7800: what to check.

[astroguy]

I picked up a very dirty 7800 for relatively cheap that was listed as broken that came with it's power supply.  After a through cleaning the only obvious problem I found that was the front buttons where in very poor shape.  So I replaced all four with nice clicky switches and the console is now working -- hurrah. 

 

It's been a while since I've run RF to my TV, so maybe I'm spoiled by HDMI and I have just forgotten how poor RF is.  The picture okay, but I get moderate flicker where there is detail on the screen, especially with blue lines.   I don't see much in the way of interfere.   I've fiddled with TV controls to basically turn off as much processing as possible (auto tune, auto colour, autoblack, etc.)   So here are my naive questions:

 

1) Is the behaviour I'm describing normal?

2) Are the specific parts of the board I should do a closer inspection of?  I suspect it's been dormant for a while. 

3) The electrolytic appear to be okay.  No sign of leakage and no sign of corrosion.  Is there a symptom that indicates when the caps are poor.  I try not to desolder if I don't need to. 

 

Thanks! 

Edited February 24 by astroguy
typos

[astroguy]

just in case anyone is following along.  Playing with old video systems is new (and fun!) so I'm learning a lot as I go.

 

- I learned that the RF on my TV is much better than the one on my VCR.  That helped with the picture.

- Since the board was quite dirty there were signs of corrosion.  The 2n3904 transistor near CD4013B flipflop was hanging on by a thread and a slight touch broke off a leg.  So that was an easy swap.

- As I have everything out, I did a recap as well.  I found corrosion under the 2000uF cap.  So I guess it was good to do.   Interestingly after the recap the picture seems better, but maybe more related to the near-to-dead transistor.

 

Now I'm looking at the 7805 voltage regulator which also shows a bit of corrosion on the legs.  I'm guessing I should swap it as well with some new thermal paste.  Should I do a straight 7805 swap? I see some discussion of a drop-in DC-DC converter, but is a 1.5A 7805 good enough?

 

[+-^CrossBow^-]

3 hours ago, astroguy said:

Now I'm looking at the 7805 voltage regulator which also shows a bit of corrosion on the legs.  I'm guessing I should swap it as well with some new thermal paste.  Should I do a straight 7805 swap? I see some discussion of a drop-in DC-DC converter, but is a 1.5A 7805 good enough?

 

a 7805 that can provide 1.5A is more than enough for the 7800. Technically a 1A is enough but at the 7800 with a pokey audio based game in it will want about 720 - 750mA of current so the 1A is about the lowest output I would go. I actually replace the 7805s in the ones I service with 2A variants just because the price difference is pretty much non-existent and that way I only have to keep on type on hand. The console will only take as much as it needs anyway so the 2A output ones I use are WAY overkill. But again, the cost difference isn't enough for me to bother with keeping and user lower output variants on the console.

 

I also have a Traco 2-2450 DC-DC switching regulator in my 7800 and it has been solid for a couple years now. Not a cheap replacement as those DC-DC regulators can run about $15 shipped by itself from sources like Mouser or Digikey. It is also a 2A output variant hence the 2- at the beginning of the part number.

 

[+x=usr(1536)]

2 hours ago, -^CrossBow^- said:

I also have a Traco 2-2450 DC-DC switching regulator in my 7800 and it has been solid for a couple years now. Not a cheap replacement as those DC-DC regulators can run about $15 shipped by itself from sources like Mouser or Digikey.

 

I'll second the above.  Great upgrade, but not as cheap as a 7805.  It does run considerably more efficiently than a 7805, however, and puts out a ton less heat while consuming far less current.  More:

 

2 hours ago, -^CrossBow^- said:

It is also a 2A output variant hence the 2- at the beginning of the part number.

 

IMHO, this is the one to get, especially if using multicarts, installing video mods, running POKEYs, etc.  2A is definitely overkill even in these scenarios, but at least you'll have enough overhead for stable operation.

[astroguy]

Thank you both for the advice.  In Canada the 2-2450 is about $35 including shipping (!!).  I'll pick up one with my next larger order to offset the ($20) shipping cost.

[+-^CrossBow^-]

11 hours ago, x=usr(1536) said:

 

I'll second the above.  Great upgrade, but not as cheap as a 7805.  It does run considerably more efficiently than a 7805, however, and puts out a ton less heat while consuming far less current.  More:

@astroguy like X= stated in the quote above it is more efficient. My 7800s with DC-DC switchers in them only draw half as much current as their linear 7805 VRs siblings do. I've posted these before, but this is my bench power supply set to about 11.5v output (top meter) to simulate what the OEM PSUs provide to the 7800. The bottom meter is how much current is being drawn when the 7800 is powered on. In this test I was using a ballblazer cartridge running in demo mode.

 

7805 linear voltage regulator power draw = 720 - 750mA (Upper line scale reading)

 

Here is the same 7800 but with an older Traco 1-2450 (1A variant) installed to handle the voltage regulation. Again while running ballblazer. (320mA current)

 

Now I will state that since my bench supply is old and uses analog needles for indicators that I'm going off my best guess for the readings. But it still gives a good visual indication of the difference. One advantage to using a switching regulator like this in the consoles, besides them running cooler on the inside, is that you can then use smaller output power supplies since the console doesn't require as much to operate. I actually use a 1.2A output power supply to power both my DragonFly and the 7800 even though a 2A output is advised. That is because with the 7800 drawing half as much current as it would otherwise, I can use a lower output PSU and still have plenty of power to everything.

 

[astroguy]

That's a great demonstration.  I've added Traco 2-2450 to my growing project list for a future purchase. 

 

I did a test on the old capacitors, and they tested great (see attached photo).  Very interesting, but I'm  still glad I swapped them out, especially when finding some corrosion underneath the 2200uF cap. 

[fluxit]

3 hours ago, astroguy said:

That's a great demonstration.  I've added Traco 2-2450 to my growing project list for a future purchase. 

 

I did a test on the old capacitors, and they tested great (see attached photo).  Very interesting, but I'm  still glad I swapped them out, especially when finding some corrosion underneath the 2200uF cap. 

In my experience, that type of tester only has limited utility when testing old capacitors.  It's good for telling you if you have a completely defective new cap when populating a board.  Basically, I've had many times when the tester showed good results for capacity and ESR on a questionable cap that had been removed from the board, and when the cap was replaced with a new one, it fixed the problem.

[astroguy]

okay.. next problem. 

 

I can only get the system to start if I touch a probe to the base pin (2) of the transistor (2n3904) attached to Q11.  Nearby resistors, diodes and capacitors seems fine.

 

It's not clear what this part of the board is doing. 

[astroguy]

or.. I can touch pins 5 or 7 on the 6502 (so the R/W) and it will also boot and work.

[+x=usr(1536)]

Ooops; realised after posting that you had already replaced the switches.  My bad.

[astroguy]

Here's the matching part of the schematic. 

 

The Q12 signal looks a weird, so I'm gonna find another 2n3904 and swap it...

[astroguy]

one more note: I'm getting 5.3V on the main rail.  Changing the voltage regulator does not change that value.   Maybe voltage is too high?

[+-^CrossBow^-]

53 minutes ago, astroguy said:

one more note: I'm getting 5.3V on the main rail.  Changing the voltage regulator does not change that value.   Maybe voltage is too high?

Is that with a DC-DC switching regulator installed? Because I've only ever seen right at 5v with the ones I've installed and original linear style are typically around 4.9 - 5.03 on their output. What is the voltage input on the regulator?

 

[astroguy]

hmm.. I checked the voltage again with a multi-meter, and I see 4.94.    Looks like my bench oscilloscope needs to be calibrated.

 

power supply with no load is just over 13V.

Edited March 9 by astroguy
more info

[+-^CrossBow^-]

2 hours ago, astroguy said:

hmm.. I checked the voltage again with a multi-meter, and I see 4.94.    Looks like my bench oscilloscope needs to be calibrated.

 

power supply with no load is just over 13V.

Okay that sounds correct. what is the voltage at the ferrite bead show? That will be the input with a load on it.  And...wait I thought this console was essentially working previously as you were talking about video quality etc? What happened?

 

[astroguy]

11V at the ferrite bead with the system running.

 

The console seems to have broken from just inserting and removing cartridges.  So maybe flex on the board has broken an old solder joint?

 

I did a reflow of the cartridge pins and any other suspect spots, but no change in behaviour.  The R/W signal going into the 2n3904 at Q12 looks square, but comes out 'V' shaped, so I'm sourcing a new 2n3904.   I also reseated the 6502 and Maria chips.

 

Here's where I'm at:

- 2600 cart inserted: put probe on R/W pin, turn on power and game works

- 7800 cart inserted: put probe on R/W pin, turn on power and wait for bios/atari screen to finish, game works. 

 

Without the pin the R/W pin I see some initial address/data line activity but nothing loads. 

[astroguy]

I took a 2n3904 from another project and replaced Q12.  No difference, but I think I have a better understanding now.

 

The R/W line on Q12 is very noisy and distorted.   I thought maybe it was another bad transistor, but nope.  I'm thinking now that a capacitor must be tired.  That would explain the noise on the RW line and it may also explain why probing the RW makes the system work as it adds a small charge to get the system going.  To make things weirder, if I use a 7800 Gamedrive, the system just works.  This is usually how I play and I use cartridges mostly for testing or for games that don't work on the GameDrive and probably why I didn't notice until I swapped a cartridge.

 

Here are my observations:

- from a cold start:

  1) 2600 and 7800 games do not load unless I probe the RW line.

  2) 7800 GameDrive works

 

- from a warm start (so power down, swap cartridge)

  1) 2600 and 7800 games works

  2) If I wait maybe 30 seconds I go back to the 'cold' start scenario. 

 

This just seems like a capacitor issue to me.  Closest capacitor on the RW line is C63 which is listed as 0.01 uF Ceramic axial.  

Here's a link to what I think will work:

 

https://www.mouser.ca/ProductDetail/Vishay-BC-Components/A103K15X7RF5TAA?qs=l6VgrYIiCVRkew57SOIcsA%3D%3D

"Multilayer Ceramic Capacitors MLCC - Leaded Axial 0.01uF 50volts X7R +/-10%"

 

Is 10% tolerance fine? and I'm guessing 50V is also fine?

 

 

 

[astroguy]

So no luck.  I swapped out CR11, CR12 and CR13 diodes and the C63 capacitor with no changes in behaviour. 

 

The MARIA shows a lot of noise of different pins so I swapped the capacitors on the 6502, Maria, RAM, 2333 and TIA.  The noise is definitely reduced, but I still need to probe the R/W channel to get a game to load. 

 

As I said earlier, the 7800 GameDrive just works.  Maybe because it connects to the R/W?

[+x=usr(1536)]

9 hours ago, astroguy said:

So no luck.  I swapped out CR11, CR12 and CR13 diodes and the C63 capacitor with no changes in behaviour. 

 

The MARIA shows a lot of noise of different pins so I swapped the capacitors on the 6502, Maria, RAM, 2333 and TIA.  The noise is definitely reduced, but I still need to probe the R/W channel to get a game to load. 

 

As I said earlier, the 7800 GameDrive just works.  Maybe because it connects to the R/W?

 

Dumb question: have the pins on the sockets and cartridge port been reflowed?

 

I'm kinda grasping at straws here - corrosion on the R/W trace, perhaps?  Fingers in the cartridge slot out of alignment, oxidised, cracked, or no longer springy and making poor contact?

 

It's possible that the 7800GD's PCB may be just a fraction of a millimetre thicker than the 2600 / 7800 carts use, and it's making better contact with the cartridge port.

[astroguy]

1 hour ago, x=usr(1536) said:

Dumb question: have the pins on the sockets and cartridge port been reflowed?

 

I'm kinda grasping at straws here - corrosion on the R/W trace, perhaps?  Fingers in the cartridge slot out of alignment, oxidised, cracked, or no longer springy and making poor contact?

 

It's possible that the 7800GD's PCB may be just a fraction of a millimetre thicker than the 2600 / 7800 carts use, and it's making better contact with the cartridge port.

I have reflowed the cartridge port, but not the sockets as the current solder joints looked fine, but I might as well give em a touch.  I've traced out the entire R/W line with no disconnects.  I guess I should do the same for ground, especially for capacitor connections. 

 

I'm grasping as well.  I did a clean of the cartridge port, checked continuity of pins.  Really hard to check when a cartridge is inserted, maybe I need to stick in a pcb outside of the cartridge case to check connections.  hmm...

 

[+x=usr(1536)]

8 minutes ago, astroguy said:

I have reflowed the cartridge port, but not the sockets as the current solder joints looked fine, but I might as well give em a touch.  I've traced out the entire R/W line with no disconnects.  I guess I should do the same for ground, especially for capacitor connections.

 

Can't hurt.  At least they can be checked off the list if nothing else.

 

8 minutes ago, astroguy said:

I'm grasping as well.  I did a clean of the cartridge port, checked continuity of pins.  Really hard to check when a cartridge is inserted, maybe I need to stick in a pcb outside of the cartridge case to check connections.  hmm...

 

If you have a set of digital calipers (note: not endorsing that particular item, just using it for reference), take a reading of the various cartridge PCBs and check the variances they have.  Be sure to measure both the board thickness as well as the thickness at the fingers; there can be variance in the traces used.

[astroguy]

16 minutes ago, x=usr(1536) said:

If you have a set of digital calipers 

I do... good idea. 

[astroguy]

Some good news...

 

I traced the problem down to a failing logic chip, specifically the U12 74LS32.  So I

• chopped out the old one
• killed a via getting the legs out (oops! but these things happen)
• inserted a socket
• added a bodge and slotted in a new chip.. 

and voila the system is back to 100% working.  

 

Using a oscilloscope I noticed that the 2600/7800 switch logic was being very intermittent, which would resolve with the R/W line being probe.  So I followed the hunch and it worked.  I've tested lots of carts and so far everything is good.

 

Attached a pic of the current state:  new Caps. new power reg, new clicky buttons, new power-led, new 2N3904 transistors and now a new 74LS32.  (buttons, led, transistors and logic had all failed, rest are refreshes)

[+-^CrossBow^-]

@astroguy Excellent work! Although I've never had that particualr 7432 go bad on me before. I have had the other one just above the MARIA IC go bad before causing video output issues. Interesting panel buttons you have there. What are their exact specs?