7800 Sound Problem, bad RF module?

[Rust-O-Matic]

Hi fellas, new member here. Been reading these forums for quite a while, though. This place is a wealth of information!

 

I pulled my old 7800 out of storage recently and hooked it up directly to my tv via a known good cable and an RCA-to-F type adapter. I get video, but no sound except for loud static.

 

So, I opened up the unit, cleaned the cobwebs out, cleaned the corrosion off of the RF jack on the back, cleaned the cartridge slot and a few games really well with De-oxit, and tried again. Still no sound, just static. Next, I ran the RF cable through a VCR, then output to my TV via composite. Same result. So I dragged out my old CRT TV, and hooked it directly to it... same issue. Then, I tried a regular 2600 cartridge (instead of 7800), but still got the same result.

 

I did manage to hear a very faint trace of the opening music on Choplifter, but it was extremely quiet compared to the static noise on top of it. This leads me to believe maybe the board is still producing sound, but the RF modulator is messed up.

 

Any idea what my next line of attack should be? I am an engineering/computer technician, can solder, and am pretty handy. I'm not an electronics expert, but am tech savvy and can follow instruction. From what I have read, there doesn't appear to be any capacitors in the audio circuit that are known to die (like the 2600 had). Are there any other parts that would cause the audio issue I'm describing? Would doing a cheap ebay composite AV mod fix the issue?

 

I've read a ton of threads about sound issues, but can't seem to find a whole lot of info specific to the 7800. Any help is much appreciated.

[+-^CrossBow^-]

Really on two things come to mind. The fact you have picture and static audio, tells me that the RF modulator likely needs to be adjusted just slightly. You have to use a plastic hex adjustment tool for the coil though. There are also two poly caps that I suppose could cause something like this if they are bad. You could easily wire a separate RCA off the resistors from the audio but before the caps to verify this I believe.

[+SmittyB]

Have you tried simply fine tuning the signal on your TV? It might just need a very minor adjustment to get the audio back, also keep in mind that the frequency will drift as it warms up.

Edited October 4, 2017 by SmittyB

[Rust-O-Matic]

Really on two things come to mind. The fact you have picture and static audio, tells me that the RF modulator likely needs to be adjusted just slightly. You have to use a plastic hex adjustment tool for the coil though. There are also two poly caps that I suppose could cause something like this if they are bad. You could easily wire a separate RCA off the resistors from the audio but before the caps to verify this I believe.

Just to clarify, are you referring to the red cylindrical piece near the modulator, that looks like it's wrapped in copper wire? Sorry to sound like a dummy, but what is this piece exactly? I'd like to google more info on what this does and how to adjust it (or you can feel free to educate me), but I'm not exactly sure what term to search for.

 

As far as the poly caps, could you tell me where these are? I have a schematic, but I'm not great at following the circuits on it. I was under the impression, based on other posts, that the 7800 does not use filter caps like the older models. Do these serve some other purpose?

[Rust-O-Matic]

Have you tried simply fine tuning the signal on your TV? It might just need a very minor adjustment to get the audio back, also keep in mind that the frequency will drift as it warms up.

I should state first that one TV I tried is a new 65" LCD, and the other is a 90's CRT. I have no idea how to adjust the signal on the CRT, and I don't know if the LCD even has such an adjustment.

 

I presume (maybe wrongly, I don't know...) that since I got the same result out of both TVs, as well as when I used the VCR in between to demodulate the signal, that the problems lies within the Atari. Does that make sense?

 

Might also be worth mentioning that I played several games last night for a couple hours, so I presume everything was warmed up.

[+SmittyB]

The modulator has a hole in the side of it for adjustment, I think the red part is the colour adjustment.

The reason I mentioned the fine tuning is that most devices are likely to be expecting a signal within the typical range of the selected channel but the 7800 can drift outside of this, for example mine is half way between UK channels 36 and 37.

[Rust-O-Matic]

Based on this thread, it looks like the red cylinder is the adjustment I'm after:

 

http://atariage.com/forums/topic/40498-atari-7800-adjustments/

 

I'm going to take a closer look at it after work, and report back.

 

Thanks guys.

[+-^CrossBow^-]

The poly caps are side by side and found right under where the channel select switch is or just under that. Can't really miss them as they are the only two semi transparent axial caps there. But I do think the adjustment is needed. And I believe the RF modulator can also be adjusted further within it as well. But I was referring to the red coil by the cartridge port/connector.

[Trebor]

I'm going to take a closer look at it after work, and report back.

 

Here's a closer look, labelled to assist:

 

 

Adjustment on the RF modulator is for the channel tuner frequency, similar to the "fine tuning" knob on older CRT TVs.

The tall red one below the left side of the cartridge slot of is for adjusting the sound frequency.

The two similar-to-identical ones in the upper center of the board are for color adjustment. They increase and decrease the degrees delta between hue ranges for their respective system.

Typically delta between hue ranges is ~25.7 thru ~27.7 degrees. ~26.7 degrees after system warm up provides a full 256 color palette; however, many/most consoles warm up to ~27.7 degrees after being factory set @~25.7 degrees when the system is cold.

Excellent details and explanation regarding the 7800 color palette can be found at the 7800.8BitDev.org site under Color Documentation.

[Rust-O-Matic]

The poly caps are side by side and found right under where the channel select switch is or just under that. Can't really miss them as they are the only two semi transparent axial caps there. But I do think the adjustment is needed. And I believe the RF modulator can also be adjusted further within it as well. But I was referring to the red coil by the cartridge port/connector.

Here's a closer look, labelled to assist:

 

7800_MB_Adjustments.png

 

Adjustment on the RF modulator is for the channel tuner frequency, similar to the "fine tuning" knob on older CRT TVs.

 

The tall red one below the left side of the cartridge slot of is for adjusting the sound frequency.

 

The two similar-to-identical ones in the upper center of the board are for color adjustment. They increase and decrease the degrees delta between hue ranges for their respective system.

 

 

 

Typically delta between hue ranges is ~25.7 thru ~27.7 degrees. ~26.7 degrees after system warm up provides a full 256 color palette; however, many/most consoles warm up to ~27.7 degrees after being factory set @~25.7 degrees when the system is cold.

 

Excellent details and explanation regarding the 7800 color palette can be found at the 7800.8BitDev.org site under Color Documentation.

 

 

Couldn't ask for more clear responses. Thanks!

[Rust-O-Matic]

You guys are awesome! Adjusting the red audio piece (pot?) and fine tuning the one on the side of the modulator seems to have fixed the audio issue as well as slightly sharpened the image.

 

It might still be nice to upgrade the outputs to composite one of these days, but age and all other factors considered, it's working pretty darn good as-is. I'm super pumped.

[Arkeagood]

ive had issue with the red audio frequency cap as well, unfortunately every time i adjusted it, after 1-5 days it would come back pure static, i have had to fiddle with it so much that now it is partialy fixed, i have trust issue with a mod, but want to permanently fix it, any advice???

[Jinks]

Glad your RF is working! Use a expensive wire from your 7800 to a coax adapter and put sheilds on the ends and enjoy classic Atari the way it was meant to be played!

[ChildOfCv]

ive had issue with the red audio frequency cap as well, unfortunately every time i adjusted it, after 1-5 days it would come back pure static, i have had to fiddle with it so much that now it is partialy fixed, i have trust issue with a mod, but want to permanently fix it, any advice???

 

The sound is modulated by a voltage-controlled oscillator. So if the frequency is always wandering, that indicates a power issue.