No Signal Output From 7800

[rasch_C]

I'm going down the rabbit hole of trying to get my 7800 resurrected. I bought it non-working some years ago and took an initial look at it w/ no luck. I now have it on my new bench after my move and am diving in.

 

I have a good working power supply (9V 1.5A that I use in my home lab for testing), I have a couple of cleaned 7800 carts and my library of known working/clean 2600 carts. I have cleaned the cartridge slot. When I turn on the 7800 I get the power led on, but no video/audio signal from the RF. I have this hooked up to an old VCR (for demodulation) and then piped into my Sony PVM in my lab that I use for testing (this setup is known working for everything else including feeds from 2600s). I have tried both channel 2 and 3 as selected on the 7800.

 

I do not know much about video signals as of yet, but am measuring ~50-52 ohms on the RF modulator output connection. So, that does not seem to be shorted. There is not even a flicker of signal or audio - nothing. 7805 voltage regulator output is 4.92V. Pin 1 of U10 (4013 IC) is 4.54V, pin 10 is 4.54V when power button is off and 0V when power button is on, and pin 13 is 0V when button off and 4.54V when button is on. So, it does not seem that this is getting stuck in a 'reset' state. The reset button itself does test out working well. However, I do not notice any impact on pins 10 or 13 of U10 when pressing reset. I also don't seem to get any sound result on rf or hooked up to composite when connecting direct to where a composite mod would tie into the audio (non-POKEY). I also read 4.92V at that connection point when the power button is on.

 

I am still learning the architecture of the 7800 and still going through schematics. I have no usage familiarity at all with the 7800. These are the same symptoms and test results with either a 7800 cart or 2600 cart. Any assistance in getting this up and working would be most appreciated. I do not know where to go next, yet. Maybe there is a simple solution I have not found digging through the forums.

 

 

Thanks,

-Allen

[+Stephen Moss]

Pressing the Reset button will not have any effect on U10 because there is no connection between the two, U10 provides a system wide reset to the IC's, the reset button is a software reset to restart the game.

The 7800 uses Maria for 7800 games the TIA for 2600 so if neither are games are providing audio or video I would start with sections common to both 2600 & 7800 games.

 

It sound like the power stage and system reset is operating correctly so start simple...

1) Check that each IC is getting power by either measuring voltage across its VDD & VSS pins or checking continuity (power off) between the GND/0V/VSS pins and the GND pin of the voltage regualtor and between the +5V/VDD pins and the output pin of the regulator.

2) Check that the reset pin on each IC that has one is following the reset output from U10 (none suck in reset).

3) If you can check the output from the clock oscillators and that if running the clocks are getting to the corresponding pins in the IC's.

[rasch_C]

Thanks for the suggestions. It is obviously a common problem between both sides of the system (2600 & 7800) as I have done tests with both carts and have the same results.

 

I did not know how the reset worked, but as you describe the soft reset nature of it, it makes sense not to be tied to U10 - I'm still learning the 7800 layout. It makes good logical sense to test the power pins at each IC as a next step and I will do that. Probably won't be tonight as I have a long, busy workday ahead of me, but sometime this weekend.

 

Thanks again for the direction!

-Allen

 

Sent from my Pixel 2 using Tapatalk

[ChildOfCv]

 

I also don't seem to get any sound result on rf or hooked up to composite when connecting direct to where a composite mod would tie into the audio (non-POKEY).

Wait... are you saying you DO at least get video on the composite port?

 

Looking at the schematic, they generate the video timing signals and luminance through the OR gates in U3, and sum in the color signal from each individual video chip. So if you hook the output of pin 3 of the RF modulator straight to a TV in composite mode, you should see a good picture, at least. J2-3 claims to be a peripheral output port and should have the composite signal on pin 3.

 

The audio, sampled from that same port, pin 17, should have your tunes. It gets fed into the FM modulator below and mixed into the composite signal at J2-3, though your average TV in composite mode won't expect to see the audio as part of the video signal, so won't care.

 

If you have an oscilloscope, look at the composite output to see if it actually looks like a TV signal.

[ChildOfCv]

If you're saying you get no signal at all, anywhere, then check the clock input to U1 on pin 3. If it's not clockin' you ain't rockin' on either cartridge type. Looks like MARIA takes the clock, and divides it for her TIA. So that's one common point.

[rasch_C]

Just a quick update. I did test Vcc, Vdd, etc. On the major ICs (TIA, RIOT, Sally, Maria). I thought I might have narrowed it down to being held in reset because the reset pin on Sally is always high when power switch is on, but going through the schematics and my understanding of how U10 works this is normal (pin 13 on U10 goes high and then goes through an OR gate and then to the reset pin on Sally).

 

All power at major ICs seems to be appropriate via the testing done so far. I'm still going through the schematics and am starting to understand better the layout. I will do some more probing yet, test the logic chips, etc.

 

Is there a better (high-res) schematic for this somewhere? I have the one(s) off AtariAge printed out, but it is still fuzzy.

 

Thanks,

-Allen

 

Sent from my Pixel 2 using Tapatalk

[rasch_C]

I forgot to mention, this is an NTSC console w/o expansion port.

 

-Allen

[ChildOfCv]

How's the signal at U1 pins 3, 6, 7, and 8?

[rasch_C]

How's the signal at U1 pins 3, 6, 7, and 8?

Pins 3 & 8 of MARIA are steady at 1.246V & 4.92V respectively (0V w/ power button off). Pin 6 starts at ~1.939V and decays at rate of about 1mV every 3-4 seconds. This is also 0V w/ power button off. Pin 7 starts at ~0.745V and also decays at rate of about 1mV every 3-4 seconds. This is ~0.7V w/ power button off and shows similar decay rate.

 

W/ 2600 cart in, I checked LUM and Color lines from TIA and MARIA. TIA was showing ~21mV on most these lines (I believe LUM was lower if memory serves) and MARIA was showing ~15mV.

 

I think I'm going to check U3 next. I do not currently have a scope up.

 

Also, can anyone inform me of the pin numbers for the RF connector (J3) as I am not seeing labels for that on the board?

 

 

Thanks again,

-Allen

 

Sent from my Pixel 2 using Tapatalk

[rasch_C]

After looking more closely at my pic that I just uploaded in my last post, I noticed something strange. Some weird resistor is in place of the Q4 2N3904 transistor.

 

Board is CA025234- Does anyone know why someone would replace Q4 with a resistor? I have gone over the board and nothing else looks out of place. I will see if I have an equivalent transistor in my lab to replace.

 

Thanks,

-Allen

 

Sent from my Pixel 2 using Tapatalk

[rasch_C]

Forgot to add.... It measures 1ohm

 

Sent from my Pixel 2 using Tapatalk

[rasch_C]

I currently have the board in my Panavise now. Reason? Upon further inspection, it looks like 10 components have been replaced. I believe spots on the board are a mixture of resistors and capacitors, but they have all been replaced w/ resistors. This includes the aforementioned Q4 location.

 

I will start desoldering these components and then I'll be able to read the locations (why didn't they mark the board next to the component instead of underneath?) I do have a 2N3904 transistor so I should be good there.

 

Probably will stop for the night and doubtful I'll get to it tomorrow. Idk if maybe it was somebody's attempt at a mod that went wrong or what, but the best immediate course is to get it correct first.

 

-Allen

 

Sent from my Pixel 2 using Tapatalk

[ChildOfCv]

Well, the pins I'm asking about on U1 are timing signals, so you need to be in AC mode to read them. And that's assuming that your VOM is happy with reading voltages at that frequency. It's typically meant for reading wall power (60Hz), not MHz.

 

If you don't have an oscilloscope and don't plan to use one for more than reviving a 7800, you could buy a cheaper one that has at least 20MHz bandwidth. You could also buy something called a "logic probe" for pretty cheap. Something like this: https://www.amazon.com/Precision-DP21-Digital-Logic-Probe/dp/B009D6O4X6/ref=sr_1_5?s=industrial&ie=UTF8&qid=1547344674&sr=1-5&keywords=logic+probe It will give you colored lights for high/low and also an indication of oscillation. Handy to have for basic "yes or no" troubleshooting, and would tell you if the clock is indeed clocking.

 

As for the missing transistor, Q4 is just an amplifier for the clock generator. Even though the Rev A schematics they have show it there, it's possible that they decided that MARIA can work without the boost. In fact, R25 and R7 are missing too, and I'm guessing that C18 is the other missing component above that 74LS08 chip.

 

As for pin numbers on the RF box, it appears that the one on the left in your first picture is pin 1. It has a wide trace that runs to the RF shielding outline, as well as running under the row of resistors. That is a pretty strong indicator of ground connection.

 

It appears that J2 is just that double-row of connections there, with no pins soldered in to connect to. You could still use that as probe points. In fact, there is a composite video output that you could connect directly to your TV or VCR to see if you get anything before the RF. Most of those are obscured by J3 in the picture, but a VOM should easily let you figure out which one connects to RF pin 3.

 

Just saw your latest post. Are you sure those components are after the fact? In other words, can you tell that there has been rework done (ie residual flux or something?)

[ChildOfCv]

Oh BTW, one common failure that I've seen on both my ColecoVision and my Nintendo is that the output RF plug goes bad. Typically the plug part begins to separate from its mount. Then the repeated plugging in and removing the RF cable breaks the solder point of the inner plug contact on the board. Look at the board where the RF plug is, preferably with a magnifying glass, to see if the solder point has pulled away from the board.

[rasch_C]

Well, the pins I'm asking about on U1 are timing signals, so you need to be in AC mode to read them. And that's assuming that your VOM is happy with reading voltages at that frequency. It's typically meant for reading wall power (60Hz), not MHz.

 

If you don't have an oscilloscope and don't plan to use one for more than reviving a 7800, you could buy a cheaper one that has at least 20MHz bandwidth. You could also buy something called a "logic probe" for pretty cheap. Something like this: https://www.amazon.com/Precision-DP21-Digital-Logic-Probe/dp/B009D6O4X6/ref=sr_1_5?s=industrial&ie=UTF8&qid=1547344674&sr=1-5&keywords=logic+probe It will give you colored lights for high/low and also an indication of oscillation. Handy to have for basic "yes or no" troubleshooting, and would tell you if the clock is indeed clocking.

 

As for the missing transistor, Q4 is just an amplifier for the clock generator. Even though the Rev A schematics they have show it there, it's possible that they decided that MARIA can work without the boost. In fact, R25 and R7 are missing too, and I'm guessing that C18 is the other missing component above that 74LS08 chip.

 

As for pin numbers on the RF box, it appears that the one on the left in your first picture is pin 1. It has a wide trace that runs to the RF shielding outline, as well as running under the row of resistors. That is a pretty strong indicator of ground connection.

 

It appears that J2 is just that double-row of connections there, with no pins soldered in to connect to. You could still use that as probe points. In fact, there is a composite video output that you could connect directly to your TV or VCR to see if you get anything before the RF. Most of those are obscured by J3 in the picture, but a VOM should easily let you figure out which one connects to RF pin 3.

 

Just saw your latest post. Are you sure those components are after the fact? In other words, can you tell that there has been rework done (ie residual flux or something?)

That's good to know about the clock being A/C. I can try to retest those knowing that.

 

I have an Extech 430 meter. I do have an old scope that I've been meaning to recap and use to learn. I currently have it on my other bench in order to go through with the thought I could use it for this project. I thought I already bought the caps for it, but it appears not.

 

Yes, the components I am talking about are obvious reworks including the Q4 position. I'm going to start with the absolutely known reworks and go from there. There is also a resistor soldered directly to pin 21 on one of the RAM chips. I'm not sure where that goes or what it's for.

 

Thanks for your help,

-Allen

 

Sent from my Pixel 2 using Tapatalk

[ChildOfCv]

Wow.

 

Well, pin 21 is the write enable pin for the SRAM chip. U12, which is a 74LS08 74LS32, feeds the signal. It's possible that this chip went sort of bad and they used the resistor as a crutch. But it ought to be able to run both SRAM chips with no problem by itself.

Edited January 13, 2019 by ChildOfCv

[ChildOfCv]

Also, that Extech meter has a frequency mode. It can only read up to 10MHz, so the main clock is too fast at 14.xxx, but MARIA's clock outputs are divided by certain values to feed the TIA and CPU. So you should be able to get MHz readings on them in frequency mode, and if so, then that means the main clock must be working.

[rasch_C]

Fresh set of testing eyes today. Thank you ChildOfCv for turning me on to the freq. feature of my meter! Pin 3 of MARIA measures at 14.13Mhz -don't know how accurate/valid that is as that is outside the range of my meter. Pin 6 is 1.192Mhz. Pin 7 is 3.578Mhz. Pin 8 is 1.789Mhz.

 

Pin 3 of the RF modulator connector to the board measures ~3.308V. Grounding for RF modulator is good. Pin 3 to output (center) measures 7.52 kOhm, pin 2 to output measures 0.618 kOhm, and pin 4 to output measures 52.5 Ohm.

 

-Allen

 

Sent from my Pixel 2 using Tapatalk

[ChildOfCv]

The timings look reasonable. TIA expects a 3.58MHz clock on pin 11, which MARIA provides on pin 7. TIA outputs a 1.19MHz clock on pin 4, but nobody on the 7800 cares. This would be the CPU clock on a 2600. Instead, MARIA provides the CPU clock on pin 8, which is 1.78MHz in 7800 mode and 1.19MHz in 2600 mode. The CPU then creates a phase 2 clock signal that MARIA sees on pin 6 and TIA sees on pin 26, though I would expect it to match the CLK0 frequency. Maybe MARIA switched modes between your pin readings?

 

Unfortunately, pin 3 to output isn't a useful measurement. The RF box has a bunch of electronics inside that take the composite signal and modulate it to somewhere around 60MHz for transmission to the TV, depending on switch position for output channel. Unfortunately they left all those electronics out of the schematic. RF boxes are kinda hard to repair correctly, and most companies just tell you to replace the whole thing at once rather than trying to fix individual components inside. Is it at least possible to open the RF box up without removing the whole thing?

 

Does your VCR have a different look for "no signal" vs "blank screen"? Most VCRs I ever had to use showed blue when there was no signal on the channel. But that's the first test of a dead RF box. But if you have "something" rather than "nothing", you could try injecting a signal from a DVD player or something on pin 3 to see if it shows up on screen.

 

Oh, also, get a frequency reading on pin 3. I don't know what to actually expect since this won't be a simple timing signal, but the video composite signal. But anything significantly above zero shows that at least something is happening.

Edited January 13, 2019 by ChildOfCv

[+-^CrossBow^-]

After looking more closely at my pic that I just uploaded in my last post, I noticed something strange. Some weird resistor is in place of the Q4 2N3904 transistor.

 

Board is CA025234- Does anyone know why someone would replace Q4 with a resistor? I have gone over the board and nothing else looks out of place. I will see if I have an equivalent transistor in my lab to replace.

 

Thanks,

-Allen

 

Sent from my Pixel 2 using Tapatalk

 

This is quite normal and pretty much what all 7800s have in place of the second 3904 is this resistor after the about '86 or so. I see more 7800s with that removed and replaced with this resistor.

 

Also..what you are thinking to be resistors where you think should be caps? Those are caps..they are banded capacitors and Atari used them like made on all these older systems. I think it was to partly prevent reverse engineering if photos of the boards were taken since they would look like resistors butch actually weren't. Also that resistor you that saw bridged between the ram..

 

TOTALLY normal. I think every single 7800 that has passed by me (And I've seen a few...) all have that in place.

 

I do own an original '84 made unit that appears to have gone through factory rework with bodge resistors mounted on the bottom side of the PCB (not normal...).

[ChildOfCv]

So, a thought about the RF modulator. I haven't found any internal schematics, but from the number of pins I'm guessing it's similar to the 2600's RF box design. Pretty simple considering what it has to do. But by looking at that schematic, I see no way that you should have any continuity between the various pins (other than ground) and the output jack. There should be 100 ohms between the inner part of the plug and ground, and that's it (that is, assuming that this is closely based on the 2600 RF box). This would seem to indicate that an 8-legged pest or something has built a nest inside of there and created connections that shouldn't be there. If it's possible to remove the shielding, you should look at that.

[rasch_C]

Yes, I have still been working on this, but it's been cold here and the lab is in the garage which is a pain to get up to acceptable working temp w/ my kerosene heater. Also, I have been rebuilding stuck my laptop.

 

I would have to remove the RF box to gain access to it (cover is on the PCB side). I did try pin 3 of the RF-PCB connection to the composite video input of my PVM directly and only had brief interference show up from actual touching of the signal pin. PVM does not indicate lock of signal of any type. This is w/ and w/o cart. I also measured freq. of pin 3 and it varied between ~12.5kHz - ~13.5kHz.

 

I'll have to take pics of the reworked components and post. I do believe you guys are correct about Q4 and even the resistor on the RAM chip (didn't look like a rework to me, I was just curious on that one). There are a few components that have been reworked, however.

 

Thank you,

-Allen

[+-^CrossBow^-]

Take pics of the rework and I can tell you if it looks normal or not. Again, Atari looked to do a bunch of rework on the 7800s bitd and then resell them as new. That was the case with my original 7800 with expansion port but in a later '87 case shell and box that my folks bought new from Kaybee back in '87.

[ChildOfCv]

Yes, I have still been working on this, but it's been cold here and the lab is in the garage which is a pain to get up to acceptable working temp w/ my kerosene heater. Also, I have been rebuilding stuck my laptop.

 

I would have to remove the RF box to gain access to it (cover is on the PCB side). I did try pin 3 of the RF-PCB connection to the composite video input of my PVM directly and only had brief interference show up from actual touching of the signal pin. PVM does not indicate lock of signal of any type. This is w/ and w/o cart. I also measured freq. of pin 3 and it varied between ~12.5kHz - ~13.5kHz.

 

I'll have to take pics of the reworked components and post. I do believe you guys are correct about Q4 and even the resistor on the RAM chip (didn't look like a rework to me, I was just curious on that one). There are a few components that have been reworked, however.

 

Thank you,

-Allen

Well, horizontal frequency should be closer to 15.7KHz, but a generic meter may not get a perfect reading on a signal either--after all it's not exactly a sine wave. But since we're in the ball park, it sounds more and more like the RF modulator really is the culprit. I guess it was too much to hope for, that the composite signal be directly compatible with a TV. Actually that makes sense after thinking about it because there is such an impedance mismatch between the signal mixer and the TV. That's why the various AV mods include an impedance matching amplifier.

[rasch_C]

Well, horizontal frequency should be closer to 15.7KHz, but a generic meter may not get a perfect reading on a signal either--after all it's not exactly a sine wave. But since we're in the ball park, it sounds more and more like the RF modulator really is the culprit. I guess it was too much to hope for, that the composite signal be directly compatible with a TV. Actually that makes sense after thinking about it because there is such an impedance mismatch between the signal mixer and the TV. That's why the various AV mods include an impedance matching amplifier.

I was going to ask about the French of that. I was reading up today on the constriction of an NTSC composite signal and then though about my reading and wondered about the mismatch. So, what I should be reading at this location is in-fact the horizontal frequency?

 

My plan is to composite mod this. So, if the RF modulator is bad, I don't really care. I'm just hoping to verify a working console before modding.

 

-Allen

 

Sent from my Pixel 2 using Tapatalk