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7800: what to check.


astroguy

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1 hour ago, -^CrossBow^- said:

@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?

Here's a link to the buttons I used:
https://www.pishop.ca/product/colorful-square-tactile-button-switch-assortment-15-pack/

They are labelled as 'Omron B3F' and were purchased for a different project.  I really like them for the clicky feedback and the yellow really works even if I don't see the board very often.  I had to clip off the stem for the button caps to get the correct height. 

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47 minutes ago, Bratwurst said:

They're probably standard tactile switches with the square pegs clipped off, they come molded in yellow as well as black. I think the color denotes a different rating of pressure-force.

 

I got a bunch myself some time ago from Aliexpress: https://www.aliexpress.us/item/2251832701258721.html

Yes I use something similar but order mine direct from Mouser. I order some quality Alps branded ones as my new standard switch replacements but they cost more than the ones from Console5. I have some other Alps ones I've started to order recently that cost a bit more but have metal tops on them with a larger red colored actuator and rated for like 10million cycles.

 

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  • 2 weeks later...

I'm going to wrap up my repairs with one final saga. 

 

I was always seeing diagonal noise in the RF output no matter what I tried to cut out interference (cables, power, room location, thoughts and prayer, ...)  .  This lead me to rigging up a simple composite mode (the few resistors + cap + 2n3904).  Composite has significantly less random noise, but the diagonal banding persisted and because of the cleaner picture the banding, in my opinion, was much worse.  In the end, looks like I have a very noisy TIA, and specifically the Tsync from Pin 2. 

 

1) First problem is that when playing 7800 games the Tsync line would only decrease in amplitude to about 0.8V, which is right at the cusp of TTL logic and may causing problems with U3.  So I added a resister to drop the peak-to-peak down to 0.4 V, but still keep a valid Tsync range for 2600 games.

 

2) Second problem was a ~0.5Mhz ripple on Tsync when the signal is high and was polluting the 74LS32 (U3) signal.  After some experimentation, I found that tying a 10 nF cermanic cap to ground from Pin 3 of U3 filtered out a large portion of the noise.  

 

It was definitely a journey, but I'm quite happy with the result.  There is still some minor (very minor) flicker left that I'll likely tune out with additional caps (looking at Pins 6, 8 and 11 on U3).  This 7800 definitely has it's own character but I believe it's been tamed. 

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44 minutes ago, astroguy said:

I'm going to wrap up my repairs with one final saga. 

 

I was always seeing diagonal noise in the RF output no matter what I tried to cut out interference (cables, power, room location, thoughts and prayer, ...)  .  This lead me to rigging up a simple composite mode (the few resistors + cap + 2n3904).  Composite has significantly less random noise, but the diagonal banding persisted and because of the cleaner picture the banding, in my opinion, was much worse.  In the end, looks like I have a very noisy TIA, and specifically the Tsync from Pin 2. 

 

1) First problem is that when playing 7800 games the Tsync line would only decrease in amplitude to about 0.8V, which is right at the cusp of TTL logic and may causing problems with U3.  So I added a resister to drop the peak-to-peak down to 0.4 V, but still keep a valid Tsync range for 2600 games.

 

2) Second problem was a ~0.5Mhz ripple on Tsync when the signal is high and was polluting the 74LS32 (U3) signal.  After some experimentation, I found that tying a 10 nF cermanic cap to ground from Pin 3 of U3 filtered out a large portion of the noise.  

 

It was definitely a journey, but I'm quite happy with the result.  There is still some minor (very minor) flicker left that I'll likely tune out with additional caps (looking at Pins 6, 8 and 11 on U3).  This 7800 definitely has it's own character but I believe it's been tamed. 

Did you by chance install one of the DC-DC switching regulators into it? One thing I didn't mention, is that even though the Tracos I use are supposed to have filtering caps in them to reduce or prevent switching noise...it still causes the interference like you are talking about on RF output when I've installed it into 7800s. However, since those consoles also get a UAV, the switching noise doesn't show up on the composite and s-video output since the UAV is able to filter everything out using a separate encoder as it does. 

 

One thing that I've done in a few cases where I noticed it was more obvious than I would like is to add a 220 - 330µf 25v cap off the input and ground from the switching regulator. Just to try and smooth it out more. Not my idea, but it was something I noticed Tim Worthington included in his RGB kits for the 2600 and had as part of the required install process. This was because due to the extra load of the RGB boards, Tim included a dc-dc switching regulator to install to drop the current requirements of the 2600 so that even with the RGB board in place, the 2600 only required about 320mA of current. That meant that owners installing the kits didn't have to get beefier PSUs since the stock adapter with its 500mA output rating was still adequate.

 

Anyway, just something to maybe try and see if it helps if you did add one in. Sharp eyed folks might see one attached on the bottom of the main boards for pictures I've posted on some I've installed where the RF was still being used.

 

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I did swap to the DC-DC switching regulator.  I can certainly try another cap.  When you say to put a cap on the 'input' do you mean the 9V Input (pin 1) or the 5V to the system (Pin 3). 

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5 minutes ago, astroguy said:

I did swap to the DC-DC switching regulator.  I can certainly try another cap.  When you say to put a cap on the 'input' do you mean the 9V Input (pin 1) or the 5V to the system (Pin 3). 

I placed it on the input 9v side. The 5v is the output pin. So it was installed with the + lead on the 9v and the - lead on the center ground.

 

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Thanks for clarifying.  I did a quick test and adding a 220 µf cap made no difference.  This noise is something I remember being present even when using a 7805 regulator.

 

From the TIA, it's the Tsync pin that is strange.  There is always a ripple when the signal goes high. Rest of the pins look fine.

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3 hours ago, astroguy said:

Thanks for clarifying.  I did a quick test and adding a 220 µf cap made no difference.  This noise is something I remember being present even when using a 7805 regulator.

 

From the TIA, it's the Tsync pin that is strange.  There is always a ripple when the signal goes high. Rest of the pins look fine.

What make is the TIA out of curiosity?

 

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16 minutes ago, -^CrossBow^- said:

What make is the TIA out of curiosity?

AMI 8637MGJ

CO10444D-01

CO4075

Philippines

 

I have a picture of the board before comp mod posted one page back.

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