodes.
S12 (which is Zener 3 in your list): 8.62V.
S05: 0.56V
S06: 15.20V
S09: 5.85V
S10: 4.30V
S11: 4.93V
S08: 3.30V
S07: varies a lot between 3V and 4V.

Does this make any sense?
Also, the caps look perfect at a visual inspection.
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blanka
Atari Cosmos


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PostPosted: Mon Jun 24, 2013 3:58 am    Post subject: Reply with quote

S10 is the 12V zener, so that one is way off. (Check the value on the zener. I'm not 100% sure it is a 12V on this game in that position. They usually show the volt value in 2 digits black print)
S09 is the 6.2V zener, so the reading is right
S12 is the 9.1V zener, so the reading is right.

Measure Vic to the ground (pin 42 on the main chip). It should be around 10V (or -10V)

VFD micro's have some weird voltages around them.
The +5/6V is only found on the batteries/AC connector
The 0V is actually the highest voltage in the game!
The IC usually has its ground on the VIC line, which is at -10V
The VFD floor is in the -25V -35V range (Vdisplay)
The fillament voltages is just above the VFD floor (5V above, that is what zener S09 does), but it is an alternating current, block shaped, switching around 20-100Khz with an amplitude of 2-3V (the center is 5V above the Vdisp)
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Quaxo76
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PostPosted: Mon Jun 24, 2013 4:30 am    Post subject: Reply with quote

blanka wrote:
S10 is the 12V zener, so that one is way off. (Check the value on the zener. I'm not 100% sure it is a 12V on this game in that position. They usually show the volt value in 2 digits black print)

That will require desoldering, since the zener is placed so that I can't read the digits. I'll do that when I get back home. For now I can only read a "7" and a dot.
Quote:

S09 is the 6.2V zener, so the reading is right

Are you sure it's S09? I just traced it with the schematic you gave me, and it looks like it should be S11, and so its voltage would be a little low.
S09 also looks like a "regular diode", while S11 has the black markings. I can make out a "5" without desoldering.
Quote:

Measure Vic to the ground (pin 42 on the main chip). It should be around 10V (or -10V)

Measured between pin 42 and pin 21 of the main uP, I read 5.85. Too low.
The power supply is still at a stable 6.0V.
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Quaxo76
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PostPosted: Tue Jun 25, 2013 1:10 am    Post subject: Reply with quote

I desoldered the diodes this morning.
The diode between pin 5 of the CPU and the transformer (identified as 6.2V in the schematic), has the markings "5.83" or "5.B3".
The diode between Vic and the 6.8K resistor, identified as 12V in the schematic, has the markings "7.583" or "7.5B3".
Does this mean they are actually 5.0V and 7.5V diodes? Should I replace them with 6.2V and 12V ones, or maybe the schematic has the wrong values?


EDIT: I just noticed that in the PCB, there are markings for the actual values of the zeners. S11 is indeed a 5V, S10 is 7.5V, S12 is 9.1V. So maybe the schematic should be corrected?
Anyway, knowing this, it looks like voltage across S10 is too low - should be around 7.5V but it's 4.30.
I'll try replacing that when I can find one, unless there is something else I could try... Smile
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blanka
Atari Cosmos


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PostPosted: Tue Jun 25, 2013 9:57 pm    Post subject: Reply with quote

Yes, the values can be wrong. Thanks for finding them out, I will change them.
Guess it has to do with the double-chip layout. Normally it has just one MCU and often that one has around 10V on it, but that one is probably on -9V here with the third zener..
It's quite easy to get a fresh zener. Try a new one yes.
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Quaxo76
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PostPosted: Tue Jun 25, 2013 11:10 pm    Post subject: Reply with quote

blanka wrote:

It's quite easy to get a fresh zener. Try a new one yes.

I was running low on electronic components, so I ordered a whole set (you know, like 20 pieces for each value of each kind of common components) but it's from China, so it'll be here in a couple of weeks...

But I'm wondering: if the zener is broken and regulates to a *lower* voltage, then changing it will increase the voltage... won't that make the chip run even hotter?
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blanka
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PostPosted: Wed Jun 26, 2013 12:49 am    Post subject: Reply with quote

Yes, but in this case it might work the opposite. These VFD games have very weird voltage lines. You input 6V and 0 at the ground, but the operating voltages are -10V for the IC, -25V as average for the fillament, and -30V as idle voltage for the VFD segments (they are connected to 0V when active). This specific zener "pushes" the IC voltage of the 1771 above the Vdisp, and when it does not push it far enough, the net voltage over the IC may become too high. I'm not totally sure how this one works (3 zeners instead of 2 and 2 Vic lines instead of 1), but it surely could cause the problem.

About getting the component: last year RS-Online had free shipping for every order. It made it possible to have components like this in a 10 piece package next day for like 50 cents! They quit the free shipping unfortunately.
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Quaxo76
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PostPosted: Wed Jun 26, 2013 1:36 am    Post subject: Reply with quote

blanka wrote:
This specific zener "pushes" the IC voltage of the 1771 above the Vdisp, and when it does not push it far enough, the net voltage over the IC may become too high.

That makes sense, thank you. I've never worked before with circuits where all the voltages are negative, and the batteries' ground is the highest voltage... I'm not used to them! Smile
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Quaxo76
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PostPosted: Tue Jul 23, 2013 2:43 am    Post subject: Reply with quote

OK, I'm back. The new zeners arrived and I replaced all three.

The problem isn't solved. Now I measure 5V across the 5.1V zener S11 (which is good); 9.1V across the 9.1V zener S12 (which is great); but I still measure 4.3V across the 7.5V zener S10.
And the sound chip still gets hot.

What can be the next likely culprit? :/
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