7800 Composite Mod question on the 6.8k Pokey

[NISMOPC]

So I did the composite mode on my 7800 and everything worked,  but was curious about the 6.8k resistor used to balance volume between Pokey and TIA output.

 

I opted to remove the R5 and R6 resistors and bridge the bottom solder points as well as inline a 6.8k resistor and all is fine, but after looking at the schematics I noticed something intersting.

 

Appears the stock resistor in R5 is already a 6.8k (which I removed) and the stock resistor in R6 is a 18K (also removed).

 

Is there any reason I would want to have those resistor values on each feed instead of just a 6.8K on the TIA feed and nothing on the Pokey feed?

[+-^CrossBow^-]

The reason is that if you don't have the resistors in place, you will get unbalanced audio volume between the TIA and POKEY audio. Taking it from the bottom of the resistors especially R5 means that you are taking the audio before the resistors and will result in the TIA audio overpowering the POKEY. When I do these kinds of installs, I clip the legs of R5 and R6 facing the rear of the console and then tie the legs together with a 10µf cap off the tied leads out to the audio jacks.

 

If you leave the RF modulator in place and want it to work fully, then I actually solder in a second 6k8 and 18k resistors from the bottom legs of R5 and R6 and then tie the leads of the new resistors together followed by the filtering cap.

 

Also to answer your last question. If you only have a 6k8 off of R6 and nothing from the POKEY, you will still have the TIA being louder in volume as compared to the Pokey. In a game like Ballblazer you won't notice this because that game only uses Pokey. But in Commando or another game that uses both, you will notice that the volume of the TIA sounds are much louder than the pokey. If that doesn't bother you then I guess your fine. But for most, it drowns out the pokey to the point where it is hard to hear the background melodies etc. that the pokey is providing.

 

[NISMOPC]

25 minutes ago, -^CrossBow^- said:

Also to answer your last question. If you only have a 6k8 off of R6 and nothing from the POKEY, you will still have the TIA being louder in volume as compared to the Pokey. In a game like Ballblazer you won't notice this because that game only uses Pokey. But in Commando or another game that uses both, you will notice that the volume of the TIA sounds are much louder than the pokey. If that doesn't bother you then I guess your fine. But for most, it drowns out the pokey to the point where it is hard to hear the background melodies etc. that the pokey is providing.

 

The unit is back together now, so I can't recall 100% which feed I did, but my audio output is balanced well between TIA sounds and POKEY music on Commando. I'm going to take a look at it again tonight. 

[NISMOPC]

So, here are some pics. The 6.8k is inline on the wire coming from R6 under heat shrink. R5 is connected to R6 after the resistor and then both go to audio in on small board.

[desiv]

Good to know about the 6.8k value...  When I did mine quite a bit back, I noticed it was off so I grabbed a variable resistor I had, hooked it up, and moved it to get it about right.  I was planning on then taking it off and measuring it to see what I needed, and put in a regular resistor; but never got around to doing that...

 

Years later, that variable resistor is still in there... 

 

But one of these days.. 

Edited February 5, 2021 by desiv

[+-^CrossBow^-]

14 minutes ago, NISMOPC said:

So, here are some pics. The 6.8k is inline on the wire coming from R6 under heat shrink. R5 is connected to R6 after the resistor and then both go to audio in on small board.

So if I'm reading this right, you have a 6k8 from both R5 and R6? Because if you were take the signal after R5 closest to the rear of the console, then you wouldn't get any pokey at that point. Both of the audio inputs route from the bottom of the resistors up to the poly caps and then into the RF modulator section. 

 

Eitherway, with only 6k8 off the TIA that is half of what the original circuit was using to keep the volume on it down next to the pokey. Granted it isn't nearly as bad as just taking the audio from before they hit the resistors as that is nothing but pure TIA driving the signal at that point but still.

 

I actually use mil spec 1% resistors in the 7800 services I do. You have to read them though because they don't have color bands and are just brown colored resistor pills with their values printed on the sides.

 

[NISMOPC]

5 minutes ago, -^CrossBow^- said:

So if I'm reading this right, you have a 6k8 from both R5 and R6?

 

Not sure if you can tell in the pics, but the resistor is inline on the black wire from R6. The brown wire comes off of R5 then connects to the end of the 6.8K resistor before going to the small board. So no, the R5 & R6 are not bridged before the resistor. R5 bridges to R6 after the resistor.

 

Also, I am only knowledgeable enough to be dangerous, but here is my thinking of what is going on. 

 

Removing the 6.8k from Pokey and removing 18k from TIA, then only putting a 6.8K back on to the TIA would essentially net the same results wouldn't it? By not restricting the Pokey anymore and restricting the TIA approx .1/2 of what it originally was restricted to seems accurate to me??

Edited February 5, 2021 by NISMOPC

[+-^CrossBow^-]

19 minutes ago, NISMOPC said:

Removing the 6.8k from Pokey and removing 18k from TIA, then only putting a 6.8K back on to the TIA would essentially net the same results wouldn't it? By not restricting the Pokey anymore and restricting the TIA approx .1/2 of what it originally was restricted to seems accurate to me??

This is a valid point, but I believe the reasons for the resistors in the first place is that the audio without the resistors would be too hot otherwise and the resistors are there to lower the amplitude of the signal. But you are correct in your thinking. If you really wanted to test it, you could do what others usually do with doing this mod. They take the pokey from the top where C10 is (Removing C10 in the process). As that is literally just the first via from the external audio line off the cartridge slot.

 

[NISMOPC]

Since I bought a couple of these empty pcb boards and After reading your post about audio possibly being to hot, I tried a different approach. I did not install the 75k over ground/video this time. I installed 6.8k on the Pokey R6 and 18k on the TIA R5. Tested on my 20" CRT and then for shniggles on my game room 32" lcd. I don't notice much difference in sound and no difference in video, but did a cleaner install this time.

[NISMOPC]

Actually. The more I play, video looks much better on the CRT without the 75k resistor. Much crisper color. Think this is a winner!!! 

[+-^CrossBow^-]

18 hours ago, NISMOPC said:

Actually. The more I play, video looks much better on the CRT without the 75k resistor. Much crisper color. Think this is a winner!!! 

I have a few come to me with complaints of the picture being a too dark on LCDs as opposed to CRTs and removing the 75k off the video/gnd output has taken care of it. I understand why it is there, but it seems most displays seem to compensate without it fine and in many cases, better.

 

[marauder666]

It would be very dark with 75k, i assume you meant 75 ohm.

[NISMOPC]

19 hours ago, marauder666 said:

It would be very dark with 75k, i assume you meant 75 ohm.

Ha! Yes indeed. I can't edit the post...

 

It was 75Ω as can be seen in the original set-up I had. What a gooey mess. Glad I replaced whole unit with a new one and used thicker gauge wire as well.

 

[NISMOPC]

I'm all confused. It WAS a 75k Ω and did not interfere with the brightness at all. With and without, same brightness.